DISEASE PROCESSES OF THE PARASITE PERKINSUS-MARINUS IN EASTERN OYSTERCRASSOSTREA-VIRGINICA - MINIMUM DOSE FOR INFECTION INITIATION, AND INTERACTION OF TEMPERATURE, SALINITY AND INFECTIVE CELL DOSE

Experiments were conducted to: (1) test the response of oysters to dif ferent doses of the oyster parasite Perkinsus marinus and to 2 stages, meronts or prezoosporangia; and (2) investigate the synergistic effec ts of temperature, salinity and infective cell concentration on P. mar inus infection in oysters. A dose-dependent response of P. marinus inf ection was found in oysters inoculated with 0, 10, 10(2), 10(4), and 1 0(5) meronts or prezoosporangia per oyster and maintained at 22 to 25 degrees C and 14 to 21 ppt for 8 to 12 wk. The minimum dose required t o infect oysters was 10(2) meronts or prezoosporangia per oyster throu gh shell cavity inoculation. Interactive effects between temperature, salinity, and infective cell dose on P. marinus prevalence was insigni ficant in the experiment, in which oysters were challenged by 0, 2.5 x 10(3) or 2.5 x 10(4) meronts per oyster and held at 9 temperature-sal inity regimes (10, 15 and 25 degrees C at 3, 10 and 20 ppt). However, there was a significant positive interaction relevant to infection int ensity between temperature and salinity, and between temperature and m eront dose. Temperature was the most important factor followed, respec tively, by the infective cell dose and salinity in determining the sus ceptibility to P. marinus in oysters. Reduced condition index was obse rved in moderately to heavily infected oysters and in oysters at 25 de grees C.