Human monocytic ehrlichiosis is an acute Lone Star tick-borne bacterial infection caused by the obligate intracellular bacterium Ehrlichia chaffeensis. The organism infects monocytes and macrophages, proliferates in cytoplasmic membrane-bound morulae, uses type IV secretion effectors to preserve its intracellular niche and acquire iron, and produces a febrile illness with thrombocytopenia, leukopenia, and elevated hepatic transaminases.
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name: Human Monocytic Ehrlichiosis
creation_date: "2026-09-25T10:29:22Z"
category: Infectious Disease
parents:
- Tick-borne disease
- Ehrlichiosis
synonyms:
- HME
- Human ehrlichiosis due to Ehrlichia chaffeensis
- Human ehrlichial infection, human monocytic type
description: >-
Human monocytic ehrlichiosis is an acute Lone Star tick-borne bacterial
infection caused by the obligate intracellular bacterium Ehrlichia
chaffeensis. The organism infects monocytes and macrophages, proliferates in
cytoplasmic membrane-bound morulae, uses type IV secretion effectors to
preserve its intracellular niche and acquire iron, and produces a febrile
illness with thrombocytopenia, leukopenia, and elevated hepatic
transaminases.
disease_term:
preferred_term: human monocytic ehrlichiosis
term:
id: MONDO:0000225
label: human monocytic ehrlichiosis
classifications:
harrisons_chapter:
- classification_value: INFECTIOUS_DISEASES
evidence:
- reference: PMID:39093857
reference_title: "Human monocytotropic ehrlichiosis-A systematic review and analysis of the literature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Human monocytotropic ehrlichiosis (HME) is a tick-borne bacterial
infection caused by Ehrlichia chaffeensis.
explanation: >-
The systematic review defines HME as a tick-borne E. chaffeensis
infection, supporting placement under Harrison's infectious-diseases
part.
pathophysiology:
- name: Amblyomma-Borne Ehrlichia Inoculation
role: trigger
biological_scale: ORGANISM
description: >-
Infected Amblyomma americanum ticks inoculate E. chaffeensis and initiate
the mononuclear-phagocyte infection that defines HME.
biological_processes:
- preferred_term: symbiont entry into host
term:
id: GO:0044409
label: symbiont entry into host
downstream:
- target: Mononuclear Phagocyte Ehrlichia Entry
causal_link_type: DIRECT
description: >-
Tick inoculation exposes monocytes and macrophages to E. chaffeensis
organisms capable of intracellular replication.
evidence:
- reference: PMID:12525424
reference_title: "Ehrlichia chaffeensis: a prototypical emerging pathogen."
supports: SUPPORT
evidence_source: OTHER
quote_role: REVIEW_SYNTHESIS
snippet: >-
Ehrlichia chaffeensis is an obligately intracellular, tick-transmitted
bacterium that is maintained in nature in a cycle involving at least one
and perhaps several vertebrate reservoir hosts.
explanation: >-
Review synthesis establishing E. chaffeensis as the tick-transmitted
obligate intracellular organism initiating HME.
- reference: PMID:12525424
reference_title: "Ehrlichia chaffeensis: a prototypical emerging pathogen."
supports: SUPPORT
evidence_source: OTHER
quote_role: REVIEW_SYNTHESIS
snippet: >-
This animal serves as a keystone host for all life stages of the principal
tick vector (Amblyomma americanum) and is perhaps the most important
vertebrate reservoir host for E. chaffeensis.
explanation: >-
Identifies Amblyomma americanum, the Lone Star tick, as the principal
tick vector of E. chaffeensis.
- name: Mononuclear Phagocyte Ehrlichia Entry
biological_scale: CELLULAR
description: >-
E. chaffeensis predominantly infects monocytes and tissue macrophages, then
multiplies in cytoplasmic membrane-bound vacuoles that contain clusters of
bacteria called morulae.
cell_types:
- preferred_term: monocyte
term:
id: CL:0000576
label: monocyte
- preferred_term: macrophage
term:
id: CL:0000235
label: macrophage
biological_processes:
- preferred_term: symbiont entry into host cell
term:
id: GO:0046718
label: symbiont entry into host cell
downstream:
- target: Ehrlichia Vacuolar Niche
causal_link_type: DIRECT
description: >-
Entry into the mononuclear-phagocyte lineage creates a host cell
compartment in which the organism can preserve an early-endosome-like
vacuole.
evidence:
- reference: PMID:27172113
reference_title: "Diagnosis and Management of Tickborne Rickettsial Diseases: Rocky Mountain Spotted Fever and Other Spotted Fever Group Rickettsioses, Ehrlichioses, and Anaplasmosis - United States."
supports: SUPPORT
evidence_source: OTHER
snippet: >-
E. chaffeensis, the pathogen that causes human monocytic ehrlichiosis,
predominantly infects monocytes and tissue macrophages
explanation: >-
CDC national recommendations identify monocytes and macrophages as the
predominant E. chaffeensis host cells. Evidence source is OTHER because
this is a national recommendations report.
- name: Ehrlichia Vacuolar Niche
biological_scale: CELLULAR
role: intrinsic_resistance
conforms_to: >-
intracellular_pathogen_persistence#Intracellular Niche and Beta-Lactam
Exclusion
description: >-
E. chaffeensis replicates in membrane-bound vacuoles that resemble early
endosomes, sequestering the organism inside host phagocytes where it can
deploy type IV secretion effectors.
cell_types:
- preferred_term: monocyte
term:
id: CL:0000576
label: monocyte
- preferred_term: macrophage
term:
id: CL:0000235
label: macrophage
biological_processes:
- preferred_term: biological process involved in interaction with host
term:
id: GO:0051701
label: biological process involved in interaction with host
downstream:
- target: Etf-2 RAB5 Vacuole Maturation Arrest
causal_link_type: DIRECT
description: >-
Vacuolar E. chaffeensis delivers Etf-2 to RAB5-GTP on the
organism-containing vacuole.
- target: Etf-3 Ferritinophagy Iron Acquisition
causal_link_type: DIRECT
description: >-
Vacuolar infection delivers Etf-3, which redirects host ferritin to
ferritinophagy.
- target: Ehrlichia Ribosomal Translation (Doxycycline Target)
description: >-
Intracellular E. chaffeensis remains dependent on bacterial ribosomal
translation.
evidence:
- reference: PMID:27172113
reference_title: "Diagnosis and Management of Tickborne Rickettsial Diseases: Rocky Mountain Spotted Fever and Other Spotted Fever Group Rickettsioses, Ehrlichioses, and Anaplasmosis - United States."
supports: SUPPORT
evidence_source: OTHER
snippet: >-
The organisms multiply in cytoplasmic membrane-bound vacuoles, forming
tightly packed clusters of bacteria called morulae.
explanation: >-
CDC national recommendations describe E. chaffeensis multiplication in
membrane-bound cytoplasmic morulae. Evidence source is OTHER because this
is a national recommendations report.
- reference: PMID:40797321
reference_title: "Development of mRNA-lipid nanoparticle intrabodies against rickettsial infection."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: >-
E. chaffeensis replicates in membrane-bound vacuoles resembling early
endosomes in human monocytes/macrophages.
explanation: >-
Establishes the early-endosome-like vacuole as the replicative
compartment within human monocytes and macrophages.
- name: Etf-2 RAB5 Vacuole Maturation Arrest
biological_scale: CELLULAR
description: >-
The type IV secretion effector Etf-2 binds RAB5-GTP on
E. chaffeensis-containing vacuoles and delays their maturation into late
endosomes, preserving the replicative niche.
biological_processes:
- preferred_term: symbiont-mediated suppression of host phagosome maturation
modifier: INCREASED
term:
id: GO:0141158
label: symbiont-mediated suppression of host phagosome maturation
downstream:
- target: Mononuclear Ehrlichia Persistence
causal_link_type: DIRECT
description: >-
Blocking RAB5-dependent vacuole maturation helps intracellular
E. chaffeensis avoid progression toward a degradative compartment.
evidence:
- reference: PMID:40797321
reference_title: "Development of mRNA-lipid nanoparticle intrabodies against rickettsial infection."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: >-
The type IV secretion system effector Ehrlichia translocated factor-2
(Etf-2) directly binds to RAB5-GTP on E. chaffeensis-containing vacuoles.
