Epizootic Ulcerative Syndrome (EUS) Technical Handbook

Aetiologysnakehead fry died when challenged at 20 o C, but no mortality was recordedat 29 o C, or in other species of fish (including EUS-susceptible fish) at eithertemperature.If viruses have a role in the pathogenicity of EUS, their most likely effect isto cause skin lesions sufficient to allow entry of the fungus, A. invadans.Kanchanakhan (1996b) subjected snakehead juveniles to i.m. injections ofT9412 rhabdovirus or L15 medium, followed by bath challenges with A.invadans spores at 20 o C. EUS was induced in 100% of fish given rhabdovirusinfections and only 35% in fish given control L15 injections. This providessome evidence that T9412 may help to predispose fish to infection by A.invadans, but co-immersion challenges with the virus as well as the fungusare required to demonstrate that this can occur under more natural conditions.ParasitesSeveral metazoan (Dactylogyrus sp., Gyrodactylus sp.) and protozoan (Chilodonellasp., Trichodina sp., Costia sp., Henneguya sp., Ichthyophthirius sp.) parasiteswere identified from 273 EUS infected fish during the 1982-3 epizootic inThailand (Reungprach et al., 1983) Several fish examined before the secondoutbreak, and thought to be at an early stage of the disease, showed tiny redspots on the skin. Examination revealed a large number of Epistylis sp.protozoans (Tonguthai, 1985).In Australia, Callinan and Keep (1989) and Pearce (1990) found protozoan andmetazoan parasites present on some affected fish, but concluded that noparasite species was intimately associated with lesions and there was noevidence to suggest that parasites initiate ulcers. In their survey of affectedcountries in southeast Asia, Roberts et al. (1986) found that diseased fishcarried no more than the expected parasite load for wild rice paddy or riverinefish.It therefore appears unlikely that any parasite acts either as a pathogen ora vector for a pathogen of EUS. However, parasites may at times induce stressin fish and predispose them to infection. For example, Subasinghe (1993)demonstrated a clear association between parasite burden of Trichodina sp.on gills and susceptibility of striped snakeheads to EUS infection. It is alsopossible that external parasites may, in some circumstances, induce mildskin lesions which would allow propagules of the fungal pathogen, Aphanomycesinvadans, to attach and infect the fish host.BacteriaAvailable evidence suggests that bacteria may be important, but not essential,at two distinct stages in the pathogenesis of EUS.1. Current evidence indicates Aphanomyces invadans must attach to thedermis before it can invade underlying tissues. Cutaneous bacterialinfections (e.g. Flexibacter) may predispose fish to EUS by inducing skinlesions which provide an entry for the fungus (Figure 3).23