Avian Chlamydiosis

Chlamydia psittaci, the primary pathogen that causes avian chlamydiosis, is an obligate intracellular bacterium. Eight avian serotypes are recognized, based on binding of monoclonal antibodies to epitopes of the major outer membrane protein; of these, 6 (A–F) infect avian species and are distinct from mammalian Chlamydia serotypes.

Strains of C psittaci have been classified into 9 genotypes using genetic differences in the omp1 gene. Seven of these (A, B, C, D, E, F, and E/B) are found in avian species and usually correspond to the equivalent serotype. Six additional genotypes were described later.

Each avian serotype/genotype tends to be associated with certain types of birds (see the table Associations Between Avian Genotypes of Chlamydia psittaci and Types of Birds). Serotypes A and D are highly virulent in turkeys and can cause mortality rates ≥ 30%. Serotypes B and E are most frequently recovered from wild birds. Avian serotypes (particularly A, B, and E/B) can infect humans and other mammals.

The life cycle of C psittaci involves 4 stages:

• elementary bodies (environmentally resistant, extracellular contagious particles; see elementary bodies image)

• intermediate bodies (intracellular transition phase with a morphology between elementary body and reticulate body)

• reticulate bodies (intracellular replicative stage)

• aberrant bodies (intracellular nonreplicating particles formed in certain stressful circumstances [eg, antimicrobial treatment] that revert to reticulate bodies when the stressor disappears)

Chlamydia psittaci, elementary bodies

Image

Courtesy of Dr. Jean Sander.

After elementary bodies are inhaled or ingested from the environment, they attach to mucosal epithelial cells and are internalized by endocytosis. Elementary bodies within endosomes in the cell cytoplasm inhibit phagolysosome formation and differentiate into metabolically active, noninfectious reticulate bodies that divide and multiply by binary fission, eventually forming numerous infectious, metabolically inactive elementary bodies. Newly formed elementary bodies are released from the host cell by lysis.

Other Chlamydiaceae species usually have a more limited number of avian hosts. Additional Chlamydia spp infecting birds include the following:

• Chlamydia gallinacea: common in chickens; also infects turkeys and guinea fowl

• Chlamydia avium: pigeons and psittacine species

• Chlamydia buteonis: birds of prey

• Candidatus Chlamydia ibidis: ibises

Infection with C gallinacea is subclinical but can result in decreased weight gain in broilers and death in experimentally infected embryonated chicken eggs. Infection with C avium is usually subclinical; however, rare cases of respiratory disease in psittacine species and one report of fatal disease in picazuro pigeons have been reported. Chlamydia abortus strains closely related to C psittaci occur in poultry and wild birds and have not been associated with disease. The zoonotic potential of these Chlamydia spp remains to be determined.

Avian chlamydiosis is a reportable disease; state and local governmental regulations should be followed wherever applicable. The disease has a worldwide geographic distribution.

Chlamydial infections have been identified in at least 465 avian species, particularly caged birds (primarily psittacine species), colonial nesting birds (eg, egrets, herons), ratites, raptors, and poultry. Among domestic species, turkeys and ducks are most often affected.

Transmission is mainly by the fecal-oral route or by inhalation. Respiratory discharge and feces from infected birds contain elementary bodies that are resistant to drying and can remain infective for several months when protected by organic debris (eg, litter and feces). Airborne particles and dust spread the organism.

After elementary bodies are inhaled or ingested, the incubation period is typically 3–10 days but can be up to several weeks in older birds or after low-level exposure. Host and microbial factors, route and intensity of exposure, and treatment determine clinical course.

Arthropod-borne transmission by blood-sucking ectoparasites is possible. Vertical transmission has been documented in several bird species, including turkeys, chickens, and ducks.

Possible sources of C psittaci include the following:

• contact with infected, clinically ill birds or clinically normal carriers

• vertical transmission from infected birds

• infected mammals

• infected arthropods

• contaminated environments

Stressors (eg, transport, crowding, breeding, cold or wet weather, dietary changes, or decreased food availability) and concurrent infections, especially those causing immunosuppression, can initiate shedding in latently infected birds and cause recurrence of clinical signs. In turkeys, C psittaci and Ornithobacterium rhinotracheale infection are often concurrent. Carriers often shed the organism intermittently for extended periods. Persistence of C psittaci in the nasal glands of chronically infected birds may be an important source of persistent shedding.

