Avian chlamydiosis can be an inapparent subclinical infection or acute, subacute, or chronic disease of wild and domestic birds characterized by respiratory, digestive, or systemic infection. Infections occur worldwide and have been identified in at least 150 avian species, particularly colonial nesting birds (eg, egrets, herons), ratites, caged birds (primarily psittacines), raptors, and poultry. Among poultry, turkeys, ducks, and pigeons are most often affected; infection of chickens is infrequent. The disease is a significant cause of economic loss and human exposure in European duck flocks. Longterm inapparent infections lasting for months to years are common and considered the normal chlamydia-host relationship; 10-30% of surveyed avian populations may be found positive. The same strain may cause mild disease or asymptomatic infection in one species, but severe or fatal disease in another species.
Avian chlamydiosis is a zoonotic disease that can affect people following exposure to air- or dustborne organisms when infected birds are in flocks or processed, or when organisms are shed from the digestive or respiratory tracts of infected birds confined in breeding aviaries, lofts, or wholesale or retail outlets. Human disease most often results from exposure to psittacines or pigeons and can occur even if there is only brief proximity to a single infected bird. When workers are exposed to infected turkeys or ducks at processing, increased absenteeism due to acute respiratory disease often occurs ~1 wk after a flock with a high condemnation rate due to airsacculitis has been processed. Some individuals, especially pregnant women and those with impaired immunity, are more susceptible than others. The illness in people is usually respiratory and characterized by abrupt onset of flu-like symptoms; pneumonia, organ failure, and death can result if the disease is severe or left untreated. Precautions should be taken when examining a dead infected bird (eg, detergent disinfectant to wet feathers, fan-exhausted examining hood, dust mask or plastic face shield, and gloves) to avoid exposure.
A recent taxonomic revision resulted in the causative organism being renamed Chlamydophila psittaci (formerly Chlamydia psittaci). The name of the disease resulting from infection with C psittaci remains avian chlamydiosis.
C psittaci is an obligate intracellular bacterium. All strains of chlamydia share an identical genus-specific antigen in their lipopolysaccharide but often differ in the composition of other cell-wall antigens, providing a basis for serotypic identification. Currently, 8 serotypes are recognized; 6 (A-F) infect avian species and are distinct from mammalian chlamydia serotypes. Each avian serotype tends to be associated with certain types of birds. Serotype D is highly virulent for turkeys and can cause mortality of 30% or higher. Serotypes B and E are most frequently recovered from wild birds. Avian serotypes are capable of infecting people and other mammals.
Respiratory discharges or feces from infected birds contain elementary bodies that are resistant to drying and can remain infective for several months. Airborne particles and dust spread the organism. After inhalation or ingestion, elementary bodies attach to microvilli on mucosal epithelial cells and are internalized by endocytosis. Elementary bodies within endosomes in the cell cytoplasm differentiate into metabolically active, noninfectious reticulate bodies that divide and multiply, eventually forming numerous infectious, metabolically inactive elementary bodies. Newly formed elementary bodies are released from the host cell by lysis.
Possible sources of C psittaci include infected birds, asymptomatic carriers, vertical transmission from infected hens, infected rodents, and contaminated feed. Stressors and concurrent infections, especially those causing immunosuppression, can initiate shedding in latently infected birds and may cause recurrence of clinical disease. 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 organisms. Transmission is fecal-oral or by inhalation. The incubation period typically is 3-10 days but may be up to 2 mo in older birds or following low exposure.
Host and microbial factors, route and intensity of exposure, and treatment determine clinical course.
Severity of clinical signs and lesions depends on the virulence of the organism and susceptibilty of the bird; asymptomatic infections are common. Nasal and ocular discharges, conjunctivitis, sinusitis, green to yellow-green droppings, fever, inactivity, ruffled feathers, weakness, inappetence, and weight loss can be seen in clinically affected birds. Necropsy findings in acute infections include serofibrinous polyserositis (airsacculitis, pericarditis, perihepatitis, peritonitis), pneumonia, hepatomegaly, and splenomegaly. Multiple pale foci and/or petechial hemorrhages can be seen in the liver and spleen. Similar lesions are seen in other systemic bacterial infections and are not specific for avian chlamydiosis.
Multifocal necrosis in the liver and spleen is associated with large, granular, basophilic intracytoplasmic inclusions, occasional heterophils, and increased mononuclear cells (macrophages, lymphocytes, plasma cells) in hepatic sinusoids and splenic sinuses. Necrosis results from direct cell lysis or vascular damage. The latter is also the source of the generalized serofibrinous exudation. Enlargement and discoloration of the spleen or liver characterize chronic infections. Necrosis and inclusions are not seen, but the mononuclear cell response is present in these birds. Lesions are usually absent in latently infected birds, even though C psittaci is often being shed.
