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

Abstract

1 Background 1. Tuberculosis (TB) is a multi-host disease affecting humans, domesticated and wild animals including cattle species [1]. It is a contagious disease spread both by direct contact but also by indirect means. While the usual route of infection is by inhalation, oral infection also occurs. TB is exacerbated in all species by the impact of malnourishment, poor sanitation, poorly ventilated housing, close contact and overcrowding. 2. The primary causal agent of TB in cattle, Mycobacterium bovis, is a member of the Mycobacterium tuberculosis complex (MTBC). This complex also includes Mycobacterium tuberculosis, the main causal agent of TB in humans. Members of the complex are closely related species in their genetic make-up. While the different members of the complex have a predilection for specific species, they also have the ability to transmit to other species, e.g. M. bovis can infect humans and M. tuberculosis sometimes infects cattle. 3. Progression of infection is protracted from initial exposure to clinical signs, taking months or even years. However, observations of clinical signs become uncommon once an eradication programme, including live animal testing and removal of those infected, commences. 4. The complex epidemiology of TB in cattle is well recognised, with the role of wildlife in transmitting infection to cattle acknowledged as an impediment to eradication [2-4]. However, a detailed understanding of inter-host transmission dynamics and their relative importance has been a recognised major knowledge gap. This has been mainly due to the slowly evolving nature of M. bovis and the lack of practical, cost-effective methods to discriminate between isolates. In cattle, it is also hindered by the frequency of skin testing where disclosure of a reactor does not necessarily reveal the precise timing and location of the infection event. It only points to an historic transmission of infection that could have occurred in a time frame ranging from weeks to years previously. 2 Evidence, in general, that cattle-to-cattle, badger-to-cattle and deer-to-cattle transmission each plays a role in the transmission of TB to cattle in Ireland 5. It is important to consider this question within the framework of recognised sources of TB infection in an Irish context [4]. As highlighted in the figure below, these include: • From cattle, either: o introduced infection, o residual infection (that is, infected animals that test negative to current diagnostic tests), or o infected cattle in neighbouring farms. • From badgers • From deer