Symptom-free crayfish plague a threat to the recovery of the noble crayfish populations


<div>Crayfish plague is the biggest threat to the indigenous noble crayfish. At Evira’s Kuopio research unit, tests have shown remarkable differences in mortality rates between the two different types of crayfish plague. The plague type carried by signal crayfish causes rapid illness and death in noble crayfish after they have been infected. On the other hand, some strains of the crayfish plague type that has existed in Finland for over 100 years in noble crayfish cause symptoms very slowly. According to the results, the crayfish plague would not always lead to a total mortality of crayfish in the waterway, but could leave behind crayfish that are carriers of the disease. Symptom-free noble crayfish that carry the crayfish plague complicate restockings and stockings with feral crayfish.</div>

Crayfish plague strains that cause a mild form of the disease are troublesome, because they do not destroy the whole crayfish population of the waterway. In this way the crayfish plague can remain permanently in the waterway and cause mass mortalities only after the crayfish population has increased to a sufficient level. The occurrence of noble crayfish is therefore not a guarantee that there is no plague in the waterway. Crayfish plague of the noble crayfish type has been found in crayfish that had survived many years after the crayfish plague had caused mass mortality.

At this stage, there are no methods available by which laboratory tests could reliably ensure that the crayfish meant for restocking are free of the plague. It is recommended to study the history of the crayfish plague in the lake of origin and perform long term trials in crayfish cages. Evira’s Kuopio research unit studies the ability of different crayfish plague strains to cause disease in a network project focusing on the epidemiology and diagnostics of the crayfish plague. The project is jointly run by the Finnish Game and Fisheries Research Institute, Evira and the University of Kuopio, and is financed by the European Community under the Financial Instrument for Fisheries Guidance programme. One of the objectives of the project is to find better methods for identifying the crayfish plague.

Crayfish plague is a serious parasitic disease that threatens the noble crayfish populations and will generally destroy the crayfish in the waterways that have been infected. It is caused by Aphanomyces astaci, an oomycete organism. In Finland there are two types of crayfish plague, one of which is spread mainly through signal crayfish, which is resistant to the parasite. The other type, which has been found so far only in noble crayfish, appears mainly outside the stocking areas of signal crayfish in Eastern, Central and Northern Finland.

Annually Evira diagnoses cases of crayfish plague in about a dozen different waterways.


For additional information, please contact:

Veterinarian Satu Viljamaa-Dirks, Evira, Kuopio Research Unit, tel. 020 77 24962

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