VIMS research reveals new causes for intensification of oyster disease

A new paper from Scientific Reports, led by researchers at the Virginia Institute of Marine Sciences at William & Mary, provides a well-established account of the decades increase in prevalence and deadlines of major oyster diseases in coastal waters: salt and seawater. I am disagreeing with the rise in temperature. Mid-Atlantic.

Dr. Ryan Carnegie, lead author of the treatise, “introduces a whole new lens that allows you to see the history of oysters in the Chesapeake Bay region over the last 35 years. There was no significant intensification of Delmo disease in the 1980s. I understand that. ” It was simply due to the drought. More fundamentally, it was due to the emergence of a new and highly toxic form of Perkinsus marinus, the parasite that causes dermo. “

With a rare twist, the team’s evidence is that this native parasite change is brought about by the effects of another non-native oyster parasite known as MSX, first seen in Baywater in 1959. It suggests that it has responded to the pressure of evolution. With decades of over-harvesting, habitat destruction, and the early devastation of MSX, the bay’s traditional oyster fishery has fallen to historic lows.

Carnegie said his team’s findings will help better manage the modern oyster industry in Bay, which is currently on the rise due to disease-resistant farms, reef restoration, and wild harvest restrictions. say. “The lesson is that pathogens like P. marinus are very dynamic and our disease monitoring needs to pay attention to possible changes. This includes more virulent strains and Includes the emergence of different forms of mutants, or life history than we would expect. Management needs to be continually adjusted to changes in the dynamics of the disease. “

The team’s findings also give scientists and fisheries managers a better understanding of Bay Oyster’s “solid” era from the late 1980s to the early 2000s. “The persimmons have reached this level of devastation not because they have been unable to cope with Delmo after decades of exposure, if not centuries or thousands of years,” says Carnegie. “They bottomed out because they took time to challenge and adapt to a whole new form of parasite. And now they are adapting. This is the recent oysters in the area. The key to recovery. “

Along with Carnegie, the other author of this treatise is the late Susan Ford of the Haskin Shellfish Institute at Rutgers University. Peter Kingsley-Smith of the Department of Natural Resources, South Carolina; VIMS Rita Crockett, Lydia Bienlien, Lucia Saffy, Laura Whitefleet Smith, Eugene Balleson. This study is funded by VIMS Foundation A. Marshall Acuff, Sr., and Memorial Endowment for Oyster Disease Research.

Evidence of the more toxic Delmo parasite

Skin disease occurs when the common persimmon Crassostrea virginica is infected with the protozoan parasite Perkinsus marinus. Infected oysters grow slowly, are ill, and are less successful at breeding than healthy oysters. Severe infections lead to oyster death and release of parasites into the surrounding water, which can infect other nearby oysters when filtering water for food. The disease does not affect shellfish eaters.

The Gulf native Delmo was first recorded in the Chesapeake Bay in 1949, but may have been there for much longer. Prior to the 1980s, it usually occurred as a chronic disease that killed about 30% of oysters each year, primarily older animals that had been exposed to parasites for several years. However, “around 1986, Dermo suddenly became an acute, serious and devastating illness that could kill more than 70% of host oysters within months of infection,” Carnegie said. .. The increasing pathogenicity of Perkinsus parasites continues today.

Scientists initially said that the pathogenicity of Delmo surged in the mid-1980s over the years that hit the central Atlantic Ocean, as the infectivity of parasites is known to increase with higher salinity. It is due to the drought, and it is believed that the coastal salinity increased as the inflow of freshwater from the river decreased. Rising seawater temperatures also promoted an increase in Delmo disease, and since the 1980s, ocean warming has been attributed to a northward increase in the range of Delmo. However, over time, Carnegie and other researchers have begun to realize that salt and temperature alone cannot fully explain the sustained increase in oyster mortality associated with Delmo infection along the East Coast. I did.

“We began to ask why the longer and more intense droughts of the early pre-1980s did not cause a similar intensification of the disease, and why the subsequent moist period did not return the parasites to low levels. The characteristics of early infections. “

Motivated by these questions, Carnegie et al. Collected samples from modern Bay Oyster in 1960 using paper-thin tissue slices glued to slides for microscopic observation. And compared with the sample stored in VIMS. We found striking and unexpected differences and comprehensively examined over 8,000 tissue samples collected from the Chesapeake Bay, South Carolina and oysters, New Jersey between 1960 and 2018.

“Our analysis clearly showed the emergence of new parasite variants between 1983 and 1990 at the same time as the historic mid-1980s outbreak of Dermo,” says Carnegie. .. Changes included changes in the site of infection (from deeper connective tissue to the lining of the gastrointestinal tract), changes in reproductive strategy, and a sharp decrease in cell size. The Chesapeake Bay saw the most striking changes between oysters sampled in 1985 and 1986. At this time, modern varieties increased in frequency from 22% to 99% of observations.

“The picture that emerges is the emergence of a new toxic form of the Delmo parasite Perkinsus along the Mid-Atlantic coast in the mid-1980s, which disperses from it into a previously widely distributed form. Replaced. At the Atlantic estuary. Pathogenic changes in pathogens have been documented in other systems, but the range of changes we have seen and their widespread and rapid spread. Is unusual. “

“Our work emphasizes the importance of long-term environmental monitoring,” he adds. “Without it, and without the maintenance of the associated natural history collection, this new perspective would not have been possible.”

Evolutionary pressure

The type of change observed in the Delmo parasite provides a new but predictable response to the devastating effects of MSX, a disease caused by the non-native parasite Haprosporidium nelsoni, first reported in Baywater in 1959. Suggests to represent. By 1961, 90% of Virginia’s farmed oysters had reduced the yield of planted oysters from 3,347,170 bushels in 1959 to 361,792 bushels by 1983, resulting in the loss of an estimated 1.8 billion animals. It was. This reduction may have been exacerbated by simultaneous losses from the wild population.

Parasites that kill the host quickly effectively destroy their home. Therefore, over the evolutionary time scale, natural selection is characterized by the types of minor and long-term Perkinsus infections historically observed in oysters in the Chesapeake Bay and low rates of Delmo mortality, native parasites and their hosts. Often leads to a balance between.

However, as new parasites arrive at the scene, their evolutionary balance can change. Carnegie and his team dramatically disrupted the long-established equilibrium between Derumo and Classostrea with the catastrophic arrival of exotic MSX parasites in Baywater, creating a new, more toxic disease. I presume that they were directly connected.

“A significant reduction in oyster abundance, as caused by the arrival of MSX, will have serious implications for parasites such as P. marinus, which are completely dependent on a single host,” says Carnegie. As evidence, he points out that Perkinsus began in 1959 and declined abundantly and sharply. Recent theoretical modeling emphasizes that increased parasite pathogenicity may be the result of such reduced host resources.

“The changes seen in the Delmo parasite may have adapted to the decline in oyster abundance and longevity after the rapid establishment of Haplosporidium nelsoni and MSX in 1959,” says Carnegie. “We assume that our findings indicate a new ecosystem response to marine parasite invasion, an increased pathogenicity of native parasites.” An interesting possibility is death from MSX. It is possible that the changes observed by the researchers may represent a shortened life cycle of the parasite, as they have adapted to the shorter-lived oysters.