Relation between pathologic findings and cellular immune responses in sheep with naturally acquired paratuberculosis.

Objective: To investigate the relation between lesions and cellular immune response-based tests in sheep with naturally acquired paratuberculosis and to evaluate efficacy of an interferon-gamma (IFN-gamma) assay and the intradermal skin test.
Animals: 110 and 88 adult sheep were used to evaluate the IFN-gamma assay and skin test, respectively.
Procedure: Before euthanasia, heparinized blood was collected and incubated with avian purified protein derivative (PPD). Interferon-y production was assessed, using a commercial enzyme immunoassay. Skin testing in the caudal fold also was performed, using avian PPD. Sensitivity and specificity of these techniques were evaluated, using histopathologic findings as reference.
Results: Of the sheep examined, 61 (55.4%) had lesions that were classified in 5 categories, according to location, extension, cellular types, and presence of acid-fast bacilli. Regarding cellular immunity test results, 55.4% of sheep with and 6.1 % without pathologic changes had positive results of the IFN-gamma assay. Skin test sensitivity (55.6%) and specificity (100%) were slightly greater.
Conclusion: Close association between pathologic findings and cellular immune response was apparent. Most sheep with focal (tuberculoid) and some with diffuse lesions (borderline tuberculoid), all with scant or no acid-fast bacilli in the intestine, had positive results of the IFN-gamma assay and skin test. Most sheep with diffuse lesions and high numbers of bacilli (borderline lepromatous) were test negative.
Clinical Relevance: Cellular immune response-based and serologic tests are useful tools for seeking evidence of paratuberculosis. Each technique detects different types of immune-reacting sheep, and their simultaneous use may detect most infected animals.