The farm environment: an overlooked source of Mycobacterium bovis?

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Bovine tuberculosis (bTB) is a cattle disease which can be transmitted to people (although human infections are rare in the UK). Despite decades of efforts to eradicate it, the disease has spread across much of western Britain. The disease seems to persist because the tests used to detect and remove infected cattle miss some infected animals, and because wildlife (especially badgers) can also catch the disease and pass it on to cattle.Most cattle catch bTB by breathing in specks of mucus or saliva from other cattle. However, new research shows that badgers avoid cattle, and very seldom come close enough to transmit (or catch) the disease in this way. Because there is very strong evidence that badgers give bTB to cattle, and that cattle give bTB to badgers, this finding strongly suggests that cattle can both transmit and catch bTB without coming into close contact with another animal.Both cattle and badgers excrete bTB bacteria, which can survive in the environment for weeks or months. We can quantify these bacteria in the environment by using genetic methods which count the number of copies of bacterial DNA in a sample of soil, faeces, or water. Our preliminary studies have detected such bacteria in cattle slurry, and in the soil of fields where cattle graze, as well as in badger faeces collected on cattle farms. It is likely that cattle can catch bTB from these sources; however, current bTB management does little to address the risk posed by bTB in the environment.Our project aims to address six research questions:(1) Where in the farm environment are bTB bacteria concentrated?(2) Are the bTB bacteria found in the environment still alive and hence likely to infect cattle?(3) To what extent do cattle come into contact with bTB bacteria in the environment?(4) Does spreading slurry onto fields contribute to bTB bacteria in the environment?(5) Do the bTB bacteria found in the environment come mostly from cattle or from badgers?(6) How might farms be managed to avoid cattle catching bTB from the environment? Our project will seek to answer these six questions by a careful study of 20 farms located in Cornwall, where rates of cattle bTB are very high. Our team will collect thousands of samples of cattle dung, slurry, soil and badger faeces, as well as swabs from inside barns and cattle troughs. These samples will be brought back to our laboratory where we shall extract DNA from them and count the numbers of copies of bTB genetic code from each one. These analyses will allow us to map where on the farms bTB bacteria are most abundant, and to explore whether the numbers and distribution of bacteria vary between seasons. At the same time, we shall track the movements of both cattle and muckspreaders (used to spread slurry) using technology similar to that present in vehicle satnav systems. Tracking muckspreaders will help us to assess whether bTB is more abundant in fields where slurry has been spread recently, and tracking cattle will help us to assess where they are at greatest risk of encountering bTB bacteria in the environment.We shall test whether bTB found in the environment is still alive and also explore how long bTB bacteria survive in cattle dung and badger faeces held inside small open-air enclosures inaccessible to animals.To assess where environmental bacteria come from, we plan to compare the DNA of bTB bacteria found in soil, barns and troughs with that of bacteria known to have come from cattle (found in fresh cattle dung) and from badgers (found in fresh badger faeces). If, for example, the DNA strains found in soil were similar to those found in cattle dung, but different from those found in badger faeces, it would suggest that most environmental contamination comes from cattle.To ensure that our research findings can be translated into practical guidance for farm management, our project steering committee includes representatives from the farming industry and the veterinary profession.