VIMS team wins grant to study blue crab disease

Professor Jeffrey Shields of the Virginia Institute of Marine Science has received a five-year, $2.4 million federal grant to study how fishing pressure and declines in water quality affect the emergence and spread of a blue crab disease in the seaside bays of Virginia’s Eastern Shore.

Shields is joined on the project by VIMS researchers Kimberly Reece and Harry Wang, along with Dr. Mark Butler of Old Dominion University. The grant supports three newly hired post-doctoral fellows and three graduate students at VIMS as well as a graduate student at ODU.

The grant is one of only eight such awards made to research teams nationwide. It comes through the Ecology of Infectious Diseases (EID) Program, a joint effort of the National Science Foundation (NSF) and the National Institutes of Health (NIH).

Shields notes that fishing pressure is known to affect the movement, aggregation, feeding, and mortality of marine organisms, and therefore the transmission of disease. Despite this, he says, “The effect of fishing pressure on disease has received little attention.”

Shields and his team will use the grant to study how fishing pressure and declining water quality within coastal bays may promote outbreaks of Hematodinium in blue crabs. Hematodinium is a parasite that infects the blood of crabs and other crustaceans worldwide.

In blue crabs, Hematodinium outbreaks are most common in fall among juveniles within the relatively warm, salty waters of coastal bays along the Atlantic seaboard, particularly in Virginia, Maryland, Georgia, and Florida. Hematodinium was first reported from the Eastern Seaboard in the mid 1970s. The first Virginia outbreak was reported in 1994.

Shields suspects that trapping of water within these shallow bays amplifies the parasite’s spread, thus leading to seasonal outbreaks of the disease in blue crabs.  He also suspects that the blue-crab fishery, by targeting adults, increases the relative number of juveniles, which are more susceptible to the parasite.

During disease outbreaks, crab mortality can reach 50% in crab pots, and 75% in shedding facilities on Virginia’s Eastern Shore. Infections are generally fatal with crabs dying from energy depletion or disruption of bodily tissues. The disease is not harmful to humans.

Shields estimates that Hematodinium outbreaks cost Virginia’s blue crab fishery from $500,000 to $1 million in losses per year. By investigating how the parasite is transmitted and causes disease, Shields’s study may help reduce these losses by identifying how fishing practices may promote disease transmission in coastal bays.

“Anecdotal evidence suggests that some fishing practices may help to spread the disease,” says Shields. “These include culling of the catch between locations, re-baiting with infected animals, and in some cases using male crabs as bait to attract pre-molt females for the soft-shell fishery.”

Shields notes that the results of his team’s study will have far-reaching implications. “While we are focusing on a protozoan disease in the blue crab, our work has broad application to other fisheries. We see the blue crab-Hematodinium system as a general model to gauge how fishing, by removing adults, affects the spread of diseases most prevalent among juveniles. Few if any studies have produced models capable of integrating local environmental change and fishing pressure with disease dynamics for mobile marine organisms. This has broad application to several fish and shellfish populations.”

James Collins, Asst. Director for Biological Sciences at the National Science Foundation, one of the federal agencies supporting Shield’s work, echoes Shield’s assessment of the study’s broader significance. Says Collins, "Understanding the causes and consequences of emerging infectious diseases is among the grand-challenge questions in the environmental sciences."