Prions and Chronic Wasting Disease

21 April 2020

During the BSE crises of the 80’s and 90’s, a rogue protein devastated British cattle farming. Almost unheard of outside the US, a similar disease called chronic wasting disease has infected deer across the country. It is lethal, highly contagious, untreatable and persists in the soil for years.

Prions

Over 4 million cows were destroyed in an attempt to contain the BSE outbreak. The disease was caused by a prion (protein infection); a misfolded version of a common protein. The healthy form is found throughout the body in people and animals but is most abundant in the central nervous system. In man, different folding errors in this protein induce different diseases. These include chronic familial insomnia, CJD (Creutzfeldt-Jacob Disease), its variant vCJD and Kuru. In animals it causes scrapie (sheep), chronic wasting disease (deer) and BSE (cattle).

The origin of most prion diseases is unknown. A few are known to originate from a mutation in the gene (PRNP) which expresses the healthy protein. In CJD this mutation occurs in about 1 in one million people a year who go on to develop the disease.1 When healthy proteins made by the PRNP gene come into contact with the misfolded protein, they are modified by the misfolded protein into more prions. The misfolding of proteins increases their tendency to stick to themselves and the prions join together to form fibrous strings called amyloids (plaques).2 It is thought that these fibres grow in length until they break, thus multiplying the number of fibres and surfaces onto which more prions can stick to. The number of fibres grows until it disrupts the cellular structure and cell-to-cell communication within the nervous system. This disruption is characterised by holes in the cellular structure causing the characteristic ‘spongy’ appearance. Amyloids are also associated with more than a dozen other incurable human diseases including Parkinson’s, Alzheimer’s and Type-2 diabetes. It is increasingly thought possible that all amyloid diseases are related to prions.3

After infection occurs there is an ‘incubation’ period before symptoms appear. Depending on the disease this period could be measured in months or decades (kuru).4 Once symptoms do appear, they progress rapidly and death always occurs. The symptoms result from the damaged neural cells and tissue, and include personality changes, memory loss, confusion, spasms and loss of coordination. There is no cure for a prion disease and treatment is only supportive. Investigational treatments in animals include attempts to prevent amyloid construction using drugs or antibodies that will bind to the prion attachment site and stop them from sticking together.5 Recently a class of chemicals called conjugated luminescent polythiophenes were discovered that bind to prions and prevent plaques in mice.6 Preventative measures such as the proper sterilisation of surgical tools and regulation of food products have rendered the acquisition of prion diseases in humans a rarity. The sterilization of prions is a challenge however due to their resilience to most forms of attack including both high and sub-zero temperatures, ionizing radiation, protein-digesting enzymes, and even chemicals such as benzene, alcohol, formaldehyde and acid. Prions also stick fast to stainless steel which is very troubling for surgeons and meat processors. Proven methods of prion sterilization include high-pressure autoclaving and immersion in hypochlorus acid (a chemical also produced by immune cells), or bleach; which can be corrosive to metal instruments. If sterilization were to fail, the long, symptomless incubation period could make isolating the source of infection very difficult.

An article by the Standard-Examiner highlights the fear over prion contamination and the difficulty in its neutralization. The article addresses concerns over the waste disposal company, Stericycle, and the perceived risk posed by one of its hospital incinerators. Stericycle had been given permission to accept prion contaminated substances, however, some kinds of prions are thought able to survive the 1000°C incineration process. Particles would likely exit through the smoke stack, disperse over several miles and potentialy cause infection by inhalation or contamination of grasses used by animals. The spread of prions in the environment has become a key concern in North America where chronic wasting disease (CWD) infects large numbers of cervids (deer) and causes environmental contamination.

Chronic Wasting Disease

Chronic wasting disease was first recognised in 1967 in Northern Colorado, and has since spread continuously to 26 US states.7 Cases have also appeared in Finland, Norway, Sweden and South Korea. It is unknown how the disease entered Europe, but entry into South Korea was gained via the importation of infected live deer. The disease has a profound effect on populations with infection rates of over 40% in unmanaged areas.8 Within captive (farmed) populations infection rates can be as high as 100%.9


Distribution of Chronic Wasting Disease in North America
Credit: Bryan Richards, USGS National Wildlife Health Center. Public domain.

