Equine strongyle communities are constrained by horse sex and species dipersal-fecundity trade-off.

BACKGROUND: Equine strongyles are a major health issue. Large strongyles can cause death of horses while cyathostomins (small strongyles) have shown increased resistance to anthelmintics worldwide. Description of strongyle communities have accumulated but little is known about the diversity of these communities and underpinning environmental factors.

METHODS: Strongyles were recovered after ivermectin treatment from 48 horses located in six premises in Poland. Correlation between previously published species fecundity and the observed relative abundance and prevalence were estimated. Significance of horse sex was determined at the species level (prevalence, relative abundance) and at the community level (species richness and dissimilarity between communities).

RESULTS: Strongyle species fell into two groups, contrasted by their prevalence and relative abundance. Six to nine horses were necessary to sample at least 90% of strongyle community diversity, providing a minimal cut-off to implement sampling trial in the field. Strongyle communities entertained a network of mostly positive interactions and species co-occurrence was found more often than expected by chance. In addition, species fecundity and prevalence were negatively correlated (Pearson's r = -0.71), suggesting functional trade-offs between species dispersal abilities and fecundity. This functional trade-off may underpin species coexistence. Horse sex was also a significant constraint shaping strongyle communities. Indeed, mares generally displayed more similar strongyle communities than stallions (P = 0.003) and Cylicostephanus calicatus was more abundant in stallions suggesting sex-specific interactions (P = 0.006).

CONCLUSIONS: While niche partitioning is likely to explain some of the positive interactions between equine strongyle species, coexistence may also result from a functional trade-off between dispersal ability and fecundity. There is significant evidence that horse sex drives strongylid community structure, which may require differential control strategies between mares and stallions.