Longterm inapparent infections lasting for months to years are common and are considered the normal Chlamydia-host relationship. The prevalence of infection varies considerably between species and by geographic location:

• Psittacine species most commonly develop clinically apparent infections.

• Infection is endemic in commercial turkey flocks; no clinical signs or mild respiratory signs and low mortality rates are common. Outbreaks are rare.

• Chickens are relatively resistant to developing clinical signs of chlamydiosis, and subclinical infection is frequent.

• Wild birds are often seropositive for C psittaci.

Severity of clinical signs and lesions of avian chlamydiosis depends on the virulence of the organism, infectious dose, stress factors, and susceptibility of the bird species. Subclinical infections are common.

Clinical signs include the following:

• nasal and ocular discharge

• conjunctivitis

• sinusitis

• green to yellow-green, soft to watery droppings (diarrhea)

• fever

• inactivity

• ruffled feathers

• weakness

• inappetence

• coelomic distension from hepatosplenomegaly

• weight loss

• drop in egg production

In parrots, respiratory signs, listlessness, weakness, green droppings, and coelomic distension are common. Many parrots are infected subclinically.

Respiratory signs predominate in turkeys and chickens. Watery diarrhea is often present in ducks. Young birds are more likely to develop severe disease. Chickens are relatively resistant to disease, with death occurring mainly in young birds. Ducklings may show trembling, imbalanced gait, watery diarrhea, conjunctivitis, rhinitis, anorexia, and weight loss.

Conjunctivitis and rhinitis may be observed in adult pigeons. Fibrinous polyserositis, hepatitis, and enteritis are observed in squabs.

Clinicopathological test results vary with the organs affected and severity of disease. Hematologic changes most often present are anemia and leukocytosis with heterophilia and monocytosis. Plasma bile acid concentration, AST activity, LDH activity, and uric acid concentration may be increased.

Radiographs, CT scan, ultrasonographic examination, and laparoscopy can reveal an enlarged liver and spleen and thickened air sacs.

Necropsy findings in acute chlamydiosis infections in birds include the following:

• serofibrinous polyserositis (airsacculitis, pericarditis, perihepatitis, peritonitis)

• bronchopneumonia

• hepatic necrosis

• hepatomegaly (see hepatitis image)

• splenomegaly

See images of acute chlamydiosis with polyserositis in pigeon and chlamydiosis with hepatomegaly in lorikeet.

Chlamydiosis, hepatitis, cockatiel

Image

Courtesy of Dr. A. J. Van Wettere.

Acute chlamydiosis, pigeon

Image

Courtesy of Dr. A. J. Van Wettere.

Chlamydiosis, lorikeet

Image

Courtesy of Dr. A. J. Van Wettere.

Similar lesions are present with other systemic bacterial infections and are not specific for avian chlamydiosis. Small granular basophilic intracytoplasmic bacterial inclusions may be observed in multiple cell types (eg, epithelial cells, macrophages) on cytological and histological examination.

In chronic infections, pallor and enlargement of spleen or liver may be noted. Necrosis and bacterial inclusions are not typically observed. Lesions are usually absent in latently infected birds, even though C psittaci may be shed in respiratory secretions and feces.

• For flocks: serological testing, necropsy, and PCR assay

• For individuals: PCR assay or culture, antibody titers from paired samples, or a combination of serological testing and PCR assay or culture

Because of the variety of clinical signs and common occurrence of latently infected carriers, no single diagnostic test can reliably determine infection with Chlamydia spp. Procedures to detect the organism or antibodies are used.

In general, the more acute the disease, the greater the number of infective organisms and the easier it is to make a diagnosis. When birds are acutely ill, clinical findings, including hematologic testing, biochemical analysis, and radiological evaluation or typical gross lesions, are adequate for a tentative diagnosis.

