Epizootic ulcerative syndrome (EUS) is an infection caused by the oomycete Aphanomyces invadans also known as A. piscicida.

Symptoms and pathology

The disease is characterized by sudden high mortality in wild and farmed fish. A wide range of species and sizes of fish are afflicted and many fish show abnormal behaviour. Deep erosions and big wounds, which can be found both in the head and on the body, can be seen in more chronic cases. The brain can be bared. Histologic sections will reveal extensive, serious, mycotic granulomatic changes and distinct flocculent necrotic muscle fibres. The invasive oomycete can grow the whole way through the muscle tissue into the spinal cord, kidney and peritoneum. In late stages very compact fibromatous stroma containing granulomatous tissue and dead hyphae can be seen. Such fish can survive but are often deform. By microscopy of a tiny piece of muscle tissue, cut from an area deep under a lesion, thick walled bifurcated hyphae without cross walls can be seen.


As sequela to EUS secondary infections with Gram negative bacteria is common. The disease hits both wild and farmed fish, with the following genera as most sensitive: Channa (snakeheads), Mastacembelus (spiny ell), Puntious (barbs), Trichogaster (gourami), Catla, Mugil (mullet), Labeo. Fish belonging to genera Oreochromis (tilapia), Chanos (milkfish) and Cyprinus (carps) are not or only less susceptible. Both fish in fresh and brackish water are susceptible. The disease was first reported in Japan and Australia in the start of the 1970s and has since spread all over Asia. The disease is also reported from North America and in 2007 it was reported from Africa (Botswana). When the disease is introduced in a population, outbreaks may be seen every year. EUS is primarily a problem at temperatures 18-22°C and after periods with heavy rain.   


Precautions EUS is listed by the OIE.

10 DECEMBER 2018