Epizootic ulcerative syndrome: Exotic fish disease threatens Africa’s aquatic ecosystems

Abstract

In late 2006 an unusual ulcerative condition in wild fish was reported for the first time in Africa
from the Chobe and upper Zambezi Rivers in Botswana and Namibia. Concern increased
with subsistence fishermen reporting large numbers of ulcerated fish in their catches. In April
2007 the condition was confirmed as an outbreak of epizootic ulcerative syndrome (EUS).
The causative agent, Aphanomyces invadans, is a pathogenic water mould of fish that shows
little host specificity. Ulcers follow infection of tissues by oomycete zoospores, resulting in
a granulomatous inflammation associated with invading oomycete hyphae. Granulomatous
tracts surrounding oomycete hyphae within the necrotic tissues characterise the diagnostic
histological picture. The upper Zambezi floodplain at the confluence with the Chobe River
spans the four countries of Botswana, Namibia, Zambia and Zimbabwe, making disease control
a challenge. The floodplain ecosystem supports a high fish diversity of around 80 species,
and is an important breeding and nursery ground. The annual cycle of flooding brings about
changes in water quality that are thought to favour the infectivity of A. invadans, with diseased
fish appearing soon after the plains become flooded. Since 2006 the disease has spread rapidly
upstream along the upper Zambezi and its tributaries. By 2010 the disease was reported from
the Okavango Delta in Botswana and in 2011 from the Western Cape Province of South Africa.
EUS has the potential to disrupt floodplain ecosystems elsewhere in Africa where high fish
diversity forms the basis of subsistence fisheries and local economies, and is a direct threat to
freshwater fish culture.