The farm environment: an overlooked source of Mycobacterium bovis?

牛结核病(BTB)是一种可以传播给人的牛疾病(尽管人类感染在英国很少见)。尽管几十年来一直在努力根除这种疾病，但这种疾病已经蔓延到了英国西部的大部分地区。这种疾病之所以持续存在，是因为用于检测和清除受感染牛的检测方法漏掉了一些受感染的动物，也因为野生动物(尤其是獾)也会感染这种疾病，并将其传染给牛。大多数牛通过吸入其他牛身上的粘液或唾液颗粒而感染牛结核病。然而，新的研究表明，獾避开牛，很少有足够近的距离通过这种方式传播(或感染)疾病。因为有非常有力的证据表明，獾会把结核病传染给牛，而牛又会把结核病传染给獾，这一发现有力地表明，牛可以在不与另一种动物近距离接触的情况下传播和感染结核病。牛和獾都会排泄结核病细菌，这种细菌可以在环境中存活数周或数月。我们可以通过使用遗传方法来量化环境中的这些细菌，这些方法计算土壤、粪便或水样本中细菌DNA的拷贝数。我们的初步研究已经在牛的泥浆中，在牛吃草的田地的土壤中，以及在牛场收集的獾的粪便中发现了这种细菌。牛很可能会从这些来源感染牛结核病；然而，目前的牛结核病管理几乎没有解决牛结核病在环境中造成的风险。我们的项目旨在解决六个研究问题：(1)牛结核病细菌在农场环境中集中在哪里？(2)在环境中发现的牛结核病细菌是否仍然活着，从而可能感染牛？(3)牛在多大程度上接触到环境中的牛结核病细菌？(4)将泥浆撒到田里对环境中的牛结核病细菌有贡献吗？(5)在环境中发现的牛结核病细菌主要来自牛还是来自獾？(6)如何管理农场以避免牛从环境中感染牛结核病？我们的项目将通过对位于康沃尔的20个农场的仔细研究来寻求这六个问题的答案，那里的牛牛结核病发病率非常高。我们的团队将收集数千份牛粪、泥浆、土壤和獾粪便的样本，以及牛舍和牛槽内的拭子。这些样本将被带回我们的实验室，在那里我们将从它们中提取DNA，并计算每个样本的BTB遗传密码副本的数量。这些分析将使我们能够绘制出农场中BTB细菌最丰富的地方，并探索细菌的数量和分布是否会随着季节的变化而变化。与此同时，我们将使用类似于车载卫星导航系统的技术来跟踪牛和粪便撒布器(用于散布泥浆)的移动。追踪粘液传播者将帮助我们评估在最近传播泥浆的田野中是否存在更多的细菌，跟踪牛将帮助我们评估它们在环境中遇到细菌的风险最大的地方。我们将测试在环境中发现的细菌是否仍然活着，并探索牛粪和獾粪便中的细菌存活多长时间，这些粪便保存在动物无法接触的小型露天围栏中。为了评估环境细菌来自哪里，我们计划将在土壤、谷仓和水槽中发现的细菌的DNA与已知来自牛(在新鲜牛粪中发现)和从獾(在新鲜獾中发现)的细菌的DNA进行比较。例如，如果在土壤中发现的DNA菌株与在牛粪中发现的相似，但与在獾粪便中发现的DNA菌株不同，这表明大多数环境污染来自牛。为了确保我们的研究结果能够转化为对农场管理的实际指导，我们的项目指导委员会包括来自养殖业和兽医专业的代表。

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.