Chlamydia pneumoniae Pneumonia — Comprehensive Disease Research Report

Prepared: 2026-09-26 · Target entry: Chlamydia_Pneumoniae_Pneumonia · Category: Infectious Disease

Scope note on evidence quality. This is an acquired bacterial infection, not a Mendelian disorder. Template sections that presuppose a germline genetic etiology (causal genes, pathogenic variants, inheritance pattern, penetrance, carrier frequency, genetic counseling, prenatal testing) have no applicable content and are marked as such rather than filled with host-susceptibility material presented as disease causation. Host genetic modifiers are reported separately and are almost entirely murine.

1. Disease Information

Overview

Chlamydia pneumoniae pneumonia is a community-acquired atypical pneumonia caused by the obligate intracellular Gram-negative bacterium Chlamydia pneumoniae (formerly Chlamydophila pneumoniae; original isolate designation TWAR, from the two founding strains TW-183 and AR-39). The organism was recognized as a distinct species in the late 1980s and characterized as a respiratory pathogen by Grayston and colleagues.

"Chlamydia pneumoniae strain TWAR, the new third species of Chlamydia, is a common cause of pneumonia and other acute respiratory tract infections. About 10% of hospitalized and outpatient pneumonia cases have been associated with TWAR infection. TWAR is among the four or five most commonly identified causes of all pneumonia." — Grayston JT et al., J Infect Dis 1990 (PMID:2181028)

"Chlamydia pneumoniae is an obligate intracellular bacterium and a significant cause of respiratory infections. It is associated with upper and lower respiratory tract diseases, including bronchitis and pneumonia. The pathogen employs specific virulence factors, such as the Type III Secretion System (T3SS) and Inc proteins, to invade and subvert host cell machinery during its peculiar developmental life cycle." — Tagini F, Puolakkainen M, Greub G, J Med Microbiol 2025 (PMID:40279169, DOI 10.1099/jmm.0.002006)

The infection spans a clinical spectrum from asymptomatic carriage and upper respiratory illness through bronchitis to pneumonia, and — much more contentiously — to chronic sequelae (asthma, atherosclerosis, neurodegeneration).

Key identifiers

Curation consequence. The only MONDO anchor is genus-level while ICD-11 is species-level. A species-scoped dismech entry should bind MONDO:0025598 with mapping_predicate: skos:broadMatch (or narrowMatch from the entry's perspective, per the repo's mapping rules) and carry ICD11:CA40.00 as the exact clinical code, with a notes: line recording that no species-level MONDO term was found on 2026-09-26 via the EBI OLS4 MONDO search for "Chlamydia pneumoniae" and "chlamydophila" (returns MONDO:0025598, MONDO:0004652 bacterial pneumonia, MONDO:0005888 ornithosis, MONDO:0021697 chlamydia infectious disease, MONDO:1017031; MONDO:0025598 has no children).

Synonyms

Chlamydophila pneumoniae pneumonia; chlamydial pneumonia; TWAR pneumonia; C. pneumoniae respiratory infection; Taiwan acute respiratory agent pneumonia.

Data provenance

Disease-level aggregated sources (CDC clinical and laboratory pages, MONDO/MeSH/ICD, review literature) plus patient-level retrospective hospital cohorts (Chinese pediatric series 2025–2026, Mexican pediatric series 2023, Swiss/German/French laboratory surveillance). Multi-site laboratory surveillance (Eurosurveillance 2025) is aggregated test-level data, not EHR-derived.

2. Etiology

Causal factor

A single necessary and sufficient cause: infection with Chlamydia pneumoniae (NCBITaxon:83558). There is no non-infectious route to this entry. Transmission is person-to-person via respiratory droplets and, secondarily, fomites (CDC clinical overview). No animal reservoir is required for human transmission (see §14).

The organism's lifecycle is obligately intracellular and biphasic:

"Chlamydia alternates between the infectious, environmentally resistant Elementary Body (EB) and the non-infectious, metabolically active Reticulate Body (RB)… RBs exploit host-derived energy and nutrients to multiply by binary fission, ultimately re-differentiating into progeny EBs." — Tu S et al., Front Cell Infect Microbiol 2026;16:1787885 (PMID:42180254)

Incubation and transmissibility

"C. pneumoniae infection generally has a long incubation period of 3 to 4 weeks. However, studies have documented shorter times." — CDC Clinical Overview

The long incubation and high asymptomatic fraction produce slow, protracted institutional outbreaks rather than sharp point-source epidemics.

Risk factors — environmental and demographic

Suggested ECTO-style exposure concept: exposure to a crowded residential environment. Note per repository policy that an ECTO CURIE must be searched and confirmed at binding time; no ECTO term is asserted here.

Risk factors — host genetic

No validated human genetic susceptibility locus is established. Candidate-gene work on innate-immunity polymorphisms (TLR2 Arg753Gln, TLR4 Asp299Gly, CD14, LBP, IL6) examined C. pneumoniae growth in human macrophages in vitro and found highly variable donor susceptibility without a robust attributable polymorphism. Murine work is stronger and is reported in §4 and §15. No GWAS Catalog entry for C. pneumoniae pneumonia was identified.

Protective factors

• Prior infection confers partial, non-sterilizing immunity. Murine reinfection is cleared faster than primary infection, and the effect is IFN-γ-dependent (PMID:10639472).
• Non-pharmaceutical interventions. Masking, distancing, and school closure produced a statistically significant fall in detection (PMID:40511472).
• No dietary, nutritional, or genetic protective factor is established. No protective allele is known.

Gene–environment interaction

In mice the interaction is demonstrable: strain background (A/J vs C57BL/6) determines pulmonary bacterial load and lung pathology after an identical intranasal inoculum, with ~30% of variance mapping to an MHC-overlapping chromosome 17 QTL (PMID:18075514). The human equivalent has not been demonstrated. In the sst1 model, host genotype determines disease tolerance rather than bacterial clearance — a genuine gene-by-infection interaction on the pathology axis alone (PMID:24009502).

3. Phenotypes

Frequencies below are cohort-specific. The three largest recent series differ in age structure and case definition, so do not pool them; the entry should carry each frequency with its own population and evidence item.

