THE ROLE OF CATTLE-TO-CATTLE TRANSMISSION OF MYCOBACTERIUM-BOVIS IN THE EPIDEMIOLOGY OF TUBERCULOSIS IN CATTLE IN THE REPUBLIC-OF-IRELAND - A REVIEW

Bovine tuberculosis eradication programmes, based on the identification of infected cattle by means of the tuberculin test and their removal before they become infections, have succeeded in eradicating the disease in many countries. However, there has been little discernible improvement in bovine tuberculosis levels in the Republic of Ireland since 1965 in spite of the implementation of an intensive eradication programme. The main route of M. bovis infection for cattle is the aerogenous one with the oral route being of lesser importance. There is some variation in the reported minimum dose required to set up infection in cattle by the respiratory route. Cattle-to-cattle transmission trials and field studies of tuberculosis out-breaks showed that the majority of in-contact cattle did not display evidence of infection, based on tuberculin test results and postmortem examination. Furthermore, the rate of transmission of M. bovis from animals which were infectious was low. This suggests that cattle in the early stages of disease do not commonly transmit M. bovis to in-contact animals. On the basis of the scientific evidence currently available and the progress of the National Eradication Programme in the Republic of Ireland to date it would appear that bovine-to-bovine transmission of M. bovis is no longer the primary source of new out-breaks here. Effective means of addressing the other factors involved in the transmission of tuberculosis will be required to achieve the final eradication of the disease in the Republic of Ireland.