A practice-based survey of the frequency of Johne's disease in south west England.

The frequency of Johne's disease in cattle in south west England was estimated from data collected by telephone interviews with veterinarians and farmers. The response rate was 81.6 per cent. The disease frequency was expressed as the proportion of farms with clinical disease and the cumulative incidence in the infected herds. The proportion of farms affected was 1.0 per cent and the cumulative incidence on those farms was 1.9 per cent per year. Similar values were obtained when diagnosis by faecal examination, post mortem examination and histology was taken into consideration; 0.9 per cent of farms were affected and the cumulative incidence in the infected herds was 2.0 per cent per year. The survey was validated against three external reference points. There was good agreement between the use of vaccine and MAFF records, and the total number of holdings and census data. When the responses of the veterinarians were compared with those of farmers there was also good agreement on the use of vaccine (kappa = 77.8 per cent), the number of cases reported in the last year of diagnosis (r = 0.78) and the total number of cattle in the herds (r = 0.75). However, the results suggested that the total number of cattle holdings was overestimated and consequently the proportion of farms affected may have been underestimated.