Parasites are important members of the ecological web within which an animal lives, and can be used as indicators of ecosystem health. However, few baseline parasitological data are available for free-ranging animals, particularly for the black howler monkey (Alouatta pigra). In this study a total of 283 fecal samples were collected from 50 individually identified A. pigra during 2003 and 2004 and examined for parasites. The samples were processed using standard quantitative centrifugation concentration techniques, with sugar and zinc sulfate used as flotation media. The slides were examined using bright-field and phase microscopy. Antigen capture enzyme-linked immunosorbent assays (ELISAs) were used to detect protozoa. Four parasites were detected: 1) Controrchis biliophilus (Dicrocoeliidae), 2) Trypanoxyuris minutus (Oxyuridae), 3) Giardia sp. (Hexamitidae), and 4) Entamoeba sp. (Endamoebidae). Controrchis biliophilus was detected in 80% (wet season) and 81% (dry season) of the A. pigra samples; Trypanoxyuris minutus was detected in 8% (wet season) and 27% (dry season) of samples; and Giardia sp. was detected in 40% (wet season) and 27% (dry season) of samples. For the first time, DNA from Giardia sp.-positive fecal samples was extracted from A. pigra. Alouatta pigra individuals that lived near human settlements in Belize were infected with Giardia duodenalis (syn. G. lamblia, G. intestinalis) Assemblages A and B. These results suggest that G. duodenalis is transmitted from people and/or domestic animals to A. pigra.

We examined the effects of demographic and ecological variables on the prevalence of intestinal parasites in free-ranging black howlers, Alouatta pigra, from Belize and Mexico. We collected 253 fecal samples from 50 individually identified monkeys during 2003. We processed all samples via standard centrifugation concentration techniques with sugar and zinc sulfate as flotation media. We used antigen capture enzyme-linked immunosorbent assays to detect protozoa. We analyzed data for each season separately. The most important factor in predicting whether an Alouatta pigra would be infected with a parasite was its membership in a particular social troop. It was not possible to isolate the effects of human presence, forest fragmentation, primate density, and the absence of Ateles geoffroyi on the prevalence of parasites in Alouatta pigra because the factors covaried. We detected few species of gastrointestinal parasites, possibly due to Alouatta pigra's arboreal and herbivorous lifestyle and small geographic range. The prevalence of each parasite had a different pattern, with Controrchis sp. (presumed to be C. biliophilus), a fluke, tending to be more prevalent in Alouatta pigra that inhabited disturbed habitats, and Trypanoxyuris minutus, a pinworm, tending to be more prevalent in primates from undisturbed habitats. Giardia sp. tended to be more prevalent in primates at high densities. These results indicated that it is important to examine each parasite's infection pattern separately to obtain a more accurate representation of the dynamics among the host, parasites, and ecology.