Detection of BCD infections in snow crabs

Jeffrey ShieldsVirginia Institute of Marine Science

Dave TaylorDepartment Fisheries and Oceans, Newfoundland

What is BCD?

• Bitter crab disease is caused by a parasitic dinoflagellate, Hematodinium, that lives in the blood of the snow crab.

• The disease causes a aspirin-like, bitter flavor in cooked crabs.

• Late stages of the disease cause snow crabs to turn pink as if partially cooked.

• Early stages can only be detected microscopically.

Wet smear (phase contrast) showing vegetative stages of Hematodiniumand rounded plasmodial forms.

Detection methods

• Visual examination• Heart dissection• Microscopic determination

– wet smears– neutral red– fixed preparations– histology

Histological preparation of the heart. Multinucleate plasmodial stages and single cell trophonts are abundant.

Photo credit: Paul Collins, DFO

VISUAL DETECTION:The crabs on the right are infected. Note the barnacles on the crab on the left. Molting stress may contribute to the onset or transmission of the disease, but infected crabs turn color from the severity of infection.

Photo credit: Paul Collins, DFO

VISUAL DETECTION:The crabs on the left are infected. Note the opaque white or pink of the shell (carapace). The muscle is severely degraded in these heavily infected animals, and their metabolic reserves (glycogen) are depleted.

Heart dissection

• Onboard visual determinations can sometimes be questionable.

• In such cases gross examination of the heart can confirm diagnosis of heavily infected crabs. – Remove the carapace off the animal in question.– Locate and examine the heart.– If the heart is milky white, not translucent beige or

white, then the crab most likely has Hematodinium.

Microscopic determination 1

• Trained personnel can make determinations using wet smears of hemolymph.

• However, more reliable results can be obtained by making permanent, stained preparations.

Vegetative trophonts of Hematodiniumfrom the snow crab. Host blood cells arevirtually nonexistent in this sample, but the parasites resemble hemocytes to the untrained eye.

Microscopic determination 2

• Neutral red is a useful vital stain for Hematodinium.– Mix one drop 0.25% neutral red

(in filtered seawater) with one drop hemolymph. Observe at 400x.

– Vacuoles in Hematodinium are bright red.

– Vacuoles in hemocytes rarely stain light orange.

Dinospores of Hematodinium in the snow crab. This stage is rarely seen as it is ephemeral. Sporulation often kills the crab because parasites exit through the host’s gills.

Histological determination 1

• Histology is the best method for diagnosis of infections.

• Hematodinium is like other dinoflagellates. The chromatin is condensed within the nucleus. Nuclei appear as if in arrested metaphase.

Plasmodial and prespore stages of Hemato-dinium from the heart of an infected snow crab. Note the deeply blue-black staining chromatin within the nuclei of the parasites.

Histological determination 2

Heavy infection of Hematodinium in a snow crab (left). The chromatin stains deeply blue-black within the nuclei of the parasites. The picture on the left shows sporulation of the parasite with the nuclei of the dinospores virtually black. The picture on the right shows the hemolymph of an uninfected crab.

Infected snow crab Healthy snow crab

Histological determination 3

Sheets of parasite cells (sheet-like plasmodia) from within the hepatopancreas (digestive organ) mature and break off into the bloodstream where they undergo division and sporulation.

The muscle of heavily infected snow crabs are degraded and shrunken. This is a good example of “pressure” pathology induced by the large number of parasites in the hemolymph.