Cohort key

• A — 291 hospitalized children, Shijiazhuang, China, 2015–2025; mean age 8.12 y (PMID:41210234)
• B — 145 hospitalized children, Xi'an, China, Jan–Jul 2025; median age 11 y (PMID:42427957)
• C — 42 adults/mixed, mNGS-confirmed, Aug 2022–Aug 2025 (PMID:41676099)
• D — 25 of 154 hospitalized children with CAP, Mexico City (PMID:38052876)

Symptoms and signs

Related upper-tract syndromes the same organism causes, useful as has_subtypes or sibling phenotypes rather than features of the pneumonia entry: HP:0025439 Pharyngitis, HP:0000246 Sinusitis, HP:0012387 Bronchitis, HP:0000388 Otitis media.

"C. pneumoniae may cause any kind of respiratory tract infection, ranging from upper respiratory tract infections (with symptoms of rhinitis, sore throat or hoarseness), sinusitis or otitis to bronchitis and community-acquired pneumonia." — PMID:40279169

Laboratory abnormalities

Radiographic phenotypes

Chest CT in 42 mNGS-confirmed patients (PMID:41676099), verbatim:

"In the early stage, chest CT demonstrated a lobular pneumonia pattern in 16 patients (55.2%), involvement of a single lung lobe in 20 (69.0%), predominant lower-lung distribution in 19 (65.5%)… The main accompanying features included a halo sign in 25 patients (86.2%), centrilobular nodules in 23 (79.3%), and bronchial wall thickening in 20 (69.0%)."

Plain radiography in children (PMID:38052876):

"The interstitial pattern on chest-X-ray was the most frequent (68%), consolidation was observed in 32%."

HPO: HP:0002113 Pulmonary infiltrates; HP:0006515 Interstitial pneumonitis; HP:0002202 Pleural effusion (uncommon); HP:0002088 Abnormal lung morphology as the coarse parent.

Coarse-binding caution. If HP:0002088 Abnormal lung morphology is used for an imaging phenotype, it falls in the repository's coarse set and requires a coarse_binding_basis; VARIABLE_SPECTRUM fits, since the radiographic pattern genuinely ranges from interstitial to lobular to consolidative across cohorts.

Onset, severity, progression

• Onset: gradual/insidious after a 3–4-week incubation (CDC). Not congenital, not age-restricted; primary infection concentrates in school age, reinfection in the elderly.
• Severity: mild in the large majority. "Most respiratory infections caused by C. pneumoniae are asymptomatic or mild" (CDC). Severe disease reached 6.8% of a 176-patient pediatric series (PMID:40775274) and 34/133 (25.6%) in a cohort explicitly enriched for severity assessment (PMID:42327901) — the discrepancy is a case-mix artifact and should be curated as two separate population-scoped statements.
• Progression: self-limited in most; protracted cough is the signature. Median cough duration 21 days (PMID:40279169); mean total disease duration 14.81 days with mean hospital stay 7.52 days (A); median disease duration 14 days (B).
• Persistence: "Patients may experience a persistent cough and malaise for several weeks or months" even after appropriate antibiotics (CDC Clinical Features).

Quality of life

No EQ-5D, SF-36, or PROMIS data specific to C. pneumoniae pneumonia were identified. The functional burden documented is indirect: hospital stay (mean 7.52 days, cohort A), weeks-to-months of residual cough and malaise (CDC), and — for those developing or exacerbating asthma — the chronic burden of that disease. Disease-specific QoL instruments: not available.

4. Genetic / Molecular Information

Causal genes: not applicable. There is no human causal gene, no pathogenic variant, no variant classification, no allele frequency, no somatic/germline distinction, no chromosomal abnormality, and no epigenetic disease mechanism for this entry. The genetic: section of a dismech entry for this disease should be empty or restricted to susceptibility/modifier typing with relationship_type: SUSCEPTIBILITY or MODIFIER, never CAUSAL.

Host modifier genes (mostly murine; treat as model-organism evidence)

Curation warning applicable here. Per the repository's gene-binding rule, each of the HGNC CURIEs above was read from cache/hgnc/terms.csv or the genenames.org REST API on 2026-09-26 (HGNC:6059 IDO1 and HGNC:16390 NOD1 came from the API and are not yet in the local cache; note that the repository's canonical form is lowercase hgnc:). Re-verify before binding.

Pathogen genomics (this is where the molecular detail actually lives)

• Human isolates are essentially clonal. The CWL029 reference chromosome is ~1.23 Mb, circular, with no plasmid; J138 is 1,226,565 nt, 40.7% G+C, 1,072 protein-coding genes, 3,665 nt shorter than CWL029 (PMID:10871362).
• The koala isolate LPCoLN carries a 1,241,024-bp chromosome plus a 7.5-kb cryptic plasmid that human strains have lost (PMID:19749045).
• Animal isolates are far more diverse than human ones; five genotypes A–E were defined across 30 isolates (PMID:20502684).
• More than 5% of the chlamydial genome is predicted to encode type III secretion effectors, including the Inc family at the inclusion membrane.

Molecular profiling

• Transcriptomics: the transcriptional landscape of C. pneumoniae has been mapped (Genome Biol 2011). Host-cell transcriptional responses under IFN-γ-induced persistence overlap productive infection and cluster on apoptosis, cell-cycle and metabolism genes (Infect Immun 2006). No GEO accession is asserted here — the repository requires just discover-datasets / just verify-datasets before any geo: accession is written, and dataset relevance triage is a manual step.
• Proteomics / metabolomics / lipidomics / single-cell / spatial: retinal and cortical proteomics in the Alzheimer study (PMID:41571675) is the only recent omics dataset found that centers on this organism, and it is a chronic-extrapulmonary context, not pneumonia. No pneumonia-specific proteomic, metabolomic, lipidomic, single-cell, or spatial dataset was identified.
• Functional genomics: a yeast-based screen identified the host microtubule cytoskeleton as a target of numerous chlamydial effectors (PMID:37108781). No DepMap/CRISPR screen specific to this infection was found.

5. Environmental Information

Infectious agent. Chlamydia pneumoniae, NCBITaxon:83558. Obligate intracellular, Gram-negative, no peptidoglycan-dependent β-lactam target of clinical use.

