Molecular epizootiology of Perkinsus marinus and P. chesapeaki infections among wild oysters and clams in Chesapeake Bay, USA.

Perkinsus marinus and P. chesapeaki host ranges among wild Chesapeake Bay, USA, region bivalves were examined by surveying Crassostrea virginica oysters and members of several sympatric clam species from 11 locations. Perkinsus genus- and species-specific PCR assays were performed on DNA samples from 731 molluscs, and species-specific in situ hybridization assays were performed on a selected subset of histological samples whose PCR results indicated dual or atypical Perkinsus sp. infections. PCR assays detected P. marinus in 92% of oysters, but the P. chesapeaki PCR assay was positive for only 6% of oysters, and P. marinus was detected by PCR in only one clam. The very low prevalence of P. marinus infections in clams is noteworthy because all surveyed clams were sympatric with oyster populations showing high P. marinus infection prevalences. P. chesapeaki commonly infected Mya arenaria, Macoma balthica, and Tagelus plebeius clams, as well as the previously unreported P. chesapeaki host clams Mulinia lateralis, Rangia cuneata, and Cyrtopleura costata. Among 30 in vitro isolates propagated from surveyed hosts, 8 P. marinus isolates were exclusively from Crassostrea virginica oysters, and all 22 P. chesapeaki isolates were from clam hosts of 5 different species. Although both P. marinus and P. chesapeaki were previously both shown to be experimentally infective for oyster and clam hosts, this survey of wild bivalves in the Chesapeake Bay region reveals that P. marinus infections occur almost exclusively in oysters, and P. chesapeaki infections predominate among members of at least 6 clam species.