The incubation period for CWD is around 18 – 24 months. Prions are infective during this long period so there is great potential for disease transmission. Symptoms of salivation and increased drinking and urination, aid transmission as infectious prions are shed in deer urine, faeces and saliva. Infection potential is compounded by the prions hard-to-kill nature; persisting in the soil for many years. Laboratory studies have also shown prion uptake by plants and grasses and distribution into leaves and stems.10 Once an infection has occurred in an area the soil becomes contaminated and it should not be repopulated by deer again. Attempts to eradicate CWD from cervid research facilities have repeatedly failed, due to the environmental contamination.11

CWD transmission to humans is not thought likely, but uncertainty remains. Lab studies have shown that disease transmission to other primates (macaques) is possible and that CWD prions can be forced to adapt to humans.12 The ongoing economic consequences of CWD will be considerable; in large part due to the impact of CWD on the hunting industry. Hunting in the USA is a $40 billion industry.13 Purchases of hunting licences drop in areas affected by CWD14 with knock-on effects in the local economy as fewer hunters purchase lodgings, equipment, food and other local services. Losses also result from the cost of increased regulation, testing and laboratory costs, a lower desire for game meat and government payouts to farms whose deer had to be culled.15

Current management strategies for wild populations include the study and surveillance of deer, and restrictions on the movements of infected live animals and carcasses. Identification of animals displaying symptoms is rare in the wild as they are picked off by predators and hunters or fall to other diseases. Hunters have thus proved useful in detection by submitting samples from kills for testing.16 Strategies for captive populations include extermination and quarantine. The UK has so far remained free of CWD. The vulnerability of British deer to CWD varies between species but infection is thought possible in all. Early detection was a key factor in the successful containment of CWD from Norway but currently no monitoring of deer occurs in the UK. Monitoring programs are currently hampered by a lack of an effective test for live animals. Though the chances are considered low, should an outbreak occur in the wild population of the UK, eradication would be unfeasible and the consequences for deer severe.17


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  • Scrapie prions have been measured to endure for 16 years in the environment.

  1. National Prion Clinic at the University College London (UCL) - Prions and Prion Disease.
  2. Wikipedia.com - Proteopathy.
  3. TheScientist.com - the Bright Side of Prions. Jan 2014.
  4. ReliasMedia.com - How Long is the Incubation Period for Prion Diseases? Aug 2006.
  5. JBC.Org (Journal of Biological Chemistry) - Human Anti-prion Antibodies Block Prion Peptide Fibril Formation and Neurotoxicity. Feb 2012.
  6. LiveScience.com - Simple Chemical Stops Prion Disease. Aug 2015.
  7. National Wildlife Health Centre - Expanding Distribution of Chronic Wasting Disease. Mar 2020.
  8. Chronic Wasting Disease Alliance - CWD the Basics.
  9. Department for Food, Environment and Rural Affairs (UK) - What is the Risk of a Cervid TSE Being Introduced from Norway into Great Britain? Qualitative Risk Assessment. Sep 2016.
  10. Chronic Wasting Disease Alliance - Overview of Chronic Wasting Disease. Jul 2019.
  11. Chronic Wasting Disease Alliance - Overview of Chronic Wasting Disease. Jul 2019.
  12. Chronic Wasting Disease Alliance - Overview of Chronic Wasting Disease. Jul 2019.
  13. Montgomery Advertiser - 'Zombie' Disease 'inevitable' in Alabama and Could Kill State's $1.8 billion Deer-hunting Industry. Dec 2019.
  14. MinnPost - Lawmakers Propose Series of Measures to Stop March of Chronic Wasting Disease in Minnesota. Feb 2019.
  15. Researchgate.net - Potential Economic Impacts of Chronic Wasting Disease in Colorado. Aug 2010.
  16. Chronic Wasting Disease Alliance - Overview of Chronic Wasting Disease. Jul 2019.
  17. Department for Food, Environment and Rural Affairs (UK) - What is the Risk of a Cervid TSE Being Introduced from Norway into Great Britain? Qualitative Risk Assessment. Sep 2016.

Image Credits

  1. Homepage photo of brown deer by Lina White from Unsplash.

  2. Brown deer photo by Riccardo Chiarini from Unsplash.

  3. Map showing the distribution of chronic wasting disease in North America by Bryan Richards from the United States Geological Health Survey National Wildlife Health Center.


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