Abstract
Parasitism in wild mammals can vary according to myriad intrinsic and extrinsic factors, many of which vary seasonally. However, seasonal variation in parasitism is rarely studied using repeated samples from known individuals. Here we used a wild population of individually recognized red deer (Cervus elaphus) on the Isle of Rum to quantify seasonality and intrinsic factors affecting gastrointestinal helminth parasitism over the course of a year. We collected 1020 non-invasive faecal samples from 328 known individuals which we then analysed for propagules of three helminth taxa: strongyle nematodes, the common liver fluke Fasciola hepatica and the tissue nematode Elaphostrongylus cervi. Zero-inflated Poisson models were used to investigate how season, age and sex were associated with parasite prevalence and count intensity, while Poisson models were used to quantify individual repeatability within and between sampling seasons. Parasite intensity and prevalence varied according to all investigated factors, with opposing seasonality, age profiles and sex biases between parasite taxa. Repeatability was moderate, decreased between seasons and varied between parasites; both F. hepatica and E. cervi showed significant between-season repeatability, while strongyle nematode counts were only repeatable within-season and showed no repeatability within individuals across the year.
Highlights
Gastrointestinal helminths include a range of nematode, trematode and cestode species and are an important selective force in wild vertebrate populations (Poulin, 2007)
Both F. hepatica and E. cervi were repeatable between seasons once age and sex category were accounted for using Poisson models, while strongyle counts were only repeatable within-season
An overriding feature of this study is the contrasting and asynchronous effects shown by the different parasite taxa: despite all being helminths and having relatively similar life cycles, they exhibited substantially different seasonality and intrinsic trends as well as showing different levels of repeatability which are not attributable to their different detection assays
Summary
Gastrointestinal helminths include a range of nematode, trematode and cestode species and are an important selective force in wild vertebrate populations (Poulin, 2007). Faecal egg counts (FECs) are often found to correlate well with burden (McKenna, 1981; Budischak et al 2015), despite egg output being a complex product of both host and parasite biology (Sargison, 2013) which can fluctuate over time (Turner et al 2010). In both livestock and wild mammals, one striking feature of FEC is their distribution, which is typically strongly right-skewed with a small number of individuals with relatively high counts (Wilson et al 2004). We aim to use repeated counts at multiple timescales to separate real between-season differences in parasitism from baseline variation arising from experimental error or small-scale temporal fluctuations
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