Chronic Wasting Disease
Thank you for taking an interest in learning about Chronic Wasting Disease (CWD as it will be referred to from here on out). The goal with this information is to systematically break down CWD as a whole, and to bring awareness to this devastating illness. There will be five points I will focus on with this educational piece:
1.) With my background of working in the medical field, I will break down CWD as it pertains to the disease, and explain how the anatomy and pathophysiology of CWD works.
2.) I am going to talk about the different known strains in which CWD is a part of, and the host fauna in which they affect.
3.) I will explain how the disease affects the host fauna. I will also cover the life cycle of the disease and the signs/symptoms of the illness.
4.) I will go over the growing concern of CWD as a health hazard to humans.
5.) Finally, I will focus on measures being done to combat this disease and also touch on methods that may not have been considered yet as a possible positive control measure.
1.) CWD is a medical condition that is in the family of TSE’s (Transmissible Spongiform Encephalopathy) which is a form of prion disease. Prion is derived from “proteinaceous infectious particle.” It is important to note here, that a prion forms when a regular protein particle folds over and becomes structurally abnormal. Although it is not known scientifically why or how this happens, it is known once this takes place it blocks regular function of cells in which the protein molecule is inside. Pathogens are estimated to have over one trillion individual species, and it is also estimated that less then 99.999% have been discovered. Like bacteria, viruses, fungi, and protozoa, prions are their own species of pathogen. This is important to understand because unlike the aforementioned they do not contain any genetic material. Unlike the other well-known pathogens, prions do not require DNA or RNA to form or survive, and therefore are able to spread, infect, and EVOLVE at its leisure. Now I typed evolve that way for a specific reason which will be directly important to this publication later on as you will hopefully see.
TSE’s gain their names because of the condition the brain is left in postmortem. TSE’s create countless microscopic lesions in the brain’s tissue, which closely resembles the holes of a sponge. Depending on the area of the brain being infected will determine the symptoms the host will experience. Although TSE’s affect the entire body of the host these small sponge-like holes are only found in brain tissue. This has only recently been discovered by scientists, and it is now known why these lesions create this condition in the tissue of the brain. The protein in question here is known as “PrP”, which directly causes the prion disease in the host. The disease happens when a misfolded version of PrP enters the body and then into a cell which binds with a normal PrP. This is a crucial find as they have discovered once the binding has taken place the normal PrP is forced to convert into misfolding and becomes an abnormal PrP. This is essentially how the disease spreads in the body, literally one cell molecule at a time. This directly correlates to the reason why TSE’s have a rather short onset (time from symptoms appearing to the conclusion of the disease) but a long duration (time from the beginning of the disease to the conclusion of the disease), relatively speaking. Almost all infected hosts do not show signs or symptoms for months to years after the infection has started, however, once symptoms appear mortality is not far behind (we will cover this topic more in the section on the lifecycle of the disease). Finally, there are twenty-one amino acids in the body that bind together to create the thousands of different proteins in the body. Just like origami, they bind and then twist into a certain shape that make up that particular protein. However, when the PrP protein folds into an unrecognizable shape it becomes abnormal in the body and in turn becomes toxic to the cell creating the disease.
2.) There are currently 12 known forms of TSE in the family. In this section I am going to cover all forms and how they correlate to the type of fauna they infect. Out of the 12 known species of TSE’s, currently there are 6 found in non-human mammals and there are 6 found in humans. We will start by going over the strains found in other mammals and then finish with the ones that have been discovered to directly infect humans. The first 6 TSE’s are: Scrapie (Sc), Transmissible mink encephalopathy (TME), Chronic wasting disease (CWD), Bovine spongiform encephalopathy (BSE), Feline spongiform encephalopathy (FSE), and Exotic ungulate encephalopathy (EUE). On the human side of the disease there is: Kuru (Kuru), Creutzfeldt-Jakob disease (CJD), Variant Creutzfeldt-Jakob disease (vCJD), Gerstmann-Sträussler-Scheinker syndrome (GSS), Fatal familial insomnia (FFI), and Variably protease-sensitive prionopathy (VPSPr). It is important to note here that the different strains mentioned above all seem to have a certain species they attack and although they seem to each affect one, they do pose a serious risk to affect all the species in the family whether captive or wild.
The subgroup TSE Scrapie (PrPSc), is a prion disease that infects sheep and goats. Although it was discovered almost three hundred years ago, very little is known about this prion. It derives its name from the relentless scratching domestic sheep and goats demonstrate when contracting this illness. In fact, the animal seems to experience an excruciating and unsatisfiable itch to the point they continually rub against objects till they literally rub their own wool off their bodies. In the livestock industry due to the extreme health risks facing the public, when one of these strains is found in and amongst the herds all animals are killed as a precaution to stop the spread of the disease. Unfortunately, because a diagnoses can not be confirmed until after an autopsy has been performed, many animals are found to not have the illness. These strategies have been implemented here in the U.S. and many other places abroad after the mad-cow disease epidemic. Very recently it has been theorized by scientists that in wilderness areas where domestic sheep and goats are able to graze freely, these animals may pose a serious risk of spreading the pathogen to wild bighorn sheep and mountain goats.
