Laboratory transmission studies of QPX disease in the hard clam: Interactions between different host strains and pathogen isolates

Abstract

Quahog Parasite Unknown (QPX) disease is attributed with hard clam mortalities from Atlantic Canada down to Virginia. Prior field investigations discovered that resiliency to QPX infection varied considerably among different clam stocks. QPX descriptions and pathologic severity have also varied among field surveys and diagnostic reports, raising the question of whether there is more than one strain of the parasite throughout its known geographic range. In this study we used a recently developed experimental transmission methodology to test the hypothesis that genotypic variability in the host and/or in the pathogen was responsible for differences in the severity of QPX infections. Inoculation methods were applied in a trial that utilized three QPX isolates, geographically or morphologically distinct, and naïve juvenile clams obtained from culture facilities in Massachusetts (MA), New York (NY), Virginia (VA), and Florida (FL). Trends in prevalence and disease severity were significantly associated with seed origin and QPX isolate. Results show clams from FL and VA to be noticeably more susceptible toward QPX infection than clams from MA or NY. QPX isolated from infected NY clams appeared more virulent than the QPX isolated from infected MA clams. Hard clam stock susceptibility differences are important considerations for aquaculture applications in the field, it is also important to be aware that different QPX strains represent different potential threats of disease.