The biology of crayfish plague (Aphanomyces Astaci: Schikora) in Great Britain
Aspects of the biology of the crayfish plague fungus (Aphanomyces astaci : Schikora) were studied and a chronology of the spread of crayfish plague in the rivers of Europe was constructed. A study of the spread of crayfish plague in the River Lea and its tributaries indicated a decline in crayfish populations throughout the system with residual populations of Austropotamobius pallipes remaining in the Mimram and in the Stort. Populations may also exist in the Ash and at two sites in the Lea. The occurrence of five Pacifastacus leniusculus introductions in the system may explain the occurrence of the disease in these waterways. Six tested invertebrate species (including: Gammarus Pulex) did not appear to be suitable alternative hosts for the fungus and fish did not appear to act as transport hosts, growth and sporulation of the fungus occurred on plant substrates but could not be demonstrated under non-sterile conditions. Putative sexual stages were noted both on sterile hemp seeds and during a laboratory infection of Astacus leptodactylus. polyplanetIsm was demonstrated In culture and a maximum of five generations of zoospores occurred at 15*C. Histopathological examinations of infected animals showed the spread of the fungus within the host to be limited. Locally cuticle, epidermis and connective tissue showed most involvement. Although nervous tissue was infected, gross involvement of the nerve chord was not seen and nervous involvement was not believed to be the major cause of death. The infection of certain tissues may explain exhibited behaviour patterns. Attempts to demonstrate an effect of infection on urine production were unsuccessful. Infection of the proximal leg joints could be enhanced by wiping with a mixture of chloroform and methanol in Ast. leptodactylus but not in P. leniusculus. Solvent extracts of both crayfish species and of G. pulex prevented termination of Aph. astaci zoospores and affected the growth of sporelings. Calcium chloride extracts of crayfish enhanced termination and increased sporeling growth. The Ast. leptodactylus extract appeared to kill motile zoospores, whilst its effect on encysted spores could be reversed by replacing the medium. None of the effects demonstrated appeared to account for the differential susceptibility of the two species to Aph. astaci.
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