Infected and dead crayfish are the main source of A. astaci zoospores in the environment. Frequently, the introduction of resistant carriers with no or few visible lesions is responsible for transfer of the disease to susceptible crayfish that inhabit the same water body. On other occasions, when crayfish and zoospore densities are low, a long-term, low-grade pattern of mortality may occur because there are fewer crayfish to act as reservoirs of infection).
Mammals, such as otters, mink or muskrats, and waterbirds have sometimes been blamed for spreading crayfish plague in Europe, but scientific studies have found that zoospores do not survive the temperatures of the gastrointestinal tract of mammals or birds. The movement of fish may be of greater concern in the spread of infection because zoospores remain viable in fish mucus and fish intestinal tracts. The cleaning and gutting of fish from other water bodies is another potential source of infection.
|Red Claw Crayfish (Cherax quadricarinatus)|
|Yabby (Cherax destructor)|
North American species of crayfish appear to have a host parasite balance that ensures continuity of both species without major population crashes. Overt disease resulting in crayfish mortality only occurs in North American species after stressful events, such as overcrowding or unseasonable weather. This sporadic pattern of disease outbreaks is typical of long-term coexistence of host and parasite.
Geographical location rather than phylogenetic origin is important in determining susceptibility to crayfish plague. For example, although Pacifastacus leniusculus is in the same family as European crayfish, it is resistant to the disease, whereas Cambaroides japonicus from Japan is susceptible despite being phylogenetically related to crayfish from eastern United States. Stress factors that predispose crayfish to crayfish plague include suboptimal water quality, high stocking density and intra- and interspecies aggression. The presence of abrasions on the exoskeleton also increases the likelihood of A. astaci gaining entry into the crayfish cuticle. This is more likely to occur immediately after moult, when the exoskeleton is still soft.
|Marron (Cherax tenuimanus)|
Characteristics of pathogen substrain There are at least four strains of Astaci, each having different growth, sporulation and zoospore characteristics and different temperature tolerances. The substrain found on Procambarus clarkii in Spain grows and sporulates better and has greater zoospore motility between 18°C and 25°C than other substrains. This appears to be an adaptation of P. clarkii to Spain’s climate, which is warmer than that of eastern United States. More strains with adaptations to different environmental factors will probably be identified in the future.