Ecology of Some of the Major Trichostrongylid Parasites of Sheep

Abstract

<p>Long term sustainable control of gastrointestinal nematode parasites of sheep necessitates the use of a combination of control options. Formulating an appropriate integrated parasite management approach requires a thorough understanding of nematode epidemiology and the ecology of their free-living lifecycle stages. Temperature and water availability have previously been identified as the two most important environmental factors effecting the dynamics of the free-living stages of the nematode lifecycle. Factors directly affecting water availability (e.g. rainfall, soil moisture, relative humidity) exert their impact through the modulation of faecal water content (FWC) which represents the microclimate in which nematode eggs develop to infective larvae. Initial incubation studies investigating the impact of temperature (0, 10, 20, 30 or 40°C) and water addition (1–10 mL) on gravimetric FWC (%) resulted in the formulation of a mathematical model predicting the dynamics of FWC postdeposition (<i>R²</i> = 0.93). Further validation studies were used to determine whether this model could adequately predict the quantity of water addition required per day to maintain target FWCs of 0, 20, 40, 60 or 80% (<i>R²</i> = 0.67). Subsequently, faecal samples containing monospecific infections of <i>Trichostrongylus vitrinus</i>, <i>Trichostrongylus colubriformis</i> and <i>Teladorsagia circumcincta</i> were incubated at constant target temperatures (0, 10, 20, 30 and 40°C) and target FWCs (0, 20, 40, 60 and 80%) for 8 days. Temperature and FWC had a significant effect (<i>p</i><0.05) on the daily counts of eggs, pre-infective larvae, intra-pellet infective larvae and extra-pellet infective larvae. Development of eggs to intra-pellet infective larvae were observed at target temperatures of 20 and 30°C for all species with few <i>T. colubriformis</i> also developing at 40°C. To elucidate the survival and development/migration rates of each lifecycle stage, the experimental data was used to parameterise the mathematical model of Laurenson and Kahn (2018) for <i>T. vitrinus</i> (<i>R²</i> = 0.70), <i>T. colubriformis</i> (<i>R²</i> = 0.68) and <i>T. circumcincta</i> (<i>R²</i> = 0.73). The predicted optimum temperature (and FWC, %) for the development and migration of <i>T. vitrinus</i>, <i>T. colubriformis</i> and <i>T. circumcincta</i> eggs to extra-pellet infective larvae were 23°C (73%), 25°C (60%), and 24°C (71%), respectively. The results from this study highlight the importance of temperature and FWC for the development of eggs to infective larvae and conform to the predominance of <i>T. vitrinus</i> and <i>T. colubriformis</i> in cooler and moist climatic conditions and <i>T. colubriformis</i> in warmer and drier regions.</p>