Eradication of paratuberculosis in dairy herds: Determination of the initial herd prevalence and modelling of prevalence development.

A prerequisite for the success of any eradication programme is the accurate determination of the initial herd prevalence as well as a herd-specific pr ediction of prevalence development. This prerequisite is not currently give n for the eradication of paratuberculosis in infected herds. In the work pr esented a method to predict the initial paratuberculosis prevalence in infe cted herds is presented; it is based on the formation of two groups (ELISA- positive and negative) and the determination of generally applicable factor s (positive predictive value [ppv(n)] of the ELISA and sensitivity of fecal culture in the ELISA-negative group [sen,]). The ppv(n) of the ELISA was d etermined to be 0.6 based on the cultural examination of the ileocaecal lym ph node of 64 ELISA-positive animals; the value for senF was set to be 0.64 based on the cultural examination of feces and ileocaecal lymph nodes of 4 0 ELISA-negative animals. To calculate the initial herd prevalence the numb er of animals in each of the groups was multiplied with the ppv(n) of the E LISA or with the reciproke value of sen(F) (1.5). The values were added and divided by the size of the herd. The practicability of this model was exam ined on nine herds with a total of 708 animals. The development of herd pre valence was modelled based on the examination scheme given in the paratuber culosis control programme of the,Niedersachsische Tierseuchenkasse" (local bord for infectious disease control in food animals in the state of Lower S axony, Germany). For the calulation a yearly turnover-rate of 33% with rest ocking from within the herd and a possibility of paratuberculosis diagnosis only in animals two years and older were assumed. The development of herd prevalence is examplary presented for four herds with different initial pre valences.