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Which species are susceptible or affected?
from What you need to know about epizootic ulcerative syndrome (EUS) – An extension brochure for Africa
What does epizootic ulcerative syndrome (EUS) do to the fish?
Clinical signs of EUS in affected fish showing the range of lesions
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EUS causes unsightly lesions in affected fish. EUS-affected fish may swim slowly, show abnormal swimming movements, and in rivers may be observed near the riverbanks swimming with the current where healthy fish swim against the current. Sick fish will also seek out shallow vegetated areas of still waters.
©FAO/D. Huchzermeyer, Rhodes University
Figure 5. Labeo lunatus – upper Zambezi labeo with early lesion of epizootic ulcerative syndrome (EUS). Note red appearance of the lesions and swelling of the surrounding tissue, Chobe River, Botswana, 2007 (Andrew et al., 2008).
Figure 6. Clarias gariepinus – Sharptooth catfish affected by epizootic ulcerative syndrome (EUS) showing both the red spots that are typical of early infection and the large, deep ulcers that develop later. Fish sampled during an EUS outbreak, Bangweulu swamps, north Zambia, 2014.
©FAO/D. Huchzermeyer, Rhodes University
Lesions can range from small pinpoint red spots, haemorrhagic spots, localized swelling and localized raised areas on the body surface with protruding scales or scale loss to skin erosion, exposure of underlying musculature and extensive ulceration.
Ulcers or ‘wounds’ can be found over a broad area of the body with the center of the lesions containing dead tissue.
Lesions are observed most often on the lateral surface but can also occur on any part of the body.
Figure 7. Enteromius poechii – Dashtail barb with typical early lesion of epizootic ulcerative syndrome (EUS), Kabompo River, upper Zambezi, 2010 (Huchzermeyer and Van der Waal, 2012).
Figure 8. Marcusenius macrolepidotus – Northern bulldog. Note the unilateral ulcerating lesion typical of epizootic ulcerative syndrome (EUS) at the base of the pectoral fin, Equateur Province, Democratic Republic of Congo (DRC), 2015.
©FAO/R. Bills, South African Institute of Aquatic Biodiversity
©FAO/D. Huchzermeyer, Rhodes University
©FAO/O. Weyl South African Institute of Aquatic Biodiversity
Figure 9. Hydrocynus vittatus – Tigerfish. Extensive dermal ulceration typical of epizootic ulcerative syndrome (EUS), Okavango River, 2010 (Huchzermeyer and Van der Waal, 2012).
©FAO/B. Van der Waal, Namibia
Figure 10. Brycinus lateralis - silver robber with early lesions typical of epizootic ulcerative syndrome (EUS) possibly associated with preceding predator injury to the skin, Kafue River, Zambia, 2007.
©FAO/B. Van der Waal, Namibia
Figure 11. Serranochromis macrocephalus - purpleface largemouth with an advanced lesion typical of epizootic ulcerative syndrome (EUS) showing extensive ulceration, fish farm, Caprivi, Namibia, 2008.
©FAO/D. Huchzermeyer, Rhodes University
Figure 12. Parachanna obscura – Snakehead. Note ulcerating lesions typical of epizootic ulcerative syndrome (EUS) in the tail region of a fish sampled, Equateur Province, Democratic Republic of Congo (DRC), 2015.
Following on reports of large numbers of fish with ulcers in the Chobe River on the border between Botswana and Namibia in 2006 (Figure 13), surveys initiated in February 2007 revealed the condition to be present in fish in the Chobe River in the vicinity of the Chobe Game Reserve in Botswana, the Caprivi Region of Namibia and on the Zambian side of the Zambezi River above Victoria Falls. The disease was subsequently confirmed as the first outbreak of EUS on the African continent. In this area, EUS-infected fish have been observed seasonally at the end of subsequent summers, but with a lower prevalence. Reports from Zambia have indicated a rapid spread upstream in the Zambezi River and some of its tributaries, in some cases associated with large-scale fish mortality, as well as in the Kafue River. In 2010, outbreaks of EUS in fish were reported from the Okavango Delta in Botswana, and in 2011, from Lake Liambezi in Namibia. Three outbreaks of EUS were reported from Western Cape Province of South Africa in 2011 and 2012. The disease was first noticed affecting a number of fish species in an artificial impoundment on the Palmiet River, and in a farm dam on the Eerste River. A further outbreak in the Limpopo catchment in the North West Province of South Africa was reported in 2014.
Other cases were reported from estuarine fish in the Western Cape Province in 2015, and in 2016 from a river in the Western Cape Province and from an impoundment in the Northern Cape Province. A single case was reported from the Kruger National Park in Limpopo Province in 2017. The first occurrence of EUS in the headwaters of the Congo River was reported from the Bangweulu Wetlands in the north of Zambia in 2014, where in subsequent years, the disease reappeared annually at the end of the rainy season. The first outbreak of EUS in the Democratic Republic of Congo, affecting the Equateur Province in the north west of that country, was confirmed and reported in 2015. Zimbabwe reported two outbreaks of EUS for the first time in 2016 from Mashonaland East and Midlands Provinces with further outbreaks reported subsequently. Reports of EUS from Malawi were made for the first time in July 2020 from the Bua River in the Central Province. Following no reports of disease from Botswana since 2010, an outbreak of EUS associated with high mortality was again reported from the Chobe River in July 2020.
In Southern Africa, EUS has had its greatest impact on floodplains, and flood plain fisheries have been particularly hard hit. The annual flooding cycle in the floodplains of the Zambezi system and elsewhere creates conditions particularly favourable to outbreaks of EUS.
Location of the first recorded occurrence in Africa
Map conforms to United Nations Map No. 4170 Rev. 18.1 UNITED NATIONS February 2020.
Figure 13. First recorded occurrence of epizootic ulcerative syndrome (EUS) in the Chobe River on the border between Botswana and Namibia, 2006.
©FAO/C. Huchzermeyer, Bangweulu Wetlands, African Parks
Figure 14. Climbing perch, Ctenopoma multispine, a species frequently affected by epizootic ulcerative syndrome (EUS) during the annual outbreaks of epizootic ulcerative syndrome (EUS) that have occurred in the Bangweulu swamps, north Zambia, since 2014 (Huchzermeyer et al., 2018).
