Southern Veterinary Services | Deer & Wildlife Stories

Chronic Wasting Disease of Cervids: Putting the Agent in Perspective

R.Greg Stewart DVM MS PhD
Southern Veterinary Services, Inc.

As ranchers and stockmen, we are familiar with the terms mad cow disease and scrapie. These diseases are caused by several members of a rarely occurring and unique group of agents collectively called transmissible spongiform encephalopathies(TSEs). TSEs can cause pathology in a number of warm blooded species. To understand the origins of these agents it is imperative that we review what they are and how they come about. All affected species have in their nervous tissue a normal protein that has a particular conformation or shape known as an alpha helix or coil. These proteins have specific functions and are required by the animal to live and move about in normal ways.

If the conformation of the protein is changed to another shape called a beta pleated sheet then normal functions are impaired. These sheets accumulate in the nervous tissue and when they pile up neurodegenerative processes ensue.

How does this change in shape and function occur? TSEs are acquired in three confirmed ways. Classical or infectious form-this moves from one animal to another by urine, saliva, feces.

This form appears as clusters of animals. Spontaneous (sporadic) or atypical form- this does not move from animal to animal and is the result of a spontaneous mutation.

Inherited or Germline form-a mutation occurs and is passed from generation to another. The onset of clinical signs is usually a very long time after the acquisition of the agent. Most sporadic cases are in elderly animals.


Spongiform Encephalopathy (BSE) or Mad Cow Disease is a disease that is acquired through consumption of infected neural tissues from the inclusion of ruminant source offal in feeds. This practice is now illegal in North America.

A second form of the presentation is called atypical and results from a spontaneous mutation that occurs in elderly animals. The four cases on record in the USA are all classified as atypical or sporadic.

There are several forms.

Gerstmann-Straussler-Scheinker Syndrome (GSS) – always inherited or germline transmission.

Familial insomnia-almost always inherited or germline transmission. A case of sporadic noted.

Creutzfeldt – Jakob disease (CJD) – Sporadic caused 87%, 8%germline, 5% man caused or iatrogenic. Humans who care for CJD patients are no more at risk for acquiring the disease than the general population unless they could by some rare circumstance come in contact with infected neural tissue.

Variant Creutzfeldt-Jakob Disease (vCJD)-acquired through human consumption of infected tissues from BSE infected cattle or from blood transfusions from vCJD infected persons. The elimination of ruminant source meat and bone meal from cattle diets has halted the creation of new cases.

Sheep and Goats
Scrapie-primarily classical or infectious forms. Now an atypical or sporadic form is known as Norway 98 or Nor 98.

Likely it was the classical scrapie agent in the offal fed to cattle in Europe that initiated the BSE outbreak that eventually led to some 198 cases of human vCJD over three decades. Scrapie is not known as an agent that infects cattle under natural conditions. It does not affect or infect humans.

Consumption of scrapie positive sheep meat likely has occurred for two hundred years without consequence.

Chronic Wasting Disease (CWD) is primarily the classical or infectious form. One case of sporadic or atypical has been noted in North America for which we have good information. There are likely others. This TSE, chronic wasting disease, generally appears in clusters. It is a rare disease that is much more rare than deer rabies. It does not affect other livestock or nonCervids. It does not affect humans. This disease first originated in a research facility in Colorado and has radiated out from there geographically. Research institutions likely have moved the agent between states. Trophy movement and carcass movement may also be contributing to movement between states. State government movements to other states in restocking programs have gone on long before adequate testing procedures came into being and since.

Free ranging deer do not recognize state lines and have walked across boundaries from one state to another.

All farmed Cervids that are moved from one state to another require the producer to enroll in a federal monitoring and herd certification program and permits are issued for each movement.

The agent is present in some twenty four states. Some states like Wyoming have elected to do nothing. Their mule deer populations and harvest rates are not being influenced by CWD and haven’t been for at least ten years. Wisconsin on the other hand elected to spend 82.9% of their Department of Natural Resources budget to try to eradicate the disease in a specific area and between DNR and three other agencies in the state they spent $32.3 million in their effort.

At the end of the “project” there were more deer in the killing fields (the eradication attempt zone) than at the start and the incidence of CWD was increased. Not a very effective use of taxpayer funds! The worst part of the situation was that after the gun smoke cleared from the killing fields, an eleven (11%) percent decline in the number of persons buying hunting licenses was documented. So CWD hysteria is effectively pushing people away from hunting. This is perhaps the greatest travesty in all of this.

