How does epizootic ulcerative syndrome (EUS) occur?
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easonal climatic conditions can create environments that favour occurrence of EUS. In many parts of Africa, for instance, the drop in temperature at the end of the rainy season, seasonal draining of floodplains during the winter months and flood-drought cycles that favour acidification of surface waters have been associated with outbreaks of EUS. Infection in fish leading to EUS occurs when motile spores (zoospores) of A. invadans are released into the water from infected fish or other carriers/vectors. The zoospores are attracted to the skin of the fish. ©FAO/K. Hatai and FAO Fisheries Technical Paper 402/2
The spores penetrate compromised areas of skin and germinate, forming fungal filaments or hyphae. The hyphae invade widely into the surrounding skin and deeply into the underlying muscle, resulting in extensive destruction of tissues leading to ulceration. The hyphae will produce sporangia that eventually release further motile spores. The sporangia are narrow, with diameters similar to that of the hyphae.
Figure 15. Typical characteristic of Aphanomyces sporangium (Japanese isolate).
©S. Kanchanakhan (June 2007)
A single row of primary zoospores formed within a zoosporangium are then released through the sporangium to encyst at the apical tip of the sporangium to form achlyoid clusters. From here, the main freeswimming stage of Aphanomyces spp., the secondary zoospore, is discharged into the water to seek out a further fish to infect.
Figure 16. Sporulation of the Botswana oomycete isolate successfully identified as Aphanomyces by the Aquatic Animal Health Research Institute (AAHRI), Thailand, June 2007.
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