Quahog Parasite Unknown (QPX), An Important Disease of Hard Clams (Mercenaria mercenaria)
IAAAM 1999
Roxanna Smolowitz1, DVM; Dale Leavitt2, PhD
1University of Pennsylvania, Rinehart Coastal Research Center, Woods Hole Oceanographic Institution and 2Rinehart Coastal Research Center, Woods Hole Oceanographic Institution, Woods Hole, MA, USA


Quahog Parasite Unknown (QPX) was identified in aquacultured hard clams grown in Provincetown and Duxbury, MA, USA in the summer and fall of 19951. The disease continues to devastate the clam industry in those areas. Recently, disease and resultant mortality in hard clams were identified in Virginia2 and New Jersey3 cultured animals demonstrating that the disease is not limited to the Northeast.

QPX is a protistan organism that has been placed in the phylum Labyrinthomorpha4,5. Only rarely have organisms in this group caused disease in animals. In both infected tissues and cultures of QPX four life forms can be identified including: thalli, sporangia, endospores and flagellated zoospores.1,6.

The disease caused by QPX is slowly progressive. Animals do not die until approximately 22 years after infection, just below market size. Mortality is most severe in the spring and summer as moribund clams unearth themselves and lie on the top of the sediment. Nodules up to 1 cm can be noted in the mantle edges of severely infected animals. Microscopically, infections are most common in the mantle and gills suggesting QPX invades those organs directly exposed to seawater.

Little is known about the life cycle of QPX, pathogenic components of the organism or methods of amelioration of its effect on cultured animals in endemic areas. Our present studies are examining methods of control and disease reduction by developing possible resistant strains of clams, rotation of plots, plot pretreatment, and laboratory studies of the organism's ability to cause disease in clams. Results of these studies to date will be presented.


1.  Smolowitz R, D Leavitt, F Perkins. 1998. Observations of a protistan disease similar to QPX in Mercenaria mercenaria (hard clams) from the coast of Massachusetts. Journal of Invertebrate Pathology 71: 9-25.

2.  Ragone Calvo LM, JG Walker, EM Burreson. 1998. Prevalence and distribution of QPX, Quahog Parasite Unknown, in hard clams Mercenaria mercenaria in Virginia, USA. Diseases of Aquatic Organisms 33: 209-219.

3.  Kraeuter J. Haskins Shellfish Lab., Rutgers University, personal communication.

4.  Whyte SK, RJ Cawthorn, SE McGladdery. 1994. QPX (Quahog Parasite X), a pathogen of northern quahog Mercenaria mercenaria from the Gulf of St. Lawrence, Canada. Diseases of Aquatic Organisms 19: 129-136.

5.  Moss P, R Smolowitz, S Kleinschuster, M Dykstra. Molecular identification of QPX (Quahog Parasite Unknown), a pathogen of Mercenaria mercenaria. In preparation.

6.  Kleinschuster SJ, R Smolowitz, J Parent. 1998. In vitro life cycle and propagation of quahog parasite unknown. Journal of Shellfish Research 17: 75-78.

Speaker Information
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Dale Leavitt, PhD
Woods Hole Oceanographic Institution
Woods Hole, MA, USA

Roxanna Smolowitz, DVM
Woods Hole Oceanographic Institution
Woods Hole, MA, USA

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