Reindeer CWD prion ecology: Risk of dissemination by sheep

In this project, the primary objective is to investigate if reindeer chronic wasting disease (CWD) is transmissible to sheep, and to elucidate which role sheep may play in further spread of the disease. In 2016, CWD was diagnosed in a reindeer from Nordfjella. This was the first time CWD was diagnosed in Europe. To minimize the risk of further spread of the disease, the Norwegian Government decided to cull all reindeer in zone 1 of Nordfjella, which was completed during winter 2018. The area is quarantined for at least five years before restocking of reindeer. Approximately 70000 sheep will continue to graze in the area in the quarantine period. In September 2020, the first case of contagious CWD in reindeer was found in Hardangervidda, and in October 2022 the second case was found in the same area. Therefore, the disease is no longer confined to one area and under less control. Sporadic, or spontaneous, CWD have been found in moose and red deer. CWD is highly contagious and transmits directly between cervids or via the environment. Prions are very persistent and can reside in the environment for many years. Therefore, sheep grazing in Nordfjella are very likely to be exposed to CWD. Salt lick spots in Nordfjella were visited by both sheep and reindeer and are considered high risk areas for environmental contamination of prions. The old salt lick areas are now fenced off and made inaccessible to sheep. In work package 1, we are studying if reindeer CWD can be transmitted to sheep and cause clinical disease, or if sheep may be silent carriers of the disease. In two different experiments, 12 lambs (and two controls) were inoculated with reindeer prions to experimentally study transmission between reindeer and sheep. Blood, feces, and biopsies from lymphoid tissue in rectum have been collected regularly. One of the experiments have been finished and the other experiment is soon ending. A range of samples from different tissues are taken at the study endpoint of each sheep. We have started analyzing samples from this part of the project, but still no results are ready for publication. In work package 1, new and sensitive laboratory methods are being established in addition to traditional methods used for prion detection. In work package 2, we have investigated lymphoid rectal tissue from over 500 lambs and sheep, and in addition lymphoid tissue in the small intestine from 37 lambs, grazing in Nordfjella. We did not find prions in any of these samples using two different methods (immunohistochemistry and ELISA). We also analyzed historical GPS-data (WP3) from reindeer and sheep (also included in the gut investigations) to see if there was overlapping use of grazing areas. We showed that there had been close contact between reindeer and sheep sharing pasture areas. The GPS-data also included movements of a CWD-diseased reindeer. This work was published in 2022. In work package 3, we use camera traps to investigate sheep behavior at salt licks, and to study if sheep and reindeer have visited the same areas. We have access to images from 16 salt lick spots placed in Nordfjella and further 16 camera traps placed in the reindeer areas Knutshø and Forolhogna. Over 500.000 images are analyzed by AI (artificial intelligence). This work is partially published as student work at Swedish University of Agricultural Sciences. In cooperation with international experts, soil samples from Nordfjella are analyzed for prions with highly sensitive methods. The soil analyses show evidence of small quantities of prions from a few salt lick places in Nordfjella zone 1 (where the reindeer were extinct) and in one single sample from Nordfjella zone 2, where reindeer are still present. In work package 4, we performed laboratory experiments to closer investigate reindeer, moose and red deer CWD isolates and a sheep scrapie isolate. We studied the in vitro conversion ability of these prions towards the normal cellular prion protein (PrP) of different ruminant species including Norwegian cervids, sheep, and goat. This part of the study informs about species barriers and zoonotic potential. We collected brain material from all these species through ordinary hunting and slaughter. The brains were PrP genotyped, and both susceptible genotypes towards CWD (cervids) and scrapie (sheep and goat), and less susceptible genotypes were included. Reindeer CWD and classical sheep scrapie isolates were similarly propagated in brains from different species, suggesting the potential for spillover of these contagious diseases. The moose and red deer isolates amplified in few brains and only in cervid species. Furthermore, the roe deer brain supported conversion of three isolates suggesting that this species may be vulnerable to prion disease. This work was published in 2023. Knowledge about the potential exposure for CWD and risk of transmission to sheep is essential for the management of shared pasture resources, mitigation and public information.

In April 2016, the prion disease chronic wasting disease (CWD) was diagnosed for the first time in Europe in a Norwegian reindeer from Nordfjella. Culling of over 2000 reindeer in Nordfjella zone 1 was completed during winter 2018. The area is quarantined for at least five years before restocking. During this period, stringent measures and intensified surveillance are undertaken to prevent CWD from spreading out of Nordfjella. Approximately 70 000 sheep will continue to graze in the area during the quarantine period. Prion diseases are fatal brain disorders and encompass Creutzfeldt-Jakob disease in humans, bovine spongiform encephalopathy in cattle, CWD in cervids and scrapie in sheep and goats. CWD is highly contagious and transmits directly between animals or via the environment where prions can reside for many years. Traditional salt lick spots are most likely high-risk areas for environmental exposure of CWD prions. In zone 1 of Nordfjella, all salt licks are now fenced in, but until late July 2018 sheep had access to these spots. However, the surrounding areas might be contaminated and are still accessible. A carrier state of the disease in sheep cannot be excluded and raises questions regarding spread of CWD prions as well as food safety. This interdisciplinary project will investigate the possible uptake and transmission of reindeer CWD to sheep, and elucidate which role sheep that graze in Nordfjella may play in further spread of the disease. To address this we have; designed experimental studies were young lambs are exposed to CWD, collected material from sheep that have been grazing in Nordfjella, monitoring of sheep and reindeer behavior and collected environmental samples at salt lick spots, and designed in vitro studies that will explore agent characteristics. The future management of CWD has to be based on research. In this project, we highlight issues of great importance for Norwegian authorities, EU legislation, and sheep industry among others.