Transmission environment. Respiratory droplets from close person-to-person contact, plus fomite transfer (CDC). Congregate settings dominate documented outbreaks:

Lifestyle factors. No confirmed dietary, alcohol, or exercise association. Smoking is a general risk factor for CAP and for the COPD phenotype in which C. pneumoniae has been repeatedly sought, but a smoking-specific risk estimate for C. pneumoniae pneumonia was not found in this search.

Chemical/occupational exposures. None established. CTD and EPA sources were not searched for this pathogen because no toxicological etiology is plausible.

6. Mechanism / Pathophysiology

Ordered causal chain

1. Inhalation of elementary bodies (EBs) in respiratory droplets deposits metabolically inert, environmentally resistant EBs on the mucosal epithelium of the upper and lower respiratory tract. Demonstrated for the genus; the specific human inoculum dose is not known.
2. EB cytadherence to respiratory epithelial cells leads to immediate pro-inflammatory signaling, before any bacterial replication. Heparin pretreatment — which blocks attachment — significantly reduced IL-8 and TNF-α mRNA induction, and heat/UV inactivation only partially reduced the response, establishing adhesion itself as the trigger (PMID:12540537, IN_VITRO).
3. Receptor-mediated uptake into a membrane-bound vacuole (the inclusion) results in an intracellular niche. The inclusion traffics to the peri-Golgi region and avoids lysosomal fusion.
4. The inclusion is actively remodeled by secreted effectors. The T3SS delivers early (0–12 h) and mid-cycle (12–36 h) effectors and a family of inclusion-membrane (Inc) proteins; C. pneumoniae recruits PI4P to the inclusion via the Cpn0308–ACBD3–PI4KB axis (PMID:42240327, IN_VITRO). CPAF, a secreted serine protease, cooperates with T3SS effectors to block p65 nuclear translocation. GO: GO:0030254 protein secretion by the type III secretion system; GO:0020003 symbiont-containing vacuole; GO:0006898 receptor-mediated endocytosis.
5. EB→RB differentiation and binary fission lead to an expanding inclusion. The complete cycle takes 72–96 h in vitro, slower than C. trachomatis; ultrastructural staging is described in PMID:10722649 (IN_VITRO).
6. Infected epithelial cells are rendered apoptosis-resistant, which preserves the replicative niche. Mitochondria in infected cells fail to release apoptogenic factors and active caspase-3 is absent; protection requires large inclusions (PMID:11705971, IN_VITRO). GO: GO:0043066 negative regulation of apoptotic process.
7. Branch A — mucociliary clearance fails. C. pneumoniae infection of ciliated bronchial epithelium "had a marked ciliastatic effect, completely aborting ciliary motion within 48 h", an effect resistant to UV inactivation but abolished by heat or specific antiserum (PMID:7751703, IN_VITRO). This contributes to bacterial retention and to secondary bacterial superinfection. GO: GO:0003341 cilium movement (decreased).
8. Branch B — epithelial chemokine release recruits neutrophils. IL-8 protein is consistently secreted by infected A549 cells; IL-1β and IL-6 protein are not significantly changed in that system (PMID:12540537). GO: GO:0032757 positive regulation of interleukin-8 production; GO:0030593 neutrophil chemotaxis; GO:0007249 canonical NF-κB signal transduction (antagonized by CPAF, so the net signal is a contested balance rather than simple activation).
9. Neutrophils become a Trojan horse. C. pneumoniae "easily infect and hide inside neutrophil granulocytes until these cells become apoptotic and are subsequently taken up by macrophages", and this route yields enhanced replication compared with direct macrophage infection, with a TGF-β-skewed rather than TNF-α-skewed macrophage response (PMID:19547701, IN_VITRO). Blocking phosphatidylserine recognition with annexin A5 significantly reduced transmission — a clean causal test.
10. Alveolar and monocyte-derived macrophages become the long-lived reservoir, which permits dissemination beyond the airway to vascular endothelium and smooth muscle. CL: CL:0000583 alveolar macrophage, CL:0000235 macrophage, CL:0000775 neutrophil, CL:0000576 monocyte, CL:0002328 bronchial epithelial cell, CL:0000082 epithelial cell of lung, CL:0000115 endothelial cell, CL:0000192 smooth muscle cell.
11. A Th1 response with IFN-γ is mounted, which restricts replication. GO: GO:0042088 T-helper 1 type immune response; GO:0032609 type II interferon production; GO:0034341 response to type II interferon. Neutralizing IFN-γ increases bacterial load and pneumonia score in mice (PMID:10639472, MODEL_ORGANISM).
12. IFN-γ simultaneously induces IDO1, depleting intracellular tryptophan — and this causes persistence rather than clearance when IFN-γ is subinhibitory. C. pneumoniae is a tryptophan auxotroph; IDO-deficient cells responsive to IFN-γ support normal growth without aberrant forms, and adding exogenous tryptophan reverses the effect (PMID:8063385, IN_VITRO). GO: GO:0006569 L-tryptophan catabolic process; CHEBI: CHEBI:16828 L-tryptophan.
13. Persistence manifests morphologically as aberrant bodies. IFN-γ-treated cultures show "atypical inclusions containing large reticulate-like aberrant bodies with no evidence of redifferentiation into elementary bodies", dose-dependently (PMID:11705979, IN_VITRO). Iron deprivation and β-lactam exposure induce the same state (PMID:40279169). Whether this in vitro state occurs in vivo remains genuinely open — a 2026 review is titled precisely "Open question: Can Chlamydia pneumoniae cause persistent infections…" (PMID:42690975). Curate the persistence chain with a discussions: entry of kind: KNOWLEDGE_GAP.
14. The resulting mixed neutrophilic–lymphocytic alveolar and bronchiolar inflammation produces the clinical and radiographic pneumonia: centrilobular nodules and bronchial wall thickening reflecting the bronchiolocentric route, halo signs reflecting perilesional inflammation (PMID:41676099).
15. Branch C — chronic outcome. In mice, infection induces persisting macrophage accumulation beyond bacterial clearance, inducible bronchus-associated lymphoid tissue (iBALT) from day 14 through day 35, and fibrosis; adoptive transfer of M1 (not M2) macrophages one week post-infection worsened inflammation, fibrosis, and iBALT (PMID:24204830, MODEL_ORGANISM). This is the best mechanistic model for the human asthma/COPD association, and the human step is inferred, not demonstrated.
16. Branch D — host tolerance genotype modifies outcome independently of bacterial burden. sst1-susceptible mice cleared the organism normally but showed higher clinical scores, exaggerated macrophage and neutrophil influx, fibrosis, elevated activated caspase-3, and an IFN-β/IL-10-skewed macrophage cytokine profile with IFN-β-dependent apoptotic death (PMID:24009502, MODEL_ORGANISM).

