August 2000

Polyomavirus infection and psittacosis [Chlamydia psittaci infection] in psittacine birds, and pasteurellosis [Pasteurella multocida infection] in rabbits can be challenging diseases to accurately diagnose.

Antech Diagnostics is offering new, state-of-the-art diagnostic profiles for each of these infections. These profiles combine serology and polymerase chain reaction [PCR] technology to better define the disease state of the patient. Test results should be evaluated in conjunction with clinical signs, bloodwork abnormalities, and sometimes with biopsy results to ensure accurate interpretation.

A positive antibody titer indicates exposure to the organism in question, but does not specify whether the host is actively infected. A negative titer in the absence of very recent exposure or peracute disease indicates that there has been no exposure, therefore ruling out the possibility of disease. (A patient with a negative titer may have recent exposure and insufficient time to seroconvert or may be significantly immunocompromised and unable to mount a detectable antibody response.) Detection of IgM directed against a particular organism usually indicates recent infection. A rising titer in samples collected two or three weeks apart (and ideally tested simultaneously) also indicates active infection or exposure.

PCR is an exceptionally sensitive method of detecting organism-specific sequences of nucleic acid and is typically done on whole blood, or swabs taken from the cloaca, from secretions, or from tissues at necropsy. Although a positive PCR result indicates the presence of organism-specific nucleic acid sequences, the significance of this finding needs careful consideration. A positive test result could be from a patient with clinical disease, exposure of the patient without infection or disease, from a sub-clinically infected patient, or due to a contaminated sample.

The combination of serology and PCR testing enables more complete interpretation of the test results. Negative test results on both serology and PCR, even in an ill patient, reduce the likelihood that the specific agent is involved, whereas a positive PCR test in a patient with a negative antibody titer may indicate an immunocom-promised animal, insufficient time to convert, or an environmental contaminant (in which case environmental testing is warranted).

Polyomavirus can cause disease in both psittacine and passerine species. Feather abnormalities and increased neonatal mortality in budgerigars, and sudden death in young parrots [including amazons, African greys, macaws, eclectus, ring-neck parrots, cockatoos, and conures] and finches have been ascribed to polyomavirus infection. These birds typically are serologically positive and have positive PCR tests on blood and/or cloacal swabs prior to death. Necropsy findings include hemorrhages, hepatomegaly and splenomegaly. On histology, intranuclear inclusion bodies can be seen, but are not necessarily present in all cases. Testing for polyomavirus nucleic acid in tissue (in situ hybridization) may be necessary to confirm the diagnosis.

Clinical disease in adult parrots due to polyomavirus infection is uncommon, although death of adult birds has been described either alone or in conjunction with Chlamydia psittaci or psittacine beak and feather disease virus infection. Adult parrots develop subclinical infections and may subsequently serve as a reservoir of infection through intermittent viral shedding in their droppings. Either a positive antibody titer or a positive PCR test result on whole blood supports viral exposure and/or viremia. Documentation of viral shedding can be done via PCR testing of cloacal swabs. However, shedding may be intermittent and repeated cloacal swab PCR tests may be needed to confirm the presence of a carrier state.

In a closed, polyomavirus-free aviary, testing for polyomavirus should be done prior to the introduction of new birds. The carrier status of new birds should be investigated during quarantine.

Psittacosis refers to infection of parrots with Chlamydia psittaci. Ornithosis is the same disease in other avian species, and infection in people is referred to as chlamydiosis. (Chlamydiosis is the preferred term and can be used to describe all infections with C. psittaci.) There are numerous strains of C. psittaci that are infective across a wide range of hosts. Many mammals, as well as reptiles and insects, can be infected by C. psittaci.

In parrots, ruffled feathers with lime green droppings (biliverdinuria) and a history of exposure to a new bird is the classical presentation for chlamydiosis. The clinical course usually is several weeks before either death or recovery (resulting in a potential carrier bird), although acute deaths may also occur. Affected birds typically show leukocytosis and an increase in the enzyme aspartate transaminase (AST). Protein electrophoresis usually shows hypoalbuminemia and hypergammaglobulinemia. The diagnosis can be confirmed in a sick parrot with a positive or rising antibody titer, and a positive PCR test from a combined choanal/cloacal swab. In clinically normal parrots, a negative titer and negative PCR test result [on whole blood or combined choanal/cloacal swab] indicates absence of C. psittaci infection. A positive result with either serology or PCR testing in a normal parrot with no increase in either white cells or AST indicates exposure, rather than clinical disease. A carrier state exists in many adult birds which serve as the reservoir for further disease outbreaks. There is no definitive testing to confirm a carrier bird, but a persistent IgG titer is a common finding.

Testing for chlamydiosis should be done prior to introducing new birds to an aviary, and as a screening test for a new pet, breeding bird, and for birds being exposed to ill or immunosuppressed individuals.

Pasteurella multocida is commonly found in the upper respiratory tract of rabbits and has been reported to cause clinical disease involving many organs. The classical presentation is an initial serous, and later, a white to yellow mucopurulent, nasal discharge and sneezing. Other signs are usually related to abscess formation in a wide range to tissues including dental roots, bone, skin, and other organs. These signs are not pathognomonic for pasteurellosis, however, as other bacteria may cause similar signs with respiratory disease and abscesses.

There are no consistent changes on routine blood testing. A positive culture is diagnostic, but false negatives limit the sensitivity and therefore the usefulness of cultures. A positive or rising titer and a positive PCR test from a nasopharyngeal swab confirms P. multocida as the causative agent, whereas a negative result on both serology and PCR indicates that P. multocida is not involved. Nasal swabs are less sensitive than nasopharyngeal swabs for PCR testing.

Regardless of the type of diagnostic testing being performed, proper sample collection and handling are paramount for accurate test results. Refer to the table below for guidance in collecting the appropriate specimen. Further questions can be directed toward our specially trained Avian and Exotics Customer Service representatives or Veterinary Consultants.