Crayfish plague (Aphanomyces astaci)

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.

2021

In 2021, a total of 58, 39, 20 and 36 water samples were collected from selected sites in the Halden-, Mosse-, Glomma watercourse regions and in the Eidskog region, respectively. Sampling locations were strategically selected and focused on both control zones and the risk areas adjacent to crayfish plague control zones. The presence/absence of the three target species was determined simultaneously through screening with species-specific qPCR assays.

In the Halden watercourse, eDNA of A. astaci was detected in the south of Lake Rødnessjøen and in River Hølandselva, further upstream than previously detected. No spread of the pathogen was observed in any of the other areas monitored with the eDNA methodology. However, one sample from River Lierelva (risk zone) was positive for signal crayfish eDNA in June. This coincided with the cessation of positive noble crayfish eDNA detections at this site in September. Also two other locations in the risk zone yielded no noble crayfish eDNA, which is a decline in positive noble crayfish detections from previous years.
In the Mosse watercourse, no eDNA of A. astaci or signal crayfish was detected, while noble crayfish eDNA was detected upstream of Lake Langen.
In the Glomma watercourse, eDNA from noble crayfish was detected at one station - Oppstadåa south. No eDNA was detected from signal crayfish or A. astaci.
In Eidskog, all samples were negative for signal crayfish and A. astaci, while several samples were positive for noble crayfish eDNA in the rivers Vrangselva and Finnsrudelva.

In August 2021, the river Mysenelva that is part of the Hæra watercourse was struck by crayfish plague. The Norwegian Veterinary Institute confirmed the detection of A. astaci from dead noble crayfish. This further increases the number of A. astaci infected areas in Norway. The Norwegian Food Safety Authority extended the crayfish plague zone for Glomma to incorporate Mysenelva downstream of Rustadfossen east of Mysen.

In summary, the single positive detection of signal crayfish eDNA in Lierelva is a non-conclusive result and requires close follow-up in 2022 regarding the possible spread of crayfish plague. The observed crayfish plague outbreak in Mysenelva increases the number of infected water systems in Norway, and will be integrated in the crayfish plague surveillance in 2022.

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Crayfish plague (Aphanomyces astaci)

2021

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