Blastocystis surface antigen is stable in chemically preserved stool samples for at least 1 year.

Blastocystis spp. refer to a group of prevalent enteric protists found in humans and animals. Detection of Blastocystis spp. in fecal samples is often performed by clinicians with direct microscopy, which provides low sensitivity, or with culture and polymerase chain reaction testing, a method which is problematic when used with formalin-preserved stool samples. Prior study of Blastocystis and other enteric protists suggests that immunofluorescence antibody (IFA) stain could provide sensitivity and compatibility with formalin, but no information is available on the longevity of Blastocystis sp.'s surface antigens in formalin. We collected fecal samples from animals at a country fair held in the summer of 2009 in Oregon, USA. Samples were tested for the presence of Blastocystis infection using an IFA stain shortly after collection, and again after 1 year, with samples stored refrigerated at 4-8 °C. Most samples collected from steer, pigs, and goats were found to be Blastocystis positive. All fecal samples that were Blastocystis positive initially remained positive after 1 year. Blastocystis-negative samples remained negative. Minimal degradation was observed in stained slides. Blastocystis surface antigens detected by a polyclonal stain remained stable in formalin for a period of at least 1 year.