Consequently, Etf-2 hinders the engagement of RAB5 GTPase-activating
protein with RAB5-GTP, delays maturation of Ehrlichia vacuoles to late
endosomes, thus facilitates infection.
explanation: >-
Mechanistically links the Etf-2 effector to RAB5-GTP binding and delayed
maturation of E. chaffeensis vacuoles.
- name: Etf-3 Ferritinophagy Iron Acquisition
biological_scale: CELLULAR
description: >-
The Etf-3 type IV secretion effector binds host ferritin light chain and
induces ferritinophagy, increasing the labile iron pool available for
intracellular ehrlichial proliferation.
biological_processes:
- preferred_term: ferritinophagy
modifier: INCREASED
term:
id: GO:7770069
label: ferritinophagy
downstream:
- target: Mononuclear Ehrlichia Persistence
causal_link_type: DIRECT
description: >-
Ferritinophagy-derived labile iron supports intracellular Ehrlichia
proliferation.
evidence:
- reference: PMID:39705831
reference_title: "Development of Etf-3-specific nanobodies to prevent Ehrlichia infection and LNP-mRNA delivery in cellular and murine models."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: >-
Ehrlichia translocated factor-3 (Etf-3) is a type IV secretion system
effector that binds host-cell ferritin light chain and induces
ferritinophagy, thus increasing cellular labile iron pool for Ehrlichia
proliferation.
explanation: >-
Establishes the Etf-3 ferritinophagy effector pathway that expands
cytosolic labile iron for ehrlichial growth.
- name: Mononuclear Ehrlichia Persistence
biological_scale: CELLULAR
description: >-
Effector-mediated preservation of the early-endosome-like vacuole and
diversion of ferritin iron allow E. chaffeensis to persist and amplify
inside infected monocytes and macrophages.
cell_types:
- preferred_term: monocyte
term:
id: CL:0000576
label: monocyte
- preferred_term: macrophage
term:
id: CL:0000235
label: macrophage
biological_processes:
- preferred_term: biological process involved in interaction with host
modifier: INCREASED
term:
id: GO:0051701
label: biological process involved in interaction with host
downstream:
- target: TNF-Neutrophil Immunopathology
causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
description: >-
Persistent monocytotropic infection elicits T-cell and neutrophil
inflammatory responses that can become injurious in severe disease.
- target: Acute Febrile Cytopenic HME
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
description: >-
Intracellular replication and inflammatory host responses produce the
characteristic fever, cytopenias, and elevated hepatic transaminases.
evidence:
- reference: PMID:39705831
reference_title: "Development of Etf-3-specific nanobodies to prevent Ehrlichia infection and LNP-mRNA delivery in cellular and murine models."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: >-
Ehrlichia chaffeensis is an obligatory intracellular bacterium that
infects monocytes and macrophages and causes human monocytic
ehrlichiosis.
explanation: >-
Connects E. chaffeensis intracellular infection of monocytes/macrophages
to the human disease.
- name: TNF-Neutrophil Immunopathology
biological_scale: CELLULAR
description: >-
In fatal monocytotropic ehrlichiosis models, excessive TNF-alpha-producing
CD8 T-cell responses and neutrophils amplify immune-mediated liver injury
and toxic-shock-like disease.
biological_processes:
- preferred_term: tumor necrosis factor production
modifier: INCREASED
term:
id: GO:0032640
label: tumor necrosis factor production
downstream:
- target: Acute Febrile Cytopenic HME
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
description: >-
Dysregulated TNF-rich and neutrophil-amplified inflammation contributes to
the severe systemic inflammatory arm of HME.
evidence:
- reference: PMID:14734762
reference_title: "Overproduction of TNF-alpha by CD8+ type 1 cells and down-regulation of IFN-gamma production by CD4+ Th1 cells contribute to toxic shock-like syndrome in an animal model of fatal monocytotropic ehrlichiosis."
supports: SUPPORT
evidence_source: MODEL_ORGANISM
snippet: >-
In conclusion, uncontrolled TNF-alpha production by CD8(+) T cells
together with a weak CD4(+) Th1 cell response are associated with
immunopathology and failure to clear IOE in the fatal model of HME.
explanation: >-
Mouse-model evidence links fatal monocytotropic ehrlichiosis to
dysregulated TNF-alpha-producing CD8 T-cell inflammation.
- reference: PMID:23478316
reference_title: "Neutrophils mediate immunopathology and negatively regulate protective immune responses during fatal bacterial infection-induced toxic shock."
supports: SUPPORT
evidence_source: MODEL_ORGANISM
snippet: >-
depletion of neutrophils from lethally infected mice enhanced bacterial
elimination, decreased immune-mediated pathology, and prolonged survival.
explanation: >-
Mouse-model evidence identifies neutrophils as amplifiers of
immune-mediated pathology during fatal Ehrlichia infection.
- name: Acute Febrile Cytopenic HME
biological_scale: ORGANISM
role: outcome
description: >-
The organism-level syndrome presents as an undifferentiated fever with
thrombocytopenia, leukopenia, and elevated hepatic transaminases; severe
cases may progress to ARDS, acute renal failure, multi-organ failure, or
secondary hemophagocytic lymphohistiocytosis.
downstream:
- target: Fever
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Thrombocytopenia
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Leukopenia
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Elevated Hepatic Transaminases
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Headache
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Myalgia
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Skin rash
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Acute Respiratory Distress Syndrome
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Acute kidney injury
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Multi-organ failure
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Hemophagocytosis
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Lymphopenia
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Hyponatremia
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Anemia
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Vomiting
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Diarrhea
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Meningoencephalitis
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
evidence:
- reference: PMID:39093857
reference_title: "Human monocytotropic ehrlichiosis-A systematic review and analysis of the literature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
HME primarily presents as an unspecific febrile illness (95% of the
cases), often accompanied by thrombocytopenia (79.1% of the cases),
leukopenia (57.8% of the cases), and abnormal liver function tests (68.1%
of the cases).
explanation: >-
Pooled human systematic-review data identify the febrile, cytopenic, and
hepatic-laboratory syndrome that characterizes HME.
- name: Ehrlichia Ribosomal Translation (Doxycycline Target)
biological_scale: MOLECULAR
role: therapeutic_vulnerability
conforms_to: >-
bacterial_protein_synthesis_inhibition#Bacterial mRNA Translation by the
Ribosome
description: >-
E. chaffeensis depends on bacterial 70S-ribosome translation; doxycycline
exploits this vulnerability by binding the 30S ribosomal subunit and
arresting bacterial protein synthesis.
biological_processes:
- preferred_term: translation
term:
id: GO:0006412
label: translation
evidence:
- reference: PMID:24336183
reference_title: "Ribosome-targeting antibiotics and mechanisms of bacterial resistance."
supports: SUPPORT
evidence_source: OTHER
quote_role: REVIEW_SYNTHESIS
snippet: >-
The ribosome is one of the main antibiotic targets in the bacterial cell.
explanation: >-
Review synthesis of the bacterial ribosome as an antibiotic target,
supporting this conserved therapeutic-vulnerability node.