The combination of a serological test with an antigen detection test, PCR assay, or culture is a practical diagnostic scheme to confirm chlamydiosis. In individual birds, the preferred sample for bacterial culture or PCR assay is conjunctival, choanal, and cloacal swabs. Multiple samples collected throughout 3–5 days are recommended for detection of intermittent shedding by subclinically affected birds.

Antibodies may not be detectable, depending on the test used and on the level and stage of infection. Antibody testing often is used to distinguish past exposure from current infection. Interpretation of titers from single serum samples is difficult. A 4-fold increase in titers between paired acute and convalescent samples is diagnostic, and high titers in a majority of samples from several birds in a population are sufficient for presumptive diagnosis.

Serological methods include direct and modified direct complement fixation, elementary body agglutination, antibody ELISA, and indirect immunofluorescence. The elementary body agglutination test detects IgM and is useful to determine recent infection. Complement fixation methods are more sensitive than agglutination methods. High antibody titers can persist for years after treatment and complicate evaluation of subsequent tests.

Infection with other Chlamydiaceae species such as C gallinacea in chickens and C avium in pigeons and psittacine species can complicate the interpretation of serological tests because antibodies detected are likely not specific for C psittaci.

Antigen detection methods include immunohistochemical analysis (eg, immunofluorescence, immunoperoxidase) and ELISA. In situ hybridization (ISH) can also be used to detect nucleic acids in tissue sections.

The specificity and sensitivity of ELISA kits developed for detection of Chlamydia trachomatis in humans is uncertain when the tests are used for detection of C psittaci in birds. They appear to have good specificity but somewhat low sensitivity, and they are not recommended for diagnosis. These kits are most useful when birds are clinically ill.

PCR assay is the most sensitive and specific test. Chlamydiaceae and C psittaci PCR assays are available. However, results can differ between laboratories because of the lack of standardized PCR primers and laboratory method variations. False-positive results can be a concern with PCR assay because cross contamination can occur relatively easily during sampling of individual birds in an aviary.

Gross and histological lesions are not pathognomonic. The organism can sometimes be identified in impression smears of affected tissues (eg, liver, spleen, and lung). Chlamydiae stain purple with Giemsa and red with Macchiavello and Gimenez stains. Immunohistochemical testing is more sensitive than histochemical stains to detect bacteria in tissue; however, cross reactivity with nonchlamydial organisms can occur.

In situ hybridization and electron microscopy can also be used to confirm diagnosis.

Isolation and identification of C psittaci can be performed in chick embryo or cell cultures (eg, BGM, L929, Vero) at a qualified laboratory. Cloacal, choanal, oropharyngeal, conjunctival, or fecal swabs (in a special Chlamydia transport medium) from live birds or tissues (liver and spleen preferred) from dead birds should be refrigerated and submitted promptly to the laboratory. Freezing, drying, improper handling, and improper transport media can affect viability. The laboratory should be contacted for directions for submitting samples.

Concurrent infections with other, more easily diagnosed diseases (eg, colibacillosis, pasteurellosis, herpesvirus infections, mycotic diseases) can mask chlamydial infection. Laboratory and clinical findings should be correlated. Chlamydiosis must be distinguished from other respiratory and systemic diseases of birds.

• Tetracyclines

• All-in all-out management, biosecurity, sanitation, and quarantine

Treatment of avian chlamydiosis prevents death and shedding but cannot be relied on to eliminate latent infection; shedding may recur.

Tetracyclines (chlortetracycline, oxytetracycline, doxycycline) are the antimicrobials of choice. Drug resistance to tetracyclines is rare; however, decreased sensitivity requiring higher dosages is becoming more common. Tetracyclines are bacteriostatic and effective only against actively multiplying organisms, making extended treatment times necessary (from 2–8 weeks, during which minimum inhibitory concentrations in blood must be consistently maintained).

Doxycycline is the current drug of choice because it is better absorbed and has less affinity for calcium, better tissue distribution, and a longer half-life than other tetracyclines. Doxycycline added to feed or water can also result in adequate blood levels and has less effect on normal intestinal flora than does chlortetracycline.

The dosage and duration of the treatment varies between species. Protocols derived from controlled studies performed in the particular species treated should be used when available. Also see information in the Compendium of Measures To Control Chlamydophila psittaci Infection Among Humans (Psittacosis) and Pet Birds (Avian Chlamydiosis), 2017, from the National Association of State Public Health Veterinarians.

