Blood parasites of fish from the northwestern Atlantic Ocean

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Abstract
Examination of thick and thin blood smears from 5013 marine fish from the northwestern Atlantic Ocean revealed 25 of 59 species to be infected with one or more of the following blood parasites: trypanosomes (27% of 3610), trypanoplasms (9% of 588), piroplasms (26% of 2584), and haemogregarines (36% of 1708). Higher prevalences of infection were observed in cold-water, benthic, sedentary fish than in warm-water, littoral, epipelagic, or midwater species. Trypanosome infections were more prevalent among fish taken off the Labrador – east Newfoundland coasts, lower in those from St. Pierre Bank, Grand Bank, and Gulf of St. Lawrence, and lowest in forms from the Scotian Shelf. Seven of nine species of hematophagous leeches harboured asexual stages of haemogregarines, whereas natural infections of trypanosomes occurred in one leech, Johanssonia arctica. Using five species of laboratory-reared leeches and laboratory-initiated infections of trypanosomes in piscine hosts, development of Trypanosoma murmanensis was observed only in J. arctica. It is suggested that the distribution of trypanosomes of marine fish is related to that of J. arctica, a cold-water species, whereas the other haematozoan infections might be attributed to those leech species that are associated with their respective piscine hosts.