Direct transmission of the haemoflagellate Cryptobia salmositica among Pacific salmon (Oncorhynchus spp.)

Abstract

Laboratory-reared juvenile Oncorhynchus nerka were marked, inoculated with Cryptobia (= Trypanoplasma) salmositica, and held in tanks with equal numbers of untreated fish. About 85% (116 of 137) of the untreated fish (held in freshwater or seawater) exposed by brief out-of-water contact with heavily infected fish in a dipnet became infected. Without out-of-water contact, only 4% (3 of 85) of the untreated fish held in the same tanks as inoculated fish became infected. The flagellates were shown to escape from the coelom to the body surface of infected fish via ruptured areas in the region of the abdominal pores. The escaped flagellates were similar in morphology to those in the blood and were equally sensitive to lysis in water but the mucus from the fish provided the flagellates with short-term protection. Supportive evidence that direct transmission may occur in the field was obtained when juvenile Oncorhynchus kisutch held in seminatural rearing channels in a hatchery showed an increase in the prevalence of infection after they were moved to an adjacent channel. Leech vectors (Piscicola salmositica) were not observed to account for the increase in the number of infected fish, and the prevalence of infection remained low in fish that were not moved. Indirect versus direct life history is currently the only criterion used in distinguishing Trypanoplasma (parasitic in blood and other fluids of fish) from Cryptobia (parasitic in molluscs and externally on fish). The evidence of direct transmission in a blood form demonstrates that this criterion is not reliable. The genus Trypanoplasma Laveran and Mesnil, 1901 is therefore considered to be a junior synonym of the genus Cryptobia Leidy, 1846.