The diversity and host specificity of gastrointestinal parasites infecting free-ranging sloths is poorly known. We characterized gastrointestinal parasites of two sloth species from Costa Rica, three-fingered sloth (Bradypus variegatus) and two-fingered sloth (Choloepus hoffmanni), for the first time in both a primary forest and an urban habitat. We asked whether host-parasite interactions were predominantly structured by host identity, the habitats in which hosts occurred, or both. Protozoa and nematode eggs were present in both species, but cestode eggs were recorded only in C. hoffmanni. We found eight parasitic morphotypes, which matches the total number of parasite taxa described in sloths over the past 100 years, and these first reports of gastrointestinal parasites of sloths from primary forest radically expand our knowledge of the general diversity and host specificity of sloth parasites. We found no significant difference in overall parasite richness between sloth species or habitats, but the parasite richness of C. hoffmanni was 2-fold greater in the primary forest vs. urban habitat. As no parasite sharing was observed between sloth species, we found strong and significant differences in parasite composition between host species regardless of habitat. In B. variegatus, we observed eggs of four nematode taxa (Spirocercidae, Subuluroidea, Spirurida, Ascarididae) and cysts of Eimeriidae (Apicomplexa). By contrast, in C. hoffmanni, we observed cestodes (Anoplocephalidae), a different nematode from the family Spirocercidae and also cysts of Eimeriidae (Apicomplexa). Many rare taxa were recorded only in samples from the primary forest, and these did not match any sloth parasites that had been previously described in the literature, suggesting that at least some could be undescribed species. Together, these results highlight the need for further research in comparative wildlife parasitology, the characterization of host-parasite transmission networks, and the identification of any intermediate hosts that may be relevant to sloth health.
