Carp fall victim to Koi sleepy disease

Koi sleepy disease, caused by carp oedema virus, only affects carp, both ornamental and common. Up to 80% of the population in a pond or lake can be wiped out by the virus, leading to several tonnes of dead bodies floating at the surface. First detected in Japan in the 1970s, the virus has since spread to several continents. The World Organisation for Animal Health (WOAH) recently defined Koi sleepy disease as an emerging and notifiable disease.

The disease has been present in France for the past 10 years or so and affects bodies of water of all sizes throughout the country. It is more common at the end of winter, when waters warm up, but outbreaks have been observed all year round.

There is no cure for the disease, so it is important to detect it to be able to take measures to prevent its spread, such as banning fishing and transfers of fish from infected ponds. To enable this detection, the Fish Virology, Immunology and Ecotoxicology Unit (VIMEP) of ANSES's Ploufragan-Plouzané-Niort laboratory has adapted a PCR test developed in Great Britain. It has transferred it to three volunteer departmental veterinary testing laboratories, which carry out analyses whenever the disease is suspected (see box). These tests are able to detect the virus, as the clinical manifestations of the disease can be mistaken for those of other infectious diseases.

Searching for factors that favour the disease

The unit is also participating, alongside another unit of the Ploufragan-Plouzané-Niort laboratory, the Epidemiology, Health and Welfare Unit (EPISABE), in an ongoing research project called CEVIRAL. Financed by the European Maritime and Fisheries Fund (EMFF), it is coordinated by the Technical Institute for Poultry, Rabbit and Fish Farming (ITAVI). It aims to better understand the disease, particularly the factors that encourage its spread, such as water temperature, and identify sources of contamination. "For example, by sequencing the viral DNA, we were able to show that three lakes in the Tarn département had been contaminated in 2022 by the same strain, following the introduction of carp from the same batch of fish” explains one of the study's authors. The project is also intended to raise the awareness of fish farming and fishery professionals who are not yet familiar with the disease. They are encouraged to participate in research by collecting observations and sending in samples for analysis if the disease is suspected.