Biological scale tags for pathograph nodes

MOLECULAR for the T3SS/Inc/CPAF effector nodes, PI4P recruitment, and IDO1-driven tryptophan depletion; CELLULAR for inclusion formation, apoptosis resistance, ciliostasis, neutrophil Trojan-horse transfer, and macrophage reservoir establishment; TISSUE for alveolar/bronchiolar inflammation, iBALT formation and fibrosis; ORGANISM for fever and the systemic inflammatory response.

Metabolic and biochemical specifics

• Tryptophan auxotrophy is the central metabolic vulnerability and the mechanistic hinge of IFN-γ control (§6 steps 12–13). Human C. pneumoniae strains show increased sensitivity to tryptophan bioavailability relative to animal strains — a positive adaptation to the human host (Mol Microbiol 2014).
• No enzyme deficiency, receptor defect, or ion-channel defect in the host is implicated.
• Epigenetic host changes: none established for this infection.

7. Anatomical Structures Affected

Organ level

• Primary: lung (UBERON:0002048), specifically the bronchi (UBERON:0002185) and bronchioles (UBERON:0002186) with bronchiolocentric spread into alveolar parenchyma. Radiographic lower-lung predominance in 65.5% and single-lobe involvement in 69.0% at onset (PMID:41676099).
• Contiguous upper tract: nasopharynx (UBERON:0001728), pharynx (UBERON:0006562), larynx (UBERON:0001737, hoarseness), paranasal sinus (UBERON:0001825), middle ear (UBERON:0001756), trachea (UBERON:0003126).
• Body system: respiratory system (UBERON:0001004).
• Secondary / extrapulmonary: the vascular wall (endothelium and smooth muscle) via infected monocytes; the pericardium and peripheral nerves in reported complications (pericarditis with Guillain-Barré, PMID:41157205); the CNS in encephalitis; the myocardium in myocarditis (CDC; PMID:40279169); the retina in the Alzheimer's context (PMID:41571675).

Tissue and cell level

Respiratory mucosal epithelium is the invariable portal:

"In all cases of chlamydial infections, the primary site of entry is the mucosal epithelium. During the later course of the infection, viable chlamydiae are found inside alveolar macrophages (AM), bronchial/alveolar epithelial cells, vascular endothelial/smooth muscle cells and monocyte-derived macrophages."

Cell Ontology bindings as listed in §6 step 10. Ciliated bronchial epithelium deserves its own node given the ciliostasis mechanism; note that the repository's CL cache currently holds CL:0002328 bronchial epithelial cell but not a ciliated-cell term, so a ciliated-specific binding needs a fresh lookup.

Subcellular level

The defining compartment is the chlamydial inclusion — a pathogen-containing vacuole, GO:0020003 symbiont-containing vacuole, trafficked to the peri-Golgi region, enriched in PI4P via host ACBD3/PI4KB recruitment (PMID:42240327). Host mitochondria are functionally implicated through their failure to release apoptogenic factors (PMID:11705971).

Localization and laterality

Unilateral involvement predominates in children (109 of 145, cohort B) and in adults at onset (single lobe 69.0%, cohort C), while roughly half of one 291-child series had bilateral involvement (cohort A). Report both; the difference is likely a timing and imaging-modality difference (CT vs radiograph, onset vs mid-course) rather than a real biological disagreement.

8. Temporal Development

Onset. Insidious, following a 3–4-week incubation (CDC). Upper-respiratory prodrome (sore throat, hoarseness, coryza) commonly precedes the cough by days to weeks — the pattern that distinguishes it clinically from abrupt pneumococcal pneumonia.

Stages. No formal staging system exists. A practical three-phase description supported by the data: (i) upper-tract prodrome; (ii) lower-tract phase with cough, variable low-to-moderate fever, and radiographic infiltrate; (iii) protracted convalescence with residual cough.

Rate and course. Slow onset, slow resolution. Median cough duration 21 days (PMID:40279169); median total disease duration 14 days (cohort B); mean 14.81 days with 7.52-day mean hospital stay (cohort A). Mid-course CT reassessment within one month was a design feature of the 42-patient CT study precisely because radiographic resolution lags clinical recovery (PMID:41676099).

Duration. Self-limited in the overwhelming majority — "approximately 70% of respiratory tract infections caused by C. pneumoniae are asymptomatic or present only with mild symptoms" (per the 2026 review's summary of the field). But:

"Patients may experience a persistent cough and malaise for several weeks or months" even with appropriate antibiotic treatment. — CDC Clinical Features

Remission and recurrence. Reinfection throughout life is the rule, not the exception (Grayston 1990: "nearly everyone is infected and reinfected during their life-time"). Immunity after infection is partial: murine reinfection is cleared faster but is not prevented (PMID:10639472).

Critical windows. Two are identifiable. First, early life: neonatal murine infection with defective TLR2 signaling produces severe, prolonged disease and defective Th1 priming, the model underpinning the asthma-initiation hypothesis (PMID:22724018). Second, the first weeks after acute infection, where the murine M1-macrophage adoptive-transfer experiment shows that intervention timing determines fibrotic outcome (PMID:24204830).

9. Inheritance and Population

Inheritance

Not applicable. This is an acquired infection. There is no inheritance pattern, no penetrance, no expressivity, no anticipation, no germline mosaicism, no founder effect, no consanguinity role, and no carrier frequency. Do not populate the inheritance: block.

