Simulating economic effects of improving the sensitivity of a diagnostic test in Paratuberculosis control

摘要

The perspectives of improving the sensitivity of a milk-antibody ELISA (hereafter: milk-ELISA) used in test-&-cull strategies against paratuberculosis in dairy cattle were investigated by simulations. The current sensitivity varies between 10% and 80%, increasing with increasing lactation stage, parity and infection stage. We simulated the effects of improving this sensitivity to 80% (independent of these factors). By using a paratuberculosis model called PTB-Simherd, 20 scenarios were simulated in order to study five test-&-cull strategies in each of four herds with 200 dairy cows. The test-&-cull strategies were based on either no testing, the milk-ELISA test alone (positive cows culled immediately) or on confirmation of ELISA positive cases witha faecal culture test (current sensitivity). Test-&-cull strategies were simulated with both the current sensitivity and an improved sensitivity. The four herds were defined by a moderate (25%) or high (80%) initial true herd-prevalence (including youngstock), and a poor or good reproduction management (heat detection rate 40% or 60%). These factors were assumed to influence the effect of improving the sensitivity of the milk-ELISA. Management - both concerning calf management and in general - was specified to represent a typical Danish herd. Results of the simulations indicated that with an improved milk-ELISA test and cullings based only on the ELISA-results it was possible to reduce the prevalence of paratuberculosis effectively. Economically all scenarios benefited from the improvement of the test over a ten-year period. By using the improved test, a test-&-cull strategy combined with a faecal culture test resulted in an immediate increase in net annual revenue per cow of 15 to 20 Euro. The advantage of the improved test stayed at that level the following years. If instead test-&-cull strategies were based only on the milk-ELISA, the improved test reduced net annual revenue per cow with 15 Euro (on average) the first four years, but after that net annual revenue increased continuously, and after ten years it was 70 to 90 Euro higher than if the current milk-ELISA was used. Also, the milk-ELISA test with its current sensitivity seemed to be profitable already after 2 years in high-prevalence herds using a test-&-cull strategy based on the milk-ELISA alone. By combining the milk-antibody test with a confirmatory faecal culture test, the effect of improving sensitivity compared to 'no control' only had a limited effect on prevalence and economy.In herds with poor reproduction, the economic advantages of improving sensitivity were only found in herds with moderate initial prevalence, and which only used milk ELISA in test-&-cull strategies.