Monitoring programmes for paratuberculosis-unsuspected cattle herds, based on quantification of between-herd transmission

A mathematical model was developed to calculate the between-herd transmission of paratuberculosis, expressed as the reproduction ratio Rh, which is the average number of new infected herds caused by one (initial) infected herd. In a monitoring programme herds are visited regularly and a certain number of animals is sampled and tested. The model developed here to design effective monitoring programmes consists of three parts: (a) within-herd dynamics of the infection after (accidentally) introducing the infection in a herd, (b) the detection probability of the infected herd, and (c) the final equation for Rh. The model is age structured. The most important input for part (a) is the within-herd reproduction ratio (R0) of the infection, assumptions about the infectious period, culling rate of cattle, herd size, and time between subsequent visits. Output is number of infected cattle at each visit. For part (b) input is number of infected animals in the herd at each visit, number (and age) of animals sampled, and the (infection-age dependent) sensitivity of the diagnostic test. For part (c) Rh is calculated by multiplying the detection probability at a certain visit by the cumulative number of infected cattle at that moment, accumulated for all visits of the herd in time, and then multiplied with the rate with which animals are sold from one herd to another. This yields the average number of infected animals sold from the infected herd (before detection) to other herds, i.e. Rh. The current monitoring programme in the Netherlands for paratuberculosis-unsuspected herds (yearly pooled faecal culture of all cows > 2 year) yields an Rh value of 0.58, which is sufficiently below 1. Other effective alternatives will be shown. Testing all animals > 1 year with the pooled faeces test every two years is an attractive, because cheaper, alternative for which Rh is sufficiently below 1 as well.