Transmission of crayfish plague.

Two possible means of transmission of crayfish plague were investigated: via fish (as vectors), and via crayfish (as hosts or vectors when dead). The crayfish transmission experiments focussed on both the viability of the fungus in dead crayfish when kept in simulated field conditions, and on the treatments which kill viable forms of Aphanomyces astaci within the recently dead host (cadaver). It was found that A. astaci remains viable for 5 d, and possibly longer in crayfish kept in water at 21 degrees C after dying of crayfish plague. Heating (boiling for 1 min) was the quickest way of decontaminating crayfish cadavers. Freezing took considerably longer: after 48 h at -20 degrees C, viable stages were still present. It was also found that A. astaci is unlikely to survive passage of the gastrointestinal tract of either mammals or birds as no viable stages were found after 12 h at 37 degrees C. Two basic modes of transmission of crayfish plague via fish were investigated: (1) after passage of initially viable forms of A. astaci through the digestive tract of fish and (2) via fish skin. If A. astaci was fed to fish as infected abdominal cuticle, it was still viable after passage through the gastrointestinal tract. When pure mycelium or spores were fed to fish there was no indication of viable forms of A. astaci after passage through the gastrointestinal tract. Transmission via fish skin was not observed under the experimental conditions applied. The results on fish as vectors have practical importance for fish transport and stocking because the present study shows that there is a risk of transmission of crayfish plague via fish faeces. The investigation of crayfish as vectors delivers methods that could be used for treatment of crayfish imported for human consumption into crayfish-plague-free areas. This application is particularly important for the importation of American crayfish, which are in general suspected to carry A. astaci in their cuticle.