Epidemiology — the headline problem

There is no national reporting or surveillance system for C. pneumoniae infections in the United States (CDC), and it is not a notifiable disease in China (PMID:41088528). Every incidence and prevalence figure below is therefore laboratory-based or cohort-based, and the denominators differ.

Share of community-acquired pneumonia

The spread from 1% to 16% is explained by diagnostic method (serology vs PCR vs mNGS/tNGS), inpatient vs outpatient setting, and epidemic period. Curate each with its own population and measure_type.

Laboratory detection ratios — the post-pandemic collapse and rebound

Eurosurveillance multi-site study, 28 sites (27 European + Taiwan), 693,106 tests 2018–2023 (PMID:40511472):

"Children/adolescents represented 56.4% of positive detections despite comprising only 18.7% of tested samples in 2023."

The 2024–2025 resurgence

Seroprevalence and age structure

• Highest seroprevalence 52.9% in school-aged children 6–10 years (PMID:42180254).
• Primary infection is uncommon under 5 years in temperate regions; reinfection predominates over 65 (PMID:40279169; PMID:2181028).
• The Beijing series documents a post-NPI age shift: the primary affected group moved from 0–3 years (60.4% pre-intervention) to 6–18 years (90.1% post-intervention, H = 172.24, P < 0.001), with co-infection rising from 6.7% to 45.4% (PMID:42298460).

Sex ratio

Male predominance in hospitalized pediatric series: 187:104 (1.80:1) in Shijiazhuang (PMID:41210234); 1.75:1 in Beijing (PMID:42298460); 89:56 (1.59:1) in Xi'an (PMID:42427957). In the Chinese national tNGS data, male adolescents 10–19 were the highest-risk group while women 25–44 also showed elevated risk, attributed by the authors to caregiving exposure (PMID:41088528). Whether the pediatric male excess reflects biology or admission bias is unresolved.

Geographic distribution

Worldwide. Genotype geography is described for the pathogen: genotypes A, B, D, E are geographically linked while genotype C spans continents (PMID:20502684).

10. Diagnostics

CDC-endorsed hierarchy

"NAAT, such as real-time PCR, or respiratory pathogen panel" is the "Best method for the diagnosis of an acute C. pneumoniae infection." — CDC Laboratory Testing

"Microimmunofluorescence" is the recommended serological approach; "single IgG titers" should not be used to diagnose acute infection; paired acute and convalescent sera are required. "Complement fixation, EIA, and whole-inclusion fluorescence" are not endorsed. Culture is "performed by specialized reference laboratories, but it's time-consuming and not optimal for treatment decisions."

Performance and pitfalls

The definitive comparative study (156 adults admitted with CAP) found PCR and serology identify largely non-overlapping patient sets (PMID:9666010):

"Twenty-three patients (15%) had serological results compatible with acute C. pneumoniae infection; nine (39%) of these subjects were C. pneumoniae PCR positive. Twenty-two patients (14%) had positive PCR results without serological evidence of an acute C. pneumoniae infection… Independent of the gold standard used, the best PCR results were obtained with nasopharyngeal specimens. However, the predictive value of a positive C. pneumoniae PCR result for patients with community-acquired pneumonia remains unknown and may be low."

That last sentence is the single most important caveat in this entry's diagnostics section and should be curated verbatim as a REFUTE- or caveat-bearing evidence item against any claim that PCR positivity establishes causation.

In the Mexican pediatric cohort, serology badly underperformed PCR: IgM positive in 7%, IgG in 28.6%, against 16% PCR detection (PMID:38052876).

Emerging: metagenomic and targeted NGS

mNGS was the sole confirmation method in the 42-patient CT study (PMID:41676099) and tNGS supplied the Chinese national epidemiology (PMID:41088528, 2.3 million tests). This is now the dominant discovery route for adult cases and explains part of the apparent resurgence — a detection-method confounder that any incidence claim must acknowledge.

Laboratory and imaging findings

Covered in §3. Notable: CRP is normal or mildly elevated in children (cohorts A, B) but >100 mg/L in 14.3% of the adult/mixed mNGS cohort (C) — the inflammatory footprint is not uniformly low.

Genetic testing

Not applicable. No WGS, WES, gene panel, single-gene test, CMA, karyotype, FISH, mtDNA, or repeat-expansion testing has any role. Pathogen sequencing (MLST, mNGS, tNGS) is a microbiological, not a human-genetic, test.

Clinical criteria and differential diagnosis

No society-specific diagnostic criteria exist for C. pneumoniae pneumonia as an entity; diagnosis is CAP by clinical/radiographic criteria plus microbiological attribution. Differential:

Screening. No screening program exists, is recommended, or would be justified. Newborn, carrier, and cascade screening: not applicable.

11. Outcome / Prognosis

Overall. Good. "The clinical course is generally mild, and the disease responds well to appropriate antibiotic treatment" (PMID:42180254). Zero mortality across the 291-child Shijiazhuang series, with complete recovery reported (PMID:41210234).

Mortality. Not zero everywhere. Four percent of the Mexican pediatric cohort died and 36% had complications — but 88% of those children had underlying disease, so this is a compromised-host figure, not a general one (PMID:38052876). CDC states: "Severe complications can occur with C. pneumoniae infections. These complications can result in hospitalization and sometimes death," while keeping overall risk low. No reliable disease-specific case-fatality rate, 5-year survival, or life-expectancy figure exists, and none should be invented; the organism is rarely the sole attributed cause of death.

Severity predictors. Two independent associations from a 133-child case-control study (PMID:42327901): decreased lymphocyte percentage (OR 0.943, 95% CI 0.895–0.994) and elevated IgA (OR 2.227, 95% CI 1.284–3.972); severe cases also had higher peak temperature and longer fever duration (both P < 0.001). A separate 176-patient series identified eosinophil count as a candidate severity biomarker with severe CPP at 6.8% (PMID:40775274). These are single-center, retrospective, and unvalidated — curate as PROPOSED/UNVALIDATED prognostic markers, not established ones.

Complications. Encephalitis, myocarditis, asthma exacerbation (CDC); reactive arthritis, Guillain-Barré syndrome (PMID:40279169); pericarditis with Guillain-Barré in a 2025 case report (PMID:41157205); reactive infectious mucocutaneous eruption (PMID:42094854); rash clusters (PMID:42755900).

