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Clinical impressions are rarely sufficient to differentiate canine distemper from other causes of infectious canine respiratory disease. Pre‐mortem testing options are limited; serological tests are limited by viral immunosuppression and interference due to maternal or vaccine‐induced antibodies and fluorescent antibody (FA) testing (cells from the conjunctiva, blood, respiratory tract epithelium, or urinary bladder) is very specific but has low sensitivity. Another pre‐mortem test is PCR (on urine sediment, epithelial swabs, bronchoalveolar lavage, buffy coat preps, or CSF); however, PCR may also detect vaccine virus in recently vaccinated animals and for this reason the use quantitative reverse transcriptase PCR (RT‐PCR) is recommended. If an animal dies of suspected distemper, or if the presentation of the disease is unusual and confirmation is necessary, distemper can be identified reliably on necropsy samples and histopathology by a qualified pathologist.

Gross findings: If the lungs are involved, canine distemper virus will be disseminated and affect all lobes. In most cases, oculo‐nasal discharges are thick and mucopurulent. The lungs are generally edematous or consolidated (interstitial pneumonia). Thick, foamy to mucopurulent hemorrhagic exudates may be found in the airways. Secondary (bacterial) infection is common, both because of viral damage to the airways and because of lymphoid depletion. Therefore, a cranioventral distribution of lung consolidation (bronchopneumonia) does NOT rule out distemper. Lymphoreticular tissues are characteristically involved and are the primary site for viral replication. There can be enlargement of the tonsils and/or atrophy of the thymus. Hyperkeratosis (“hardpad disease”) of the nose and/or footpads is sporadically present. There are no gross lesions of the central nervous system even when nervous signs are uniquely present. The heart should always be examined, opened, and sampled when investigating respiratory outbreaks; right heart failure (e.g. as a result of heartworm disease) often poses clinically as respiratory distress.

Histopathology: In addition to the list above for general respiratory disease, histopathologic samples useful in the diagnosis of CDV include the brain and bladder. Samples submitted for histology and paraffin‐embedded are also used for immunohistochemistry, which is one of the definitive methods of identifying CDV induced respiratory and/or neurologic disease.

Molecular diagnostics: PCR can be used to detect virus in lung, CSF, feces, or urine. False positives are possible within one to three weeks of vaccination. The diagnostic laboratory should be consulted regarding testing details.