Cellular responses of oysters infected with Haplosporidium nelsoni: changes in circulating and tissue-infiltrating hemocytes

The protozoan parasite Haplosporidium nelsoni (MSX) elicits an inflammatory-type response in oysters, Crassostrea virginica. We assayed circulating hemocytes of oysters exposed to H. nelsoni to quantify the effects of parasitism, selection for resistance, and season on total and differential counts. All factors had a significant (P < 0.02) effect on total counts, but explained relatively little of the overall variance [season (12%) > selection (4%) > infection (2%)]. Circulating hemocyte densities increased with infection intensity in resistant animals, but were depressed in susceptible oysters with light infections. Counts in both groups were lowest in August and highest in May, at which times densities in susceptible oysters were reduced (P < 0.02) compared to those in resistant animals. No differences existed in November. Susceptible oysters may be more debilitated by infections than resistant animals, resulting in impaired circulation and depressed cell counts. The number of tissue-infiltrating hemocytes increased with infection intensity (P < 0.0001), but showed no statistically significant (P > 0.05) association with numbers of circulating hemocytes in individual oysters. Size-frequency analysis with a Coulter counter indicated that the proportion of large cells (presumed to be granular hemocytes) was lower (P < 0.0001) in susceptible oysters, which were also heavily infected, compared to resistant oysters, which had very few infections. The loss of granular hemocytes may stem from degranulation associated with tissue damage and inflammation. Present evidence suggests that the principal role of the hemocyte response in MSX disease is to plug lesions, remove debris, and repair tissues and that these functions may help oysters survive infection.(ABSTRACT TRUNCATED AT 250 WORDS)