North America is home to many members of the deer family. Moose, caribou, elk, whitetail and mule deer are an important part of their respective ecosystems and at least one of these species is found in almost every habitable square mile of the continent.
Today, their very existence is threatened by Chronic Wasting Disease. CWD is highly transmissible and is always fatal. It was originally identified in 1967 in a captive research facility near Ft. Collins, CO, managed by Colorado State University and the Colorado Division of Wildlife. The disease has now spread to at least half of the states in the continental US, two Canadian provinces, Korea and Norway. CWD is unique in many ways. It is caused when a particular protein normally found in animals, called a prion, becomes misshaped. When this misshaped protein enters a host it attaches to and converts normal proteins to this aberrant shape, forming masses which the body cannot break down. These protein masses accumulate in the brain of the infected animal and interrupt normal neurological operations, a process which is very similar to Alzheimer’s disease in people. The animals become unable to carry out such basic functions such as proper ingestion and digestion. Their condition declines and they become very susceptible to other diseases, such as pneumonia, just as we see in cases of Alzheimer’s disease. The result is always death.
Perhaps the most dangerous aspects of CWD are is its high transmissibility and the many modes of transmission. The rogue prion is extremely rugged, lasting years in the environment. CWD may be passed vertically, that is, mother to offspring during pregnancy or soon after birth. It passes horizontally through animal to animal contact and through contact with contaminated environments. Infected animals pass the infectious agent to the environment through urine, feces, and saliva. When they die they become huge reservoirs of infection as their carcasses decompose. Studies have shown that the prion will tie up with clays and actually become more infectious. Plants may even take up the prions. Mammalian predators and scavengers such as coyotes, foxes, wolves, coons, weasels, lions, and bobcats, and birds such as crows, magpies, condors and vultures feed on the carcasses. Studies have proven the infectious protein is unaffected by digestive systems and is passed back into the environment through feces. Coyotes and wolves have been shown to transport the disease in excess of 50 miles; crows can transport the disease hundreds of miles. No part of the continent is safe from contamination.
Because the disease is caused by a protein, a non living structure, conventional disease treatments are not applicable. Proteins are not living organisms, so they cannot be killed. Antibiotics which are effective against bacteria simply do not work against the rogue proteins. Vaccines work against viruses by causing an animal to build natural resistant to a disease. Prion vaccines have not been successfully developed despite years of work at great expense.
Proteins are made up of a chain of amino acids. The prion protein is a chain of 256 amino acids in a specific sequence. Each species of the deer family has this same basic copy of the prion. The misshaped CWD infectious prion easily converts this standard copy to the misshaped form causing the disease. Of significance, however, is each species has some individuals with naturally occurring genetic mutations resulting in a slightly different sequence of amino acids making up their prions. These prions with slightly different configurations in many cases resist conversion to the misshaped CWD infectious prion. Animals with these mutations usually represent a very small percentage of the general population. The promotion of these resistant animals may be the best way to manage CWD.
Past efforts to eliminate or contain CWD for the most part have been fruitless. A scorched earth policy was implemented for years regarding captive animals. If CWD was discovered, entire herds were destroyed and the carcasses burned and buried or put through a chemical digester. The property’s enclosure then had to remain intact with no resident cervids for a period of 5 years. Herds that had sold animals to these properties, known as “trace back” herds, were aggressively examined to ensure they did not contain CWD positive animals. Animals that left the positive herd (“trace forwards”) were euthanized and tested. The captive industry developed a very aggressive testing and monitoring program. Brain stems and lymph nodes were tested from all animals that died from any cause. A captive herd must have a 5 years surveillance program with an extremely high percentage of animals testing negative before a facility is certified so that animals can be moved to other facilities.
These efforts have enjoyed limited success. Facilities which have been depopulated, left empty for 5 years and then repopulated have so far been clean from the disease. A couple of captive herds which had one CWD positive were quarantined, aggressively managed and tested. The animal which was positive had been identified by the post mortem test and was likely removed from the herd when it was in the early stages of CWD. These two facilities went through a five-year quarantine without a new case of CWD and are presumed to be CWD free. However, in every other case once CWD was found, the herd quarantined and monitored, the infection rate geometrically increased. In one case the infection rate escalated to 80 percent before the herd was euthanized, less than three years after the initial discovery of CWD on the premises.
