Aetio-Pathological Investigations Among Fish Species presenting Epizootic Ulcerative Syndrome (EUS) In the Zambezi river Basin in sesheke District of Zambia

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Songe, Mwansa Mathilda
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This study was conducted in Sesheke district of Zambia to investigate the etio pathological changes that occur in fish species presenting with Epizootic Ulcerative Syndrome in the Zambezi River Basin. The disease is endemic in Asia, but it is being reported for the first time in Africa. A total of two hundred and seventy (270) fish belonging to sixteen species were sampled from seven (7) major fishing camps in Sesheke district of Zambia during the study period. These species were: Clarias ngamensis, C. gariepinus, Barbus poechii, Tilapia sparmanii, Seranochromis angusticeps, Brycinus lateralis, Micralestes acutidens, Sargochromis carlottae, Hydrocinus vittatus, Phryngochromis acuticeps, Schilbe intermedius, Hepsetus odoe, Labeo lunatus, Oreochromis andersonii, B. unitaeniatus and B. paludinosus. Among the fish samples collected from the field, only Tilapia sparmanii did not show any macroscopic or microscopic lesions.The disease was diagnosed by the histopathological technique. Hematoxylin and Eosin standard stains were used. Grocott stain was used to confirm the presence of fungal hyphae in the tissue sections. Furthermore, samples were cultured for isolation of Aphanomyces invadans, the causative agent of EUS.Following isolation, the infectivity and role of A. invadans in the etiology of fish skin ulceration in Oreochromis niloticus, Oreochromis andersonii and Barbus paludinosus were investigated in the laboratory through the pathogenicity and challenge studies. Of these species, O. niloticus did not show any visible lesions, even after 32 days post-inoculation. The other two species, on the other hand, were successfully infected with the fimgus after 15 days post-inoculation, upon exposure after disruption of the epidermis. The deeply penetrating ulcers observed in O. andersonii and B. paludinosus after the challenge were characterized by dermatitis, myofibrillar degeneration and necrotizing granulomatous myositis. The experimentally induced lesions also exhibited invasiveness, often involving the kidney, confirming Koch's postulates. Natural infections were characterized by dermatitis, hemorrhages, severe muscular necrosis, necrotizing granulomatous inflammation and myofibrillar degeneration. Invasiveness involving internal organs was also observed. The study has demonstrated that EUS can be experimentally induced in O. andersonii and B. paludinosus after exposure to A. invadans zoospores isolated from the Zambezi river basin.The challenge experiments are significant in establishing which fish species are susceptible to EUS. In this case O. niloticus would be an excellent culture species for the emerging fish farmers in Zambia, owing to its comparative resistance to the disease. It is, therefore, important that these studies are continued in a number of species. This will enhance the true understanding of which species can be affected by EUS in the Zambezi river basin.
Fish species-Zambezi River , Fish pathology-Zambezi River