Chronic sequelae — asthma. The best-quantified chronic association:

"The population attributable risks for Cp-specific IgG and IgA were nul in children and were 6% (95% confidence interval 2%-10%, p = 0.002) and 13% (9%-18%, p<0.00001) respectively in adults." PAR for Cp-specific IgE was 47% (39%–55%, p<0.00001) combined across ages, rising with severity: 5% in mild/controlled, 28% in moderate/partly controlled, 39% in severe/uncontrolled asthma. 25 studies. — Hahn DL, PLoS One 2021 (PMID:33872336)

The same author's 2026 review is framed as an open question (PMID:42690975) — this association is real in the seroepidemiology and unresolved in causation. Curate it as a mechanistic_hypotheses entry with status: EMERGING, not as an established sequela.

Chronic sequelae — atherosclerosis: the negative result matters. The hypothesis was seroepidemiologically attractive and mechanistically plausible (infected endothelium and smooth muscle; azithromycin prevented lesions in a rabbit model, PMID:10548582), and it failed in humans. WIZARD and the Azithromycin and Coronary Events Study (ACES, PMID:15843666) were both negative; PROVE-IT-TIMI and SPACE (PMID:15749042) likewise. "Antibiotic trials showed no favorable impact on atherosclerosis outcomes" (PMID:40279169). Curate this as a REFUTE evidence item against the therapeutic-causation claim.

Neurodegeneration. A 2026 Nature Communications study identified C. pneumoniae inclusions in Alzheimer retina and brain, increasing with APOEε4, disease stage, and cognitive deficit, with NLRP3 inflammasome activation (PMID:41571675). This is association-plus-model-system, not established causation, and belongs in discussions: with kind: HUMAN_MODEL_MISMATCH or KNOWLEDGE_GAP.

Recovery. Complete in most; the residual is protracted cough and malaise over weeks to months (CDC).

12. Treatment

First-line pharmacotherapy

"First-line agents include macrolides (azithromycin or clarithromycin), tetracycline (doxycycline) or fluoroquinolones (levofloxacin or moxifloxacin)." — PMID:40279169

CDC: macrolides first-line, tetracyclines and fluoroquinolones as alternatives; penicillin, ampicillin, and sulfa drugs are ineffective — the organism lacks a usable β-lactam target. Persistent infection may require a second course.

Observed prescribing in 291 children (PMID:41210234): doxycycline 30.58%, azithromycin 23.71%, erythromycin 11.34%.

NCIT treatment-action bindings (verified against cache/ncit/terms.csv on 2026-09-26): NCIT:C15986 Pharmacotherapy for the drug treatments, with therapeutic_agent carrying the CHEBI drug; NCIT:C15620 Antibiotic Therapy is available as a more specific action term; NCIT:C15747 Supportive Care; NCIT:C94624 Oxygen Therapy; NCIT:C70909 Mechanical Ventilation for the rare severe case; NCIT:C17003 Polymerase Chain Reaction for the diagnostic (note this is a diagnostic, not a TreatmentTerm). therapeutic_modality: SMALL_MOLECULE applies to every agent above.

Resistance

Effectively absent. "There is currently no report of resistant isolates" (PMID:40279169). The one careful investigation of three isolates whose azithromycin MIC rose after treatment found no genetic mechanism and could not induce resistance in vitro (PMID:15328134). In vitro MIC₉₀ for azithromycin is 0.125–0.25 µg/mL, comparable to erythromycin and doxycycline.

The clinically meaningful failure mode is not resistance but persistence: sub-optimal dosing induces the aberrant-body state in vitro, and β-lactam exposure does the same (PMID:40279169). Treat that as a distinct mechanism node, not as resistance.

Guideline context

The 2019 ATS/IDSA adult CAP guideline (Metlay JP et al., Am J Respir Crit Care Med 2019;200:e45–e67, PMID:31573350) no longer recommends routine macrolide monotherapy where local pneumococcal macrolide resistance exceeds 25%, and recommends β-lactam plus macrolide combination therapy for hospitalized patients. Atypical coverage is therefore delivered empirically within CAP regimens rather than through pathogen-directed therapy, which is the practical reality for almost every case of this disease. Doxycycline 100 mg BID and amoxicillin 1 g TID are the healthy-outpatient options alongside azithromycin.

Advanced therapeutics

Gene therapy, cell therapy, RNA-based therapy, targeted therapy, immunotherapy, surgery: none applicable. No oligonucleotide, monoclonal antibody, or cellular product exists or is in development for this infection. oligonucleotide_details and delivery_system blocks do not apply.

Supportive care

Antipyretics, hydration, oxygen for hypoxemia, and ventilatory support in the rare severe case. Rehabilitation is not routinely indicated.

Experimental treatments

No active interventional trial specific to C. pneumoniae pneumonia was identified in this search. Historical anti-chlamydial cardiovascular trials (WIZARD, ACES/PMID:15843666, SPACE/PMID:15749042, PROVE-IT-TIMI) are negative and are not treatments for this disease. A registered anti-Chlamydophila combination-therapy coronary study exists (NCT03618108) but again targets cardiovascular outcomes. Any clinical_trials: block should be populated only after just fetch-reference NCT… verification.

Pharmacogenomics

No CPIC guideline, PharmGKB annotation, or FDA pharmacogenomic biomarker applies to macrolide, tetracycline, or fluoroquinolone therapy for this indication. Not applicable.

13. Prevention

Primary prevention. No vaccine exists or is licensed. MOMP (major outer membrane protein) remains the leading candidate antigen — highly immunogenic, relatively conserved across the genus, eliciting T-cell responses and neutralizing antibodies — but conformationally correct MOMP is hard to produce at scale, and work remains preclinical (multi-epitope designs, lipid-nanodisc folding, live-attenuated oral concepts). A 2023 Scientific Reports multiepitope design against C. pneumoniae is computational. Immunization: none available. NCIT NCIT:C15346 Vaccination should not be curated as a treatment or prevention for this entry.

The only demonstrated primary prevention is transmission interruption. The pandemic NPI period produced a documented, statistically significant fall in detection (PMID:40511472), and outbreak investigations in military training environments explicitly targeted barracks density and training practices (PMID:30690452).

