Monte Carlo simulation of flock-level sensitivity of abattoir surveillance for ovine paratuberculosis.
We used Monte Carlo simulation to estimate distributions for flock-level sensitivity of abattoir-based surveillance for ovine paratuberculosis as currently practised in New South Wales, Australia. Probability distributions were used as input variables for within-flock prevalence, years-infected and individual animal-level sensitivity and specificity of gross pathology as a screening test for the presence of paratuberculosis. Distributions used as inputs for the size of abattoir-slaughter groups were based on existing abattoir-surveillance data from NSW. Predicted flock-level sensitivity depended on within-flock prevalence and the number of animals examined and was sensitive to estimates of animal-level sensitivity and specificity. The median probability of detection of an infected flock based on the examination of one abattoir line was predicted not to exceed 0.95 unless the within-flock prevalence was > or =7%. If the within-flock prevalence was 2%, the probability distribution of flock-level sensitivity had a median of 0.73, with 80% of values lying between 0.55 and 0.84. Improvement in the flock-level sensitivity could be achieved by submitting more than three gross pathology-positive specimens per line, if available-but the degree of improvement depended on the number of sheep slaughtered (line size) and the within-flock prevalence. At 2% prevalence, a median flock-level sensitivity of 0.95 could be obtained in lines of >390 sheep if six gross pathology-positive specimens were submitted. We concluded that abattoir surveillance based on identification of gross pathology as a screening test is not a sensitive tool for detecting recently infected flocks or flocks which have a moderate or lower prevalence of infected animals. But-with relatively minor modifications of the protocol currently in use-it could become a key component of a surveillance programme which included additional testing strategies for small flocks.
Abbott
K A
KA
Royal Veterinary College, Hawkshead Lane, North Mymms, AL9 7TA Herts, UK. kabbott@rvc.ac.uk
Whittington
R J
RJ
D000003:Abattoirs; D000818:Animals; D009010:Monte Carlo Method; D009517:New South Wales / Q000453:epidemiology; D010283:Paratuberculosis / Q000453:epidemiology* / Q000209:etiology; D011159:Population Surveillance / Q000379:methods; D015995:Prevalence; D012680:Sensitivity and Specificity; D012756:Sheep