- name: Cell-Penetrant Antimicrobial Requirement
biological_scale: CELLULAR
role: therapeutic_vulnerability
conforms_to: >-
intracellular_pathogen_persistence#Requirement for Cell-Penetrant
Antimicrobials
description: >-
Because E. chaffeensis replicates inside monocytes and macrophages, HME
treatment must use agents that achieve intracellular concentrations rather
than poorly cell-penetrant antibiotics.
biological_processes:
- preferred_term: response to antibiotic
term:
id: GO:0046677
label: response to antibiotic
evidence:
- reference: PMID:28639230
reference_title: "Intracellular Pharmacokinetics of Antibacterials and Their Clinical Implications."
supports: SUPPORT
evidence_source: OTHER
quote_role: REVIEW_SYNTHESIS
snippet: >-
Therapeutic efficacy against intracellular pathogens has been correlated
mainly with the intracellular concentrations achieved by the different
antimicrobial agents.
explanation: >-
Review synthesis establishing intracellular drug accumulation as the
pharmacologic requirement this HME node conforms to.
phenotypes:
- name: Fever
description: >-
Fever is the dominant presentation of HME, reported in 95% of reviewed
cases.
frequency: VERY_FREQUENT
phenotype_term:
preferred_term: Fever
term:
id: HP:0001945
label: Fever
evidence:
- reference: PMID:39093857
reference_title: "Human monocytotropic ehrlichiosis-A systematic review and analysis of the literature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
HME primarily presents as an unspecific febrile illness (95% of the
cases), often accompanied by thrombocytopenia (79.1% of the cases),
leukopenia (57.8% of the cases), and abnormal liver function tests (68.1%
of the cases).
explanation: >-
The systematic review found fever in 95% of HME cases, supporting a
very-frequent fever phenotype.
- name: Thrombocytopenia
description: >-
Thrombocytopenia is frequent in HME and was reported in 79.1% of reviewed
cases.
frequency: FREQUENT
phenotype_term:
preferred_term: Thrombocytopenia
term:
id: HP:0001873
label: Thrombocytopenia
evidence:
- reference: PMID:39093857
reference_title: "Human monocytotropic ehrlichiosis-A systematic review and analysis of the literature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
HME primarily presents as an unspecific febrile illness (95% of the
cases), often accompanied by thrombocytopenia (79.1% of the cases),
leukopenia (57.8% of the cases), and abnormal liver function tests (68.1%
of the cases).
explanation: >-
The systematic review found thrombocytopenia in 79.1% of HME cases,
just below the 80% very-frequent threshold.
- name: Leukopenia
description: >-
Leukopenia is frequent in HME and was reported in 57.8% of reviewed cases.
frequency: FREQUENT
phenotype_term:
preferred_term: Leukopenia
term:
id: HP:0001882
label: Decreased total leukocyte count
evidence:
- reference: PMID:39093857
reference_title: "Human monocytotropic ehrlichiosis-A systematic review and analysis of the literature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
HME primarily presents as an unspecific febrile illness (95% of the
cases), often accompanied by thrombocytopenia (79.1% of the cases),
leukopenia (57.8% of the cases), and abnormal liver function tests (68.1%
of the cases).
explanation: >-
The systematic review found leukopenia in 57.8% of HME cases, within the
30-79% frequent band.
- name: Elevated Hepatic Transaminases
description: >-
Abnormal liver function tests are frequent in HME; the HPO binding captures
the common elevated-transaminase manifestation of this laboratory
abnormality.
frequency: FREQUENT
phenotype_term:
preferred_term: Elevated hepatic transaminases
term:
id: HP:0002910
label: Elevated circulating hepatic transaminase concentration
evidence:
- reference: PMID:39093857
reference_title: "Human monocytotropic ehrlichiosis-A systematic review and analysis of the literature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
HME primarily presents as an unspecific febrile illness (95% of the
cases), often accompanied by thrombocytopenia (79.1% of the cases),
leukopenia (57.8% of the cases), and abnormal liver function tests (68.1%
of the cases).
explanation: >-
The systematic review found abnormal liver function tests in 68.1% of HME
cases; elevated circulating hepatic transaminases are the HPO-bound
laboratory abnormality represented by that clinical phrasing.
- name: Acute Respiratory Distress Syndrome
description: >-
ARDS is an occasional severe complication of HME, with higher reported
incidence in immunocompromised than immunocompetent patients.
frequency: OCCASIONAL
phenotype_term:
preferred_term: Acute respiratory distress syndrome
term:
id: HP:0033677
label: Acute respiratory distress syndrome
evidence:
- reference: PMID:39093857
reference_title: "Human monocytotropic ehrlichiosis-A systematic review and analysis of the literature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
The incidence of complications is higher in immunocompromized compared to
immunocompetent cases, with ARDS (34% vs 19.8%), acute renal failure (34%
vs 15.8%), multi organ failure (26% vs 14.9%), and secondary
hemophagocytic lymphohistiocytosis (26% vs 14.9%) being the most frequent
reported.
explanation: >-
The systematic review identified ARDS as a recurrent HME complication,
with a higher proportion among immunocompromised reviewed cases.
- name: Hemophagocytosis
description: >-
HME can trigger secondary hemophagocytic lymphohistiocytosis, particularly
among severe reported cases.
frequency: OCCASIONAL
phenotype_term:
preferred_term: Hemophagocytosis
term:
id: HP:0012156
label: Hemophagocytosis
evidence:
- reference: PMID:39093857
reference_title: "Human monocytotropic ehrlichiosis-A systematic review and analysis of the literature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
The incidence of complications is higher in immunocompromized compared to
immunocompetent cases, with ARDS (34% vs 19.8%), acute renal failure (34%
vs 15.8%), multi organ failure (26% vs 14.9%), and secondary
hemophagocytic lymphohistiocytosis (26% vs 14.9%) being the most frequent
reported.
explanation: >-
The systematic review identified secondary HLH as a recurrent severe HME
complication; hemophagocytosis is the HPO histologic process represented
by that syndrome.
- name: Headache
description: Headache is a common presenting symptom of HME.
frequency: FREQUENT
phenotype_term:
preferred_term: Headache
term:
id: HP:0002315
label: Headache
evidence:
- reference: PMID:35986240
reference_title: "\"Leopards do not change their spots:\" tick borne disease symptomology case report."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
quote_role: BACKGROUND
snippet: "Patients infected with Human Monocytic Ehrlichiosis often present with symptoms including fever, headache, myalgia, and occasionally a macular rash."
explanation: HME commonly presents with headache among its symptom complex.
- name: Myalgia
description: Myalgia is a common presenting symptom of HME.
frequency: FREQUENT
phenotype_term:
preferred_term: Myalgia
term:
id: HP:0003326
label: Myalgia
evidence:
- reference: PMID:35986240
reference_title: "\"Leopards do not change their spots:\" tick borne disease symptomology case report."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
quote_role: BACKGROUND
snippet: "Patients infected with Human Monocytic Ehrlichiosis often present with symptoms including fever, headache, myalgia, and occasionally a macular rash."
explanation: HME commonly presents with myalgia among its symptom complex.
- name: Skin rash
description: A macular rash occurs in a substantial minority of HME cases and helps distinguish it from anaplasmosis.
frequency: OCCASIONAL
phenotype_term:
preferred_term: Skin rash
term:
id: HP:0000988
label: Skin rash
evidence:
- reference: PMID:35986240
reference_title: "\"Leopards do not change their spots:\" tick borne disease symptomology case report."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
quote_role: BACKGROUND
snippet: "Patients infected with Human Monocytic Ehrlichiosis often present with symptoms including fever, headache, myalgia, and occasionally a macular rash."
explanation: HME can present with a macular rash.
- reference: PMID:9200384
reference_title: Human monocytic ehrlichiosis in children.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "fever (100%), rash (67%), myalgias (58%)"
explanation: >-
In a 12-patient pediatric HME case series, rash was reported in 67% of
children, supporting the point that rash is more common in pediatric than
adult disease.
- name: Acute kidney injury
description: Acute renal failure is a severe HME complication, more frequent in immunocompromised patients.
frequency: OCCASIONAL
phenotype_term:
preferred_term: Acute kidney injury
term:
id: HP:0001919
label: Acute kidney injury
evidence:
- reference: PMID:39093857
reference_title: "Human monocytotropic ehrlichiosis-A systematic review and analysis of the literature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
The incidence of complications is higher in immunocompromized compared to
immunocompetent cases, with ARDS (34% vs 19.8%), acute renal failure (34%
vs 15.8%), multi organ failure (26% vs 14.9%), and secondary
hemophagocytic lymphohistiocytosis (26% vs 14.9%) being the most frequent
reported.
explanation: The systematic review lists acute renal failure among the most frequent HME complications.
