Detection of Infection and Susceptibility of Different Pacific Salmon Stocks (Oncorhynchus spp.) to the Haemoflagellate Cryptobia salmositica

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Abstract
The hematocrit centrifugation technique, modified by keeping the hematocrit tubes cold (between 1.degree.-10.degree. C), was sensitive for detecting light infections of C. salmositica (as few as 75 flagellates/ml blood). In wet mount preparations, infections lighter than 7.5 .times. 103 flagellates/ml blood could not be detected consistently. Different Pacific salmon stocks from British Columbia [Canada] demonstrated differences in susceptibility to C. salmositica in experimental studies using laboratory reared juvenile fish. O. keta and O. tshawytscha from the Big Qualicum River stocks (Vancouver Island) and O. nerka from the Fulton River stock (Skeena River system), were all equally susceptible and suffered high mortalities at low exposures (100 flagellates in 0.1 ml physiological saline inoculated intraperitoneally per fish). O. nerka from the Weaver Creek stock (Fraser River system) was the most resistant with no mortalities even at exposures of 106 flagellates (in 0.1 ml physiological saline) per fish. O. kisutch seemed to be slightly less resistant than the Weaver Creek O. nerka, but fewer than 16% of the inoculated fish died. O. kisutch from the Big Qualicum River seemed to be slightly more resistant than O. kisutch from the Capilano River stock (a coastal river near Vancouver), with fewer mortalities and lighter infections when the experiments were terminated. Differences in susceptibility are believed to be associated with innate, genetically transmitted resistance.