Mixed strongyle parasite infections vary across host age and space in a population of feral horses

Abstract

Identifying factors that drive among-individual variation in mixed parasitic infections is fundamental to understanding the ecology and evolution of host–parasite interactions. However, a lack of non-invasive diagnostic tools to quantify mixed infections has restricted their investigation for host populations in the wild. This study applied DNA metabarcoding on parasite larvae cultured from faecal samples to characterize mixed strongyle infections of 320 feral horses on Sable Island, Nova Scotia, Canada, in 2014 to test for the influence of host (age, sex and reproductive/social status) and environmental (location, local density and social group membership) factors on variation. Twenty-five strongyle species were identified, with individual infections ranging from 3 to 18 species with a mean richness (±1 s.d.) of 10.8 ± 3.1. Strongyle eggs shed in faeces were dominated by small strongyle (cyathostomins) species in young individuals, transitioning to large strongyles (Strongylus spp.) in adults. Egg counts were highest in young individuals and in the west or centre of the island for most species. Individuals in the same social group had similar parasite communities, supporting the hypothesis that shared environment may drive parasite assemblages. Other factors such as local horse density, sex, date and reproductive/social status had minimal impacts on infection patterns. This study demonstrates that mixed infections can be dynamic across host ontogeny and space and emphasizes the need to consider species-specific infection patterns when investigating mixed infections.

Keywords: DNA metabarcoding; ITS2; disease ecology; gastrointestinal parasite; mixed infection; nemabiome.

Graphical abstract

Figure 1.

Map of Sable Island National Park Reserve, Nova Scotia, Canada (from Gold et al., 2019).

Figure 2.

Strongyle parasite species richness, Inverse Simpson diversity index and Shannon diversity index for 320 horses on Sable Island, Nova Scotia, Canada, across host age and standardized longitude. Each point indicates an individual sample. Boxplots on age plots show mean values for each age (in years). Lines in longitude plots describing the conditional means with a 95% confidence interval.

Figure 3.

Prevalence of 25 strongyle species identified by DNA metabarcoding across host age for 320 Sable Island horses in 2014. Lines show the conditional means with a 95% confidence interval. Each point indicates the mean prevalence for each age with error bars estimated as 95% bootstrap confidence intervals.

Figure 4.

Prevalence of 25 strongyle species identified by DNA metabarcoding across a west to east gradient on Sable Island (standardized to a mean of 0, s.d. of 1) for 320 Sable Island horses in 2014. Each point indicates an individual sample, with lines describing the conditional means with a 95% confidence interval.

Figure 5.

Fecal egg counts (FEC) of 25 strongyle species across host age for 320 Sable Island horses in 2014. Species-specific FEC was calculated by multiplying aggregate strongyle FEC by species-specific relative abundance estimated from DNA metabarcoding. Each point indicates an individual sample, with lines describing the conditional means with a 95% confidence interval. Note that independent y-axis were used per species to better visualize trends.

Figure 6.

Fecal egg counts (FEC) of 25 strongyle species identified by DNA metabarcoding across a west to east gradient on Sable Island (standardized to a mean of 0, s.d. of 1) for 320 Sable Island horses in 2014. Species-specific FEC was calculated by multiplying aggregate strongyle FEC by species-specific relative abundance estimated from DNA metabarcoding. Each point indicates an individual sample, with lines describing the conditional means with a 95% confidence interval. Note that independent y-axis were used per species to better visualize trends.

Figure 7.

Principal coordinate analysis of the (A) Jaccard and (B) Bray-Curtis dissimilarity matrix of parasitic strongyle community composition among 320 Sable Island horses in 2014. Each point represents a strongyle parasite community, with colour indicating horse age class (Juvenile, Subadult, Adult) with ellipses for the 95% confidence interval for each age class. Discrimination between samples of different ages (in years) are available in the Supplementary Materials.