Crayfish plague is a destructive waterborne disease that affects freshwater crayfish populations globally. This fast-acting affliction is particularly devastating to native crayfish species outside of North America, often wiping out entire populations in a matter of weeks. The disease represents a significant threat to freshwater biodiversity, causing severe ecological disruption in the areas it invades. It is a major factor contributing to the decline of native crayfish species across Europe and Asia.
The Causative Agent and Global Spread
The agent responsible for the crayfish plague is Aphanomyces astaci, a microscopic, fungus-like organism classified as an oomycete, or water mold. Originating in North America, the pathogen co-evolved with native North American crayfish species, resulting in a generally non-lethal host-parasite relationship. The global spread of this organism began in the mid-19th century, with the first reported mass mortalities occurring in Italy around 1860.
The subsequent rapid invasion across Europe and Asia was largely linked to the introduction and farming of North American species. Invasive carriers, such as the American signal crayfish, were intentionally moved across continents for aquaculture or to replace depleted native stocks. These carrier species introduced the pathogen into new environments lacking natural resistance. The zoospores of A. astaci are motile and released into the water, where they can travel short distances to find new hosts.
Recognizing Signs of Infection
In susceptible crayfish species, clinical signs appear rapidly, often leading to death within a few weeks, depending on water temperature. The pathogen preferentially targets the soft, non-calcified areas of the exoskeleton, particularly the joints of the legs and the ventral abdominal cuticle. As the oomycete’s filamentous structures, called hyphae, invade the host, they cause visible damage.
Outward signs include a whitening of the musculature beneath the thin cuticle of the tail and abdomen, sometimes accompanied by areas of browning where the host attempts to contain the invader. As the infection progresses, the hyphae penetrate the nervous system, leading to distinct behavioral changes. Infected crayfish lose their natural aversion to light, may be seen stumbling in daylight, and exhibit a loss of coordination described as a “walking on stilts” gait before becoming paralyzed.
Why Some Crayfish Species Survive
The difference in mortality rates between North American and non-North American crayfish stems from a long co-evolutionary history. North American species, such as the signal crayfish, developed an effective immune response that allows them to become asymptomatic carriers. Their immune system walls off the invading hyphae primarily through a process called melanization.
This process involves immune cells encapsulating the pathogen in a layer of melanin, restricting the infection to the outer cuticle, which is shed during molting. Susceptible species, like the native European noble crayfish, lack this strong, localized defense. In these non-co-evolved hosts, the A. astaci hyphae breach the basal lamina beneath the epidermis.
Once past this protective layer, the pathogen spreads rapidly throughout the connective tissues and internal organs, ultimately leading to systemic failure and death. This biological vulnerability means that when a resistant carrier species is introduced, it acts as a mobile reservoir, ensuring the pathogen’s continuous presence and devastating susceptible native populations.
Essential Biosecurity Measures
Preventing the spread of the crayfish plague relies entirely on strict biosecurity protocols, as there is no known treatment for the disease in the wild. The most effective action for water users, including anglers, boaters, and hikers, is to follow the “Check, Clean, Dry” procedure after leaving any water body.
Users must visually check all equipment, footwear, and clothing for mud, plant fragments, or living organisms and remove them at the site. All items that contacted the water must then be thoroughly cleaned and disinfected before being used elsewhere. If drying is not feasible, equipment should be treated with an approved disinfectant to eliminate lingering zoospores. Finally, equipment must dry completely for a minimum of 48 hours, as the pathogen’s spores are poorly resistant to desiccation. It is also imperative that no crayfish or water from one catchment area be moved to another, as this represents a direct pathway for transfer.