Sensitive and specific polymerase chain reaction assay for detection of Loma salmonae (Microsporea)

Ribosomal DNA sequence of the microsporidian parasite Loma salmonae, including portions of the small subunit (SSU), large subunit (LSU), and internal transcribed spacer regions (ITS), was determined. Based on L. salmonae-specific regions of sequence, a polymerase chain reaction (PCR) assay was developed for detection of this parasite in chinook salmon Oncorhynchus tshawytscha. The specificity of the assay was verified by the lack of amplification with DNA from 3 other microsporidians that parasitize fishes in British Columbia, Canada. A dilution study assessed the sensitivity of the test, showing that the PCR product (derived from multiple copies of rDNA per cell) was routinely detected from as few as 0.01 spores per 50 mu l reaction (or 40 spores g(-1) of infected gill tissue). The assay detected L. salmonae infections in the gills, kidneys, and ovaries of broodstock females from a seawater netpen on Vancouver Island, British Columbia, and infections in these organs were confirmed by histology. This is the first report of L. salmonae infections in ovarian tissue. Although neither PCR nor histology detected L. salmonae within the eggs themselves, it is possible that the progeny of infected females may become exposed to the parasite through contaminated ovarian fluid.