The subgroup TSE Transmissible mink encephalopathy (PrPTME), is a very rare form of the prion disease that affects farm raised minks. It was first discovered in Wisconsin in the mid 1940’s. The last major outbreak was also witnessed in Wisconsin in the mid 1980’s. However, there have now been confirmed cases in Canada, Finland, Germany, and the former Soviet Union. Although this illness has never been discovered in a wild specimen, it is extremely important to bring to light the fact that experimental infections have been conducted and successfully established in raccoons, striped skunks, ferrets, American pine martens, and beech martens. I feel it would be safe to conclude after seeing how many different members of the Mustelidae family have successfully been infected, most if not all members of the family could be susceptible to the disease (otters, wolverines, and fishers) to name a few more.
The subgroup TSE Chronic wasting disease (PrPCWD), is a form of prion disease that directly affects the Cervidae family (deer). It has received the nickname “zombie deer disease.” This is a unique member of the TSE family as it is the reason this publication is being constructed. CWD has been discovered in both farm-raised and wild populations of deer. Many members in the family have been confirmed to carry the pathogen: white-tailed deer, mule deer, red deer, sitka deer, rocky mountain elk, shiras moose, and caribou have all been found to be susceptible to the pathogen. Spread of the disease has been found in the United States, Canada, Norway, and South Korea. Recent research shows a large and rapid spread of the disease here in North America, with 24 states and 2 provinces of Canada confirming cases.
The subgroup TSE Bovine spongiform encephalopathy (PrPBSE), is a prion disease that affects cattle. This is the pathogen that has received the nickname “mad cow disease.” This disease gained international fame when it was first discovered in Great Britain in 1986. Between 1986 to 2001 156 people had died after consuming tainted bovine meat. Now although they did not die from BSE, it was directly linked to variant Creutzfeldt-Jakob disease which is a human form of TSE. We will cover this form of TSE when we go over vCJD. The total number of fatal cases of vCJD that is directly linked to consuming beef infected with BSE is now at 231 at the end of 2018. Because of this discovery and the ability for one form of TSE to initiate another, scientist have cautioned to the consumption of suspected CWD infected meat, as the possibility for the disease to initiate vCJD is at a high probability.
The subgroup TSE Feline spongiform encephalopathy (PrPFSE), is a rare form of a prion disease that is found in both captive and domestic felines. This form has not been found in a wild specimen, however, scientist do believe that the possibility is likely as the pathogen can easily pass to wild individuals in a captive situation. Researchers note that infected individuals experience ataxia (loss of voluntary coordination) for approximately 8 weeks before the disease became fatal. It is important to note here that FSE was first discovered in Great Britain in 1990 and is directly correlated to BSE.
The subgroup TSE Exotic ungulate encephalopathy (PrPEUE), is by far the rarest form of TSE’s found in non-human mammals. In fact, there is only one reported case of an outbreak in history. It took place at a zoo located in Great Britain during the 1990’s and is directly linked to the BSE outbreak. Research showed that consumption of meat and bone meal made up of cattle was ingested by a number of residential animals. The infected animals included all listed species: greater kudu, nyala, gemsbok, common eland, Arabian and Scimitar Oryx, an Ankole-Watusi cow, and an American bison. It is important to note the vast array of species that were directly impacted by BSE. Although many of these animals are not closely related in terms of an evolutionary standpoint, all being of an ungulant or ruminant descent made them all susceptible to the pathogen.
The subgroup TSE Kuru (PrPKuru), is our first human form of the prion disease. Kuru is an extremely rare form of the pathogen that has only been found in the south Pacific on the island of Papua New Guinea. It was very common among the indigenous tribe of the Fore people for many years, obviously with the lack of modern scientific study the disease was thought to be that of spirits possessing members of the tribe. They attempted to cure the infected by feeding the sick, pork and casuarinas bark. It was not until 1957 that the theory of cannibalism as the cause of the TSE was suggested. By 1960 Australia banned the practice of cannibalism in that country and the number of Kuru cases steadily decreased over time. It is estimated that roughly 2,700 people contracted and died from the disease between 1957 to 2004. The last fatality from Kuru is believed to be in 2009.
The subgroup TSE Creutzfeldt-Jakob disease (PrPsCJD), is another form of TSE that infects humans. This disease is found worldwide at an average rate of 1 confirmed case per 1,000,000 people, making it the most common form of the human strains. This disease was first investigated by German neurologist, Hans Gerhard Creutzfeldt in which the subgroup is named for. Later more research was created by Alfons Maria Jakob, in which the name was expanded. This form of the disease is the most common of the human forms of TSE’s and in turn have led to recent discoveries of variant forms of this disease. This form tends to not show symptoms until approximately the age of 60 in the infected person. It is strongly believed most people are infected by medical surgeries where infected tissue is surgically implanted into the new host.
The subgroup TSE Variant Creutzfeldt-Jakob disease (PrPvCJD), also known as new variant Creutzfeldt-Jakob disease, is a relatively new discovered form of the prion disease (PrPsCJD). This pathogen rose to fame when it was discovered that consuming beef infected with BSE would raise the chances of forming PrPvCJD exponentially. Since its discovery there have been less than 250 confirmed cases worldwide, and most of these were located in Great Britain during the “mad cow disease” outbreak during the 1990’s and early 2000’s. However, it is important to note here there has been a discovery that unlike sporadic CJD, vCJD does show prions in the bloodstream of the host and it is transmittable through blood transfusions. Due to this finding the UK has put a ban on blood transfusions from anybody who ha