The first rule of the medical practitioner is “DO NO HARM!” Secondly, we believe that all disease control programs should be RISK BASED. What happens when a state like Texas or Arkansas finds CWD in the wild? The typical response is “go in there and cordon off the area and kill some animals”. “Kill them and harvest the tissues from 300 subjects”, seems to be the paradigm nowadays. Look at the incidence of CWD positives. Then what? Usually it goes like this: wait a year and go kill another 300 and test the incidence. Then what? Well, you can take it from here. The documentation of a CWD positive animal in the wild is handled much differently
than one found in a farmed Cervid location. This usually involves total depopulation.

Here are some aspects of the discussion to ponder:

The incidence of actual disease is far far less than the incidence of subjects that test positive by the methodology that requires that we kill animals in order to harvest the target tissues. The presence of clinical signs in Cervids is extremely uncommon unless the animal is protected in isolation or research pens until an advanced age.

Biopsy methodology is now available to test Cervids via ante mortem tests rather than by killing the subject.

CWD has violated many state borders in wild populations with no influence from deer farmers.

The primary mortality associated with CWD comes at the hands of regulators. 172,000 deer in Wisconsin alone and many more across the nation to satisfy the requirements of regulation.

This is not an exotic disease if it can walk across the NM-TX border and is now present in twenty five states.

It is not avian influenza. It is not foot and mouth disease. It is not exotic Newcastle.

It is not anthrax. It does not affect any other livestock species. It does not affect man either by contact or by eating positive meat. If it did so then every taxidermist, deer processor, and deer biologist etc. would show an increased incidence of neurodegenerative disease above that of the general population.

CWD hysteria has injured hunting participation. This seems curious because a lot of DNR programs are funded by hunter access fees.

Overreaction by regulators has harmed commerce and the economy in many deer farming states. Here’s a partial list: transportation and travel, hospitality and motels, restaurants, hunting licenses, fencing companies, outfitter fees, sporting goods, real estate values and real estate commissions, feed companies, agricultural entities and farmers, deer farmers and the value of deer genetics/semen/embryos just to mention a few.

Perhaps it time to come to our senses.

My priorities would be:
1) Property rights.
2) Hunting as a tradition and lifestyle choice.
3) Management of wildlife and the environment with science based decisions.
4) Affirmation of second amendment rights.
5) Preserving rural lifestyle.
6) Minimizing government intrusion and maximizing Liberty.
7) Public education using objective facts in place of subjective opinion and computer modeling.

There are many other factors that have a far greater influence on deer populations than CWD. That list would include hunting (as you can see here at, predators, epizootic hemorrhagic disease (EHD), blue tongue virus, DVCs (deer vehicle collisions), birds of prey, drought, and flood, to name a few.

It’s high time we embrace the science based facts of CWD. We should discontinue the exorbitant spending, the unnecessary loss of valuable animals and we must remove excessive regulatory burden.

CWD symposium

Beware of French Models You Meet on the Internet: Myths of Prion Infection Transmission

A Short Review: Current awareness on the transmission of prion infection through semen and embryos in ruminants with emphasis on Chronic Wasting Disease (CWD).

R Greg Stewart, DVM MS PhD, Southern Veterinary Services, Inc,

Much rumor and innuendo are circling about the recent Chronic Wasting Disease (CWD) positives on a ranch in Mountain Home Texas. Because a pair of twin brothers and another pen mate from the same sire tested CWD positive, a “giant leap in science” has been made to assume that since these bucks were the product of artificial insemination with frozen semen that the semen was the mode of transmission. This assumption is, by all scientific evidence, false.

Let me address what is known scientifically and what we have learned through recent epidemiology gained when other herds have been depopulated. First, the USDA APHIS Program Standards on Chronic Wasting Disease completed in May 2014 state in Section 2.6 and on page 17 of the document that: “At this time there is no scientific evidence that germplasm (embryos or semen) may transmit the disease”. Second, the most studied transmissible spongiform encephalopathies (TSE) in animals are scrapie in sheep/goats and bovine spongiform encephalopathy (BSE) or Mad Cow in cattle. Let’s see what is scientifically known in these species.

