Biology subjects

Hayward, A. D.

Publications and source records attributed to Hayward, A. D..

3 recordsLinked to original sources

bioRxiv2022

Genetic parameters for resistance to gastrointestinal nematodes in sheep: a meta-analysis

Gastrointestinal nematodes (GIN) are damaging parasites of global sheep populations. The key weapons in fighting GIN have been anthelmintic drugs, but the emergence of drug-resistant parasites has meant that alternative control methods are needed. One of these alternatives is to breed for enhanced host resistance to GIN, and decades of research have estimated the genetic contribution to different measures of resistance to GIN and their genetic correlations with other desirable performance traits. It is clear that parasite resistance is a heritable trait that can be selected for. Despite this consensus, estimates of both heritability of resistance and genetic correlations with other traits vary widely between studies, and the reasons for this variation have not been examined. This study provides a comprehensive and quantitative meta-analysis of genetic parameters for resistance to GIN in sheep, including measures of worm burden (faecal egg counts, FEC), anti-parasite immunity (GIN-specific antibodies), and parasite-induced pathology (FAMACHA(C) scores). Analysis of 591 heritability estimates from 121 studies revealed a global heritability estimate for resistance to GIN of 0.25 (95%CI = 0.22 - 0.27) that was stable across breeds, ages, geographical location and analytical methods. Meanwhile, analysis of 559 genetic correlations from 54 studies revealed that resistance to GIN overall has a positive genetic correlation of +0.10 (95%CI = 0.02 - 0.19) with performance traits, and that this was consistent across breeds, ages, sexes and analytical methods. Importantly, the direction of the genetic correlation varied with the resistance trait measured: while FEC and FAMACHA(C) scores were favourably correlated with performance traits, adaptive immune markers were unfavourably correlated, suggesting that selection for enhanced immune responses to GIN could reduce animal performance. Overall, the results suggest that breeding for resistance to GIN should continue to form part of integrated management programs to reduce the impact of parasites on health and performance, but that selection for enhanced immune responses should be avoided.

genetics↗

bioRxiv2022

Long-term temporal trends in gastrointestinal parasite infection in wild Soay sheep

Monitoring the prevalence and abundance of parasites over time is important for addressing their potential impact on host life-histories, immunological profiles, and their influence as a selective force. Only long-term ecological studies have the potential to shed light on both the temporal trends in infection prevalence and abundance and the drivers of such trends, because of their ability to dissect drivers that may be confounded over shorter time scales. Despite this, only a relatively small number of such studies exist. Here, we analysed changes in the prevalence and abundance of gastrointestinal parasites in the wild Soay sheep population of St Kilda across 31 years. The host population density has increased across the study, and population density is known to increase parasite transmission, but we found that density and year explained temporal variation in parasite prevalence and abundance independently. Prevalence of both strongyle nematodes and coccidian microparasites increased during the study, and this effect varied between lambs, yearlings and adults. Meanwhile, abundance of strongyles was more strongly linked to host density than to temporal (yearly) dynamics, while abundance of coccidia showed a strong temporal trend without any influence of density. Strikingly, coccidian abundance increased threefold across the course of the study in lambs, while increases in yearlings and adults were negligible. Our decades-long, intensive, individual-based study will enable the role of environmental change and selection pressures in driving these dynamics to be determined, potentially providing unparalleled insight into the drivers of temporal variation in parasite dynamics in the wild. Key findingsO_LIWe studied temporal trends in gastrointestinal parasites of wild sheep over 31 years C_LIO_LIYear and host population density explained temporal variation in parasites independently C_LIO_LIPrevalence of both strongyle nematodes and coccidia varied across the study period C_LIO_LIAbundance of strongyles was more closely linked to host density than year C_LIO_LIAbundance of coccidia increased threefold over time in lambs, but did not vary in adults C_LI

ecology↗

bioRxiv2021

Seasonal dynamics and repeatability of gastrointestinal parasite burdens in wild Soay sheep

Seasonality is a ubiquitous feature of wildlife disease ecology, but is determined by a complex interplay of environmental, parasitological and host factors. Gastrointestinal parasites often exhibit strong seasonal dynamics in wild vertebrate populations due to, for example, environmental influences on free-living or vectored life stages, and variation in the physiological and immune status of hosts across their annual cycle. At the same time, wild populations are typically infected with multiple parasites. The seasonal dynamics of co-infecting parasites may differ depending on age and reproductive status, and associations among parasites may be driven by short-term within-individual changes or longer-term interactions that are consistent among hosts. Here, we used faecal samples and egg counts collected repeatedly from individually marked and monitored wild Soay sheep that were part of a long-term study to investigate seasonal dynamics of six gastrointestinal parasite groups (strongyle nematodes, coccidian protozoa, Capillaria, Strongyloides, Nematodirus, and Moniezia). Prevalence and abundance generally tended to be higher spring and summer, and burdens were higher in lambs than adults. Within the highly prevalent strongyle nematode group, we found differences in seasonality of egg counts depending on adult reproductive status. Reproductive ewes had increased counts in spring around the time of birth followed by a drop in abundance in summer, while barren ewes showed little evidence of seasonality. Males showed a sustained rise in egg counts through spring and summer, and sex differences were only strongly apparent in summer. In contrast, in similarly prevalent coccidia we found a peak in faecal oocyst counts in spring but no differences in seasonality among males, barren and pregnant ewes. Using multivariate mixed-effects models, we went on to show that both strongyle and coccidia counts are moderately repeatable across seasons among individuals. We further show that apparent positive correlation between strongyle and coccidia counts was driven by short-term within-individual changes in both parasite burdens rather than long-term among-individual covariation. Overall, our results demonstrate that seasonality varies across demographic and parasite groups and highlight the value of investigating fluctuating susceptibility and exposure over time for understanding epidemiology of a population.

ecology↗