While the full impact of chronic wasting disease (CWD) on wildlife populations, hunting culture and conservation continues to come to light, it is already clear that this disease will be part of a new normal for the foreseeable future. Tackling this monumental conservation challenge will require everyone’s help, and one of the first dilemmas hunters often face is whether to have their harvested deer tested for the disease.
There is surprisingly little information about CWD testing available outside of the scientific literature, so it should come as no surprise if this is an area of frustration or uncertainty. Hopefully, this article will help address those concerns by providing a straightforward guide to the what and how of CWD testing.
Not the Usual Testing Methods
To understand CWD testing, it helps to know how the disease works. CWD is caused by misfolded prion proteins. Deer make normal prion proteins in their bodies, but during a CWD infection, the 3-D structure of those proteins is manipulated into a new shape: a CWD prion. These misfolded proteins are extremely durable and are not broken down by normal cellular processes. Over time, CWD prions accumulate in the deer’s tissues, kill off brain cells and cause the animal’s demise.
Unlike viruses or bacteria, these misfolded proteins can’t be detected by looking for their genetic material because proteins don’t have genetic material of their own. Instead, the three tests currently approved by the U.S. Department of Agriculture use various methods to mark and detect CWD prions directly. These tests are most often run on a piece of brainstem, a pair of lymph nodes from the neck, or both. These tissues are used because they accumulate CWD prions earliest in an infection and provide the most reliable results. While both tissues are found in the head/neck, collecting these samples does not prevent an animal from being taxidermied.
The Gold Standard CWD Test: IHC
One important testing method for CWD surveillance and research is an immunohistochemical (IHC) stain. For this method, sample tissues are chemically preserved and thin slices are placed on microscope slides. Special markers and stains are applied to make any CWD prions in the tissue appear bright pink, which you can see in the photo at the top of this article. A trained professional looks at each slide under a microscope, and if they see pink staining, the animal is considered positive. This process is time consuming, but it provides very reliable results. For that reason, IHC is considered the “gold standard” for CWD testing, meaning it is the test to which all others are compared.
ELISA: Cheaper and More Efficient
The CWD test most commonly used for surveillance may be the enzyme-linked immunosorbent assay or ELISA. This test can be run on large batches and uses an automated system, making it faster and cheaper than IHC. To run an ELISA, small pieces of tissue are broken down and portioned onto clear testing plates. Each sample is treated with markers that stick to CWD prions and absorb light. More prions hold more markers and, like a tinted window, more markers mean less light can pass through the sample. The automated system measures this, and samples with a darker “tint” are considered positive. Positive cases can be confirmed with additional rounds of ELISA or IHC testing.
Western Blot
The third approved test for CWD is the western blot. For this test, samples are exposed to enzymes that break down regular proteins but leave the durable CWD prions intact. The samples are filtered through a gel-like sheet and special markers are applied to make any remaining proteins visible. If intact prion proteins are present after the treatment, the sample is considered positive. The size of the remaining proteins determines how far they can move through the gel filter, and unique sizes can indicate unique prion strains.
The western blot is a good test with some really valuable research applications, but there are occasionally misconceptions about its use for detecting a non-contagious form of the disease, referred to as atypical, sporadic or spontaneous CWD. Wildlife managers are interested in this question because a non-contagious form of CWD may not require active management to protect wildlife populations. To date, sporadic CWD has not been reported in white-tailed deer. Researchers believe it is possible because similar conditions occur in sheep and cattle and because a small number of atypical CWD cases were reported from red deer and moose in Europe alongside contagious CWD in reindeer.
Thankfully, information from these species shows us what to look for if this condition were to occur in deer. Sporadic prion diseases are relatively rare and generally occur in older animals. These cases are thought to be non-contagious because their misfolded prions occur in the brain and spinal cord but do not spread to the rest of the body, which precludes shedding. Knowing this, managers can rule out animals that have tested positive for CWD in their lymph nodes as suspects for this condition without needing a special test. Whether the western blot could be used to diagnose sporadic CWD in deer won’t be known until a first case is identified in the species and its western blot pattern documented.
The Future of CWD Testing
The exact accuracy of current CWD testing methods is surprisingly difficult to pin down in the scientific literature, but it is generally considered to be greater than 90%. Both positive and negative results are considered reliable, but animals in the earliest stages of an infection may not be detected due to the small amount of CWD prion present. This is why laboratories report their results as “not detected” instead of “negative.” The current tests are also labor intensive and rely on a small number of accredited laboratories, which slows results. New testing methods have been invented that amplify small quantities of prion to increase their detectability. These tests (abbreviated RT-QuIC and PMCA) still need tweaking but may be approved for routine use in coming years.
Researchers are also working on tests that detect chemical changes in infected deer, methods that can be deployed in the field, and potential uses of prion-sniffing dogs. While we look forward to having better options in the future, it is still important that we keep up the fight against CWD using currently available tests to inform wildlife management and avoid human exposure.