Applied studies on epizootic ulcerative syndrome - the ecology, immunogenicity and treatment of Aphanomyces invadans.
Abstract
This project was conceived to make applied use of the findings of previous DFID funded projects which identified and characterised causal agents of epizootic ulcerative syndrome (EUS), in particular the fungal pathogen, Aphanomyces invadans. The tools and knowledge generated from these projects were utilised in a variety of research activities aimed at improving techniques for the diagnosis and control of EUS. Research was conducted jointly with a number of regional institutes to encourage contact between researchers, and help build up a consensus among fisheries workers regarding EUS causation and control.
DNA probes, designed by previous projects, were developed for use in the diagnosis of EUS. It was not possible to develop a technique to identify A. invadans DNA in the environment, so studies concentrated on using histological diagnosis for disease confirmation. A comparative study of histological techniques identified Uvitex H and E staining as the most rapid and reliable diagnostic technique. Contact with international scientists allowed verification of the presence of EUS in Nepal, Vietnam and Pakistan by histological diagnosis, and in USA by histology and genetic characterisation of A. invadans.
Studies on zoospore physiology demonstrated characteristics of the fungus relevant to the development of control strategies. In vitro studies on the fungus were also used to screen 50 compounds for antifungal activity. Candidate treatments were then tested in tank and pond trials.
Bath challenge models were developed for use in pond trials to test preventative treatments. These trials were conducted in Thailand, Bangladesh and India. A variety of low cost inputs were found to reduce the number of fish affected and/or the severity of infections, but none were able to prevent infection completely. Farmer based studies in collaboration with the CARE LIFE project showed that farmers were keen to adopt treatments, and believed them to be successful.
Immunological studies demonstrated differences in the immune reaction of EUS susceptible and EUS resistant fish. Resistant fish tend to have a more effective cellular response to fungal infection. An immunostimulant trial indicated that Salar bec is effective in boosting this nonspecific response in susceptible snakeheads. Trials also showed that snakeheads from EUS endemic areas have a strong serum response to A. invadans infection, and a passive immunisation trial showed that the antibodies involved are protective. This shows that there is potential for vaccine development, although this was not pursued further on this project.
Epidemiological studies in Bangladesh and Nepal identified a range of factors that increased the risk of EUS infection. Most significant of these was connection of ponds to natural water bodies and presence of wild fish in the pond. This was supported by prevalence data, which showed that EUS is endemic in natural water bodies in most of the areas examined. A range of management strategies to reduce the risk of EUS outbreaks was formulated.
Collaborative research, seminar tours and production and dissemination of manuals and leaflets ensured a widespread awareness of the project and its research outputs in South and Southeast Asia.
Citation
Lilley, J.W. Applied studies on epizootic ulcerative syndrome - the ecology, immunogenicity and treatment of Aphanomyces invadans. (2001) 363 pp.
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