Benson Lee, Kyriazakis Ilias, Fox Naomi, Howell Alison, Innocent Giles T, Kenyon Fiona, Williams Diana, Ewing David A
Biomathematics and Statistics Scotland, James Clerk Maxwell Building, The King's Buildings, Peter Guthrie Tait Road, Edinburgh EH9 3FD, UK.
School of Biological Sciences, Queen's University Belfast, 19 Chlorine Gardens, Belfast BT9 5DL, UK.
Int J Parasitol. 2025 Aug;55(10):509-523. doi: 10.1016/j.ijpara.2025.04.008. Epub 2025 Apr 11.
Gastro-intestinal nematode infections are considered one of the major endemic diseases of sheep on the grounds of animal health and economic burden, both in the British Isles and globally. Parasites are increasingly developing resistance to commonly used anthelmintic treatments meaning that alternative control strategies that reduce or replace the use of anthelmintics are required. We present GI-NemaTracker, a systems-level mathematical model of the full host-parasite-environment system governing gastro-intestinal nematode transmission on a sheep farm. The model is based on a series of time-varying delay-differential equations that explicitly capture environmentally-driven time delays in nematode development. By taking a farm systems-level approach we represent both in-host and environmentally-driven free-living parasite dynamics and their interaction with a population of individually modelled lambs with diverse trait parameters assigned at birth. Thus we capture seasonally varying rates of parasite transmission and consequently variable weight gain of individual lambs throughout the season. The model is parameterised for Teladorsagia circumcincta, although the framework described could be applied to a range of nematode parasite species. We validate the model against experimental and field data and apply it to study the efficacy of four different anthelmintic treatment regimes (neo-suppresive treatment, strategic prophylactic treatment, treatment based on faecal egg counts and a regime which leaves 10% of the animals untreated) on lamb weight gain and pasture contamination. The model predicts that similar body weights at a flock level can be achieved while reducing the number of treatments administered, thus supporting a health plan that reduces anthelmintic treatments. As the model is capable of combining parasitic and free-living stages of the parasite with host performance, it is well suited to predict complex system responses under non-stationary conditions. The implications of the model and its potential as a tool in the development of sustainable control strategies in sheep are discussed.
无论在不列颠群岛还是全球范围内，从动物健康和经济负担方面考虑，胃肠道线虫感染都被视为绵羊的主要地方病之一。寄生虫对常用驱虫治疗的耐药性日益增强，这意味着需要采用减少或替代驱虫药使用的替代控制策略。我们提出了GI-NemaTracker，这是一个关于绵羊农场胃肠道线虫传播的完整宿主-寄生虫-环境系统的系统级数学模型。该模型基于一系列时变延迟微分方程，明确捕捉了线虫发育过程中受环境驱动的时间延迟。通过采用农场系统层面的方法，我们描述了宿主体内和环境驱动的自由生活寄生虫动态，以及它们与一群出生时被赋予不同性状参数的个体建模羔羊的相互作用。因此，我们捕捉到了寄生虫传播的季节性变化率，以及整个季节中个体羔羊体重增加的变化情况。该模型针对环形泰勒虫进行了参数化，不过所描述的框架可应用于一系列线虫寄生虫物种。我们根据实验和现场数据对模型进行了验证，并将其应用于研究四种不同驱虫治疗方案（新抑制治疗、战略预防性治疗、基于粪便虫卵计数的治疗以及10%动物不治疗的方案）对羔羊体重增加和牧场污染的效果。该模型预测，在减少治疗次数的情况下，可以在羊群层面实现相似的体重，从而支持减少驱虫治疗的健康计划。由于该模型能够将寄生虫的寄生阶段和自由生活阶段与宿主表现相结合，因此非常适合预测非平稳条件下的复杂系统反应。本文讨论了该模型的意义及其作为绵羊可持续控制策略开发工具的潜力。
