Direct Transmission of a Hemoflagellate, Cryptobia salmositica (Kinetoplastida: Bodonina) Between Rainbow Trout Under Laboratory Conditions1

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Abstract
Transmission of Cryptobia salmositica occurred when infected and uninfected rainbow trout were held in the same tank. In tanks where infected and uninfected fish were allowed to mix freely, 67–80% of the uninfected fish acquired detectable infections by the 27th week. None of the control fish in another tank was infected. In another tank where the infected and uninfected fish were separated by a wire screen, 9 of 20 uninfected fish acquired detectable infections by the 22nd week. This is the first demonstration of direct transmission of a hemoflagellate via the water medium in aquatic vertebrates. Cryptobia salmositica was found in the mucus on the body surface of 9 of 10 fish examined 6 weeks after infection. These parasites were infective and some of them were morphologically similar to those in the blood or peritoneal fluid. It is suggested that the vascular species of Cryptobia were originally ectoparasitic on fishes and that these ectoparasitic species were descended from the free‐living Procryptobia.