Infection rates in reactors to an absorbed ELISA used in a test and cull program for bovine Johne's disease

To determine the proportion of cattle, whose sera gave positive reactions in a commercial enzyme linked immunosorbent assay (ELISA) for bovine Johne's disease, that were confirmed infected with Mycobacterium avium subsp paratuberculosis by histology and culture of tissues.

Dairy cattle (n = 493) from the Echuca district of Victoria, whose sera were positive in the ELISA, were slaughtered at an abattoir where standard specimens were collected for histology and culture. Only if samples were histologically negative were further samples submitted for culture.

The proportion of cattle in which infection was confirmed increased from 70.4% in 1996 to 89.4% in 2001 giving an overall confirmation rate of 79.9%. This was mainly because more reactors were confirmed by culture each year, the proportion increasing from 0% in 1997 to 27.5% in 2000 but decreasing to 16.7% in 2001. If all unconfirmed reactors were presumed to be uninfected, the minimum specificity of the ELISA was 99.62%. There were no significant differences between the age groups in the proportion confirmed infected.

Confirmation rates and specificity of the ELISA were high when used in a typical JD-infected Victorian dairy cattle population. Imperfect sensitivity of histology and culture and the selection of reactors which favoured more false positives, means the estimates were probably conservative. Confirmation rates were not affected by age of ELISA reactor.