For most species, treatment must be maintained uninterrupted for 45 days so that antimicrobials can reach C psittaci organisms during the replication phase of its life cycle; in its elementary body stage within macrophages, the bacterium is unreachable by antimicrobials and must be actively replicating for the drugs to be effective. When tetracyclines are administered orally, additional sources of dietary calcium (eg, mineral block, supplement, cuttlebone) should be decreased to minimize interference with drug absorption.

In budgerigars, 30 days of treatment can be effective; recent studies have shown that shorter treatment periods of 21–30 days may also be effective (1). However, given that budgerigars can carry and shed C psittaci without showing clinical signs, many clinicians still prefer to treat potentially infected budgerigars with the full 45-day course of doxycycline.

Outbreaks of clinical infection in poultry flocks are not common. Treating infected flocks with chlortetracycline (441–827 g/tonne [400–750 g/ton] of feed) for a minimum of 2 weeks has effectively decreased potential risk of infection for plant employees. Medicated feed must be replaced by nonmedicated feed for 2 days or more before slaughter and processing. Calcium supplementation must be withheld during treatment with chlortetracycline, with calcium concentration in the feed decreased to ≤ 0.7%. Medicated feed should be provided for 45 days if elimination of the organism is attempted.

Persistence of oxytetracycline residues in eggs of laying hens is 9 days, and persistence of doxycycline residues is 26 days after administration at 0.5 g/L in drinking water for 7 days. Treatment of poultry with doxycycline is extra-label and not approved for use in food-producing birds in the US. Veterinarians wanting to prescribe this drug for use in food animals must get prior governmental approval by contacting the Food Animal Residue Avoidance Databank.

Psittacine birds are the species most commonly affected clinically by chlamydiosis. Pigeons also may become infected with C psittaci; they may not show clinical signs as often as infected parrots, but they serve as an important reservoir for zoonotic spread of this disease.

Infected birds are typically treated with oral doxycycline for an uninterrupted 45-day course or long-acting injectable doxycycline. Clinicians treating chlamydiosis with long-acting injectable doxycycline should carefully research the formulation they use before using it; some formulations of injectable doxycycline have been associated with tissue reactions, and pharmacist-compounded formulations of doxycycline hyclate have been associated with anecdotal reports of sudden death (2).

Dosages for doxycycline for treatment of chlamydia in birds have largely not been validated. Treatment recommendations show oral doses starting at 25 mg/kg every 24 hours in psittacine birds, with precise dose ranges varying by species. The Compendium of Measures to Control Chlamydia psittaci Infection Among Humans (Psittacosis) and Pet Birds (Avian Chlamydiosis) provides suggested dose ranges species by species as follows:

• cockatiels: 25–35 mg/kg every 24 hours

• Senegal parrots and blue-fronted and orange-winged Amazon parrots: 25–50 mg/kg every 24 hours

• African grey parrots, Goffin's cockatoos, blue and gold macaws, and green-winged macaws: 25 mg/kg every 24 hours 

The most commonly used doxycycline dosage to treat parrots and pigeons is 25–50 mg/kg, PO, every 12–24 hours (3). This medication may cause regurgitation, especially in macaws and cockatoos, and therefore should be used at the lower dosage in these species.

In addition to doxycycline, oral azithromycin (40 mg/kg every 48 hours for the duration of therapy) has been documented to be an effective treatment for cockatiels.

Injectable doxycycline is effective in treating psittacosis when administered at 60–100 mg/kg, SC or IM, every 5–7 days for 45 days (3); however, this drug can cause tissue inflammation and necrosis in patients when administered IM. Therefore, SC administration is generally preferred.

All birds in contact with the clinically affected bird, even if they exhibit no clinical signs, should be treated to prevent reinfection by subclinical carriers.

For infected flocks, use of chlortetracycline-medicated feeds for 45 days was historically a standard recommendation for treating chlamydiosis in imported birds. Difficulties from palatability of the feed and the high level of antimicrobial necessary to achieve adequate blood levels have limited the use of chlortetracycline.