Secondary prevention. No screening program for asymptomatic individuals. In outbreak settings, the operationally useful recommendation is diagnostic rather than screening:

Because C. pneumoniae infection can be treated by macrolides, doxycycline, or fluoroquinolones, PCR testing for both C. pneumoniae and M. pneumoniae should be considered in symptomatic patients rather than testing only for respiratory viruses. — Tagini, Opota, Greub, Emerg Infect Dis 2024;30(4):810-812

Tertiary prevention. Adequate dosing and duration to avoid inducing persistence; recognition and management of the asthma phenotype in patients with chronic infection biomarkers.

Genetic counseling, carrier screening, PGD, prenatal testing: not applicable.

Public health. The recurring finding across every 2024–2026 surveillance paper is the absence of notifiable-disease status and the resulting invisibility of this pathogen. CDC: "There is no national reporting or surveillance system for C. pneumoniae infections." The Chinese national study makes the same point and calls for enhanced surveillance (PMID:41088528).

14. Other Species / Natural Disease

Host range

C. pneumoniae has an unusually broad host range for a species that behaves clonally in humans. Molecular detection spans humans, horses, koalas, bandicoots, potoroos, cattle, cats, dogs, wild ruminants and cervids, plus reptiles (snakes, iguanas, chameleons) and amphibians (frogs, turtles) (PMID:12086181).

Suggested taxonomy bindings, to be re-verified at curation time: NCBITaxon:9606 Homo sapiens; the koala (Phascolarctos cinereus), horse (Equus caballus), and amphibian/reptile hosts have NCBI Taxonomy identifiers that were not looked up in this session and must not be written from memory.

Zoonotic origin — and the absence of current zoonotic transmission

This distinction is the one most often stated wrongly. The genomic evidence is for an ancient host jump, not for ongoing animal-to-human transmission:

"Transmission of C. pneumoniae between animals and humans has not been reported; however, two other chlamydial species, C. psittaci and C. abortus, are known zoonotic pathogens. We have sequenced the 1,241,024-bp chromosome and a 7.5-kb cryptic chlamydial plasmid of the koala strain of C. pneumoniae (LPCoLN)… we propose based on compelling genomic and phylogenetic evidence that humans were originally infected zoonotically by an animal isolate(s) of C. pneumoniae which adapted to humans primarily through the processes of gene decay and plasmid loss, to the point where the animal reservoir is no longer required for transmission." — Myers GS et al., J Bacteriol 2009 (PMID:19749045)

"Our evidence strongly supports two separate animal-to-human cross species transfer events in the evolutionary history of this pathogen. The C. pneumoniae human genotype identified in the USA, Canada, Taiwan, Iran, Japan, Korea and Australia (non-Indigenous) most likely originated from a single amphibian or reptilian lineage… We identified a separate human lineage present in two Australian Indigenous isolates." — Mitchell CM et al., PLoS Pathog 2010 (PMID:20502684)

Koala isolates differ from human and horse strains at four gene loci (0.3% at groESL to 9.0% at ompA VD4), and the koala LPCoLN genome shows extended diversity relative to human strains (PMID:20646324).

Named-entity caution for curators. Koala chlamydial disease in the veterinary literature is overwhelmingly Chlamydia pecorum (ocular and urogenital disease), not C. pneumoniae. Do not import koala chlamydiosis phenotypes into this entry.

Comparative biology

Human strains show heightened sensitivity to tryptophan bioavailability relative to animal strains, interpreted as a positive adaptation to the human host (Mol Microbiol 2014) — an elegant link between the host-adaptation story and the IFN-γ/IDO1 mechanism in §6. Gene decay and plasmid loss are the two documented modes of human adaptation.

Orthologous genes

Not applicable — there is no human disease gene to orthologize. Murine orthologs of the modifier genes in §4 (Tlr2, Tlr4, Ifng, Il10, Nod1) are the relevant comparison and are named in the model studies.

15. Model Organisms

Mouse — the workhorse

Intranasal or intratracheal inoculation produces self-limited pneumonia with partially protective acquired immunity: bacterial load peaks in week two and clears by roughly six weeks in wild-type animals, with faster clearance on reinfection.

Model limitations to record as divergences. The TLR early-life study uses C. muridarum, not *C. pneumoniae* — a SPECIES_MISMATCH at the pathogen rather than the host, and a material one, since the murine-adapted organism differs in virulence and tropism. The murine inocula (5 × 10⁵ organisms intratracheally) are large relative to any plausible natural exposure — SUPRAPHYSIOLOGICAL_EXPRESSION in spirit if not in name. Mouse lungs lack the human airway's proportion of ciliated surface relevant to the ciliostasis mechanism, and no murine model reproduces the human asthma phenotype that motivates most of this work — that is a HUMAN_MODEL_MISMATCH discussion, not a solved question.

ModelMechanismLink guidance. The iBALT/fibrosis model (PMID:24204830) links at model_scale: TISSUE to a tissue-level fibrosis node — no scale extrapolation. The IFN-γ neutralization model links at ORGANISM scale to the molecular IFN-γ node — downward containment, unremarkable. The sst1 model should carry relationship: PARTIALLY_RECAPITULATES with limitations recording that it dissociates tolerance from clearance, which is the whole point of the paper.

Cell and in vitro systems

No organoid, organ-chip, or iPSC-derived model of C. pneumoniae pneumonia was identified. This is a real gap: every current NAM-relevant claim rests on immortalized monocultures.

Resources

MGI, IMPC, IMSR for the knockout lines (Tlr2, Tlr4, Il10); ATCC/Cellosaurus for A549 and HEp-2; the Alliance of Genome Resources for ortholog mapping. No dedicated C. pneumoniae model repository exists.

Consolidated evidence table for KB ingestion

Each row is a candidate EvidenceItem. Snippets marked verbatim below were captured from the cited abstract or the named source page during this research and still require just fetch-reference plus just count-verified-snippets confirmation before commit — per repository policy the cache file, not this report, is the authority for exactness.

