Microclimatic influences on the availability of gastrointestestinal nematode larvae of small ruminants

In the face of climate change and increasing anthelmintic resistance, sustainable control strategies for nematodes in livestock are needed. A prerequisite to the application of these strategies is an understanding of the ecology of the major gastrointestinal nematode (GIN) species, which forms the theme of this study. This thesis investigates the effect of microclimatic factors on the ecology and epidemiology of GIN in small ruminants. It focuses on the two most important climatic variables that determine the behaviour of the free-living stages of GIN : temperature and moisture. The rationale for the work and the methods used are summarised in Chapters 1 and 2. The effects of constant and fluctuating temperatures and moisture availability on the development and survival of infective larvae (L3) of Haemonchus contortus and Teladorsagia circumcincta were first investigated by conducting controlled experiments (Chapter 3). A significant part of the thesis then investigates the effect of moisture on the migration of L3 from faeces, which has barely been studied previously. This was first examined using controlled experiments on the effect of artificial rainfall on larval migration in laboratory environments (Chapter 4). These experiments revealed that a threshold faecal moisture content (FMC} was needed to release L3, and that adding more rainfall over this threshold had no effect on larval migration. The effect of current and future climate warming scenarios on opportunities for migration of L3 was examined using glasshouse experiments (Chapter5). Results indicated no significant difference in the rate of migration between baseline and warmed (+3°C} conditions, with migration being possible for 3-6 hours after a single rainfall event. Results of the controlled experiments were used to devise a simple model to predict the larval availability of H. contortus, which was tested in the field by comparing faecal egg counts (FEC} on sheep farms with meteorological data (Chapter 6). The field studies provided some qualitative validation of the predictions, but were limited by ability to differentiate H. contortus from other nematode species in faecal samples. Two rapid and simple methods for the diagnosis of H. contortu5 were consequently developed, based on egg morphology and differential development success in culture, to support future and more precise field studies (Chapter 7). There were three sequential steps to achieve the overall aims of the thesis, starting with laboratory experiments, then experiments on turf in controlled environments (experimental greenhouses) and finally validation with field data. The final goal of these efforts was to develop a mathematical model to predict the time course of larval availability under manipulated and natural climates, thus providing evidence to support the design of climate-driven GIN control strategies. While achieving robust and useful models for this purpose is an ongoing endeavour, the results presented in t his thesis provide new understanding of the effects of climate on larval availability, especially the role of rainfall in supporting migration of infective nematode larvae from faeces. In particular, it is clear that in drier regions of the world, water availability through rainfall will impose stronger effects on larval translation onto pasture than will higher temperatures. This thesis presents a renewed focus on micro-climatic conditions for parasite transmission, understanding of which will be essential to predicting effects of climate change on livestock disease from GIN.