Quantifying the risks of TB infection to cattle posed by badger excreta.

Despite strong circumstantial evidence to suggest that the main route of TB transmission from badgers to cattle is via contaminated badger excreta, it is unclear whether the associated risks are high enough to account for the prevalence of the disease in south-west England. To decide whether this was a viable route of transmission, cattle contact with badger excreta was investigated using a deterministic approach to quantify the risks to cattle posed by badger excreta. Levels of investigative and grazing contacts between cattle and badger urine and faeces could each account for the disease prevalence in south-west England. An infection probability of 3.7 x 10(-4) per bite from pasture contaminated with badger urine infected with Mycobacterium bovis could account for the prevalence of TB in cattle in south-west England. Infection probabilities of 6.9 x 10(-7) per investigation and 1.1 x 10(-7) per bite from badger latrines could each account for the prevalence of TB in cattle in the south-west. When considering only the high risk areas of south-west England these bounds fell by a factor of eight. However, badger excreta may still constitute a high level of risk to cattle. The levels of cattle contact with badger excreta are far higher than previously thought, suggesting that it is the probability of infection per given contact with infected badger excreta which has the greater influence on the probability of transmission and not the level of contact. The infection probability per cattle contact with infected badger excreta is in all likelihood extremely low.