Developing a New Tool for Phenotyping Tick Resistance in Cattle

在肯尼亚，大型商业农场和小农农场的牛生产是饮食中蛋白质、碳水化合物、脂肪、维生素和矿物质的重要来源。不幸的是，与在世界其他地方类似的热带和亚热带环境中饲养的牛一样，许多蜱虫攻击牛，造成蜱虫喂养损害和疾病传播的严重损失。虽然可以通过杀虫剂，甚至一些替代虫害管理工具（如抗蜱疫苗）来保护牛不受蜱虫的侵害，但大多数工具都不能为蜱虫管理提供永久解决方案，因为它们既不完全有效，也不可持续。制定新的策略来保护牛生产免受蜱虫侵害是至关重要的。牛宿主耐药性已被确定为影响蜱虫管理经济效益的最重要因素，因为它可以遗传，而且成本低，不需要额外的劳动力或资源。然而，牛对宿主的抵抗力的改善在很大程度上被忽视了，因为确定对蜱虫抵抗力的个体差异是困难和昂贵的。迫切需要一种更简单、成本效益高的方法来确定对蜱虫抗性的个体差异。一种可能的生物学机制可以用来识别蜱虫的这种抗性变化，包括牛的气味，蜱虫利用牛的气味来定位合适的宿主动物。在产卵之前，蜱虫用它们的嗅觉来探测和定位适合吸血的宿主动物。宿主动物的气味由吸引人的和驱虫剂的挥发性化合物组成，研究表明，与那些抵抗力较弱的动物相比，对蜱虫抵抗力较强的动物会产生更高水平的驱虫剂气味。这种生态相互作用可以在牲畜保护中得到利用，通过使用令人反感的气味将蜱虫从动物身边推开，并使用吸引人的气味将蜱虫拉入陷阱，甚至同时使用两种气味作为推拉策略的一部分。在虫害管理中使用合成驱避剂和有吸引力的气味已经进行了广泛的探索，但由于保护是短暂的，这种技术对于广泛的牛生产系统是不可持续的。驱虫剂气味的进一步发展是，它们可以由动物自己传递，通过选择抗蜱虫的动物来繁殖和创造产生驱虫剂气味的牲畜。这种通过寄主提供驱蚊剂的方法已在非洲的谷类作物蛾类病虫害管理中取得了显著成功，迄今已有10万多家小农场采用了这种方法。自然条件下的实验。在巴西，使用耐蜱牛、荷尔斯坦-弗里斯牛和吉罗兰多牛（对蜱敏感程度不同）的农场条件表明，耐蜱牛与驱避气味水平的增加有关。6-methyl-5-hepten-2-one。如果我们能确定肯尼亚牛品种的蜱虫抗性与驱虫气味水平之间的类似相关性，那么我们就有机会开发一种新的方法来识别牛的蜱虫抗性，这种方法比目前的方法更具成本效益，并且可以用来选择抗蜱虫的动物，以培育基于驱虫气味的蜱虫抗性。我们将在受控条件下对肯尼亚牛进行筛查，以确定它们对蜱虫的抵抗力；实地筛查肯尼亚牛对自然蜱虫种群的抵抗力；在这些受控的自然条件下，筛选驱避气味的产生，并证明蜱虫抗药性与驱避气味产生之间的相关性；向农民、牛饲养者和其他相关方解释我们基于产生驱避气味来选择抗蜱动物以培育抗蜱动物的新方法。

In Kenya, cattle production on large commercial farms and smallholder farms provides an important source of protein, carbohydrates, fat, vitamins and minerals in the diet. Unfortunately, as with cattle reared in similar tropical and sub-tropical environments elsewhere in the world, many ticks attack cattle, causing severe losses from tick feeding damage and the spread of diseases. Although cattle can be protected from ticks by pesticides and even some alternative pest management tools eg anti-tick vaccines, most of the tools do not offer a permanent solution to tick management as they are neither completely effective nor sustainable. It is vital that new strategies to defend cattle production against ticks are developed.Cattle host resistance has been identified as the single most important factor affecting the economics of tick management, as it can be inherited and is low-cost requiring no extra labour or resources. However, improvements to host resistance in cattle are largely neglected because of difficulty and expense of identifying individual variation in resistance to ticks. A simpler, cost-effective method of identifying individual variation in resistance to ticks is urgently needed. One possible biological mechanism that could be used to identify such variation in resistance in ticks consists of cattle odour, which is used by ticks to locate a suitable host animal. Ticks use their sense of smell to detect and locate suitable host animals for blood-feeding prior to egg production. The odour of host animals comprises of attractive and repellent volatile compounds, and it has been shown that animals that are more resistant to ticks produce higher levels of repellent odours, compared to those animals that are less resistant. This ecological interaction can be exploited in livestock protection by using repellent odours to push ticks away from animals and using attractive odours to pull ticks into traps, or even using both sets of odours at the same time as part of a push-pull strategy.The use of synthetic repellent and attractive odours in pest management has been extensively explored, but this technology is not sustainable for extensive cattle production systems because protection is short-lived. A further development of the use of repellent odours is that they can be delivered by the animals themselves, with tick-resistant animals being selected to breed and create livestock that produce repellent odours. This approach to the delivery of repellents via hosts has been adopted with outstanding success in Africa for moth pest management on cereal crops, with more than 100,000 smallholder farms adopting it to date. Experiments under natural ie. farm conditions in Brazil using breeds of cattle called Nelore (tick-resistant), Holstein-Friesian (tick-susceptible) and Girolando (varying in its level of susceptibility to ticks) showed that tick resistance correlated with increasing levels of repellent odours eg. 6-methyl-5-hepten-2-one. If we can identify a similar correlation between tick resistance in Kenyan breeds of cattle with levels of repellent odours, then we have an opportunity to develop a new way of identifying tick resistance in cattle that is more cost-effective than current methods, and which can be used to select tick-resistant animals for breeding tick resistance based on repellent odours.We will screen, under controlled conditions, Kenyan cattle to ascertain their resistance to ticks; screen, in the field, Kenyan cattle for their resistance to natural tick populations; screen, under these controlled and natural conditions, the production of repellent odours and demonstrate a correlation between tick resistance and repellent odour production; explain, to farmers, cattle breeders and other relevant parties, our new approach to the selection of tick-resistant animals for breeding tick resistance, based on repellent odour production.