- name: Multi-organ failure
description: Multi-organ failure is a severe HME complication, more frequent in immunocompromised patients.
frequency: OCCASIONAL
phenotype_term:
preferred_term: Multi-organ failure
evidence:
- reference: PMID:39093857
reference_title: "Human monocytotropic ehrlichiosis-A systematic review and analysis of the literature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
The incidence of complications is higher in immunocompromized compared to
immunocompetent cases, with ARDS (34% vs 19.8%), acute renal failure (34%
vs 15.8%), multi organ failure (26% vs 14.9%), and secondary
hemophagocytic lymphohistiocytosis (26% vs 14.9%) being the most frequent
reported.
explanation: The systematic review lists multi-organ failure among the most frequent HME complications; no HPO term exists for this outcome, so no term is bound.
- name: Lymphopenia
description: >-
Lymphopenia is a laboratory finding of HME; in a pediatric HME case series
it was present in 75% of children on initial examination.
phenotype_term:
preferred_term: Lymphopenia
term:
id: HP:0001888
label: Decreased total lymphocyte count
evidence:
- reference: PMID:9200384
reference_title: Human monocytic ehrlichiosis in children.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "lymphopenia (75%), hyponatremia (67%), leukopenia (58%), and anemia (42%)"
explanation: The pediatric HME case series reported lymphopenia in 75% of children on initial laboratory examination.
- name: Hyponatremia
description: >-
Hyponatremia is a laboratory finding of HME; in a pediatric HME case series
it was present in 67% of children on initial examination.
phenotype_term:
preferred_term: Hyponatremia
term:
id: HP:0002902
label: Hyponatremia
evidence:
- reference: PMID:9200384
reference_title: Human monocytic ehrlichiosis in children.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "lymphopenia (75%), hyponatremia (67%), leukopenia (58%), and anemia (42%)"
explanation: The pediatric HME case series reported hyponatremia in 67% of children on initial laboratory examination.
- name: Anemia
description: >-
Anemia is a laboratory finding of HME; in a pediatric HME case series it was
present in 42% of children on initial examination.
phenotype_term:
preferred_term: Anemia
term:
id: HP:0001903
label: Anemia
evidence:
- reference: PMID:9200384
reference_title: Human monocytic ehrlichiosis in children.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "lymphopenia (75%), hyponatremia (67%), leukopenia (58%), and anemia (42%)"
explanation: The pediatric HME case series reported anemia in 42% of children on initial laboratory examination.
- name: Vomiting
description: >-
Gastrointestinal symptoms occur in HME; vomiting was reported in a pediatric
HME case series.
phenotype_term:
preferred_term: Vomiting
term:
id: HP:0002013
label: Vomiting
evidence:
- reference: PMID:9200384
reference_title: Human monocytic ehrlichiosis in children.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "vomiting, diarrhea, and headache (25%)"
explanation: The pediatric HME case series grouped vomiting with diarrhea and headache at 25% of children.
- name: Diarrhea
description: >-
Gastrointestinal symptoms occur in HME; diarrhea was reported in a pediatric
HME case series.
phenotype_term:
preferred_term: Diarrhea
term:
id: HP:0002014
label: Diarrhea
evidence:
- reference: PMID:9200384
reference_title: Human monocytic ehrlichiosis in children.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "vomiting, diarrhea, and headache (25%)"
explanation: The pediatric HME case series grouped diarrhea with vomiting and headache at 25% of children.
- name: Meningoencephalitis
description: >-
Central nervous system involvement occurs in HME, ranging from headache to
meningoencephalitis; the HPO binding captures the infectious encephalitic
end of that spectrum.
phenotype_term:
preferred_term: Ehrlichial meningoencephalitis
term:
id: HP:0002383
label: Infectious encephalitis
evidence:
- reference: PMID:16569376
reference_title: Ehrlichia infection of the central nervous system.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
quote_role: REVIEW_SYNTHESIS
snippet: "Ehrlichiosis can present with a spectrum of neurologic manifestations, ranging in severity from headache to meningoencephalitis"
explanation: The CNS-ehrlichiosis review synthesizes the human neurologic spectrum, up to meningoencephalitis, that this phenotype records.
infectious_agent:
- name: Ehrlichia chaffeensis
description: >-
Obligate intracellular Anaplasmataceae bacterium with tropism for human
monocytes and macrophages.
infectious_agent_term:
preferred_term: Ehrlichia chaffeensis
term:
id: NCBITaxon:945
label: Ehrlichia chaffeensis
evidence:
- reference: PMID:39093857
reference_title: "Human monocytotropic ehrlichiosis-A systematic review and analysis of the literature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Human monocytotropic ehrlichiosis (HME) is a tick-borne bacterial
infection caused by Ehrlichia chaffeensis.
explanation: >-
Establishes E. chaffeensis as the bacterial agent of human HME.
transmission:
- name: Amblyomma Tick Bite Transmission
description: >-
HME is transmitted by the bite of infected Lone Star ticks, Amblyomma
americanum.
evidence:
- reference: PMID:12525424
reference_title: "Ehrlichia chaffeensis: a prototypical emerging pathogen."
supports: SUPPORT
evidence_source: OTHER
snippet: >-
This animal serves as a keystone host for all life stages of the principal
tick vector (Amblyomma americanum) and is perhaps the most important
vertebrate reservoir host for E. chaffeensis.
explanation: >-
Review evidence identifies Amblyomma americanum as the principal vector of
E. chaffeensis. Evidence source is OTHER because this is a review.
- name: Transfusion and Transplant Transmission
description: >-
E. chaffeensis can also be transmitted through blood transfusion or organ
transplantation, including donor-derived infection.
evidence:
- reference: PMID:34670661
reference_title: Ehrlichiosis and Anaplasmosis among Transfusion and Transplant Recipients in the United States.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "We identified 132 cases during 1997-2020, 12 transfusion-associated cases and 120 cases in transplant recipients; 8 cases were donor-derived"
explanation: The US review documents transfusion- and transplant-associated ehrlichiosis/anaplasmosis, including donor-derived cases.
diagnosis:
- name: Ehrlichiosis Laboratory Confirmation
description: >-
Suspected HME should be treated empirically while etiologic confirmation is
pursued by methods such as blood PCR, smear examination, culture, or
serologic testing.
evidence:
- reference: PMID:17582569
reference_title: "Ehrlichioses in humans: epidemiology, clinical presentation, diagnosis, and treatment."
supports: SUPPORT
evidence_source: OTHER
quote_role: REVIEW_SYNTHESIS
snippet: >-
Once an ehrlichiosis is suspected on historical and clinical grounds,
doxycycline treatment should be initiated concurrently with attempts at
etiologic confirmation using laboratory methods such as blood smear
examination, polymerase chain reaction, culture, and serologic tests.
explanation: >-
Review synthesis supporting PCR, smear, culture, or serology for
laboratory confirmation while empirically treating suspected HME.
prevalence:
- population: United States (national surveillance, 2008-2012)
measure_type: ANNUAL_INCIDENCE
prevalence_class: BAND_1_9_PER_1000000
rate_per_100000: 0.32
rate_denominator: POPULATION_PER_YEAR
notes: >-
US HME incidence of 3.2 cases per million population per year for 2008-2012,
a fourfold increase from 2000. Estimated case-fatality rate 2.7%.
evidence:
- reference: PMID:39093857
reference_title: "Human monocytotropic ehrlichiosis-A systematic review and analysis of the literature."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "the incidence of HME was 3.2 cases per million population in the United States"
explanation: US surveillance incidence of HME for 2008-2012, cases per million population per year.
