Evaluation of ELISA and fecal culture strategies for diagnosis of Mycobacterium avium subsp. paratuberculosis

Johne's Disease (JD) is a chronic, infectious, enteritis of ruminants caused by Mycobacterium avium subsp. paratuberculosis (Mptb). In the clinical stage of this disease, the infection causes severe diarrhea and wasting of the affected animal. The subclinical infection with Mptb also has a detrimental effect on productivity of cattle. In ELISA-positive cattle, there is a 2.4 times increase in the risk of culling, and lactational 305d milk production is decreased by at least 370 kg. Reduced slaughter value and premature culling accounts for losses of CD{dollar} 1330 per year per infected 50-cow herd. Research has failed to show a consistent association between Mptb test status and reduced fertility or risk of clinical/subclinical mastitis. Host level factors for becoming infected include age of exposure and level of exposure, along with source of exposure such as manure, colostrum or milk. Agent factors involve the dose of infectious agent and strains of bacteria. Environmental management factors influence the persistence of the bacteria and the level of contamination in the environment. Emphasizing a risk factor approach is the design of various control strategies at a national level in many countries throughout the world, specifically Australia, The Netherlands and the United States.; Control of JD historically has been based upon identification of infected animals and either segregation or removal from the herd. The focus of this study was to investigate two diagnostic methods for detection of Mptb: ELISAs and culture. The first goal was to compare the diagnostic characteristics of various ELISAs for Mptb using the most appropriate gold standard. Then a comparison was done on the two commonly used gold standards: tissue and fecal culture. Additionally, pooled fecal culturing was examined to determine sensitivity of various pools sizes and use that information to estimate herd sensitivity.; A commonly used diagnostic modality is an ELISA to test for antibodies to Mptb, however, the licensed assays in North America have poor sensitivity for sub-clinically infected cattle that are not heavily fecal shedding. A European ELISA has shown some improvement in sensitivity in testing done in Germany. In order to test this ELISA against the North American ELISAs, a slaughterhouse study was conducted to obtain serum from culled dairy cows along with tissue and feces to be cultured as a gold standard comparison.; The European ELISA had a slight advantage in sensitivity over one of the North American ELISAs, but overall there was no dramatic improvement in test accuracy and agreement among the three assays was poor. Comparison of tissue culture and fecal culture indicated that fecal culture was not as sensitive as tissue culture and therefore may be a poor gold standard for JD test evaluation.; The feces collected allowed for a unique opportunity to investigate the validity of pooled fecal culture as a herd level diagnostic tool in conjunction with control programs. This testing modality is already in place in many countries and would be a component of the proposed Canadian Voluntary Johne's Control Program. The results of this research support the validity of utilizing pooled fecal culture as a herd level testing modality and also provides some credibility to minimizing the use of ELISA testing in control programs.