To evaluate the seasonal prevalence of human intestinal parasites in the western states of Colorado, Utah, New Mexico, and Montana, fecal samples were examined as part of routine diagnostic testing from patients experiencing gastrointestinal discomfort in August (summer) 2006, January (winter), and April (spring) 2007. Parasite identification in positive samples was confirmed using light microscopy after wet mount and trichrome staining techniques. Seventy-eight of the 1,083 patients surveyed (7.2%) in August tested positive for at least 1 species of intestinal parasite. Forty-eight of 726 (6.6%) patients and 51 of 795 (6.4%) patients tested positive for at least 1 species in January and April, respectively. Blastocystis sp. was the most prevalent, followed by Giardia lamblia. Approximately 25% of the parasite occurrences were multiple infections involving fecal-oral transmitted species. Co-infections with Entamoeba spp. and Blastocystis sp. were common, suggesting a possible fecal-oral transmission for the latter parasite. Entamoeba spp. were more likely to co-occur than independently. Other species detected included Endolimax nana, Diphyllobothrium latum, Hymenolepis nana, Dientamoeba fragilis, and Iodamoeba butschlii.

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