Because of the variety of clinical presentations and common occurrence of latently infected carriers, no single diagnostic test can reliably determine infection. 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 hematology, clinical chemistries, and radiology or typical gross lesions, are adequate for a tentative diagnosis. The organism can often be identified in impression smears of affected tissues stained by Giemsa, Gimenez, or Macchiavello’s methods.
Antigen detection methods include immunohistochemistry (immunofluorescence, immunoperoxidase), ELISA, and PCR. Immunohistochemistry is accurate when done by a skilled person and the number of organisms is sufficient for detection. ELISA kits are available commercially and are relatively inexpensive, easy to use, and have good specificity, but low sensitivity. They are most useful when birds are clinically ill. PCR tests have been developed but are not widely available and require further evaluation. Multiple samples collected for 3-5 days are recommended for detection of intermittent shedding by asymptomatic birds.
Confirmation requires isolation and identification of C psittaci in chick embryos or cell cultures (BGM, L929, Vero) at a qualified laboratory. Cloacal, choanal, oropharyngeal, conjunctival, or fecal swabs from live birds or tissues (eg, liver, spleen, serosal membranes) from dead birds should be submitted. Sampling from mutiple sites and over several days will increase detection of intermittent shedding. Freezing, drying, improper handling, and certain transport media can affect viability. Refrigeration; placing specimens in sealed plastic bags or other containers; using a special buffer prepared from sucrose, phosphate, and glutamase (SPG buffer); and prompt delivery of fresh specimens are preferred. The laboratory should be contacted for directions on submitting samples before they are sent. Concurrent infections with other more easily diagnosed diseases (eg, colibacillosis, pasteurellosis, herpesvirus infections, mycotic diseases, etc) may mask chlamydial infection. Laboratory and clinical findings should be correlated with each other. Chlamydiosis must be distinguished from other respiratory and systemic diseases of birds.
Antibodies may or may not be detectable depending on the test used, degree and stage of infection, and treatment of the bird. 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 a presumptive diagnosis. Serologic methods include direct and modified direct complement fixation, latex agglutination, elementary body agglutination, and direct and competitive ELISA. ELISA provides the greatest sensitivity and specificity compared with culture. The elementary body agglutination test detects IgM and is useful for determining recent infection.
Local governmental regulations should be followed wherever applicable. No effective vaccine for use in birds is available. Treatment will prevent mortality and shedding but cannot be relied on to eliminate latent infection; shedding may recur. Tetracyclines (chlortetracycline, oxytetracycline, doxycycline) are the antibiotics of choice. Drug resistance to tetracyclines is rare, but reduced sensitivity requiring higher dosages is becoming more common. Tetracyclines are bacteriostatic and only effective against actively multiplying organisms, making extended treatment times (from 2-6 wk, during which minimum-inhibitory concentrations in blood are consistently maintained) necessary.
Outbreaks in poultry flocks are not common. Treating infected flocks with chlortetracycline at 400-750 g/ton for a minimum of 2 wk before processing has effectively eliminated potential risk of infection for plant employees.
In companion birds, use of chlortetracycline-medicated feeds for 45 days is a standard recommendation for imported birds. Difficulties in palatability of the feed itself or high level of antibiotic necessary for adequate blood levels have limited its use. Long-acting oxytetracycline at 50-100 mg/kg, IM, every 2-3 days for 30 days, provides adequate continuous blood levels and results in elimination of shedding within 24 hr. However, muscle necrosis at injection sites may be extensive, which limits the usefulness of this treatment. Doxycycline in a formulation for IM use has been given at 75-100 mg/kg as a series of 7 injections over a 6-wk period. Addition of doxycycline to feeds can also result in adequate blood levels and has less effect on normal intestinal flora than does chlortetracycline. Supportive care for acutely affected birds also aids recovery.
Appropriate biosecurity practices are necessary for controlling the introduction and spread of chlamydiae in an avian population. Minimal standards include quarantine and examination of all new birds, traffic control to minimize cross-contamination, isolation and treatment of affected and contact birds, thorough cleaning and disinfection of premises and equiment (preferably with small units managed on an all-in/all-out basis), provision of uncontaminated feed, maintenance of records on all bird movements, and continual monitoring for presence of chlamydial infection.
The organism is susceptible to heat and most disinfectants (eg, 1:1,000 quaternary ammonium chloride, 1:100 bleach solution, 70% alcohol, etc), but is resistant to acid and alkali. A voluntary cooperative improvement plan leading to certification of companion birds derived from chlamydia-free breeders has been developed.