A simulation model for the spread of bovine tuberculosis through a cattle herd: A dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Science (Honours) at Lincoln University

Abstract

Bovine tuberculosis, caused by Mycobacterium bovis, presents a major problem to New Zealand agriculture because of the risk that it poses to export market access. New Zealand research has investigated the epidemiology of the disease in wildlife reservoirs, and the mechanisms of transmission from wildlife to agricultural stock. However, relatively little is known about the dynamics of the disease in cattle. This study aimed to investigate bovine tuberculosis (Tb) dynamics at the within-herd level, by construction and application of a simple simulation model of disease transmission. Model application was aimed primarily at improving current estimates of the rate of disease transmission, and comparing these with values calculated from field measurements. It appears that the rate of Tb transmission among cattle is low in comparison with the rate of infection from other sources, such as infected wildlife or brought-in stock. Consequently, it is very unlikely that Tb would persist in herds under current testing practices if an external source of infection were not present. However, given continual infection from external sources, preliminary simulation of herds in an endemic area suggested that up to half of test reactors may become infected through within-herd spread. The rate of spread appears to be greater in dairy than in beef herds, probably reflecting differences in their management. The model provides a basis for further investigation of various Tb management issues, for example the possibility of refining current herd testing regimes. There is, however, a need for more basic information on Tb disease development in cattle, and the effects of disease development on response to Tb tests. Detailed case studies on problem farms could yield valuable new data on disease transmission rates.