The effects of badger culling on the prevalence of Mycobacterium bovis in cattle herds in Ireland

Abstract

The aim of this literature review is to investigate and review the role that European badgers (Meles meles) have on the spreading of Mycobacterium bovis within cattle herds in Ireland. Tuberculosis has been an endemic disease in Ireland for several decades causing severe disease in animals and humans. There have been multiple eradication programmes implemented, the first beginning in 1965. The first eradication programme was a test and slaughter scheme of all bovine animals using a single intradermal comparative tuberculin test (SICTT). In 1974, the Eurasian badger (Meles meles) was proven to be a reservoir of bovine tuberculosis (bTb), Mycobacterium bovis (M. bovis). Transmission and maintenance of M. bovis within the badger population can depend on a number of factors such as the population density, environment and interactions between different social groups. Identification of infected badgers can be difficult as the majority of infections in badgers are latent infections. M. bovis infections in badgers are predominately diagnosed by the isolation of the bacterium from organ pools of culled badgers. In 1989 trials were carried out in county Offaly to see the effect of culling badgers in areas within close proximity to cattle herds following severe outbreaks of bTb. Following the positive results from this trial, the programme was trialled on a larger area “the four-area trial”. The annual number of badgers being culled originally was approximately 6000 badgers. It was later implemented on a country wide basis and contributed to decreasing the badger population density from 3 – 4/Km2 to 0.5/Km2. In more recent years, due to increased fear of the negative effects that badger culling was having on the badger populations in Ireland along with the Tb reproduction ratio (R) in badgers not being reduced below a value of 1, the badger culling programme was replaced by the badger vaccination programme. The vaccination programme involves vaccinating badgers with Bacillus Calmette-Guérin (BCG) strain to provide immunity and prevent them for getting infected with M. bovis. The hope is that if badgers cannot spread M. bovis between bovine herds the total incidence rates of bTb can be reduced by testing and slaughtering any animals which test positive during the SICTT. Ever since the introduction of bTb eradication programmes the incidence levels have been dropping but the disease has never been completely eradicated in Ireland. The hope is that one day Ireland will be officially bTb free.