What is it?
Crayfish plague is a highly infectious disease of all crayfish of non North American origin. The aetiological agent is an Oomycete fungus, Aphanomyces astaci, which is now widespread in Europe as well as in North America.
The disease first occurred in Europe in the third quarter of the 19th century in the Franco-German border
region. From there region a steady spread of infection occurred, principally in two directions down the
Danube into the Balkans and towards the Black Sea, and across the North German plain into Russia and
from there south to the Black Sea and north-west to Finland and finally, in 1907, to Sweden. In the
1960s the first outbreaks in Spain were reported and in the 1980s further spread of infection to the British
Isles, Turkey, Greece and Norway was reported.
Where and when might it occur?
The European crayfish species, the Noble crayfish Astacus astacus of north-west Europe, the stone crayfish Austropotamobius pallipes of south-west and west Europe, the related Austropotamobius torrentium (mountain streams of south-west Europe) and the slender clawed or Turkish crayfish Astacus leptodactylus of eastern Europe and Asia Minor are all highly susceptible.
The reservoir for the original infections in the 19th century was never established, but the post-1960s extensions are largely linked to movements of North American crayfish introduced more recently for purposes of crayfish farming.
Transmission has also resulted from contaminated crayfish traps and other contaminated equipment.
Infected crayfish may present a wide range of gross signs of infection or none at all. Focal whitening of local areas of musculature beneath transparent areas of thin cuticle, especially of the ventral abdomen and in the periopod (limb) joints, often accompanied by even more localised brown melanisation, is the most consistent sign.
In the terminal stages of infection, animals show a limited range of behavioural signs, principally a loss of the normal aversion to bright light (they are seen in open water in daylight) later accompanied by a loss of limb co-ordination, which produces an effect that has been described as walking on stilts.
Eventually, animals lose their balance and fall onto their backs before dying.
Diagnosis requires isolation and identification of the pathogen by microscopic morphology; no molecular, biochemical or serological methods that have been adequately validated exist.
Control and treatment
Control of the spread of infection once a watershed is infected is in practical terms impossible.
Prevention of all introductions of crayfish to natural waters and into enclosed waters from which they may escape to natural waters can be effective, although movement of fish can result in the movement of infected water between watersheds and can transmit infection, as can contaminated equipment such as boots and fishing gear.
Sodium hypochlorite and iodophores are effective for disinfection of contaminated equipment. Thorough drying of equipment (>24 hours) is also effective since the Oomycetes are not resistant to desiccation.