When specific information is lacking, an empiric starting dosage of doxycycline (400 mg/L of water, or 25–50 mg/kg, PO, every 12–24 hours for 45 days) has been suggested.

No effective vaccine for avian chlamydiosis is available.

Appropriate biosecurity practices are necessary to control the introduction and spread of chlamydiae in an avian population. Minimal biosecurity standards include the following:

• quarantine and examination of all new birds

• prevention of exposure to wild birds

• traffic control to minimize cross contamination

• isolation and treatment of affected and contact birds

• thorough cleaning and disinfection of premises and equipment (preferably with small units managed on an all-in all-out basis)

• provision of uncontaminated feed

• maintenance of records on all bird movements

• continual monitoring for presence of chlamydial infection

The organism is susceptible to heat (it can be destroyed in < 5 minutes at 56°C [133ºF]) and most disinfectants (eg, 1:1,000 quaternary ammonium chloride, 1:100 bleach solution, 70% alcohol) but is resistant to acid and alkali. It can persist for months in organic matter such as litter and nest material; thorough cleaning before disinfection is necessary.

Avian chlamydiosis is a zoonotic disease that can affect humans after exposure to aerosolized organisms shed from the digestive or respiratory tracts of infected live or dead birds or from the handling of infected birds, tissues (eg, slaughterhouse), or bedding.

Disease in humans most often results from exposure to pet psittacine birds and can occur even if there is only brief contact with a single infected bird. Other people in close contact with birds, such as pigeon fanciers, veterinarians, farmers, wildlife rehabilitators, zookeepers, and employees in slaughtering and processing plants or hatcheries, are also at risk. Zoonotic transmission of C psittaci in poultry industry workers is likely underestimated.

Individuals should take precautions (eg, dust mask and plastic face shield or goggles, gloves, detergent disinfectant to wet feathers, and fan-exhausted examining hood) to avoid exposure when examining live or dead infected birds.

Infection in humans varies from asymptomatic to the development of flulike clinical signs and respiratory disease (eg, pneumonia). Rarely, endocarditis, myocarditis, hepatitis, and encephalitis occur. Immunocompromised people are at increased risk of developing clinical signs of avian chlamydiosis.

• Vanrompay D. Avian chlamydiosis. In Swayne DE, ed. Boulianne M, Logue CM, McDougald LR, Nair V, Suarez DL, associate eds. Diseases of Poultry. 14th ed. Wiley Blackwell; 2020:1086-1108.

• Balsamo G, Maxted AM, Midla JW, et al. Compendium of measures to control Chlamydia psittaci infection among humans (psittacosis) and pet birds (avian chlamydiosis), 2017. J Avian Med Surg. 2017;31(3):262-282.

• Ravichandran K, Anbazhagan S, Karthik K, Angappan M, Dhayananth B. A comprehensive review on avian chlamydiosis: a neglected zoonotic disease. Trop Anim Health Prod. 2021;53(4):414.

• Zwijnenberg RJG, Vulto AG, Van Miert ASJPAM, Lumeij JT. Evaluation of antibiotics for racing pigeons (Columba livia var. domestica) available in The Netherlands. J Vet Pharmacol Ther. 1992;15(4):364-378. 

• Gonzlaez MS. Psittacine neonatology and pediatrics. Vet Clin North Am Exot Anim Pract. 2024;27(2):263-293.

1. Guzman, DSM, Diaz-Figueroa O, Tully T Jr, et al. Evaluating 21-day doxycycline and azithromycin treatments for experimental Chlamydophila psittaci infection in cockatiels (Nymphicus hollandicus). J Avian Med Surg. 2010;24(1);2010:35-45. doi:10.1647/2009-009R.1

2. Flammer K, Papich M. Assessment of plasma concentrations and effects of injectable doxycycline in three psittacine species. J Avian Med Surg, 2005;19(3):216-224.  doi:10.1647/2004-007.1

3. Sanchez-Megallon Guzman D, Beaufrere H, Welle KR, Heatley JJ, Visser M, Harms CA. Birds. In: Carpenter JW, Harms CA, eds. Carpenter's Exotic Animal Formulary. 6th ed. Elsevier Saunders; 2023:232.