- reference: PMID:34517150
reference_title: Ehrlichiosis and anaplasmosis subcommittee report to the Tick-borne Disease Working Group.
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Human monocytic ehrlichiosis and anaplasmosis are life threatening diseases with estimated case fatality rates of 2.7 and 0.3%, respectively."
explanation: The working-group report estimates the HME case-fatality rate at 2.7%.
treatments:
- name: Doxycycline
description: >-
First-line tetracycline pharmacotherapy for suspected or confirmed HME.
therapeutic_modality: SMALL_MOLECULE
treatment_term:
preferred_term: Pharmacotherapy
term:
id: NCIT:C15986
label: Pharmacotherapy
therapeutic_agent:
- preferred_term: doxycycline
term:
id: CHEBI:50845
label: doxycycline
target_mechanisms:
- target: Ehrlichia Ribosomal Translation (Doxycycline Target)
treatment_effect: INHIBITS
description: >-
Doxycycline inhibits the Ehrlichia 30S ribosomal subunit, shutting down
bacterial protein synthesis.
- target: Cell-Penetrant Antimicrobial Requirement
treatment_effect: BYPASSES
description: >-
Doxycycline satisfies the cell-penetrant-drug requirement for an organism
that replicates inside monocytes and macrophages.
evidence:
- reference: PMID:27172113
reference_title: "Diagnosis and Management of Tickborne Rickettsial Diseases: Rocky Mountain Spotted Fever and Other Spotted Fever Group Rickettsioses, Ehrlichioses, and Anaplasmosis - United States."
supports: SUPPORT
evidence_source: OTHER
snippet: >-
Doxycycline is the drug of choice for treatment of all tickborne
rickettsial diseases in patients of all ages, including children aged <8
years
explanation: >-
National recommendations for tickborne rickettsial diseases,
ehrlichioses, and anaplasmosis establish doxycycline as first-line
cell-penetrant therapy. Evidence source is OTHER as this is a national
recommendations report.
evidence:
- reference: PMID:27172113
reference_title: "Diagnosis and Management of Tickborne Rickettsial Diseases: Rocky Mountain Spotted Fever and Other Spotted Fever Group Rickettsioses, Ehrlichioses, and Anaplasmosis - United States."
supports: SUPPORT
evidence_source: OTHER
snippet: >-
Doxycycline is the drug of choice for treatment of all tickborne
rickettsial diseases in patients of all ages, including children aged <8
years
explanation: >-
CDC national recommendations support doxycycline as first-line HME
pharmacotherapy. Evidence source is OTHER as this is a national
recommendations report.
animal_models:
- name: IOE (Ehrlichia sp. HF) fatal murine ehrlichiosis
species: Mouse
publication: PMID:33407096
description: >-
Ehrlichia sp. HF (the Ixodes ovatus Ehrlichia, IOE) causes acute fatal
infection in laboratory mice that resembles fatal human monocytic
ehrlichiosis, providing the model for the TNF/neutrophil immunopathology arm.
modeled_mechanisms:
- target: TNF-Neutrophil Immunopathology
relationship: PARTIALLY_RECAPITULATES
fidelity: MODERATE
model_scale: ORGANISM
description: >-
The IOE mouse reproduces the dysregulated TNF-alpha CD8 T-cell and
neutrophil-amplified immunopathology of fatal human ehrlichiosis.
limitations: >-
The infecting organism is Ehrlichia sp. HF (IOE), not E. chaffeensis, so
the immunopathology is a surrogate-species model of human HME.
evidence:
- reference: PMID:33407096
reference_title: "Comparative Analysis of Genome of Ehrlichia sp. HF, a Model Bacterium to Study Fatal Human Ehrlichiosis."
supports: SUPPORT
evidence_source: MODEL_ORGANISM
snippet: "causes acute fatal infection in laboratory mice that resembles acute fatal human monocytic ehrlichiosis caused by Ehrlichia chaffeensis"
explanation: Establishes the IOE mouse as a model of fatal human monocytic ehrlichiosis.
- reference: PMID:14734762
reference_title: "Overproduction of TNF-alpha by CD8+ type 1 cells and down-regulation of IFN-gamma production by CD4+ Th1 cells contribute to toxic shock-like syndrome in an animal model of fatal monocytotropic ehrlichiosis."
supports: SUPPORT
evidence_source: MODEL_ORGANISM
snippet: >-
In conclusion, uncontrolled TNF-alpha production by CD8(+) T cells
together with a weak CD4(+) Th1 cell response are associated with
immunopathology and failure to clear IOE in the fatal model of HME.
explanation: The IOE model links dysregulated CD8 TNF-alpha responses to fatal-ehrlichiosis immunopathology.
Deep research results are used as seeds for research; they do not undergo the same validation as the main records and may contain errors. How we use deep research.
Target disease: Human Monocytic Ehrlichiosis MONDO ID: MONDO:0000225 Category: Infectious Disease (tick-borne bacterial zoonosis) Causative organism: Ehrlichia chaffeensis (NCBI:txid159)
Human Monocytic Ehrlichiosis (HME) is an acute, potentially life-threatening tick-borne bacterial infection caused by the obligate intracellular bacterium Ehrlichia chaffeensis (family Anaplasmataceae, order Rickettsiales). It is transmitted primarily by the lone star tick, Amblyomma americanum, and is maintained in nature in an enzootic cycle in which the white-tailed deer (Odocoileus virginianus) serves as the keystone host and major reservoir. First recognized as a human pathogen in 1986, E. chaffeensis infects and replicates within cells of the mononuclear phagocyte lineage (monocytes and macrophages), residing in membrane-bound vacuoles that resemble early endosomes. The disease is endemic to the southeastern and south-central United States and its incidence has risen steeply and expanded geographically over the past three decades, tracking the northward spread of its vector.
Clinically, HME presents as an undifferentiated febrile illness. Fever occurs in ~95% of patients, and the disease is characterized by a distinctive laboratory triad of thrombocytopenia, leukopenia, and elevated hepatic transaminases. Additional common features include headache, myalgia, and gastrointestinal symptoms; rash is common in children (~67%) but occurs in only about 30% of adults. Severe disease is largely immunopathological — a toxic-shock-like syndrome driven by dysregulated tumor necrosis factor-alpha (TNF-α) production by CD8⁺ T cells, suppressed protective CD4⁺ Th1/IFN-γ responses, and neutrophil-mediated tissue injury. Multi-organ involvement (central nervous system, lungs, kidneys, liver, and bone marrow) and complications such as acute respiratory distress syndrome (ARDS), acute renal failure, multi-organ failure, and secondary hemophagocytic lymphohistiocytosis (HLH) are more frequent in immunocompromised and elderly patients, who bear a disproportionate share of severe and fatal cases.
HME is a non-genetic infectious disease: there are no causal human genes, pathogenic variants, or inheritance considerations — host risk is defined by exposure and immune status rather than germline genetics. Diagnosis in the acute phase relies on nucleic-acid amplification (PCR, e.g., targeting the E. chaffeensis TRP120 gene), supported by peripheral-blood smear examination for intracytoplasmic morulae (specific but insensitive) and retrospective serology (indirect immunofluorescence antibody, IFA, requiring convalescent seroconversion). Treatment is empiric doxycycline, which is first-line for adults and children and should not be delayed pending laboratory confirmation; early therapy prevents severe morbidity and death. No vaccine exists, so prevention rests on tick-bite avoidance (DEET/picaridin repellents, permethrin-treated clothing, tick checks, and prompt tick removal). Surveillance case-fatality is ~1–3%, but rises to ~10–16% in hospitalized/immunocompromised cohorts.
