Blue Crab Diseases

“Pepper Spot” Disease

"Pepper Spot" Disease

Pepper Spot Disease is caused when a crab becomes infected with a parasite, and then that parasite becomes infected by another parasite (called a hyperparasite.) The disease is easily seen as tiny black specks (approximately 0.5mm in diameter) which are visible throughout the crab's tissues.

Initially, the crab becomes infected by the parasitic flatworm (fluke) Microphallus bassodactylus. Next, the encysted fluke becomes infected by the parasitic protozoan Urosporidium crescens. The very small, brownish, protozoan multiplies inside the larval worm and increases in size until the worm is completely consumed and replaced by spores. The large number of dark spores distinguishes each cyst as a visible black speck and results in the condition called "buckshot," "pepper spots," or "pepper" crabs.

Crabs are not affected by the disease but it can affect the aesthetics of the meat by making it appear unappetizing. However, the cooking process kills the parasites and renders the crabmeat completely safe to eat.

"Pepper spot" is common on the Chesapeake Bay's Eastern Shore (affecting more than 30 percent of crabs from some locations.) It appears be related to water salinity. The disease is spread by any of four species of snails which are found in shallow low-salinity estuaries. The infected snails release the infective free-swimming fluke larva (cercaria) which penetrate the crab. Many crabs are infected with the fluke which can barely be seen without a microscope. It isn't until the fluke itself becomes infected with the protozoan hyperparasite, becoming visible, that people exhibit apprehension (Jeff Shields, VIMS.)

“Bitter” Crab Disease

"Bitter" Crab Disease
(Dinoflagellate blood disease)

Bitter Crab Disease (BCD) is caused by a blood parasite, Hematodinium perezi, a type of dinoflagellate. (Dinoflagellates are single-celled, microscopic algae.) The parasite consumes oxygen from the crab's blood and tissues which causes it to become weak and lethargic, and to eventually die.

Hematodinium sp. can produce epizootics (an outbreak of disease affecting many animals of one kind at the same time.) Epizootics have been reported in Maryland, Virginia, Georgia, and Florida. The disease is most prevalent in warm, relatively shallow, high salinity waters. The parasite is found in the ocean-side bays of the Delmarva (short for Delaware, Maryland, and Virginia) Peninsula in the spring and fall, and spreads to the lower reaches of Chesapeake Bay in the fall.

In October 1996, the prevalence of BCD along the Virginia portion of the Delmarva Peninsula varied from 20 to 50 percent in legal sized crabs. Lower prevalences (1 to 10 percent) were noted for crabs caught at the mouth of the Bay. In November, the prevalence is generally low during the pre-breeding and ovigerous (sponge crab) seasons.

Once infected, the parasite grows rapidly inside the crab (up to 100 million parasites/ml of blood) over the course of 3 to 6 weeks. The crab's blood changes to a milky-white color and loses it clotting ability.

Signs of Infection

Watermen will find dead or dying crabs in their traps or crabpots. Infected crabs are weak, lethargic, have drooping limbs and mouthparts, and often die during handling. When cooked, the crabmeat has a chalky texture and a bitter aspirin-like flavor (the disease has no known impact on people who eat infected crabs.)

Early stages of the disease can only be detected under a microscope. Late stages of the disease can cause the crab's shell to turn pink as if partially cooked (see photo above.)

“Cotton” or “Cooked” Crab Disease

"Cotton" or "Cooked" Crab Disease

The microsporidium Ameson michaelis, a parasite that invades and destroys cells, causes severe muscle disintegration that results in a condition known as "cotton" crab or "cooked" crab disease. Infected tissue appears opaque (white) while the crab is still alive. Cooked meat is cottony in texture and poorly flavored. The parasite can be transmitted via cannibalism; and since as much as 25 percent of a blue crab's diet is other blue crabs, it is surprising that the parasite is only found at low prevalences (less than one percent, Shields, pers. obs.)

Shell, “Burn Spot,” or “Brown Spot” Disease

"Shell Disease," "Burn Spot," or "Brown Spot" Disease
(Chitinoclastic Disease)

Chitinoclastic disease is caused by chitinivorous bacteria which causes unattractive lesions (or spots) on the shell. The lesions are ugly which can make the infected crabs unmarketable. The disease has no known impact on people who eat infected crabs.