Defense against infection with filamentous fungi in rainbow trout

1. Healthy rainbow trout effectively resisted infection after subcutaneous inoculation with isolates of filamentous fungi recovered from cutaneous infections in fish. 2. The organisms were capable of growth on the tissues of freshly sacrificed fish following inoculation by the same method. 3. Microscopic examination of the inoculation sites revealed a prominent cellular infiltrate with higher proportions of neutrophils than those found at control sites or in peripheral blood. Inflammatory cells were found to be adherent to the surface of fungal hyphae removed from the inoculation site after 24-72 hr. 4. Neither precipitating antifungal antibody nor plasma factors capable of inhibiting fungal growth were found in the fish used in these experiments. 5. These studies suggest that inflammatory cells, particularly neutrophils, may be involved in the defense against filamentous fungi by rainbow trout.