A bath challenge system was developed to enable further investigation of the causative factors of EUS. This looked at water quality parameters as contributory factors, examining the effects of (i) low alkalinity, low hardness water, (ii) acidified low alkalinity, low hardness water, and (iii) tapwater.
Fish were exposed to sporulating fungal wads of Aphanomyces invadans, after initial exposure to water treatments for 24 hours. Fish were examined over a 30 day period for signs of lesions. Once lesions were observed fish were sampled, and the area below the lesion was examined histologically for the presence of an intramuscular fungal invasion. The study of acidified and distilled water as causal factors in EUS revealed there was no difference between these treatment groups and tapwater. It was thought however that low alkalinity and hardness do play a part in increasing susceptibilty of snakehead (Channa striata) to EUS, as tapwater also had alkalinity and hardness values that were considered low.
Behavioural observations were also highlighted as potential factors. An in vitro method of screening potential fungicides revealed that from a number of compounds (D-limonene, E-Z-MulseTM, a D-limonene/ E-Z-MulseTM emulsion, a commercial neem seed extract, a coarse neem seed extract, and a coarse neem leaf extract), only E-Z-MulseTM and the commercial neem extract showed any reasonable activity against A. Invadans mycelium, or the secondary zoospore stage.
The combination of the developed bath challenge and screened fungicides was used to study the efficacy of a number of potentially active fungicides. These compounds included calcium oxide (CaO), CIFAX (a claimed preventative and curative treatment of EUS), and the two highlighted compounds from in vitro screening, E-Z-MulseTM and commercial neem extract. Abraded fish were exposed to sporulating wads of A. invadans and sampled after 5 days, with freshly abraded fish and mycelial wads added every 5 days, over a 15 days period. Water remained in the tanks for duration of the investigation to gauge efficacy of treatments. This exposure protocol failed to precipitate sufficient infection within controls to enable accurate examination of treatments.
Fairweather, D.J. Development of a bath challenge system to study component causes and preventative treatments of epizootic ulcerative syndrome (EUS) in snakehead fish (Channa striata). (1999) 88 pp. [MSc thesis, Plymouth University]