What is the proportional contribution of cattle‐to‐cattle, badger‐to‐cattle, and deer‐to‐cattle TB transmission to bovine TB in Ireland?

Tuberculosis (TB) caused by the Mycobacterium tuberculosis complex (MTBC), including M. bovis, affects humans and multiple animal species. Cattle are significant hosts, and their interaction with other species, such as badgers and deer, is crucial for TB transmission dynamics. This opinion explores the proportional contributions of various transmission pathways to TB in cattle in Ireland: cattle-to-cattle, badger-to-cattle, and deer-to-cattle. It was carried out by the Scientific Working Group of the TB Forum following a request from the Department of Agriculture, Food and the Marine. While cattle-to-cattle transmission remains dominant, badgers play a significant epidemiological role, especially given their close interaction with cattle. Deer, primarily in County Wicklow, have also shown potential for TB transmission to cattle. Environmental factors, host density, and inter-species interaction further complicate transmission dynamics. Achieving TB eradication requires a holistic approach, addressing every transmission source. Advanced genomic and epidemiological tools, alongside traditional methods, can provide deeper insights into the transmission dynamics. The TB Eradication Programme should address, at both national and local levels, all relevant sources of transmission of TB to cattle. Current relevant sources include all cattle sources that substantially contribute to herd infection, and all wildlife that are maintenance hosts, and particularly those with spillback of infection to cattle. This situation may change over time and should be kept under review.