Why are specific tissue samples required for detection of chronic wasting disease prions

Authors: HARM, TYLER - Orise Fellow; Cassmann, Eric; Greenlee, Justin

Submitted to: North American Deer Farmer
Publication Type: Trade Journal
Publication Acceptance Date: 11/30/2021
Publication Date: 12/20/2021
Citation: Harm, T., Cassmann, E.D., Greenlee, J.J. 2021. Why are specific tissue samples required for detection of chronic wasting disease prions. North American Deer Farmer. p. 40-43.

Interpretive Summary:

Technical Abstract: In our previous article, we discussed the cause of chronic wasting disease (CWD) and how we and other researchers are working to understand the susceptibility of deer based on genotypes and environmental exposure to the CWD prion protein. In the current article, we will discuss why certain tissue samples are required for diagnosis of CWD and how current research is shaping the future of CWD diagnostics. Clinical signs (weight-loss, neurologic dysfunction, behavior changes, etc.) observed in live deer are not always conclusive and, therefore, laboratory assessment for abnormal/misfolded prion protein is required to make a definitive diagnosis. CWD research is intensely focused on developing reliable and sensitive testing strategies with the goal to reduce or eliminate the spread of the CWD agent. Sample collection strategies for CWD are determined by the most recent and relevant CWD research. Typical sample collection strategies include samples that are taken postmortem (after death of the animal) or antemortem (before death). Traditionally, targets for testing include the brain and spinal cord and various lymphoid tissues. This sampling strategy is based on how CWD progresses in deer exposed to abnormal prion protein. After oral or nose-to-nose exposure, the first tissues to accumulate abnormal prion protein are lymphoid tissues such as retropharyngeal lymph node, tonsil, Peyer’s patches, or mesenteric lymph nodes. Abnormal prion protein accumulates to high levels in these tissues before ascending to the brain and spinal cord through nerves. Brain and spinal cord can accumulate high levels of abnormal prion protein that can eventually lead to clinical signs. . Postmortem tissue samples are currently the most accepted method for diagnosing CWD. For diagnostic purposes, a sample of brainstem at the level of the obex and the retropharyngeal lymph node are the most relevant postmortem samples for CWD testing. These tissues are most relevant because the retropharyngeal lymph nodes accumulate high levels abnormal prion protein before the protein ascends to the brain and spinal cord. The obex is first region in the brain to demonstrate abnormal prion protein after it ascends through cranial nerves from the body . Rationale for collection of these tissues is based on research studies that have examined CWD infected tissues using a laboratory test known as immunohistochemistry. Immunohistochemistry, a special staining technique performed on microscopic tissue sections, is considered the “gold standard” test for CWD. Researchers have been able to identify abnormal prion protein in the retropharyngeal lymph node and obex samples from infected deer as early as 3 months and 6 months postexposure, respectively. To date, immunohistochemistry examination of the retropharyngeal lymph node and obex samples are the earliest and most likely to positively identify a CWD infected deer. . In contrast to deer, elk demonstrate a more variable presence of CWD prion protein in the retropharyngeal lymph node. Thus, the obex sample is the most likely to positively identify a CWD infected elk, however, both tissue samples are required for a definitive diagnosis. A concerted effort is being made to develop reliable, sensitive, and early antemortem testing strategies for CWD agent detection. Early antemortem testing could significantly aid in CWD surveillance of farm-raised deer. Using various experimental laboratory tests, researchers have been able to demonstrate CWD prions in several tissues including tonsils, rectal lymphoid tissues, blood, saliva, nasal secretions, urine, feces, and cerebrospinal fluid. However, it remains unclear as to when these tissues become consistently positive during disease progression. Outside of a research setting, immunohistochemistry examination is difficult or almost impossible to perform on most of the antemortem ti