Validation of pooled faecal culture for bovine Johne’s disease with low level shedder cattle

Summary

​Cattle populations infected with M. avium subsp. paratuberculosis (Map) contain individuals  shedding a range of Map concentrations in their faeces, from clinical cases shedding in excess of 108 organisms per gram, to some subclinical cases shedding fewer than 103/g. Pooled faecal culture, which is designed to reduce testing costs for infected herds, is based on thresholds of infection that can be detected using current cultural procedures. A sensitive procedure for PFC, based on radiometric culture and IS900 PCR/REA confirmation, was used to examine pooled faecal culture of faeces from low shedder cattle, defined by slow growth of Map on initial radiometric culture (first growth index at 5 weeks or later). Eight samples (stored for up to 17 months at -80oC) of 14 selected were found to yield Map on subsequent culture, including evaluation studies when samples were mixed with normal cattle faeces at pooling rates from 1:5 to 1:50. All were considered to be shedding relatively low levels of Map, estimated at less than 105/g of faeces in seven of the eight cases and less than 5 x 105/gin the remaining animal. At pooling rates of more than 1:5, PFC sensitivity was found to be low, and an incubation period of at least 10 weeks at the 1:5 rate was needed to detect cattle shedding £ 104 Map organisms/g of faeces, representing an estimated inoculum per vial of fewer than 20 organisms. These results assisted the cattle industries to clarify acceptable pooling rates for PFC as a diagnostic tool in BJD control.   ​