How is epizootic ulcerative syndrome (EUS) spread? What factors cause the fish to become infected with EUS?
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uccessful invasion and establishment of EUS in fish requires epithelial tissue damage, a susceptible fish species and environmental conditions which favour sporulation of A. invadans. In some countries, outbreaks occur in wild fish first and then spread into fish ponds. A number of risk factors are associated with EUS occurrence; some are predisposing factors, others are environmental conditions and biological factors that contribute to the occurrence of EUS. Shipping movements, ballast water, fish migrations and ocean currents are potential pathways by which the pathogen can spread between water bodies. Cross border movement of fish for aquaculture and the ornamental fish trade are proven pathways by which the disease spreads. Some EUS outbreaks follow heavy rainfall and flood events; reductions in water temperature, alkalinity and salinity; and acidified run-off water from acid sulphate soil areas have been associated with outbreaks of disease.
©FAO/D. Huchzermeyer, Rhodes University
Parasites and rhabdoviruses have also been associated with particular outbreaks, and secondary gram-negative bacteria invariably infect EUS lesions.
Figure 32. Fishing nets may be infected with spores of Aphanomyces invadans and should be dried immediately after use, and not be left where they remain wet as in this picture, Bangweulu swamps, north Zambia, 2014 outbreak of epizootic ulcerative syndrome (EUS).
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