Relationships between Water Temperature, Fish Size, Infective Dose and Ichthyophonus Infection of Rainbow Trout

Abstract

Artificial oral inoculation was carried out in order to get information on several factors affect-ing Ichthyophonus infection of rainbow trout Salmo gairdneri with thick walled multinucleate spherical bodies of Ichthyophonus hoferi cultivated in MEM-10 and the affected internal organs of rainbow trout. The outcome of these inoculations are shown as follows: The infection in ex-perimental fish (0.9 g B.W.) was affected significantly by the water temperature. Namely, the cumulative mortality for 35 days was 100% at 20°C and 15°C, 10% at 10°C and nothing at 4°C. This experiment was carried out by feeding once with approximately 100 thick walled multinucleate spherical bodies per fish. All the survivors held at 10°C and none of the survivors held at 4°C were infected by Ichthyophonus hoferi. The cumulative mortality in the different size-fish from the same fecundity of a spawner for 35 days at 20°C was 90% in 4.5 g-fish and 45% in 1.3 g-fish. This experiment was performed by feeding once with about 3000 spherical bodies from minced affected organs per fish. When fish (2.0g B.W.) were fed once approximately 3000, 300, 30 and 3 spherical bodies originated with the minced affected organs per fish and reared at 15°C for 70 days, the cumulative mortalities were 100%, 44%, 4% and 0%, respectively. All the survivors fed 300 spherical bodies per fish, one of them fed 30 spherical bodies per fish and none of them fed 3 spherical bodies per fish were infected.