Haemonchus contortus and Trichostrongylus colubriformis did not adapt to long-term exposure to sheep that were genetically resistant or susceptible to nematode infections

Abstract

We tested the hypothesis that Haemonchus contortus and Trichostrongylus colubriformis would adapt to long-term exposure to sheep that were either genetically resistant or susceptible to H. contortus. Sheep genotypes were from lines with 10 years prior selection for low (resistant, R) or high (susceptible, S) faecal worm egg count (WEC) following H. contortus infection. Long-term exposure of H. contortus and T. colubriformis to R or S genotypes was achieved using serial passage for up to 30 nematode generations. Thus, we generated four nematode strains; one strain of each species solely exposed to R sheep and one strain of each species solely exposed to S sheep. Considerable host genotype differences in mean WEC during serial passage confirmed adequate nematode selection pressure for both H. contortus (R 4900 eggs per gram (epg), S 19,900 epg) and T. colubriformis (R 5300 epg, S 13,500 epg). Adaptation of nematode strain to host genotype was tested using seven cross-classified tests for H. contortus, and two cross-classified and one outbred genotype test for T. colubriformis. In the cross-classified design, where each strain infects groups of R, S or randomly bred control sheep, parasite adaptation would be indicated by a significant host genotype by nematode strain interaction for traits indicating parasite reproductive success; specifically WEC and, for H. contortus strains, packed cell volume. We found no significant evidence of parasite adaptation to host genotype (P > 0.05) for either the H. contortus or T. colubriformis strains. Therefore, we argue that nematodes will not adapt quickly to sheep bred for nematode resistance, where selection is based on low WEC, although selecting sheep using a subset of immune functions may increase adaptation risk. Our results support the hypothesis that nematode resistance is determined by many genes each with relatively small effect. In conclusion, selection of sheep for nematode resistance using WEC should be sustainable in the medium to long-term.

Introduction

Adaptation of disease-causing pathogens or parasites to chemical control is widespread in many important parasites of humans and livestock. Parasite adaptation is a predictable evolutionary consequence of chemotherapy; intense selection pressure gives a selective advantage to tolerant parasites, allows them to multiply in successive generations and diminishes the overall effectiveness of the chemical (Gilleard and Beech, 2007). Adaptation of sheep gastrointestinal nematodes to anthelmintics follows this general pattern and is problematic because control of infections, for both animal health and production, relies almost exclusively on chemicals. Alternative control measures, including breeding strategies, are now being scrutinised as components of sustainable parasite management systems (Sayers and Sweeney, 2005).

Interest in breeding parasite-resistant sheep began in the 1970s and since then many studies have shown a genetic basis for parasite resistance in sheep. However, the physiological and underlying genetic mechanisms conferring resistance to gastrointestinal nematodes are complex and not fully understood. The most reliable indicator of resistance has been faecal worm egg count (WEC), where the relative resistance of each animal to a nematode challenge is assessed. Using WEC the heritability of nematode resistance generally lies between 0.2 and 0.4 across a range of sheep breeds, nematode species and infection protocols (Bishop and Morris, 2007). The complexity of the physiological response assessed by WEC is reinforced by recent research that has aimed to detect molecular markers for nematode resistance. Although many studies have identified significant quantitative trait loci (QTL) for nematode resistance, when compared, these studies tend to identify different chromosomal regions (Dominik, 2005, Crawford et al., 2006, Davies et al., 2006, Beraldi et al., 2007). This suggests that either (i) different experimental protocols were invoking slightly different responses (Dominik 2005), (ii) different genes are heterozygous in different populations, or (iii) many genes each with small effect determine nematode resistance so each study lacked power and could only detect a subset of the genes involved. It appears that the intimate host–parasite relationship assessed by WEC is under the control of many different genes each with variable importance depending on the infection and host conditions.

There is concern that nematodes will adapt to hosts bred for increased nematode resistance. Co-evolutionary theory supports the notion of reciprocal adaptation (Clayton and Lee, 1999), although observing this experimentally is difficult. Webster et al. (2004) provided one of the few experimental examples of reciprocal adaptation in an animal–parasite system using the trematode Schistosoma mansoni. However, several studies in sheep have failed to detect any adaptive changes in Haemonchus contortus due to host resistance for up to 10 parasite generations of serial passage (Adams, 1988, Albers and Burgess, 1988, Saulai et al., 2001). Generally, these experiments compared nematode strains produced from immunological extremes; where the extremes were obtained by using a combination of either highly resistant breeds, repeated infections or deliberate immunosuppression. One study referred to by Windon (1990) suggested that Trichostrongylus colubriformis can adapt to Merino sheep bred for low WEC following vaccination with irradiated T. colubriformis larvae. This study found a significant WEC increase in vaccinated Border Leicester × Merino ewes infected with larvae from vaccinated high responder lambs compared with similar ewes infected with larvae from low-responder lambs. However, no study has investigated whether or not the nematodes H. contortus or T. colubriformis can adapt to hosts selectively bred using WEC for susceptibility or resistance to nematodes.

In this study, we hypothesised that H. contortus and T. colubriformis would adapt to long-term exposure to host sheep with either a resistant (R) or susceptible (S) genotype. We generated four nematode strains, one strain from each species solely exposed to R sheep and one strain from each species solely exposed to S sheep for up to 30 nematode generations of serial passage. Our hypothesis was tested using a cross-classified design where cohorts of R, S or randomly bred control sheep at pasture were infected with each nematode strain and differences assessed in WEC and, for H. contortus strains, packed cell volume (PCV). If the nematodes strains had adapted to their host genotype then the WECs from the R strain nematodes would be high in R sheep and the WECs from S strain nematodes would be high in S sheep. However, our strain tests found no evidence for adaptation in the nematodes, that is, the differences in WEC between R and S genotype sheep was maintained independent of the nematode strain with which they were infected.