Ecology of the free-living stages of major trichostrongylid parasites of sheep.

Environmental factors influencing transmission.

In the UK, recent mean temperatures have consistently increased by between 1°C and 4°C compared to the 30-year monthly averages. Furthermore, all available predictive models for the UK indicate that the climate is likely to change further and feature more extreme weather events and a trend towards wetter, milder winters and hotter, drier summers. These changes will alter the prevalence of endemic diseases spatially and/or temporally and impact on animal health and welfare. Most notable among these endemic parasites are the helminths, which have been shown to be very strongly influenced by both the short-term weather and climate through effects on their free-living larval stages on pasture. In this review, we examine recent trends in prevalence and epidemiology of key helminth species and consider whether these could be climate-related. We identify likely effects of temperature and rainfall on the free-living stages and some key parasite traits likely to determine parasite abundance under changed climatic conditions. We find clear evidence that climate change, especially elevated temperature, has already changed the overall abundance, seasonality and spatial spread of endemic helminths in the UK. We explore some confounders and alternative explanations for the observed patterns. Finally, we explore the implications of these findings for policy makers and the livestock industry and make some recommendations for future research priorities.

Climate change and infectious disease: helminthological challenges to farmed ruminants in temperate regions

The transmission success of free-living larval stages of endohelminths is generally modulated by a variety of abiotic and biotic environmental factors. Whereas the role of abiotic factors (including anthropogenic pollutants) has been in focus in numerous studies and summarized in reviews, the role of biotic factors has received much less attention. Here, we review the existing body of literature from the fields of parasitology and ecology and recognize 6 different types of biotic factors with the potential to alter larval transmission processes. We found that experimental studies generally indicate strong effects of biotic factors, and the latter emerge as potentially important, underestimated determinants in the transmission ecology of free-living endohelminth stages. This implies that biodiversity, in general, should have significant effects on parasite transmission and population dynamics. These effects are likely to interact with natural abiotic factors and anthropogenic pollutants. Investigating the interplay of abiotic and biotic factors will not only be crucial for a thorough understanding of parasite transmission processes, but will also be a prerequisite to anticipate the effects of climate and other global changes on helminth parasites and their host communities.

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The role of biotic factors in the transmission of free-living endohelminth stages.

The parasitic nematode Haemonchus contortus occurs commonly in small ruminants, and it is an especially significant threat to the health and production of sheep and goats in tropical and warm temperate zones. The main signs of disease (haemonchosis) relate to its blood-feeding activity, leading to anaemia, weakness and frequently to deaths, unless treatment is provided. Due to the high biotic potential, large burdens of H. contortus may develop rapidly when environmental conditions favour the free-living stages, and deaths may occur with little prior warning. More chronic forms of haemonchosis, resulting in reduced animal production and eventually deaths, occur with smaller persistent infections, especially in situations of prolonged, poor nutrition. The global distribution of the main haemonchosis-endemic zones is consistent with the critical requirements of the egg and larval stages of H. contortus for moisture and moderate to relatively warm temperatures, but the seasonal propensity for hypobiosis (inhibition of the fourth-stage larvae within the host) largely explains the common, though sporadic, outbreaks of haemonchosis in arid and colder environments. The wide climatic distribution may also reflect the adaptation of local isolates to less favourable ecological conditions, while an apparent increase in the prevalence of outbreaks in environments not previously considered endemic for haemonchosis – especially cold, temperate zones – may be attributable to climatic changes. Although the risk of haemonchosis varies considerably on a local level, even where H. contortus is endemic, the extensive range of ecological investigations provides a sound basis for predictions of the relative geographical and seasonal risk in relation to climatic conditions.

The Pathophysiology, Ecology and Epidemiology of Haemonchus contortus Infection in Small Ruminants

Experiments were conducted in the laboratory to investigate the ability of trichostrongylid larvae to migrate vertically on various grasses under controlled environments. Only a small proportion, 2 to 3%, of the larvae given opportunity to ascend the grass blades, under conditions considered favorable for migration, climbed. Of these, 59.2, 26.7, 9.9, 3.4, and 0.8% were found on the 1st-, 2nd-, 3rd-, 4th-, and above the 5th-inch height level from the soil, respectively. It was concluded that though infective trichostrongylid larvae can climb grasses, the greatest portion remain at the base of the vegetation. The larvae were comparatively more successful on the tall fescue than on the smooth brome. The external morphology of grasses apparently influences migration of the larvae. A film of moisture is obviously advantageous to migration. On an average 0.65% of the test larvae climbed wetted blades while only 0.04% climbed unwetted blades. Upward migration was favored by higher relative humidity. On the average 1.36% of the test larvae were successful at 95% relative humidity while only 0.06% were successful at 56% relative humidity. Lower temperature adversely affected upward migration. Only 0.13% of the test larvae climbed at 40 F while 2.54% did so at 79 to 80 F. A direct influence of light intensity was not clearly established since migration in light and darkness was essentially similar. Taylor (1938), Rogers (1940), Kauzal (1941), Dinaburg (1944), Crofton (1948), and Rees (1950) studied vertical migration of infective trichostrongylid larvae on herbage. Their techniques essentially involved placement of known numbers of infective larvae, contained in a known volume of water, on the soil at the base of the test herbage in experimental plots in the field or laboratory. Subsequent recovery of larvae on the test herbage was interpreted to indicate vertical migration of the larvae from the soil surface onto the herbage. However, except for Crofton (1948) and Rees (1950), most of these investigators did not establish the actual height on the herbage the larvae ascended. Furthermore, only in a few cases (Crofton, 1948; Rees, 1950; Rogers, 1940) was the ability of the larvae to migrate vertically on the herbage correlated with environmental factors. The present study was undertaken in order to obtain an extensive series of observations Received for publication 20 August 1963. * From the Department of Veterinary Science, Paper N.S. 400, with the approval of the Director, Wis. Agr. Exp. Sta., supported in part by the University Research Committee. on the actual height of vertical migration by trichostrongylid larvae as well as the portion of the population which does migrate, and to observe the influence of various environmental factors including moisture, temperature, relative humidity, light, type of grass, and the number of grass blades available upon movement of the larvae. MATERIALS AND METHODS The larvae for these studies were obtained from "seed" calves primarily infected with Haemonchus placei with minor populations of Cooperia onchophora and Trichostrongylus colubriformis. The larvae were cultured in sterilized sphagnum according to the method of Cauthen (1940) and recovered by using Baermann funnels. Differential counts of larvae were not made for individual tests, and the larvae, therefore, have been referred to as trichostrongylid. The larvae were 7 to 20 days old when used in the tests, but in a given trial all larvae were the same age. Grass blades were grown from seeds planted in a light silt soil or a muck soil in paper pots. The soils were sterilized to render them free of nematode life prior to the tests. The grass seedlings emerged in a tuft of blades and were used when 3 inches or more tall. Numbers of larvae used in a test were estimated by dilution counts. By restriction of volume of the suspension, a known quantity of larvae could be placed in position on the soil surface at the

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Vertical migration of trichostrongylid larvae on grasses.