Modification of a bovine ELISA to detect camelid antibodies to Mycobacterium paratuberculosis

Mycobacterium avium subsp. paratuberculosis infection, or Johne's disease, reportedly has a low prevalence in South American camelid populations. Rece ntly, however, single cases in the United States as well as an outbreak of the disease in Australian alpacas (Lama pacos) have been described. To prov ide a rapid and cost-effective method of diagnosing Johne's disease in this species, the bovine Parachek(R) Johne's Absorbed EIA (CSL, Vic., Australia ) was modified to create a camelid-specific serum antibody assay. An anti-l lama IgG conjugated to horseradish peroxidase replaced the anti-bovine immu noglobulin. Checkerboard titration of principal reagents was performed usin g serum from nine tissue and/or fecal culture-positive camelids, Optimal di lutions of key components were determined in order to provide clear discrim ination between positive and negative controls. Completion of a kinetic ass ay determined the optical density at which the enzyme-substrate reaction sh ould be stopped. A herd of 100 camelids with no history of disease or expos ure to M. a. paratuberculosis, a subset of which were tissue and/or fecal c ulture-negative, was tested to establish a cut-off value. Sample results we re expressed as a percentage of the results for control sera by transformin g optical density values to ELISA values (EV%). A preliminary EV% cut-off o f 20 was established. Using this prototype assay, culture-positive animals showed significantly different antibody responses from culture-negative ani mals. These results indicate that this camelid-specific ELISA, once refined , may be a useful tool for screening camelid herds for M. a. paratuberculos is infection. (C) 2000 Elsevier Science B.V. All rights reserved.