Contrasting Parasite Infection Patterns in Two Captive North American Wild Sheep Species: Seasonal Dynamics of Trichostrongyloids and Muellerius capillaris

Background Gastrointestinal nematodes and lungworms are important parasites that can affect the health and population dynamics of free-ranging bighorn sheep ( Ovis canadensis ) and thinhorn sheep ( Ovis dalli ). However, information on the epidemiology of these helminths in captive North American wild sheep species remains limited. In this 12-month longitudinal study, we compared seasonal patterns of trichostrongyloid and Muellerius capillaris infections in both host species maintained under the same environmental and management conditions. Results A total of 117 individual faecal samples were examined monthly using the McMaster and modified Baermann techniques, supplemented by genus-specific multiplex qPCR. Clear differences were observed between the two host species. Despite identical housing, diet, and microclimatic conditions, trichostrongyloid faecal egg counts ( p = 0.044) and M. capillaris larval counts ( p = 0.048) were significantly higher in bighorn sheep than in thinhorn sheep. The two parasite groups also showed contrasting seasonal patterns. Trichostrongyloid egg counts were positively associated with temperature ( p < 0.001), with pronounced summer peaks dominated by Haemonchus spp. (>80% of total parasite DNA). In contrast, M. capillaris larval shedding followed an inverse seasonal pattern, with the highest larval counts recorded during late autumn and winter (p < 0.001). Faecal egg counts were strongly correlated with parasite DNA quantified by qPCR (Spearman's ρ = 0.79, p < 0.001). Conclusions Our results indicate that trichostrongyloids and M. capillaris show contrasting seasonal infection patterns, supporting the need for parasite-specific monitoring throughout the year. The pronounced summer dominance of Haemonchus spp. suggests that captive conditions may favour its transmission and influence the composition of the gastrointestinal nematode community. Differences between the two host species and considerable individual variation in trichostrongyloid egg and M. capillaris larval shedding further indicate that host species and individual infection status should be considered when interpreting coprological findings. These results may contribute to better planning of parasite surveillance and timing of anthelmintic interventions in captive wild ruminants.