The "gold standard" diagnostic test for CWD is the Immunohistochemistry (IHC) test performed on the obex tissue of the brain (photo left) and specific lymphoid tissues. IHC testing is a an antibody based staining which is evaluated using light microscopy. This test is both sensitive and specific. In addition, the microscopic methods used to detect positive staining also allow confirmation that the correct tissue and location within the tissue are present to detect the earliest accumulations of the prion agent. IHC is used by the National Veterinary Services Laboratories (NVSL) to confirm the presence of CWD prions in positive tissues and is an official test of the APHIS Approved State CWD herd certification program (HCP).
In December 2012, APHIS approved the Bio-RAD™ CWD enzyme-linked immunosorbent assay (ELISA) as an official test screening test on obex and lymph node tissues collected for routine CWD surveillance of farmed cervids enrolled in the Approved State CWD HCP. Any non-negative ELISA results will be confirmed by NVSL using immunohistochemistry.
The USDA Center for Veterinary Biologics (CVB) permits the use of the Bio-RAD™ CWD ELISA test in the United States and restricts its sale and use to laboratories approved by State and Federal animal health officials. CWD testing can only be performed by approved laboratories that are part of the USDA's National Animal Health Laboratory Network system.
Other diagnostic tests and technologies that allow more rapid testing of larger numbers of samples continue to be developed. A rectal associated mucosal lymphoid tissue (RAMALT) biopsy (live-animal) test has been developed by researchers and appears to hold promise for future use in certain CWD monitoring or management situations in farmed cervids. This technique utilizes the current IHC testing technologies described above. The RAMALT is not currently approved as an official test for CWD.
Information on brain (obex) and lymphoid tissue sample collection is found in the APHIS CWD Sample Collection Guidance.