HME is caused by Ehrlichia chaffeensis, an obligately intracellular, tick-transmitted bacterium in the family Anaplasmataceae, order Rickettsiales. Its principal vector is the lone star tick, Amblyomma americanum (NCBI:txid6943), and its keystone vertebrate reservoir is the white-tailed deer, Odocoileus virginianus (NCBI:txid9874). The pathogen was first identified in 1986, and more than 1,000 patients had been reported by 2000. As stated in the foundational review, "Ehrlichia chaffeensis is an obligately intracellular, tick-transmitted bacterium that is maintained in nature in a cycle involving at least one and perhaps several vertebrate reservoir hosts" and the deer "serves as a keystone host for all life stages of the principal tick vector (Amblyomma americanum) and is perhaps the most important vertebrate reservoir host for E. chaffeensis" PMID: 12525424. The disease designation itself — "human monocytic ehrlichiosis [HME], caused by Ehrlichia chaffeensis" — is confirmed in PMID: 17582569.
Alternative transmission routes exist beyond tick bite. A review of U.S. cases from 1997–2020 identified 132 ehrlichiosis/anaplasmosis cases among blood transfusion and solid-organ transplant recipients — "12 transfusion-associated cases and 120 cases in transplant recipients; 8 cases were donor-derived" PMID: 34670661. Because Ehrlichia survives in stored blood and transplant recipients are immunosuppressed, blood transfusion and organ transplantation are recognized non-vector transmission routes.
Zoonotic maintenance: E. chaffeensis is maintained "in a complex cycle involving white-tailed deer (WTD; Odocoileus virginianus) as a primary reservoir and the lone star tick (LST; Amblyomma americanum) as a primary vector", and natural "disease has been documented in some domestic animals and wildlife including domestic dogs and ring-tailed lemurs" PMID: 19819631. The closely related E. canis is "primarily responsible for the canine monocytic ehrlichiosis and is endemic throughout the world" PMID: 18770538, and E. ewingii causes granulocytotropic ehrlichiosis in humans and dogs (white-tailed deer are reservoirs for both E. chaffeensis and E. ewingii).
HME presents as a nonspecific febrile illness. A systematic review reported that "HME primarily presents as an unspecific febrile illness (95% of the cases), often accompanied by thrombocytopenia (79.1% of the cases), leukopenia (57.8% of the cases), and abnormal liver function tests (68.1% of the cases)" PMID: 39093857. The core laboratory triad is confirmed independently: "The diseases generally present as undifferentiated fever, but thrombocytopenia, leukopenia, and increased serum transaminase activities are important laboratory features" PMID: 17582569.
Symptom spectrum differs by age. In a pediatric case series (n=12, median age 7.4 y), "Symptoms demonstrated by the patients during their illness included fever (100%), rash (67%), myalgias (58%), and vomiting, diarrhea, and headache (25%)" and laboratory abnormalities included "thrombocytopenia (92%), elevated liver function tests (91%), lymphopenia (75%), hyponatremia (67%), leukopenia (58%), and anemia (42%)" PMID: 9200384; 4 of 12 presented in shock. In adults, "Patients infected with Human Monocytic Ehrlichiosis often present with symptoms including fever, headache, myalgia, and occasionally a macular rash" PMID: 35986240 — rash being far less common (~30%) than in children and less common than in Rocky Mountain spotted fever (RMSF).
Gastrointestinal-hepatic involvement is prominent. "Signs and symptoms include abdominal pain, nausea, vomiting, diarrhea, jaundice, and hepatosplenomegaly" and "If not diagnosed and treated in a timely fashion, ehrlichiosis can progress to multiorgan failure" PMID: 10436355.
| Feature | Frequency (pooled adults) | Frequency (pediatric series) |
|---|---|---|
| Fever | 95% | 100% |
| Thrombocytopenia | 79.1% | 92% |
| Elevated LFTs / transaminases | 68.1% | 91% |
| Leukopenia | 57.8% | 58% |
| Rash | ~30% | 67% |
| Lymphopenia | — | 75% |
| Hyponatremia | — | 67% |
| Anemia | — | 42% |
Suggested HPO terms: Fever (HP:0001945), Thrombocytopenia (HP:0001873), Leukopenia (HP:0001882), Elevated hepatic transaminase (HP:0002910), Headache (HP:0002315), Myalgia (HP:0003326), Skin rash (HP:0000988), Hepatosplenomegaly (HP:0001433), Hyponatremia (HP:0002902), Anemia (HP:0001903), Nausea and vomiting (HP:0002017), Diarrhea (HP:0002014).
HME is a multi-system disease. Neurologic involvement spans a wide spectrum: "Ehrlichiosis can present with a spectrum of neurologic manifestations, ranging in severity from headache to meningoencephalitis" PMID: 16569376. Pulmonary, renal, and hepatic complications are documented; one case report noted that the patient "manifested known sequelae for this emerging disease, including dyspnea, pedal edema, increased transminases, and nephrotic syndrome" PMID: 11272720.
Complications are strongly stratified by immune status. In the systematic review, complications were more frequent in immunocompromised versus immunocompetent patients: ARDS 34% vs 19.8%, acute renal failure 34% vs 15.8%, multi-organ failure 26% vs 14.9%, and secondary HLH 26% vs 14.9% PMID: 39093857.
Affected organs / body systems (UBERON): blood (UBERON:0000178), liver (UBERON:0002107), spleen (UBERON:0002106), bone marrow (UBERON:0002371), central nervous system (UBERON:0001017), lung (UBERON:0002048), kidney (UBERON:0002113). Target cell types (CL): monocyte (CL:0000576), macrophage (CL:0000235).
Cellular subversion — upstream mechanism. E. chaffeensis replicates in membrane-bound vacuoles resembling early endosomes within monocytes/macrophages, and it deploys type IV secretion system (T4SS) effectors to remodel host-cell biology. The effector Etf-2 arrests phagosome/endosome maturation: "The type IV secretion system effector Ehrlichia translocated factor-2 (Etf-2) directly binds to RAB5-GTP on E. chaffeensis-containing vacuoles. Consequently, Etf-2 hinders the engagement of RAB5 GTPase-activating protein with RAB5-GTP, delays maturation of Ehrlichia vacuoles to late endosomes, thus facilitates infection" PMID: 40797321. A second effector steals iron: "Ehrlichia translocated factor-3 (Etf-3) is a type IV secretion system effector that binds host-cell ferritin light chain and induces ferritinophagy, thus increasing cellular labile iron pool for Ehrlichia proliferation" PMID: 39705831; the same iron-robbery mechanism is described in PMID: 34074773.
Immunopathology — downstream mechanism of severe disease. In the Ixodes ovatus Ehrlichia (IOE / Ehrlichia sp. HF) murine model of fatal ehrlichiosis, lethal infection is driven not by bacterial burden alone but by a dysregulated host response: "Lethal infections caused by high or low doses of IOE were accompanied by extensive liver damage, extremely elevated levels of TNF-alpha in the serum, high frequency of Ehrlichia-specific, TNF-alpha-producing CD8(+) T cells in the spleen, decreased Ehrlicha-specific CD4(+) T cell proliferation, low IL-12 levels in the spleen, and a 40-fold decrease in the number of IFN-gamma-producing CD4(+) Th1 cells" PMID: 14734762. Neutrophils amplify the damage: "depletion of neutrophils from lethally infected mice enhanced bacterial elimination, decreased immune-mediated pathology, and prolonged survival" PMID: 23478316.
Suggested GO / molecular terms: protein secretion by the type IV secretion system (GO:0030255), phagosome maturation (GO:0090382), autophagy/ferritinophagy (GO:0006914), tumor necrosis factor production (GO:0032640), positive regulation of inflammatory response (GO:0050729), early endosome (GO:0005769). Chemical entities (CHEBI): iron (CHEBI:18248), doxycycline (CHEBI:50845).
