Crayfish plague (Aphanomyces astaci)

Updated 20.05.2026

The Norwegian Veterinary Institute (NVI) conducts crayfish plague surveillance in control areas, combat zones and risk areas.

The purpose of the program is to monitor the infection pressure and infection development in control areas and combat zones for Aphanomyces astaci, and to substantiate disease freedom and/or provide an early warning of possible infection spread in to risk areas

Background

The crayfish plague hit Norway for the first time in 1971, and has since then devastated several Norwegian watercourses. Until 2015, crayfish plague monitoring was carried out using cage experiments with noble crayfish as live bait. When suspicious mortalities occurred in cages, the dead crayfish were sent to the NVI for disease diagnostics. From 2016, the NVI took over the responsibility for the monitoring program. The program is the first in our aquatic national surveillance portfolio that utilises environmental DNA (eDNA) as infection monitoring method.

Crayfish plague is caused by the oomycete (water mould) Aphanomyces astaci, and leads to total mortality in European freshwater crayfish including noble crayfish (Astacus astacus). Noble crayfish are therefore considered an endangered species, and are red-listed both in Norway and by the IUCN. Crayfish plague is defined as a List 3 Disease (National Disease) in FOR 2008-06-17 No. 819, and is listed as a notifiable disease by OiE.

In Norway, a total of seven watercourses have been affected by crayfish plague outbreaks one or several times. This includes Vrangselva /Veksa (1971, 1974), Glommavassdraget (1997 and 2003), Store Le (1989), Haldenvassdraget (1989, 2005, 2014-ongoing), Lysakerelva (1998), Buåa (2010) and Mossevassdraget (2016). In addition, a total of four sites have been, or are still regarded as crayfish plague infected area due to the presence of illegally introduced and A. astaci carrying North American signal crayfish (Pacifastacus leniusculus). This includes Dammane in Telemark and Ostøya in Akershus, where successful eradication measures have led to declaration of disease freedom, and Fjelna watercourse in Sør-Trøndelag and Lierne in Nord-Trøndelag where infected signal are still present.

In Norway, the Animal Welfare Act (LOV-2009-06-19-97) also covers decapods. Thus, the use of live animals for disease monitoring should be reduced to a minimum whenever possible. The NVI has developed effective and reliable tools for detecting eDNA from crayfish plague infection (spores of A. astaci) in natural water systems, and the methodology was implemented in the crayfish plague surveillance program from 2016.

2025

The surveillance programme for Aphanomyces astaci in 2025 used environmental DNA (eDNA) for monitoring of Aphanomyces astaci as well as noble crayfish (Astacus astacus) and signal crayfish (Pacifastacus leniusculus) in selected watercourses. Detection of eDNA from A. astaci in River Dalselva in Indre Østfold indicates a continued ongoing outbreak linked to the River Hæra system, while no detections were made at other monitored localities.

In the Glomma watercourse, no eDNA from A. astaci or signal crayfish was detected, whereas noble crayfish eDNA was found in the tributaries Rivers Sageråa and Smalelva. This suggests that River Smalelva has not yet been affected by nearby signal crayfish or crayfish plague.

Frequent detections of noble crayfish eDNA within the regulated A. astaci restriction zones of the Halden watercourse, Mosse watercourse, and the rivers Vrangselva and Finnsrudelva in Eidskog, suggest the presence of vital noble crayfish populations within these zones.

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Aphanomyces astaci 2025 (pdf 4mb)

Crayfish plague (Aphanomyces astaci)

2025

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