Epizootic ulcerative syndrome of fishes: rhabdovirus infection and EUS induction experiments in snakehead fish

Rhabdoviral agents have long been found to be associated with Epizootic Ulcerative Syndrome (EUS) although the presence of granulomas caused by the fungus Aphanomyces invadans is currently used as the diagnostic criterium of EUS. In the past few years, rhabdoviruses have been readily isolated from EUS-infected fish during the early period of some outbreaks in Thailand, but the role of viruses in EUS remained unclear. Rhabdovirus infection and EUS induction experiments were therefore conducted in this study. The rhabdovirus strain T9412 from EUS-diseased snakehead fish was used to infect healthy snakehead. The frequency of virus re-isolation from juvenile fish infected by a number of routes decreased to 50-75% on day 7, 0-50% on day 14 and 0-25% on day 30. The virus was more virulent at 20°C than 29°C, and caused death in snakehead fry with a LD50 equal to 2.16 Log10 TCID50/ml. EUS induction in juvenile snakehead fish was experimentally achieved using both rhabdovirus and Aphanomyces invadans type species RF-6. All (20/20) juvenile snakehead developed multiple EUS lesions by day 30 at 20°C when fish were first injected intramuscularly (i/m) with virus followed by bath challenge with fungal spores. Fish which received L-15 medium by injection and were then bathed with fungal spores also developed EUS lesions although, fewer (7/20) were affected. The rhabdovirus injection alone induced only small haemorrhagic wounds at the i/m injection site in some fish held at 20(C, and most wounds healed by the end of the experiment. A similar induction experiment conducted at 29(C failed to induce the EUS disease. Results of this study indicate that the rhabdovirus is lethal to snakehead fish fry at low temperature. The virus persists for only a short period of time in the juvenile fish and induces minor skin damage. Low temperature was found to be an important factor in facilitating the host fish to succumb to rhabdovirus and A. invadans infection in the laboratory. Therefore, one possible combination of events leading to EUS in snakehead fish is low temperature and the presence of rhabdovirus and Aphanomyces pathogens