From the extensive culture of James River oyster seed in lower Chesapeake Bay beginning in the 1800s, to the intensive culture of millions of oysters and clams throughout the region today, the production of molluscan shellfish has a long and important history in the southeastern United States.
Now, however, disease is a major threat to cultured shellfish, especially to oysters.
The causes of some disease losses—such as winter mortality in clams and summer mortality in oysters—are elusive. But some diseases are better understood.
These include dermo, MSX and SSO diseases in the oyster Crassostrea virginica, and QPX disease in the hard clam Mercenaria mercenaria. This publication addresses the management of these infectious diseases.
Dermo disease in eastern oysters:
Dermo disease in C. virginica is the most significant molluscan disease in the southeastern region. It is caused by the native parasite Perkinsus marinus (originally described as Dermocystidium marinum, to which the abbreviation “dermo” refers). Protistan parasites of this genus infect molluscs in warm waters around the world.
Perkinsus marinus occurs throughout the Gulf of Mexico and along the Atlantic coast north to New England. It causes serious mortality in all but the northernmost part of its range. Perkinsus marinus is directly transmitted between oysters, primarily when oysters with advanced infections die and disintegrate.
New infections are acquired as oysters feed, the parasite infecting its hosts through gut epithelia. Infections may remain in the gut epithelium but often become systemic, resulting in significant destruction of epithelia and connective tissues (Fig. 1).
This destruction is partly caused by the digestion of host tissues by enzymes the parasite secretes. Affected oysters are typically thin, pale and “wasted” in appearance, as tissues are overrun by explosively proliferating parasites.
Uninucleate trophozoites of 2 to 10 µm develop into multinucleate “rosette” forms (to 15 µm) that rupture to release more uninucleate trophozoites.
This process occurs most rapidly and causes the greatest mortality during the warm summer and early fall months. P. marinus proliferates at temperatures higher than 20 °C, but most rapidly at 25 °C and higher. In warm, southern waters dermo may kill more than 50 percent of oysters each year.
P. marinus thrives at estuarine salinities. Warmseason salinities as low as 12 ppt will support full dermo disease outbreaks, so areas most at risk can be identified by local salinity patterns. Salinity is a key determinant of the local distribution of dermo disease. P. marinus does tolerate periods of low salinity, however.
Even at salinities below 5 ppt the parasite can survive for extended periods. So pulses of low salinity produced by typical summer storms are unlikely to purge P. marinus from oyster populations, but fluctuations in salinity do influence the timing of intense dermo disease outbreaks. The most serious outbreaks are associated with drought years.
Seasonal morbidity and mortality are typically delayed to later summer or fall in systems greatly influenced by rivers, which generally flow most strongly and produce the lowest estuarine salinities in the spring. In the Gulf of Mexico, dermo outbreaks may be linked to the El Niño – Southern Oscillation (ENSO) cycle, with serious outbreaks regularly following the arrival of warm, dry, La Niña climatic conditions.
The effect of dermo disease in aquaculture can be minimized by culturing oysters in waters where the autumn salinity is below 12 to 15 ppt. Prospects for successful culture in waters of higher salinity are improving with the development of fast-growing, relatively disease-resistant, domesticated oyster strains.
While oysters of these strains acquire P. marinus infections, many reach market size before suffering serious morbidity and mortality, even in waters of higher salinity (more than 15 ppt). Because the risk of dermo disease increases with the age of oysters, decreasing the time to market is fundamental to profitable oyster aquaculture.
The fallowing of P. marinus-infected commercial oysters beds was recommended years ago in Virginia, but it is not clear that this strategy would be beneficial anywhere today, given the abundance of this parasite.
And because transmission can occur over long distances (kilometers), siting oyster production far from sources of infection such as reefs or structures harboring wild oysters will not necessarily protect oyster crops from dermo disease.
Author:
Ryan B. Carnegie
