Specificity of four serologic assays for Mycobacterium avium ss paratuberculosis in llamas and alpacas: a single herd study

An investigation was conducted for Mycobacterium avium ss paratuberculosis infections in a research herd of llamas and alpacas. Herd culture-negative status was established over a 23-month period by screening any individuals with any signs compatible with paratuberculosis (n = 1), high: serology val ues (n = 8), or other health and research related reasons (n = 24). There w ere no M. avium ss paratuberculosis isolates from radiometric cultures of m ultiple tissue and fecal samples from these individuals and no known source s of exposure. Paratuberculosis is uncommon in North American llamas and al pacas: only 5 cases were identified after an extensive search of the Veteri nary Medical Data Base, diagnostic laboratory records, publication database s, and personal communications. Therefore, serum samples from llamas (n = 8 4) and alpacas (n = 16) in the culture-negative herd were used to obtain pr eliminary estimates of test specificity for 3 enzyme-linked immunoassays (E LISAs) and an agar gel immunodiffusion (AGID) assay kit for detecting serum antibodies to M. avium ss paratuberculosis in South American camelids. The ELISAs were modifications of established bovine assays for antibody detect ion. With provisional cutoffs, ELISA-A had 52 false positives (specificity 48%), ELISA-B had 8 false positives (specificity 92%), ELISA-C had two fals e positives (specificity 98%), and the AGID had 0 false positives (specific ity 100%). The range of ELISA values for culture-positive llamas and alpaca s (n = 10) from other herds overlapped the range of values for culture-nega tive llamas and alpacas. The accuracy of the ELISAs may be improved by usin g age- and sex-specific cutoffs because uninfected male llamas and alpacas that were older than 1 year had higher values for some tests. These tests c an be used for either llamas or alpacas; the protein-G conjugate ELISA (ELI SA-B) may be useful for multispecies applications. These assays are best us ed for rapid presumptive diagnoses of llamas and alpacas with diarrhea and weight loss and as a screening tool for herds known to be exposed to infect ion. All seropositive results should be confirmed with culture.