Diplostomum spathaceum (Rud. 1819): effects of physical factors on the infection of rainbow trout (Salmo gairdneri) by cercariae.

The relationship between infection rates of rainbow trout (Salmo gairdneri) by Diplostomum spathaceum and cercarial concentration, water flow rate and temperature were investigated by means of controlled infections within a flume. A linear relationship was obtained between cercarial concentration and mean abundance of metacercariae/fish. A biphasic relationship occurred between flow rate and abundance of metacercariae. Within the confines of the flume, it was possible to control the infection rate of trout with D. spathaceum cercariae by manipulating flow rate, suggesting that it may be a possible method of controlling diplostomiasis on fish farms. No infection occurred in fish infected and maintained below 10 degrees C and the optimum infection temperature was approximately 17.5 degrees C. Infections became established in fish infected at 7.5 and 5 degrees C but maintained at 15 degrees C prior to examination. Trout were infected at 7.5 degrees C for 10-50 min and all attached cercariae were washed off and removed from the flume. Following infection, fish were either maintained at 7.5 degrees C or 15 degrees C prior to examination. Using this method, it was possible to ascertain that it was migration and not penetration which was inhibited at water temperatures below 10 degrees C.