Epidemiology of Epizootic Ulceration Syndrome on fish of the Zambezi river basin: A case study for Zambia

Objective: An epidemiological investigation on the fish disease Epizootic Ulcerative Syndrome (EUS) was conducted from January to December 2011 in the Zambezi River System (ZRS) of Sesheke District in the Western Province of Zambia. The study aimed at determining: factors associated with outbreaks of EUS, infection rate and distribution of the disease. EUS is a newly confirmed disease in Southern Africa caused by a fungal pathogen, Aphanomyces invadans. Material and Methods: Active surveillance was conducted where a total of 4,800 fish were inspected in February, June and October for gross EUS-like lesions and disease confirmed using histopathology. Environmental cues were also assessed monthly for one year to determine their association with disease outbreaks. A questionaire was adminstered to assess spread of EUS while Geographic Information System helped map disease distribution. Results of the study implicate several predisposing environmental factors; heavy rains preceded outbreaks resulting in excess flooding which caused water levels to rise 2m higher than normal; predominantly gleysol and arenosol soils of the ZRS resulted in low water pH (4.53 to 6.5). Other factors significantly (p<0.05) associated with EUS outbreaks were: low total alkalinity (45.13±SE 0.0418), water temperature (20.94±SE 0.2173), ambient temperature (25.85±SE 0.3058), month (June) and site (lagoons) of sampling. Infection rate was 3% (144) of the 4,800 fishes sampled. Of these, 58 (40.2%) had mycotic granulomas after histopathological analysis, representing 1.2% of the total sample. Eighty six (86) of the 144 fishes were diagnosed with healing wounds representing 1.8 % of the total fish sampled. Some of them, 4,656 (97%) more exactly, had no gross lesions. Conclusion: There is indication that EUS has affected fish in ZRS from Kazungula to Chavuma Districts of Zambia with sub optimal environmental factors being associated with disease outbreaks.