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.

The Occurrence and Diversity of Waterborne Fungi in African Aquatic Systems: Their Impact on Water Quality and Human Health

“…Aquatic fungi and water molds from freshwater sources have been reported to be responsible for the infection of aquatic species [20,21,22,23,24,25]. The presence of toxic aquatic fungi in water tends to infect aquatic animals, including those that are consumed by humans.…”

“…Various fish species have been reported to have the disease called epizootic ulcerative syndrome (EUS), which is one of the most dangerous diseases in aquatic ecosystems on the African continent. This disease is caused by the water mold Aphanomyces invadans [21]. Among the highest number of infected fish are found in African aquatic systems, and some of the fish species are of economic importance [31].…”

A brief history of freshwater fish parasitology in southern Africa

“…His most recent contribution to diseases of freshwater fish was confirmation of the presence of Epizootic Ulcerative Syndrome (EUS) in Lake Liambezi (McHugh et al 2014). This disease was first reported by Andrew et al (2008) from the Chobe and Zambezi rivers and confirmed by Huchzermeyer and van der Waal (2008), who also described the international distribution of this disease. The disease probably originated in the Zambezi and was transmitted to the Okavango when the high flood after 2010 connected the Okavango and the Zambezi systems via the Selinda Spillway to the Linyanti Swamps.…”

Histological Confirmation of Epizootic Ulcerative Syndrome in Two Cyprinid Species from Lake Liambezi, Zambezi Region, Namibia

“…In sub-Saharan Africa the first report of A. invadans and an associated EUS outbreak was from Botswana in 2007, followed by Zambia in 2008 (Andrew et al 2008). Subsequently 27 fish species from the region have been shown to be susceptible to EUS (FAO 2009;Choongo et al 2009;Songe et al 2012;Huchzermeyer & Van der Waal 2012). Lesions, suspected to be associated with EUS have previously been reported from Lake Liambezi by Van der Waal (2008;.…”