A sensitive new genetic test can now detect a cripplingdisease called QPX occurring in clam beds from Cape Cod south toVirginia and north to Canada. Although it does not affect humans and itis not as well known as red tide, the disease can have a significantimpact on a local economy by killing clams and devastating shellfishharvests and commercial aquaculture operations.
QPX - for quahogparasite unknown - is a single-celled organism related to slime mold.It was first detected in 1995 in Provincetown, MA and spread to nearbyclam beds, killing nine of ten clams in many of the beds. The diseasespreads from clam to clam, infecting the clam by secreting a thickmucus layer to insulate itself from the clam’s immune system.
RebeccaGast, an associate scientist in the Biology Department at Woods HoleOceanographic Institution (WHOI), has developed a genetic test todetect the organism not only in clams but in seawater and sediment.Since QPX also decomposes seaweed, researchers now believe it can befound in all coastal waters but doesn’t become deadly to clams until itreaches a critical concentration in the water.
Gast notes thatalthough red tide got a lot of media attention this year, QPX isactually a bigger problem. The toxins that cause red tide in clams andother shellfish in New England do not kill the shellfish and will washaway once the red tide bloom diminishes, eventually making theshellfish safe to eat. QPX kills the clams, and there is no known cure.
Gastis working with Roxanna Smolowitz, a veterinarian at the nearby MarineBiological Laboratory, to find out what triggers the organisms to reachconcentrations that become deadly, and whether that threshold variesamong clam strains. Smolowitz uses traditional microscopic examinationof tissues to determine if clams are sick.
Gast’s genetic testcan now also be used to ensure clams without visible symptoms are notcarrying the disease. With the disease spreading along the East Coastand no cure, the researchers say the best solution for shellfishermenand aquaculture operations is to keep infection levels as low aspossible and try to keep seed clams free of the disease. One possibleremedy may be rotating shellfish crops, much like farmers do on land.
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