Browsing by Author "Schiemer, Fritz"

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    Infection patterns of Nile tilapia (Oreochromis niloticus L.) by two helminth species with contrasting life styles
    (Parasitology research, 2012) Akoll, Peter; Konecny, Robert; Mwanja, Wilson W.; Schiemer, Fritz
    The larval stages of Bolbophorus sp. (digenean) and Amirthalingamia macracantha (cestode) are frequently reported in Oreochromis niloticus in Uganda. Little, however, is known about their infection patterns. This study examined the influence of habitat type, host size, and sex and weather patterns on the parasite populations in Uganda. A total of 650 fish were collected between January and November 2008 from a reservoir, cages, fishponds and a stream. The prevalence and intensity of A. macracantha and the prevalence of Bolbophorus sp. differed across the water bodies reflecting the effect of habitat characteristics on parasite transmission. Host sex did not significantly influence the infection patterns, although female fish were slightly more parasitized than male and sexually undifferentiated individuals. The fish size was positively correlated with helminth infections demonstrating accumulation and prolonged exposure of larger (older) fish to the parasites. The metacercariae population did not vary significantly across months, while monthly A. macracantha infection fluctuated markedly. With regard to rain seasons, higher prevalence and intensity of A. macracantha were recorded in wet season. For Bolbophorus sp., only the prevalence varied with seasons, with higher prevalence recorded in the dry season than in wet season. Generally, Bolbophorus sp. responded weakly to changes in water body, host sex and size and weather patterns. Rainfall appears to be an essential cue for coracidia hatching.
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    Parasite fauna of farmed Nile tilapia (Oreochromis niloticus) and African catfish (Clarias gariepinus) in Uganda
    (Parasitology research, 2012) Akoll, Peter; Konecny, Robert; Mwanja, Wilson W.; Nattabi, Juliet K.; Agoe, Catherine; Schiemer, Fritz
    An intensive parasite survey was conducted in 2008 to better understand the parasite fauna occurrence, distribution and diversity in the commercial aquaculture fish species in Uganda. A total of 265 fish collected from hatcheries and grow-out systems were examined for parasites using routine parasitological techniques. The survey yielded 17 parasite species: 11 from Oreochromis niloticus and ten from Clarias gariepinus. Four parasites—Amirthalingamia macracantha, Monobothrioides sp., Zoogonoides sp. and a member of the family Amphilinidae—were recorded for the first time in the country. The parasite diversity was similar between hosts; however, O. niloticus was dominated by free-living stage-transmitted parasites in lower numbers, whereas both trophically and free-living stage-transmitted parasites were equally represented in C. gariepinus in relatively high intensities. The patterns in parasite numbers and composition in the two hosts reflect differences in fish habitat use and diet. A shift in parasite composition from monoxenous species-dominated communities in small-sized fish to heteroxenous in large fishes was recorded in both hosts. This was linked to ontogenetic feeding changes and prolonged exposure to parasites. Polyculture systems showed no effect on parasite intensity and composition. The gills were highly parasitized, mainly by protozoans and monogeneans. Generally, the occurrence and diversity of parasites in these fish species highlight the likelihood of disease outbreak in the proposed intensive aquaculture systems. This calls for raising awareness in fish health management among potential farmers, service providers and researchers.
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    Risk assessment of parasitic helminths on cultured Nile tilapia (Oreochromis niloticus, L.)
    (Aquaculture, 2012) Akoll, Peter; Konecny, Robert; Mwanja, Wilson W.; Schiemer, Fritz
    Disease outbreaks constrain aquaculture development. Knowledge on the potential risks of pathogens to farmed fish can help in designing management strategies for increased aquaculture productivity and sustainability. This study compares the helminth infections in reservoirs and therein operated cages as well as earthen fish ponds and the feeding stream in order to assess the significance of life cycle style and water sources in parasite transmission. In addition field experiments were setup to determine loading time and transmission rate. From 650 fish examined, 8 helminth species were recorded (3 from caged- and all 8 from pond-raised fish). The parasite community was dominated by trophically-transmitted species in both culture systems indicating the importance of trophic pathway in helminth transmission. The occurrence of trophically-transmitted helminths in caged-fish was positively related to their prevalence in reservoir-dwelling hosts indicating the importance of water supply in spread of helminths. The prevalence in pond-raised fish was higher than in stream-dwelling ones suggesting the presence of local sources of infective stages within ponds. Risk assessment revealed that monogeneans are high-risk parasites while heteroxenous helminths pose low to negligible threats to farmed fish. Although, cages appeared safer to heteroxenous parasites than ponds, their location in the water body, especially the distance from shores and depth is critical.