Spreading of Chronic Wasting Disease (CWD)
Report no: 2018:16
Eradication of Chronic Wasting Disease is not completed
This is the key message from an update of previous risk assessments of CWD conducted by the Scientific Committee on Food and Environment (VKM). The update was requested by the Norwegian Food Safety Authority.
Spreading of the disease
VKM has assessed a number of different factors that can affect the spread of CWD within and out of the Nordfjella area. The area includes the entire Aurland, Lærdal and Hemsedalmunicipalities and parts of Ål, Hol, Eidfjord and Ulvik municipalities. Centrally located in the area is the Nordfjella wildlife area, divided into a zone 1in the north and a zone 2in the south. The wild reindeer population in zone 1was culled in the attempt to eradicate CWD.
- It is likely that there is contamination present in the areas previously used by the culled animals. The level of contamination is greatest in places often visited by wild reindeer populations
- There may still be unidentified infected cervids in the area and new cervids can be infected from the environment in Zone 1. It is most likely to find infected individuals in the wild reindeer population in Sone 2, among red deer in Lærdal and Aurland and in the semi-domesticated reindeer herd on Filefjell. The possibility that transport of reindeer has spread infected animals to other areas cannot be excluded.
- There is a high probability of spreading of the disease from the environment in zone 1. This may occur in a situation where the wild reindeer population from Zone 2, red deer from Lærdal and Aurland or semi-domesticated reindeer from Filefjell enter Zone 1 and becomes infected. It is slightly less likely that red deer, moose and roe deer from Hallingdal become infected this way, and even less likely that humans, predators, scavengers and others carriers would spread the infection to new cervid populations.
- Sheep grazing in Zone 1 is likely to transfer the infection to cervid populations susceptible to infection. It is unlikely that a single sheep would be of concern; however, thousands of sheep are grazing in the area.
Mineral lick sites
The infectious agent in CWD, misfolded prions, is highly resistant. Prions can remain infectious in the environment for many years, and may bind to soil being taken up by plants. Infected animals can shed prions, for example through saliva, urine and faeces, and thus infect other animals directly. The contamination can also persist in the environment and may be picked up by animals susceptible to infection.
"The number of mineral lick sites used by the culled wild reindeer population and which are still accessible to animals will have a very important impact on the likelihood of spread of infection from the environment, " says Bjørnar Ytrehus. He was the chair of project group that conducted the update.
"Making the mineral lick sites inaccessible would reduce the probability of cervids entering zone 1 from becoming infected. The absence of mineral lick sites would also reduce the likelihood that sheep spread the infection," he continues.
"If there still are infected animals in the area or new animals are infected from the environment, newly established mineral lick sites inside and outside of Zone 1 will also increase the likelihood of spread of infection," Ytrehus emphasizes.
Monitoring and preparedness
Significant uncertainty is attached to the identified factors. The likelihood of spread of infection can be reduced by maintaining intensive monitoring, alertness and preparedness.
In addition, fewer mineral lick sites and other places where animals gather would reduce the likelihood of spread of infection. "Measures such as fencing and herding, reduction of cervid population and information to people using the area would also reduce the likelihood of spreading," says Ytrehus.
VKMs Panel on Biological Hazards has prepared the assessment.