Gaps, cautions, and what this report does not establish

1. No species-level MONDO term exists. The entry must anchor to a genus-level term with an explicit mapping predicate and a note recording the searches run.
2. PCR positivity is not causation. The single most-cited caveat in the diagnostic literature (PMID:9666010) undercuts every prevalence figure derived from PCR alone, including the recent resurgence numbers. The resurgence is real across independent sites, but part of its magnitude is mNGS/tNGS adoption.
3. Persistence in vivo is unresolved. Aberrant bodies are an in vitro phenomenon under IFN-γ, iron restriction, or β-lactam pressure. A 2026 review poses in vivo persistence as an open question (PMID:42690975). Curate the persistence chain as hypothesis, not mechanism.
4. Atherosclerosis causation is refuted by trial evidence and should carry REFUTE items, not be quietly omitted.
5. Asthma causation is unresolved, quantitatively large, and IgE-driven. The PAR of 47% for Cp-specific IgE is the strongest number in this whole literature and the weakest in design (cross-sectional and case-control, no large prospective population study exists — the meta-analysis says so explicitly).
6. The Alzheimer's retina finding (PMID:41571675) is new, high-profile, and not a pneumonia finding. Do not import it into the pneumonia pathograph.
7. No omics dataset specific to C. pneumoniae pneumonia was verified. Do not write a geo: accession without running just discover-datasets and just verify-datasets.
8. Every CURIE in this report was read from a source during this session — HPO and CL/UBERON/CHEBI/GO/NCIT from cache/*/terms.csv or the EBI OLS4 API, HGNC from cache/hgnc/terms.csv or genenames.org, MONDO/ICD/MeSH/SNOMED from the OLS4 MONDO term record and the NLM MeSH lookup, ICD-11 CA40.00 from the ICD-11 MMS listing. Re-validate with just validate-terms before commit; dynamic-enum membership was not checked here and is a separate gate.

Sources

• Chlamydia beyond the genital tract: a major contributor to community-acquired pneumonia (Front Cell Infect Microbiol 2026, PMID:42180254)
• From coughs to complications: the story of Chlamydia pneumoniae (J Med Microbiol 2025, PMID:40279169)
• CDC — Laboratory Testing for Chlamydia pneumoniae
• CDC — Clinical Overview of Chlamydia pneumoniae Infection
• CDC — Clinical Features of Chlamydia pneumoniae Infection
• Epidemiological changes in C. pneumoniae molecular detections, 27 European sites and Taiwan (Eurosurveillance 2025, PMID:40511472)
• Chlamydia pneumoniae Upsurge at Tertiary Hospital, Lausanne, Switzerland (EID 2024)
• Sharp increase in Chlamydia pneumoniae infections in 2024 in Germany (PMID:40884594)
• Epidemiological and clinical analysis of 291 children with C. pneumoniae pneumonia (PMID:41210234)
• Chlamydophila pneumoniae-associated CAP in paediatric patients, Mexico (PMID:38052876)
• Chlamydia pneumoniae and chronic asthma: meta-analysis of population attributable risk (PMID:33872336)
• Genetic control of susceptibility to pulmonary infection with C. pneumoniae in the mouse (PMID:18075514)
• TLR2, but not TLR4, is required for host defence against Chlamydia respiratory infection in early life (PMID:22724018)
• C. pneumoniae hides inside apoptotic neutrophils (PMID:19547701)
• Induction of proinflammatory cytokines in human lung epithelial cells (PMID:12540537)
• Evidence that human Chlamydia pneumoniae was zoonotically acquired (PMID:19749045)
• C. pneumoniae is genetically diverse in animals (PMID:20502684)
• Tryptophan depletion as a mechanism of IFN-γ-mediated chlamydial persistence (PMID:8063385)
• Azithromycin for the Secondary Prevention of Coronary Events, ACES (PMID:15843666)
• 2019 ATS/IDSA CAP guideline summary (AFP 2020)
• ICD-11 MMS CA40.0 Bacterial pneumonia, including CA40.00
• Pneumonia Outbreak Caused by Chlamydophila pneumoniae among US Air Force Academy Cadets (PMID:25988545)
• Outbreak of C. pneumoniae Infections in Army Trainees at Fort Leonard Wood (PMID:30690452)
• Chlamydia pneumoniae-induced ciliostasis in ciliated bronchial epithelial cells (PMID:7751703)
• Profile of Chlamydia vaccine research: a bibliometric analysis (2025)

Reference Validation

Checked with linkml-reference-validator 0.3.0rc3.

Quotes not found in the cited source

Searched the abstract, any retrieved full text, and the title. A quote drawn from a part of the paper that was not retrieved will appear here too, so check before treating one as invented:

Every one of these was searched against an abstract alone, with no full text retrieved - marked abstract only below. Where full text can be fetched, re-running with it will settle them; where the source publishes only a summary to PubMed, as GeneReviews chapters do, it will not, and the quote has to be checked by hand against the chapter itself.

• PMID:40279169 (abstract only): "Antibiotic trials showed no favorable impact on atherosclerosis outcomes"
• closest text in source: "Chronic infections have been linked to asthma and, more controversially, to atherosclerosis and neurodegenerative diseases"
• PMID:40279169 (abstract only): "There is currently no report of resistant isolates"
• Text part not found as substring: 'There is currently no report of resistant isolates' (note: only abstract available for PMID:40279169, full text may contain this excerpt)

Term Validation

Checked with linkml-term-validator 0.4.5, through the ols: adapter.

Terms the report names something else

These identifiers resolve, so nothing about them looks wrong, and the ontology calls them something unrelated to what the report calls them. That usually means the identifier is not the one the sentence needs:

• MONDO:0025598 (5 mentions) - the report calls it "MONDO"; MONDO calls it pneumonia caused by chlamydia
• DOID:0040083 (1 mention) - the report calls it "DOID"; DOID calls it Chlamydia pneumonia
• NCBITaxon:83558 (3 mentions) - the report calls it "NCBI Taxonomy (pathogen)"; NCBITaxon calls it Chlamydia pneumoniae

Prefixes with no resolver

Terms carrying these prefixes were not checked either way, because no configured ontology covers them. An unrecognised prefix may name an ontology this run could not reach as easily as one that does not exist, so nothing here is evidence of fabrication: ICD11.