Perkinsus marinus the Killer of Delicious Oysters

By: GP

Imagine Red Lobster, Stella’s Fish Café, and all other seafood restaurants in the world do not have oysters.  Imagine you cannot enjoy succulent raw oysters with a side of refreshing, cold beer. These situations could happen with the help of Perkinsus marinus, a pathogen prevalent in Eastern (mostly in the Atlantic Ocean region) oysters Crassostrea virginica. Perkinsus marinus, a facultative protozoan parasite, is the causative agent of a disease among wild and farmed C. virginica called dermo (1). This parasite was originally thought to belong in the same category as Apicomplexa, because its zoospores that is similar to an apical complex (1). However, in the years following the initial taxonomic placement, a phylum Perkinsozoa was established for the parasite and other organisms alike (1). During the 1950s, P. marinus was first recognized and subsequently researched. It was not until in 1990 at Delaware Bay, where sites of infection were first seen. Delaware Bay contained oysters that were brought in from the Chesapeake Bay region in 1953 (2). From that year on, there were minor cases reported on the presence of the parasite, but no big event occurred. Yet, the year 1990 created an epizootic disease that lasted till this day because of a warming trend in that region (2). For all those who do not know what epizootic means (neither did I before writing this blog), epizootic means a disease that infects many animals within a given region (2). The disease, dermo is characterized by the infection as decaying tissues of the infected hosts (3). To understand how the parasite works its way through the host, its time to look at the pictures:

NormalOysters

InfectedOysters

Too simple? Are you guys dying to know the intricate details of this infection? Okay, I will break it down. There are two stages to this parasite: free-living motile zoospores and non-motile vegetative trophozoites (1). The trophozoites, which are considered the infective stage, enter the oysters through the paleal cavity during filter-feeding. Thus, the parasites make their way to the gills and toward the mouth. Once inside the host, galectin on the surface of phagocytic hemocytes (the good guys) recognizes the outsiders and the phagocytes digest them. However, the elusive parasite still lives and causes systemic infections and death (1). The infection can spread to neighboring oysters by the release of trophozoites from live oysters in the water (3). The zoospores have been found to be among the viable parasites in water, which move to another host (4).

The effect of this parasite on the population of Eastern oysters has become an issue. Perkinsus marinus has spread along the Atlantic and Gulf of Mexico coasts and plummeted Crassostrea virginica population since 1991 (5). The main factors involved in the prevalence of the parasite are temperature of the water and concentration of saline (3). Warm water temperatures suggest pathogenicity and mortalities of oysters were higher than the oysters in cold waters (3). Feng and Conzonier first mentioned the observation of temperature effect on the oysters in 1970. They discussed how the levels of a certain important immune agent were higher in winter than in summer. This conclusion led to the possibility that during the winter the infection did not occur as much as during summer (6). The second factor regulating the distribution of P. marinus is salinity. For example, high saline concentrations of 10 and 20 parts per million (ppt) were known to result in the parasites survival. On the other hand, 3ppt and low saline concentrations displayed the intensity of the parasite decreased (7). Along with the environmental factors, there is a concern about host vulnerability that is increased by concentrations of marine antifouling agents, like tributyltin (TBT) near ports and marinas (8). TBT influences the infection process by making the infection worse, so the oysters are more susceptible to the parasite (8).

The decline of natural oysters in the Chesapeake Bay area caused an explosive increase in the insight on the parasite and how to prevent the infection from spreading everywhere else. Even though there is an increased interest in this parasite, there is no promising method to completely eradicate the infection from the oysters (8). There has been discussion on how to manage the outbreak of the disease, but no real solution. So if in the near future there is an increase market price of oysters, we know who are the culprits: Perkinsus marinus.

Now we know:

The Future of Oyster Populations

References:

1. Joseph, Sandeep J et al. “The Alveolate Perkinsus Marinus: Biological Insights from EST Gene Discovery.” BMC genomics 11 (2010): 228. NCBI PubMed. Web.

2. Bushek, David, Susan E. Ford, and Iris Burt. “Long-Term Patterns of an Estuarine Pathogen along a Salinity Gradient.” Journal of Marine Research 70.2-3 (2012): 225–251. IngentaConnect. Web.

3. Bower, Susan M., Sharon E. McGladdery, and Iola M. Price. “Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish.” Annual Review of Fish Diseases 4 (1994): 1–199. ScienceDirect. Web. 22 Nov. 2013.

4. Casas, Sandra M, Antonio Villalba, and Kimberly S Reece. “Study of Perkinsosis in the Carpet Shell Clam Tapes Decussatus in Galicia (NW Spain). I. Identification of the Aetiological Agent and in Vitro Modulation of Zoosporulation by Temperature and Salinity.” Diseases of aquatic organisms 50.1 (2002): 51–65. NCBI PubMed. Web.

5. Reece, K. et al. “Geographic Distribution of Perkinsus Marinus Genetic Strains along the Atlantic and Gulf Coasts of the USA.” Marine Biology 139.6 (2001): 1047–1055. link.springer.com. Web. 22 Nov. 2013.

6. Chu, F L E. "Perkinsus marinus susceptibility and defense-related activities in eastern oysters Crassostrea virginica: temperature effects." Diseases of aquatic organisms 16(1993):223.

7. Chu, Fu-Lin E., Jerome F. La Peyre, and Carrie S. Burreson. “Perkinsus Marinus Infection and Potential Defense-Related Activities in Eastern Oysters, Crassostrea Virginica: Salinity Effects.” Journal of Invertebrate Pathology 62.3 (1993): 226–232. ScienceDirect. Web. 4 Dec. 2013.

8. Fisher, W.S et al. “Decreased Resistance of Eastern Oysters (Crassostrea Virginica) to a Protozoan Pathogen (Perkinsus Marinus) after Sublethal Exposure to Tributyltin Oxide.” Marine Environmental Research 47.2 (1999): 185–201. ScienceDirect. Web. 23 Nov. 2013.