Friday, September 1, 2017

Peeler Virus Threatens Tasty Chesapeake Morsels

Patient 133 is part of Schott’s decade-long quest to learn more about a virus that is killing “peelers,” the term for crabs that are about to shed their hard, outer shell and become some lucky diner’s delicious soft-shell crab meal. Though they comprise only about 5 percent of the Chesapeake Bay’s blue crab harvest, which had a dockside value of more than $100 million last year, soft crabs are extremely lucrative. When sold, they fetch three to six times more per crab than hard crabs, and they are gaining in popularity nationwide as a delicacy.

About a decade ago, Schott, who works at the University of Maryland Center for Environmental Science, discovered that a virus was causing peelers to die in the tanks where they’re kept for days until they shed and are ready for market. Called CsRV1, it’s a reovirus that most likely passed through the gills or gut into the bloodstream and infected the immune cells. The virus is not communicable to humans.
We hope. . .

Now, Schott and other researchers in Maryland, Virginia and Louisiana are trying to determine if they can predict which crabs are susceptible to the virus, and if there is a genetic component. The team is also trying to assess how long afflicted crabs take to die, and how the virus spreads from one crab to another. If they can figure out answers, they hope to save watermen the frustration and expense of losing valuable animals.

“Crabs have a very different immune system from us. They have antibodies, but no [immunological] memory,” Schott said. “That is why, when they are infected, it just kills them.”

The researchers so far have learned that the virus is present in 80 percent of crabs that die in shedding operations in Virginia. The rate is similar in Maryland, but lower in Louisiana, even though it’s the same blue crab species. Some suspect that temperature stresses the crabs, though Schott said 20-degree swings in the Chesapeake Bay from water to tank have not affected mortality. Other possible factors: crowding in shedding tanks, or too much nitrogen in the water or not enough oxygen.
. . .
Shields has his own theory about what’s killing peelers. While crabs can survive wide temperature swings, bacteria increase when the mercury rises. He said he believes that this could be a major factor in their demise. The bacteria and viruses work in concert, but the bacteria might get them before the virus does. There are a lot of things that can kill a crab, he said.

In Virginia, the number of peeler operations has decreased from 318 in 2011 to 281 last year, according to the state’s marine resources commission. Maryland doesn’t keep similar data, but anecdotal information suggests that several soft-crab operations have gone out of business in recent years.

It’s unclear why soft-crab operations would be declining, especially with growing consumer demand. But it’s a labor-intensive business, requiring round-the-clock tending of shedding tanks to remove crabs from the water as soon as they shed. If the operator doesn’t, the crabs form new carapaces, becoming what watermen call a “paper shell” that’s almost worthless.
Decent bait, though. I knew a lot of peelers died in the process of shedding, but I had no idea there was a virus involved.

Unfortunately, the virus is unlikely to be amenable to widespread treatment. A way to minimize losses in shedding operations may be found, but would be hard to institute for the population at large.

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