Efforts to eliminate or even contain CWD in free range situations have been totally unsuccessful except in perhaps one case. A very small number of deer were found in an area in New York. CWD was possibly transferred by a deer head from an infected animal which had been sent to a taxidermist. No new positive animals have been identified in the ten years since the initial report, so it is possible that the disease did not establish a foothold in the area. In all other cases once CWD has been found in an area, new cases increase geometrically as time passes. Years ago wildlife managers attempted to stop CWD by mass culling. Despite killing thousands animals at the expense of tens of millions of dollars, CWD persisted and expanded.
In order to get a handle on CWD we must develop new management programs using the best and latest science, monitor the results and be prepared to adapt as necessary. While CWD has been known to have existed for approximately 50 years there is still a tremendous amount of knowledge which is lacking. In addition, there have been a great many untruths circulated regarding CWD that must be dispelled. Some individuals are irresponsibly spreading false information which in the best case is only a theory without foundation and in most cases completely counter to scientific evidence. Most of the time there is an agenda behind this effort.
A common bogus position is that CWD has been around for thousands of years, it hasn’t hurt anything yet and it is nothing to worry about. The truth is epidemiologists are very confident about when and where the disease first started and how it is spreading geographically, as well as the increase of prevalence. Many highly respected researchers have published papers concluding that cervids (members of the deer family) cannot coexist with CWD because of the characteristics of the disease, concluding that CWD will eventually prevail and kill all the cervids. Lately, some researchers have predicted elk population trends while considering the impact of resistant animals. The general consensus is that the number of animals will decline dramatically as the infection rate increases, because the majority of animals in every herd are susceptible. The population will eventually crash and then the numbers will rebuild as the resistant elk propagate. This cycle, however, will take many decades.
Some individuals protest this concept based on an opinion that something must be wrong with the resistant genotype, because they make up such a small portion of the total population. Resistant animals have been identified and observed for decades with no known deficiencies. Unfortunately, it is hard to prove nothing is wrong with something, but it is easy to identify a problem if it exists. Question: if the genetically resistant animals have a problem, what is it?
Progress going forward will need to be made based on the latest and best science. Several characteristics contribute to the challenge:
CWD is always fatal;
It is highly transmissible;
There are no treatments;
It is spreading geographically;
The prevalence is increasing;
It is transmitted from pregnant females to offspring, and through animal and environmental contact;
The environment becomes contaminated by saliva, feces, urine and carcasses from infected animals;
The infectious prion ties up with certain types of clays, becoming more infectious, and persists for years;
It is spread by other animals which feed on infected carcasses;
Plants may take up and hold the infectious prion;
Past efforts to contain or eradicate CWD, including culling, have been largely ineffective.
Happily, there are a few favorable phenomena as well:
There is a small percentage of animals in each species which show natural resistance;
New live animal tests for some species have been developed which appear to be reliable.
Although the spread of CWD has been inexorable, the process has been slow.
Going forward, since there are no cures, the focus must be the management of CWD. It is probable that CWD will eventually infect every part of North America which will support cervids. Wildlife managers, animal health officials and private herd owners must work together. The promotion and study of resistant animals must receive priority. Perhaps the best testing grounds are captive facilities which have positive cases. Every situation will be unique and different strategies will need to be developed. Some of the factors that need to be considered in order to form the best management strategy are:
How long has CWD been known to exist in the herd?
Which individual or distinct species are involved?
What is the soil type and other enclosure factors?
What is the prevalence in the enclosure and outside the enclosure?
What is the animal density?
What is the genetic frequency of resistant animals in the herd?
What is the age profile of the herd?
What are the management practices (i.e. feeding, ability to handle animals, ability to observe animals, breeding programs)?
Is live animal testing and culling of infected animals possible?
How can resistant animals be introduced and promoted?
What are the economic considerations?
Once a strategy is implemented, constant evaluation of results will be critical. Hopefully answers will be found to such questions as is there a point where the environment become so contaminated that even the resistant animals succumb? If so, do cervids need to be removed or left to die off and the environment left fallow for a period of time to let the infection dissipate before resistant animals can then repopulate? Is there a minimum dosage which must be reached before the different genotypes become infected and is it a one-time exposure or a cumulative exposure? Can CWD be managed by culling animals of the different genotypes by age, before they contract the disease or begin shedding in the environment?
The time has come to admit that CWD is a very real threat to all members of the deer family and will likely continue its march across the entire continent. The focus must now be on how to manage CWD. The latest science indicates that the best and perhaps only option is to, if possible, identify and cull positive animals, and, most importantly, promote resistant animals. Delays in implementing sound strategies to manage CWD must not be considered in a linear context, because the disease is increasing in prevalence and geographically at a geometric rate. The sooner wildlife managers, animal health officials and the private sector work together the sooner we can make progress on managing CWD.
Copyright 2017 – Barry Dyar