Human infections are found at a rate of 5-10% in most developed countries with elevated rates for individuals that work with animals. In developing countries the rate can be as high as 50%. However, only 50% and 80% of individuals infected with Blastocystis will show any symptoms. Symptoms associated with the infection are diarrhea, nausea, abdominal cramps, bloating, and excessive gas. Most cases of the infection are diagnosed as irritable bowel syndrome, which is actually a symptom-based diagnosis with no known organic cause but characterized by the very same symptoms.
Because of a growing interest in Blastocystis as a potential enteric pathogen, and the possible role of domestic and in-contact animals as reservoirs for human infection, researchers of the College of Veterinary Medicine at Oregon State University tried to estimate the prevalence of Blastocystis spp. in shelter-resident and client-owned companion animals in the US Pacific Northwest region.
They used the proposed DNA Barcode standard for protists, 18S (or SSU rRNA) to detect the presence of Blastocystis sp. Their findings indicate that shelter-resident animals were carrying a variety of Blastocystis subtype while they couldn't detect the protist in any fecal sample from client-owned animals. In addition no relationship was seen between Blastocystis carriage and the presence of gastrointestinal disease signs in either dogs or cats. These data suggest that, as previously reported for other enteric pathogens, shelter-resident companion animals are a higher risk for carriage of Blastocystis spp. The lack of relationship between Blastocystis carriage and intestinal disease in shelter-resident animals suggests that this organism is unlikely to be a major enteric pathogen in these species.
There is more good news especially for those that want to adopt shelter animals. The majority of identifiable specimens belonged to a subtype with no evidence of carriage in human beings. Therefore, it is unlikely that shelter resident animals in the Pacific Northwest of the USA represent a potential risk for zoonotic infection of animal handlers or owners.