Paramoebiasis Associated with Mass Mortality of American Lobster Homarus americanus in Long Island Sound, USA

In the autumn of 1999, a mass mortality of American lobster Homarus americanus was reported by lobster fishermen from western Long Island Sound (LIS). At the conclusion of the 1999 season, dead lobsters were estimated at 11 million, resulting in a 90– 99% reduction of landings in western LIS and failure of the natural lobster fishery. Fishermen described moribund lobsters that were “limp,” interpreted clinically as paretic or flaccidly paralyzed. Necropsies were performed on dead or dying limp lobsters, and tissue samples were collected for histopathological, ultrastructural, microbiological, and toxicological analysis. Microbiological cultures of hemolymph and hepatopancreas failed to isolate a bacterium or group of bacteria in any significant frequency or pattern. Toxicological analyses of hepatopancreas and skeletal muscle did not identify significant amounts of trace elements, polycyclic aromatic hydrocarbons, polychlorinated biphenyl congeners, or pesticides. Microscopic analyses of hemocoelomic viscera and nervous tissues revealed a protozoan parasite in foci of hemocytic infiltration of nerves, in cytoplasm of neurons, and between nerve fibers without attendant hemocytic infiltrates as well as in micronodular aggregates of hemocytes in the subcuticular interstitium and tegumental glands. Histologic and ultrastructural features of this parasite were characteristic of protozoa of the genus Paramoeba (Schaudinn, 1896), and contact transmission experiments involving lobsters from uninfected, geographically distant sites and infected lobsters from western LIS resulted in transmission of paramoebiasis. This report represents the first description of a proximate cause of lobster mortality from the 1999 epizootic, along with additional evidence supporting the assertion that this parasitic paramoeba may be the primary cause of the 1999 lobster mass mortality.