Perkinsus marinus is a major molluscan pathogen and the causative agent of “dermo” disease in oysters. Although long present and perhaps a native oyster parasite in Chesapeake Bay, P. marinus is still the most significant oyster parasite along the East and Gulf Coasts of the USA, and it infects nearly all market-sized oysters in Virginia waters. Decades of diagnostic work have established clear expectations for how P. marinus infections present in oyster hosts. Nonetheless, presentation of P. marinus will occasionally diverge from its typical appearance. In particular, two divergent, and undescribed, presentations of P. marinus may have implications for the epizootiology and management of the parasite. One has been the atypical presence of the parasite in the gonad of oysters in which the parasite is absent from, or only modestly present in, other organs. The other presentation is infection intracellularly in oyster oocytes themselves. A better characterization of both these unusual P. marinus infections was the focus of this research, which addressed the following questions: 1) Are these “alternative” presentations definitively P. marinus? 2) Are there sex-based differences in the incidence of these atypical infections? 3) Are there differences in cell sizes and nuclear count of the aberrant gonadal infections as compared to contemporary P. marinus? 4) What patterns, if any, in geographical distribution and prevalence can be drawn from these abnormal infections?
To investigate both types of unusual tropisms of P. marinus, archived samples in the Virginia Institute of Marine Science Shellfish Pathology Laboratory (VIMS SPL) spanning 2011 to 2021 in which P. marinus gonadal tropism was noted during standard diagnostics were reanalyzed. First, a fluorescence in situ hybridization (FISH) assay was used to confirm species identification on representative samples of the noted aberrant gonadal presentation and on representative samples of infected oocytes. Then samples were re-read so infections could be more carefully characterized.
Molecular work confirmed that both unusual infections were P. marinus and not some sympatric or new species. Results for the aberrant gonadal infections indicated this type of infection mainly infected males and was geographically widespread with infections found throughout the lower Chesapeake Bay and even down in Florida. Cell size and nuclear count were different from contemporary infections raising questions about potential P. marinus strains. Infected oocytes were uncommon but samples did span several years. Both these unusual infections could challenge diagnosticians who rely on “typical” P. marinus infections and are not aware of alternative histopathological presentations. So, it is useful that these differing infections have now been described. This work also demonstrates the value in routine monitoring programs, trained experts, and detailed collection and preservation of samples. These types of resources are rare, but vital for managing marine diseases.