A study of the prevalence of Mycobacterium avium subspecies paratuberculosis (MAP) infection in dairy goats was carried out in France, using a serological ELISA. One hundred and five herds were randomly selected in different regions, and all the goats aged six months or older (11,847 goats) were sampled. An estimation had previously been made of the accuracy of the ELISA, and the Rogan-Gladen estimator was used to convert apparent prevalence values into estimates of true prevalence values. Given that sensitivity and specificity at herd level are functions of sample size, and because the sizes of the herds varied considerably, the true prevalence of infection at herd level was estimated using a weighted average depending on the herd size stratum. At the level of the individual animal, the total true prevalence was estimated by applying a weighted average in accordance with the regional distribution of the animals in the target population. At herd level, apparent prevalence and estimated true prevalence were 55.2 and 62.9 per cent, respectively; at individual animal level they were 2.9 and 6.6 per cent, respectively. In herds that were seropositive for MAP, the average within-herd apparent prevalence was 5.9 per cent and the estimated average within-herd true prevalence was 11.1 per cent.
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