Blastocystis is a genus of single-celled protozoan parasites belonging to a group of organisms known as the Stramenopiles (also called Heterokonts) that includes algae, diatoms, and water molds. Blastocystis comprises several species, living in the gastrointestinal tracts of mostly vertebrate species (including humans) although some were also discovered in insects, such as Cockroaches.
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 symptoms. Symptoms associated with the infection are diarrhea, nausea, abdominal cramps, bloating, excessive gas, and anal itching. Most cases of the infection appear to become diagnosed as irritable bowel syndrome, which is a symptom-based diagnosis with no known organic cause but characterized by the very same symptoms.
For many years, scientists believed one species of Blastocystis infected humans and called it Blastocystis hominis. Consequently they thought that different species of Blastocystis infected other animals. However, genetic analyses in recent years it have shown shown that Blastocystis hominis as a unique entity does not exist. In fact, a number of distinct genetic types of Blastocystis can infect humans and other animals. In 2007 it was proposed to drop the name Blastocystis hominis entirely and refer to Blastocystis from humans and animals as Blastocystis sp. subtype nn where nn is a number assigned to each group according to the degree of genetic identity of the Blastocystis organisms, based on small subunit ribosomal RNA sequences, essentially a DNA Barcoding based taxonomy. Today there are at least 17 genetically distinct small subunit ribosomal RNA lineages known. So far, no strict associations between those subtypes and their hosts have been reported, although moderate host specificity is seen.
A team of researchers from Columbia and Denmark have now published a study that shows the distribution of Blastocystis subtypes in different Colombian hosts. Six subtypes were identified by DNA Barcoding. Humans, domesticated mammals and rats shared the same subtypes, while birds and opossums had subtypes, which are usually rare in humans. Noticeably, some subtypes were absent in cattle and in humans, indicating significant phylogeographic differences.