1. Infected Amblyomma americanum tick bites human
-> inoculates Ehrlichia chaffeensis into dermis/blood
2. E. chaffeensis is taken up by monocytes/macrophages
-> resides in an early-endosome-like vacuole (morula)
3. T4SS effector Etf-2 binds RAB5-GTP on the vacuole
-> BLOCKS phagosome maturation to late endosome/lysosome
-> bacterium evades lysosomal killing (immune evasion)
4. T4SS effector Etf-3 binds ferritin light chain -> ferritinophagy
-> RAISES labile iron pool -> fuels bacterial replication
5. Intracellular proliferation + dissemination via mononuclear phagocytes
-> seeds liver, spleen, bone marrow, lung, kidney, CNS
-> cytopenias (thrombocytopenia, leukopenia) + transaminitis
+---------------------------- BRANCH ----------------------------+
| (a) Controlled response: |
| CD4+ Th1 / IFN-gamma + macrophage activation |
| -> bacterial clearance -> self-limited illness |
| -> prompt doxycycline accelerates resolution |
| |
| (b) Dysregulated response (severe/fatal; inferred from |
| IOE murine model + immunocompromised human cohorts): |
| Excess TNF-alpha from CD8+ T cells + suppressed CD4+ Th1 |
| (40-fold down IFN-gamma) + neutrophil-mediated injury |
| -> toxic-shock-like syndrome, HLH, ARDS, |
| renal failure, multi-organ failure, death |
+----------------------------------------------------------------+
Steps 3–4 are demonstrated mechanistically in cellular/molecular models; the branch to severe immunopathology (step b) is strongly supported by the IOE murine model and by the clinical over-representation of immunocompromised patients among severe/fatal human cases, but the precise human effector-cell dynamics remain inferred rather than directly demonstrated in patients.
HME is endemic to the southeastern and south-central United States. Historical surveillance (1986–1997) reported 742 HME cases, and "HME was most commonly reported from southeastern and southcentral states, while HGE was most often reported from northeastern and upper midwestern states"; reported cases rose sharply — "The annual number of reported cases increased sharply, from 69 in 1994 to 364 in 1997" PMID: 10511519. Genus-level surveillance shows continued increases and northward geographic expansion tracking the vector; among the related E. ewingii, "ehrlichiosis was reported more commonly among older, White, non-Hispanic, and male patients" PMID: 39983701.
Case fatality. The Tick-Borne Disease Working Group reported that "Human monocytic ehrlichiosis and anaplasmosis are life threatening diseases with estimated case fatality rates of 2.7 and 0.3%, respectively" PMID: 34517150. In pooled reported (often hospitalized) cases, "The overall case fatality is 11.6%, with a significant difference between immunocompetent (9.9%) and immunocompromized (16.3%) cases", and "Immunocompromized patients are overrepresented among reviewed HME cases (26.7%), which indicates the role of HME as an opportunistic infection" PMID: 39093857.
| Metric | Value | Source |
|---|---|---|
| Surveillance case-fatality | ~2.7% | PMID: 34517150 |
| Pooled reported-case fatality (overall) | 11.6% | PMID: 39093857 |
| Case-fatality, immunocompetent | 9.9% | PMID: 39093857 |
| Case-fatality, immunocompromised | 16.3% | PMID: 39093857 |
| Immunocompromised share of cases | 26.7% | PMID: 39093857 |
Acute-phase diagnosis relies on nucleic-acid amplification: real-time/duplex PCR targeting the E. chaffeensis TRP120 gene and multiplex qPCR panels. "Early diagnosis is essential for rapid clinical treatment to avoid misdiagnosis and severe patient outcomes" and "the duplex real-time PCR assay was more sensitive than the nested PCR assay" PMID: 24023963. Supporting laboratory findings include the CBC triad (thrombocytopenia, leukopenia) and elevated transaminases. Serology (IFA) is the reference standard but requires a ≥4-fold rise between acute and convalescent sera (collected 2–4 weeks apart) and is therefore retrospective. Peripheral blood smear for intracytoplasmic morulae in monocytes is rapid and specific but low-sensitivity.
The differential diagnosis encompasses other tick-borne illnesses. "Tickborne diseases that affect patients in the United States include Lyme disease, Rocky Mountain spotted fever (RMSF), ehrlichiosis, anaplasmosis, babesiosis, tularemia, Colorado tick fever, and tickborne relapsing fever" PMID: 32352736, and "many of the signs and symptoms can mimic other common presentations" PMID: 37465658. Distinguishing features of HME: less frequent rash than RMSF, prominent leukopenia/thrombocytopenia/transaminitis, and exposure geography/vector. Importantly, empiric doxycycline covers HME, anaplasmosis, ehrlichiosis, and RMSF simultaneously.
Doxycycline is first-line for HME in both adults and children. CDC guidelines direct clinicians to "recognize that doxycycline is the treatment of choice for suspected tickborne rickettsial diseases in adults and children" PMID: 27172113 and to "understand that early empiric antibiotic therapy can prevent severe morbidity and death" PMID: 16572105. Therapy should not be delayed pending laboratory confirmation; clinicians are advised "to promptly start therapy with doxycycline, even in young children, when rickettsial infections are suspected" PMID: 26188606 — concerns about dental staining from short courses in children are considered unfounded. Treatment is typically continued at least 3 days after defervescence (usually a 7–14 day course); defervescence usually occurs within 24–48 h of therapy. In the case series of GI/hepatic ehrlichiosis, all 8 patients responded to doxycycline PMID: 10436355. In severe HME-associated HLH, adjunctive corticosteroids/IVIG or anakinra have been used alongside doxycycline in case reports.
Suggested NCIT term: Doxycycline (NCIT:C299).
No vaccine exists for HME; prevention rests on tick-bite avoidance. The Wilderness Medical Society guidelines give strong recommendations for "the use of DEET, picaridin, and permethrin; tick checks; washing and drying clothing at high temperatures; mechanical tick removal within 36 h of attachment" PMID: 34642107. Antibiotic prophylaxis after a tick bite is not routine for ehrlichiosis: "Prophylactic treatment after tick exposure in patients without symptoms is generally not recommended" PMID: 32352736.
Because E. chaffeensis does not cause lethal disease in immunocompetent mice, the Ehrlichia sp. HF (IOE) fatal murine model fills a critical gap: it "causes acute fatal infection in laboratory mice that resembles acute fatal human monocytic ehrlichiosis caused by Ehrlichia chaffeensis" and, "As there is no small laboratory animal model to study fatal human ehrlichiosis, Ehrlichia sp. HF provides a needed disease model" PMID: 33407096. E. muris provides a non-lethal, persistent-infection model. White-tailed deer and dogs serve as models of the natural transmission cycle; ticks infected with cultured organisms transmit infection — "the transmission of both wild-type and transposon mutants of E. chaffeensis to its primary reservoir host, white tailed deer and to another known host, dog" PMID: 27729288. The canine counterpart disease, canine monocytic ehrlichiosis (caused by E. canis), is an important veterinary analogue PMID: 18770538.
Model organism / taxonomy identifiers: Ehrlichia chaffeensis NCBI:txid159; Amblyomma americanum NCBI:txid6943; Odocoileus virginianus NCBI:txid9874; Canis lupus familiaris NCBI:txid9615; Mus musculus NCBI:txid10090.
Concise overview and identifiers as above. Key identifiers: MONDO:0000225; MeSH "Ehrlichiosis" (D016873); ICD-10 A77.40–A77.49 (Ehrlichiosis); ICD-11 1C30.2. There is no OMIM entry (non-genetic infectious disease). Synonyms: human monocytic ehrlichiosis, human monocytotropic ehrlichiosis, Ehrlichia chaffeensis infection, HME. Information is derived from aggregated disease-level resources (systematic reviews, CDC/surveillance data, case series) rather than a single EHR cohort.
Primary cause is infectious (E. chaffeensis). Risk factors are environmental/behavioral: residence or activity in endemic southeastern/south-central U.S., outdoor/wooded exposure, tick bite, older age, male sex, and — for severe disease — immunocompromise (transplant, immunosuppression). No genetic (germline) susceptibility loci are established for humans. Protective factors are behavioral: repellent use, protective clothing, tick checks. Gene–environment interactions: not applicable in the human-germline sense; host–pathogen interaction is dominated by immune status.
