Chronic Wasting Disease - updated knowledge about the disease and risk factors for its spreading

This is the conclusion of the newly published report on Chronic Wasting Disease (CWD) prepared by the Norwegian Scientific Committee for Food and Environment (VKM). The report provides an updated synthesis of CWD research published between 2018 and 2020, and assesses how to manage the spread of this highly infectious disease. The update was required after a male reindeer from the Hardangervidda area was diagnosed with CWD in autumn, 2020.

Prior knowledge is still valid

VKM has assessed papers published between 2018 and 2020, and described both finalised and ongoing research projects relevant to CWD management in Norway. Diagnostic and population surveillance data for wild reindeer was used to estimate CWD prevalence in the Hardangervidda area.

Synthesis of CWD research published after previous VKM reports revealed no new information that would change the prerequisites for handling CWD in Norway. “The information provided in previous VKM reports is still relevant. However, the results of several recent studies strengthen and build on previous knowledge,” says Bjørnar Ytrehus, chair of the project group.

Studies have shown that brain material infected with prions and buried in soil in the natural environment, remains infectious for many years. Experiments where white-tailed deer have been infected with CWD-prions shows that the oral infective dose is very low.

There are also studies that have investigated the relationship between hunting of adult males and occurrence of CWD. “The conclusion is that removal of a substantial proportion of adult males may maintain low levels of occurrence of CWD,” says Ytrehus.

He further highlights studies that show that the effect of management strategies depends on the phase of advancement of the disease outbreak.

“In an early phase, there is a minor possibility of stopping the outbreak by removing all infected animals before they pass the disease on to other animals. However, when five or more animals are infected the outbreak will likely continue to grow,” according to Ytrehus.

Management strategies

VKM has previously (2017) outlined three strategies for handling CWD: a) no measures; b) limit the disease; and c) eradicate the disease. These strategies were reassessed based on updated research results.

VKM concludes that a strategy of no measures very likely will lead to an increasing prevalence of CWD in the Hardangervidda wild reindeer herd. The disease would likely spread to other populations of wild and semi-domesticated reindeer as well as to other species of cervids.

“There is no scientific evidence suggesting that the disease will peter out on its own,” says Ytrehus.

Environmental contamination will accumulate, which can lead to uncontrolled spread of the disease.

The only certain way to eradicate CWD is to cull the infected animals and keep the area free from cervids until testing reveals that there is no longer any disease present in the environment. This will take years to achieve.

“The effectiveness of these measures depends on whether CWD is present in other populations, and whether a fallowing period efficiently clears any vestiges of the disease,” says Ytrehus.

VKM also points out that the “eradicate” strategy would lead to other unwanted consequences. There would be extended ecological effects, because the wild reindeer plays an important role in the Hardangervidda ecosystem. Culling many wild animals will also pose significant challenges concerning animal welfare.

Limiting CWD involves various measures to keep its prevalence within the wild reindeer population as low as possible, whilst minimising the probability of spread.

“Initiating these limiting measures simultaneously within a short time frame (while the disease prevalence is still low) offers the best opportunity to successfully limit the occurrence of CWD. If we are lucky, the collective effect of such measures may lead to eradication of the disease,” says Ytrehus.

Measures

One of the measures expected to have a significant impact on limiting the disease is to minimise the proportion of adult males and to reduce the total number of animals in a population. Such a measure must be carried out as soon as possible.

Reducing the proportion of adult males will probably reduce the occurrence of CWD in the population, because males appear to have a tripled rate of infection than females.

”While it is uncertain how a reduction of population size will affect the number of infected animals and the rate of spread within the population, it will most likely decrease the level of environmental infection and diminish the probability of spread to other populations,” says Ytrehus.

Another possible measure would be to reduce human activity in parts of the Hardangervidda area. This measure would allow the animals access to a larger area, resulting in lower densities and reducing the probability of the animals being exposed to accumulated environmental contamination.

Fences between the Hardangervidda and surrounding areas can prevent reindeer migration and thus contribute to avoiding the spread of CWD to other populations.

“Measures directed towards altering reindeer area use will be more effective if started now and fully implemented within a short time period,” says Ytrehus.

Initiating measures to reduce accessibility of salt licks will reduce the probability of exposure to environmental infection for reindeer and other cervids. It is thus important to avoid environmental contamination in areas where wild reindeer gather, such as for example in areas with salt licks for sheep.

Uncertainties

There are many uncertainties related to the assessments presented in this report. A major uncertainty relates to the fact that the disease was found in the Hardangervidda area and is not just restricted to Nordfjella.

“It is thus uncertain whether CWD is also present in other cervid populations in Norway. Increased surveillance and mapping is important for improving our knowledge base for making decisions,” says Ytrehus.

“Another major uncertainty relates to the fact that we do not know exactly how many animals in the Hardangervidda area are infected with CWD. This is another important reason to initiate extensive mapping and surveillance in this area,” says Ytrehus.

Interdisciplinary VKM approval group

The report has been approved by an interdisciplinary group comprising members from the VKM Panels on Alien Organisms and Trade in Endangered Species (CITES), Animal Health and Welfare, Microbial Ecology and Biological Hazards. The report was commissioned by the Norwegian Environment Agency and the Norwegian Food Safety Authority.