Chronic wasting disease isolates from Norway and North America lack evidence of common strain characteristics

Authors: Cassmann, Eric; Greenlee, Justin

Submitted to: North American Deer Farmer
Publication Type: Trade Journal
Publication Acceptance Date: 1/1/2022
Publication Date: 4/20/2022
Citation: Cassmann, E.D., Greenlee, J.J. 2022. Chronic wasting disease isolates from Norway and North America lack evidence of common strain characteristics. North American Deer Farmer. p. 112.

Interpretive Summary:

Technical Abstract: In North America, chronic wasting disease (CWD) was first reported in mule deer in Colorado and Wyoming in 1967. It has since been identified in 26 states, 3 Canadian provinces, and the Republic of Korea. In 2016, CWD also was discovered in Norwegian reindeer and moose. When CWD is recognized for the first time in a new location, it can be difficult to determine how it arrived there. Perhaps CWD was previously in this location but only recently recognized due to increased surveillance. Did CWD emerge in the new deer population through a cross-species transmission event or spontaneously in a single cervid? Was CWD brought into the area through movement of infected animals or contaminated materials? These questions are difficult to definitively answer in most situations. One way to help sort out all the possibilities is to analyze the CWD isolates for differences. CWD isolates can be categorized into different strains based on characteristics that make them unique. Model systems such as the mouse bioassay are used extensively to identify strain variation in prion diseases. Strain properties may include differences in attack rate (number of mice affected/number of mice inoculated), incubation period, western blot profile, and distribution of spongiform lesions and abnormal prion accumulation in the brain. By testing different CWD isolates under the same laboratory conditions, researchers can determine if these isolates have similar strain properties. Recent research suggests that there may be multiple strains of CWD in Norway that are distinct from the strains found in North America. The original report of CWD in a single reindeer in Norway demonstrated similar western blot profiles and immunohistochemical distribution of abnormal prion in the brain and lymph nodes when compared to samples from an elk from Canada, however, complete strain typing was not performed at that time. When additional cases of CWD were identified in 3 moose from Norway, differences in western blot profiles and features of IHC staining in the brain (no abnormal prion was present in the lymph nodes) suggested that these cases were distinct from North American CWD isolates, the reindeer isolate from Norway, and the other prion diseases of ruminants. Researchers in Europe and Canada published a study comparing isolates of CWD from Norway and North America in a laboratory model using bioassays in bank voles.1 In their study, the CWD strains from Norway were clearly different from the North American CWD isolates that they tested. This means that it’s unlikely that North American CWD spread to Norway. Instead, Norwegian CWD likely originated a different way; although, the exact origins and ecology of Norwegian CWD are still incompletely understood.