Avian Chlamydiosis (Psittacosis) in Pet Birds: A Clinical Reference

Bottom line

Avian chlamydiosis is infection with Chlamydia psittaci, an obligate intracellular bacterium and a reportable zoonosis: infected — and even clinically normal — birds shed an environmentally durable elementary body that causes psittacosis (parrot fever) in people [3][6]. Because shedding is intermittent, no single antemortem test rules the disease in or out; the standard of care is to combine PCR, serology, and the clinical picture [1]. Doxycycline is the drug of choice, given for a full 45 days (roughly 30 days in budgerigars) [1]. Treat every case as a human-health event — counsel the owner, advise PPE and hygiene, and report to public health where required [1][6].

Etiology: an obligate intracellular bacterium with a two-form life cycle

C. psittaci is an obligate intracellular bacterium that replicates inside a membrane-bound vacuole and cycles between two morphological forms: the small (~0.2 μm) elementary body (EB), the infectious, environmentally stable extracellular form, and the larger (~0.8 μm) reticulate body (RB), the metabolically active intracellular form that divides; one full cycle runs about 36–72 hours [3]. The EB is the reason this disease is so hard to eradicate from a collection — it resists drying and, protected by organic debris such as faeces and bedding, remains infective for several months [3]. Merck's professional edition adds that the organism is destroyed in under 5 minutes at 56 °C but is resistant to acid and alkali [2].

The host range is enormous: avian chlamydiosis has been documented in 467 bird species [3]. As of July 2026, strain classification is based on the ompA gene, which resolves 17 genotypes (A–G, E/B, WC, M56 and others); genotype A predominates in psittacines and is considered highly virulent, with B in pigeons, C in waterfowl, D endemic in poultry, and F in parakeets [3]. Psittacines — parrots, cockatiels, budgerigars, macaws, and conures — are the classic pet reservoir, but pigeons and doves, poultry, and passerines are all susceptible [3].

How it spreads, and why subclinical carriers drive outbreaks

Birds acquire C. psittaci mainly by inhaling or ingesting aerosolized respiratory and ocular secretions, dried faeces, and feather dust [3]. The clinically decisive feature is the carrier state: latently infected birds appear healthy yet shed the organism intermittently for months to years, so a normal-looking bird can seed an entire aviary [4]. Shedding — and overt disease — is precipitated by stress such as breeding, shipping, crowding, or climatic extremes [4], which is why newly acquired birds, recently shipped stock, and pet-store and aviary populations are the highest-risk settings. The Compendium notes that pet parrots, cockatiels, parakeets, and macaws are commonly implicated sources [1]. The incubation period is commonly 3–10 days but can extend to several weeks [2].

Clinical presentation

Presentation ranges from inapparent carriage to fatal systemic disease, and the signs are non-specific. Expect lethargy, ruffled feathers, anorexia, and weight loss, with nasal and ocular discharge, conjunctivitis, and sinusitis on the respiratory side [2]. The finding that should raise your index of suspicion is bright green to yellow-green urates (biliverdinuria), reflecting hepatic involvement, frequently accompanied by diarrhoea [4]. Hepatosplenomegaly can produce coelomic distension [2], and emaciation, dehydration, and death follow in advanced cases [2]. Severity varies with host species, infecting genotype, and stress load — but because clinically normal intermittent carriers are common, the absence of dramatic signs never excludes infection [4]. Supportive clinicopathology typically shows an inflammatory leukogram and elevated hepatic enzymes and bile acids consistent with the hepatitis, though none of it is specific.

Diagnosis: why one test is never enough

No single antemortem test is definitive, so the NASPHV Compendium recommends combining methods — culture, PCR, and antibody detection — particularly when only one bird is available to test [1]. PCR on a combined choanal/cloacal or conjunctival swab (or blood) is the modern mainstay, but because infected birds may not shed detectable organism in faeces at any given moment, a single negative PCR does not rule out infection [1]; PCR also cannot distinguish viable from non-viable organisms [1].

Serology (elementary-body agglutination, IFA, complement fixation) carries its own caveats: a positive titer is evidence the bird was infected by Chlamydiaceae at some point but may not indicate active infection [1], so a single titer is hard to interpret, and a four-fold rise between paired acute and convalescent samples is far more informative [2]. In practice, a probable case is defined as compatible clinical signs plus a single high serologic titer or detection of Chlamydiaceae antigen by ELISA, PCR, or immunofluorescence [4]. Round out the workup with a CBC and biochemistry, and confirm fatal or ambiguous cases at necropsy with histopathology, immunohistochemistry, or culture.

Treatment: doxycycline for 45 days

Doxycycline is the drug of choice, and the long course — classically 45 days [1][4] — reflects the organism's protected intracellular life cycle. Oral starting dosages in the Compendium are species-specific: cockatiels 25–35 mg/kg q24h, African grey parrots 25 mg/kg q24h, Amazon parrots 25–50 mg/kg, and macaws 25–30 mg/kg q24h; direct oral (per-os) dosing is not listed for budgerigars, which are instead treated with a medicated seed diet [1]. Where injections are used, long-acting doxycycline is given at 75–100 mg/kg IM every 5–7 days for the first four weeks and then every 5 days [1]; Merck's professional edition instead lists 60–100 mg/kg SC or IM every 5–7 days for 45 days and notes SC is preferred over IM to limit tissue reaction [2]. For flock treatment via medicated water, dose by species — 200–400 mg doxycycline hyclate/L for cockatiels, 400–600 mg/L for Goffin's cockatoos, 800 mg/L for African greys, and 400 mg/L for most other species, but not for budgerigars [1].

