The false mussels (Mytilopsis leucophaeata) can be mechanical carriers of the shrimp microsporidian Enterocytozoon hepatopenaei (EHP)

Enterocytozoon hepatopenaei (EHP) is an obligate intracellular parasite causing hepatopancreatic microsporidiosis (HPM) in cultivated shrimp in Asian countries. One strategy to control EHP is to identify and eliminate biological reservoir(s) in shrimp ponds. Several marine and brackish-water organisms, including false mussels (Mytilopsis) have been reported to test positive for EHP using polymerase chain reaction (PCR) technology. Thus, we tested samples of commonly found Thai false mussel Mytilopsis leucophaeata from 6 EHP-infected shrimp ponds by PCR for the presence of EHP using the spore wall protein (SWP) gene primers. The mussel samples from all 6 ponds were positive. Subsequent bioassays carried out using naive mussels cohabitated with EHP-infected shrimp gave 100% SWP-PCR positive mussels at 20 days. One batch of such PCR-positive mussels was transferred for cohabitation with naive shrimp and gave 37.5% EHP-positive shrimp within 10 days. Tissue analysis of the EHP-PCR-positive mussels using light microscopy, in situ hybridization analysis for the SWP gene and electron microscopy did not confirm EHP infection. In summary, we obtained no evidence that Mytilopsis leucophaeata was itself infected with EHP. However, it was apparently capable carrying infectious spores for some period after ingestion and serving as a mechanical or passive carrier. The results support previous reports warning of the danger of feeding living or fresh bivalves to broodstock shrimp in hatcheries or shrimp in rearing ponds without prior heating or freezing.