See Finding 2 table with HPO suggestions. Onset is acute; severity ranges mild → severe (variable), and untreated disease can be progressive to multi-organ failure. Quality-of-life impact is acute (days–weeks) in uncomplicated disease; survivors of severe disease may have prolonged recovery, but HME is not typically a chronic condition.
Not applicable to the human host: HME has no causal human genes, pathogenic variants, modifier genes, epigenetic disease drivers, or chromosomal abnormalities. The relevant molecular biology is that of the pathogen genome (~1.15 Mb), encoding the T4SS apparatus and effectors (Etf-1/Etf-2/Etf-3), ~23 P28/OMP-1 outer-membrane protein paralogs, and ankyrin-repeat/tandem-repeat proteins (TRP120 is a PCR diagnostic target).
Infectious agent: Ehrlichia chaffeensis (NCBI:txid159). Environmental/lifestyle contributors are exposure-related (endemic geography, outdoor activity, occupational exposure such as forestry or military field exercises). No chemical toxins or pollutants are implicated.
See the ordered causal chain and Finding 4.
Primary target cells: monocytes/macrophages (CL:0000576, CL:0000235). Organs: blood, liver, spleen, bone marrow, lung, kidney, CNS (UBERON terms above). Subcellular: the pathogen resides in an early-endosome-like vacuole (GO:0005769). Involvement is systemic/bilateral where paired organs are affected.
Onset: acute, typically days after tick exposure; any age (pediatric through geriatric). Course: self-limited or rapidly resolving with timely doxycycline (defervescence within 24–48 h); progressive to multi-organ failure if untreated. Not relapsing-remitting or chronic in typical disease.
No inheritance pattern (infectious). Epidemiology per Finding 6: endemic southeastern/south-central U.S., rising and geographically expanding incidence; older, male predominance; immunocompromised over-representation among severe cases.
Per Finding 7: acute-phase PCR (TRP120) is the test of choice; IFA serology retrospective; blood-smear morulae specific but insensitive; supporting CBC and LFT abnormalities. No genetic/omics diagnostics are used clinically.
Case-fatality ~2.7% (surveillance) to ~10–16% (hospitalized/immunocompromised). Prognostic factors: immune status, age, delay to doxycycline, and severity of cytopenias/organ dysfunction. Complications: ARDS, acute renal failure, multi-organ failure, secondary HLH. Recovery is generally complete with timely treatment.
Per Finding 8: empiric doxycycline first-line; adjunctive immunomodulation (steroids/IVIG/anakinra) for HLH in severe cases (case-report level evidence).
Per Finding 9: tick-bite avoidance; no vaccine; prophylaxis not routine.
Zoonotic maintenance in white-tailed deer; natural infection of dogs and other mammals; veterinary counterpart canine monocytic ehrlichiosis (E. canis). See Finding 10.
IOE/Ehrlichia sp. HF fatal murine model; E. muris persistent model; deer and dog transmission-cycle models. See Finding 10.
The unifying theme of HME pathogenesis is a two-stage host–pathogen conflict inside the mononuclear phagocyte. Upstream, E. chaffeensis wins the intracellular battle by secreting T4SS effectors that (i) freeze phagosome maturation (Etf-2/RAB5) to escape lysosomal destruction and (ii) divert host iron (Etf-3/ferritinophagy) to fuel replication. This explains the organism's tropism for monocytes/macrophages and its dissemination through the reticuloendothelial system, producing the hallmark cytopenias and transaminitis.
Downstream, disease severity is determined by the host, not the microbe. The IOE murine model demonstrates that fatal outcomes correlate with a maladaptive immune response — excess CD8⁺-derived TNF-α, collapse of protective CD4⁺ Th1/IFN-γ immunity, and neutrophil-driven tissue injury — rather than with overwhelming bacterial load. This immunopathological framework reconciles the clinical epidemiology: immunocompromised and elderly patients, whose immune regulation is impaired, suffer disproportionately severe disease, HLH, and death. It also rationalizes the therapeutic success of early doxycycline (halting bacterial replication before the immune response becomes self-destructive) and the emerging, case-report-level role of immunomodulation (steroids, IVIG, anakinra) in the sickest patients.
| PMID | Contribution | Evidence type |
|---|---|---|
| 12525424 | Defines E. chaffeensis as obligate intracellular tick-borne pathogen; deer as keystone reservoir | Review |
| 17582569 | HME etiology; undifferentiated fever + laboratory triad | Review |
| 39093857 | Systematic review: phenotype frequencies, case-fatality by immune status | Systematic review |
| 14734762 | TNF-α/CD8⁺ toxic-shock-like immunopathology (IOE model) | Model organism |
| 23478316 | Neutrophils mediate immunopathology in fatal ehrlichiosis | Model organism |
| 40797321 | Etf-2 blocks phagosome maturation via RAB5 | In vitro/molecular |
| 39705831 | Etf-3 induces ferritinophagy / iron acquisition | In vitro/molecular |
| 34074773 | Bacterial effector-induced ferritinophagy ("iron robbery") | In vitro/molecular |
| 34517150 | Population-level case-fatality (2.7%) | Working group report |
| 27172113 | CDC: doxycycline first-line, adults & children | Guideline |
| 16572105 | Early empiric therapy prevents severe outcomes | Guideline |
| 26188606 | Prompt doxycycline even in young children | Guideline/review |
| 34642107 | Prevention: repellents, tick checks, removal | Guideline |
| 32352736 | Differential diagnosis; prophylaxis not routine | Review |
| 9200384 | Pediatric symptom/lab frequencies (rash 67%) | Case series |
| 35986240 | Adult symptom spectrum | Case report |
| 16569376 | CNS involvement | Review |
| 11272720 | Pulmonary/renal/hepatic sequelae | Case report |
| 10511519 | Geographic distribution; rising incidence | Surveillance |
| 39983701 | Demographic pattern (older, male) | Surveillance |
| 34670661 | Transfusion/transplant (donor-derived) transmission | Review |
| 19819631 | Reservoir–vector zoonotic cycle | Review |
| 18770538 | E. canis / canine monocytic ehrlichiosis | Review |
| 10436355 | GI/hepatic manifestations; multi-organ failure | Case series/review |
| 33407096 | IOE fatal murine model | Model organism |
| 27729288 | Deer/dog transmission-cycle models | Model organism |
| 24023963 | Duplex real-time PCR diagnosis | Diagnostic |
| 37465658 | Signs/symptoms mimic other illnesses | Review |
Report compiled from 16 confirmed findings and 52 reviewed papers across 5 investigation iterations. Evidence types are distinguished as human clinical (case series, surveillance, guidelines), model organism (murine IOE, deer/dog), and in vitro/molecular (effector biology).
Checked with linkml-reference-validator 0.3.0rc1.
| Outcome | Count |
|---|---|
| References checked | 28 |
| Resolved | 28 |
| Unresolved (possible confabulation) | 0 |
| Unverifiable | 0 |
| References weighed for topical relevance | 28 |
| On topic | 23 |
| Off topic | 0 |
All extracted references resolved successfully.
Checked with linkml-term-validator 0.4.5, through the ols: adapter.
| Outcome | Count |
|---|---|
| Terms checked | 31 |
| Resolved | 31 |
| Unresolved (possible confabulation) | 0 |
| Obsolete | 0 |
| Unverifiable | 0 |
| Terms whose name was checked | 1 |
| Terms named correctly | 0 |
| Terms named as a different term | 0 |
| Terms whose name is worth a second look | 1 |
The report's name for these is recognisably related to the term's own name without being one of them. A loose paraphrase reads the same way as a citation of the wrong sibling term - and so does a related synonym, which the ontology records precisely because it names something adjacent rather than the same thing - so these are listed rather than judged:
GO:0005769 (2 mentions) - the report calls it "early-endosome-like vacuole"; GO calls it early endosomeEvery term resolved, and every label the report gave matched.