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

Abstract

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 the PCR method. Thus, we tested Thai false mussel Mytilopsis leucophaeata collected from the 6 ponds with EHP-infected shrimp for the presence of EHP using SWP-PCR. Results revealed the sampled mussels from all 6 ponds were PCR positive. Subsequent bioassays were carried out to study EHP transmission between mussels and shrimp. Firstly, the naïve mussels were cohabitated with EHP-infected shrimp and all mussels were SWP-PCR positive at day 20 post cohabitation. One batch of such PCR-positive mussels was transferred for cohabitation with naïve shrimp and 37.5% EHP-positive shrimp were observed within 10 days. Tissue analysis of the SWP-PCR-positive mussels using light microscopy, in situ hybridization technique and electron microscopy did not confirm EHP infection. In summary, there was no evidence demonstrating that Mytilopsis leucophaeata was itself infected with EHP. However, the false mussels were apparently capable of 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.