Co-occurrence patterns of internal parasitic nematodes and coccidia in wild bighorn sheep depend on host geography and environment.

Heterogeneity of internal parasite communities amongst host animals is shaped by processes at multiple scales, including landscape distribution of parasites, individual host suitability, and parasite interactions within a host. As a result, pathogen co-occurrence patterns vary across space and time and among host populations. Often the challenges of wildlife sampling limit understanding of the drivers of pathogen community heterogeneity in host animals. In this study, we explore patterns and drivers of parasite community heterogeneity amongst wild bighorn sheep (Ovis canadensis) hosts over time and across populations. We describe temporal patterns including (i) general seasonal patterns of nematode and coccidia infection and (ii) taxon-specific patterns that differ by parasite life histories for bighorn sheep in southeastern Oregon and northern Nevada over a five-month period. Additionally, we describe patterns of parasite associations, many of which appear to result from co-occurrence on the landscape and others that may arise from interactions among parasites within a host. At a lower taxonomic resolution, coccidia and nematodes were positively associated with Normalized Difference Vegetation Index (NDVI), a measure of landscape vegetation greenness; at a higher taxonomic resolution, Protostrongylus was the only nematode positively associated with NDVI. Eimeria was positively associated with Ostertagia and negatively associated with Marshallagia, both abomasal nematodes. These two associations were independent of host geographical distribution and landscape vegetation greenness, suggesting that these co-occurrences may be a result of parasite interactions. This study establishes a recent and geographically expansive exploration of internal nematodes and coccidia present in a wildlife system comprising multiple populations and demonstrate the need for future investigation into parasite interactions.