Fish Diseases and Management

5.5.4 Epizootic Ulcerative Syndrome (EUS)

EUS has a long history starting as early as 1975 in Australia and progressed in south east Asian countries reaching in 1992 in Srilanka and 1994 in Pakistan. This disease can cause damage in a wide variety of fishes but Chinese carp and tilapia is comparatively resistant.

Channa striatus (striped snakehead), an economically important species is highly affected by this Infection. It starts as a red spot in the skin then it eventually becomes an ulcer. As it progresses, the ulcerative area gets eroded. Infected fish swims with a disintegrated caudal peduncle and eroded head due to it the high general resistance to infections. The smaller freshwater fishes such as minnows die much before the infection can erode any organs. It also affects other Channa sp. and Indian major carps. Ulcers can be in any place of the body. The infection is common in freshwater and brackish water but not in marine fishes. It infects cultured species, wild species and large water bodies where there is no proper management.

Viruses were isolated from the EUS infected fish in south East Asian countries like Malaysia, Indonesia. Isolated viruses belong to Rhabdovirus, Reovirus and Birnavirus.

But most often only fungus was isolated indicating the consistency of the fungal aetiology in EUS. The fungus is identified as Aphanomyces invadans. Regional council of FAO opined that EUS should have fungal hyphae in histological sections of underlying muscle of the ulcer. Grocott’s stain is used for fungal staining. It gives black colouration to the fungal hyphae. A. invadans has the ability to penetrate the body of the fish by using  proteolytic enzyme.

Mortality caused

(i) by invading fungus

(ii) the invaded area is exposed to the environment resulting in osmoregulatory failure.