In the book Prions: A Challenge for Science Medicine and the Public Health System (edited by F Holger et al, Second Edition, 2004), several detailed studies were reviewed to address the issue of transmission through germplasm (semen or embryos). Perhaps the best one was detailed on page 161 of the book. I will summarize it here. Semen from 13 bulls (8 of which were BSE positive) was used to breed 167 cows. Some of the cows carried their fetuses to term, and others were embryo flushed and the embryos either transferred to recipient cows or used in further lab studies. Some of the bred cows used in these experiments as negative controls were imported from New Zealand, a country  known to be free of BSE. Here’s what they learned:

 No BSE transmission was found in any of the embryos that were ground up and injected into susceptible mice brains. This is an abnormal route of infection but is very sensitive.
 No BSE was found in any offspring from cows bred to have their own calves.
 No BSE was found in any offspring from recipient cows carrying embryos bred with positive semen.
 Embryos from positive cows were put into negative recipients. The animals were held for seven years. No BSE was found in this group.

The summary made by the editors states that there is no evidence of transmission by sire either by natural matings or through semen.

Furthermore, in the book Emerging and Exotic Diseases of Animals (edited by AR Spickler, 2010), “The presence of BSE has not been scientifically documented in milk, semen, or embryos.” And in the book, Bovine Medicine: Diseases and Husbandry of Cattle, (edited by AH Andrews et al, 2004), “No transmission of BSE has been documented either with contaminated semen or with semen from contaminated Bulls.”

In the sheep and goat analogue of this disease, scrapie, much is known. In a paper published in Reproduction (2008 March 135(3):415-418, “Semen from scrapie infected Rams does not transmit prion infection to transgenic mice.” Sarradin P, Melo S, Barc C, Lecomte C, et al.) “Scrapie is the most common transmissible TSE in livestock…Artificial insemination is widely used in modern farming, and as large amounts of prion protein have been found in sheep sperm membrane, epididymal fluid, and seminal plasma…horizontal transmission by this route has been hypothesized since no clear information has been obtained on possible sexual transmission of TSE. We therefore tested the contamination levels of semen from scrapie-infected rams at different stages of incubation, including the clinical phase of the disease. We report here that under our experimental conditions, ram semen did not transmit infectivity to scrapie-susceptible transgenic mice over expressing…the sheep prion (PRNP) gene. These results suggest that artificial insemination and natural mating have a very low or negligible potential for the transmission of scrapie in sheep flocks.”

In a paper published in Theriogenology (2008 Sept 15;70(5):725-745, “Risks of transmitting ruminant spongiform encephalopathies (prion diseases) by semen and embryo transfer techniques.” Wrathall AE, Holyoak GR, Parsonson IM, and Simmons HA) “Research on TSE transmission via reproductive technologies in deer has not yet been done, but information on the pathogenesis and epidemiology of chronic wasting disease (CWD) of deer, and on the transmission risks in other species, provides optimism that transmission of CWD via semen or embryos of deer is unlikely.”

So then, what has been learned from the recent quarantines and depopulations?
 In the Iowa-Braake depopulations, three bucks that were produced by artificial insemination tested positive. One of these was a sire named “withheld by author” that tested negative at death (RGS unpublished personal communication). The source of the outbreak in Iowa was never confirmed. The facility had been CWD testing since 2002.

 In an investigation in Wisconsin, three bucks were tested positive for CWD. One was produced by artificial insemination and two by live cover. No sires were the same. No mothers were the same (RGS unpublished personal communication). The source of the outbreak in Wisconsin was never confirmed. The facility had been testing since 2002.

Because we find twin brothers and another half brother positive in the same pen from the same sire is NOT evidence of semen transmission. Frequently when our practice serves the industry with reproductive techniques, it is common to artificially breed many does to the same semen in the same harem or pen. This makes semen utilization and the process more efficient. The transmission of the prion agents of TSEs via germplasm, either semen or embryos is a myth. Something much repeated with no evidence. There are however valid data on horizontal transmission.

Horizontal transmission with feces, saliva, and urine are researched and are proven modes of experimental transmission. Semen and embryo transmission are NOT proven modes of transmission. Just because he told you he was a French model and you meet him on the Internet does not mean it is so. Beware of Dr. Google and others who give free advice and counsel on the Internet and those who repeat it.

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