Give the full 45 days uninterrupted; budgerigars can clear on 30 days, and the Compendium notes shorter courses of 21–30 days may also be effective in budgerigars [1]. Critically, remove all calcium and mineral supplements — oyster shell, mineral blocks, cuttlebone — during treatment, because dietary calcium chelates doxycycline and impairs absorption [1]. Perform a test of cure, and counsel owners that completing therapy does not guarantee clearance — which is exactly why record-keeping and retesting matter.

Zoonosis: your reporting and counseling duties

Treat every avian chlamydiosis case as a human-health event. C. psittaci is a nationally notifiable zoonosis — psittacosis has been notifiable in the United States since 1944 [6] — and the Compendium notes that most states require physicians to report human psittacosis to state or local public-health authorities [1]. People acquire psittacosis chiefly by inhaling aerosolized dried faeces, urine, and respiratory or ocular secretions, and through contact with plumage and tissues, mouth-to-beak contact, or bites [3].

Human disease runs from an influenza-like illness — abrupt fever, chills, headache, and dry cough — to atypical community-acquired pneumonia with hepatosplenomegaly, and severe cases progress to sepsis and multi-organ failure that is occasionally fatal [5]. Before antibiotics the case-fatality rate was roughly 15–20%, versus under 2.5% with modern treatment [5]; C. psittaci accounts for about 1% of community-acquired pneumonia worldwide [3], and the Compendium recorded 112 human cases reported to CDC during 2003–2014 [1]. Highest-risk people include bird owners, aviary and pet-store staff, poultry workers, and veterinary teams [5]. Your duties are concrete: inform the owner of the human-health risk, advise PPE (a well-fitted respirator and gloves) plus wetting surfaces before cleaning and good ventilation — avoiding dry sweeping or vacuuming that aerosolizes the agent — and report per your jurisdiction. Physicians treat human psittacosis with doxycycline, typically 100 mg every 12 hours for 7–10 days [5].

Prevention and flock management

Prevention hinges on quarantine and control of movement: quarantine and test newly acquired birds before introducing them to a collection, isolate and treat every confirmed or probable positive for the full course, minimize stress, and keep records that let you trace exposures [1][4]. C. psittaci is susceptible to common disinfectants — a 1:1000 dilution of quaternary ammonium compounds, 1% Lysol, and a freshly prepared 1:32 dilution of household bleach [1], as well as 70% alcohol and heat (under 5 minutes at 56 °C) [2] — but it resists acids and alkalis and can persist for months in organic matter such as litter and nest material [2], so remove organic debris before disinfecting.

When a wasting or dyspneic bird presents, keep the avian differentials wide: aspergillosis for lower-respiratory disease, hepatic lipidosis for a large liver and biliverdinuria, and proventricular dilatation disease (PDD) for chronic weight loss — each is covered in its own hub, and C. psittaci testing belongs in that same workup because the signs overlap so heavily.

Frequently Asked Questions

Can a bird that looks completely healthy still transmit Chlamydia psittaci? Yes. LafeberVet notes that latently infected birds appear healthy yet shed the organism intermittently for months to years, so a clinically normal bird can infect other birds and people — the central reason the disease persists in collections.

Does a single negative PCR rule out avian chlamydiosis? No. The NASPHV Compendium states that infected birds may not shed detectable organism in faeces at any given time, and that no single test is adequate — it recommends combining culture, PCR, and antibody detection, especially when only one bird is tested.

What is the doxycycline dose and duration for a psittacine? The NASPHV Compendium lists species-specific oral starting doses: cockatiels 25–35 mg/kg q24h, African grey parrots 25 mg/kg q24h, Amazon parrots 25–50 mg/kg, and macaws 25–30 mg/kg q24h, for a full 45 days (about 30 days in budgerigars). Long-acting injectable doxycycline is 75–100 mg/kg IM every 5–7 days for the first four weeks, then every 5 days.

Why must calcium be restricted during doxycycline treatment? Because dietary calcium chelates doxycycline and impairs its absorption. The NASPHV Compendium directs clinicians to remove all calcium and mineral supplements — oyster shell, mineral blocks, and cuttlebone — for the duration of treatment.

Is avian chlamydiosis reportable, and how dangerous is it to people? Psittacosis has been a nationally notifiable condition in the United States since 1944 (CDC/NNDSS), and most states require reporting. Per StatPearls, human disease ranges from a flu-like illness to atypical pneumonia and, in severe cases, multi-organ failure; case fatality was about 15–20% before antibiotics but is under 2.5% with modern treatment.

Which disinfectants inactivate C. psittaci? The NASPHV Compendium lists a 1:1000 quaternary ammonium dilution, 1% Lysol, and a freshly prepared 1:32 household bleach dilution; Merck adds 70% alcohol and heat (under 5 minutes at 56 °C). Merck also notes the organism resists acids and alkalis and can survive for months in organic matter, so remove debris before disinfecting.

How should new birds be handled before joining a collection? The NASPHV Compendium recommends quarantining and testing newly acquired birds before introduction, isolating and treating any positives for the full course, and combining diagnostic methods rather than relying on one test.