Mink, A Species Susceptible to Spongiform Encephalopathy

Mink are among the many species that fall victim to transmissible spongiform encephalopathy--a malady known in popular culture as mad cow disease. In mink, the name for this affliction is transmissible mink encephalopathy. Theories about these transmissible encephalopathies abound, but to date researchers have at least as many questions as answers.

In August of 2018 the Florida Department of Agriculture announced that a cow had been diagnosed with BSE, Bovine Spongiform Encephalopathy. This was the sixth cow, over a period of years, to be diagnosed with mad cow disease in the United States. Many more cases have been diagnosed in other parts of the world...one in the UK as recently as October of 2018.

News reports about the Florida incident varied. One article assured readers that the form of BSE detected in this cow was "not contagious". ¹ Other reports explained that the cow suffered from a form of the disease that was different from "classic" mad cow disease. ² What every report (that I read) emphasized was that the Florida cow in no way presented a danger to humans.

While it is true that the dead cow in Florida no longer represented a danger to humans, it's also true that its existence throws up red flags, because of the uncertainty surrounding spongiform encephalopathies. The lack of certainty about BSE makes every case a major news event for the agricultural industry, the health community, and research scientists. Major because the disease does cross species, and it does infect humans, sometimes in surprising ways. Fear of BSE getting into the food chain can virtually destroy the beef industry.

Various types of spongiform encephalopathies have been found in domestic animals and in wild animals. Whenever the disease strikes the course is unremittingly cruel and inevitably fatal. Below is a chart that describes how the disease may typically progress in humans. Each one of the arrows represents a set of symptoms. The size of the arrow indicates the frequency of that symptom in a cross section of patients. However, as the chart demonstrates, everyone afflicted with the disease ultimately ends up in the same dismal condition. And inevitably, the disease kills.

TSEs, Transmissible Spongiform Encephalopathies

The U. S. National Institutes of Health has a website that describes the group of diseases characterized as Transmissible Spongiform Encephalopathies. BSE is one of those. A disease in sheep and goats, called Scrapie, is another. Scrapie has been observed in animals since at least 1732.

Scrapie

The picture shows a sheep that is presenting with symptoms of Scrapie. This link takes you to a video of an animal severely affected by the disease. The video was made by a BMC veterinary research team: Simmons H, Simmons M, Spencer Y, Chaplin M, Povey G, Davis A, Ortiz-Pelaez A, Hunter N, Matthews D, and Wrathall A.

TSE: Transmissible Spongiform Encephalopathy in Humans

The most well-known form of transmissible spongiform encephalopathy in humans is Creutzfeldt-Jakob disease, CJD. The route of transmission for this disease varies. Some cases have resulted from the administration of growth hormone to children, because the material to make the drug was harvested from infected cadavers. Other routes of transmission include corneal transplants and contaminated surgical instruments. It wasn't until 1996, that the first suspected case of food-borne transmission of BSE (in humans) was reported.

Whatever species TSE affects, all manifestations of the disease have one thing in common: the presence of prions.

Human Brain Tissue Showing Damage From CJD (Variant)

This image shows spongiotic changes in the cortex, and loss of neurons. ³

What Are Prions?

An excellent article describing prions in depth may be found here. To summarize what that article says: prions aren't viruses or bacteria. They have no DNA or RNA. They're just bits of protein that become misshapen (they fold) and they influence other proteins to become misshapen. The infectious character of prions is demonstrated in the mouse model below.

The path of infection can be traced by following the red dots. This picture shows four nerve cells. The prion infection travels along neurites. In this way, a prion infection spreads throughout the body. By end-stage disease, the infectious proteins may be distributed systemically. They may be found in the eyes, the gut, and even the tongue.

It is the apparent infectious nature of prions that has led researchers to conclude prions cause TSEs. Most scientists who work with TSEs assume this to be the case. However, there is a body of research that suggests science should consider another culprit. A candidate for that role is spiroplasma.

Spiroplasmas

If you click on the picture, you can see spiroplasmas swimming. This variety of spiroplasma, S. eriocheiris, is fatal to the Chinese Mitten crab.

How Are Animals Exposed to Spiroplasmas?

This microorganism is found in many places. In insects, it can have a symbiotic relationship (help them), or it can be pathological (make them sick). One insect that carries spiroplasma as a symbiont is the leaf-cutting ant. Spiroplasmas are part of the gut biome in this ant. Spiroplasmas may also be found in termites and ticks. One 2012 study, conducted in the Ardennes, found that of 267 tick species collected, 76% carried spiroplasmas as symbionts. So, it's safe to say there are abundant sources of spiroplasmas in nature.

The case for insects as vectors of spiroplasmas was described in a journal called, Annals of Agricultural and Environmental Medicine. This article asks the question, is spiroplasma an emerging arthropod-borne disease?

Leaf-Cutting Ant

It's not surprising, given the widespread availability of spiroplasmas, that ruminants may be contaminated merely through contact with soil. One variety of spongiform encephalopathies found among wild ruminants is CWD, chronic wasting disease. The US CDC classifies this disease as a prion disease and suggests that such animals be kept out of the human food chain, although "To date, there have been no reported cases of CWD infection in people." ³

An article in the Journal of Neuropathology and Experimental Neurology explains how ingestion of spiroplasmas by ruminants may lead to the development of spongiform encephalopathy. Spiroplasmas bind to clay and create a biofilm, which is virtually impervious to chemical and physical assault. The microorganism has the ability to pass through the gut of the animal and migrate to tissue in the nervous system. Analysis of these clay-bound spiroplasmas reveals that they are identical to spiroplasmas found in the tissue of infected animals.

A Deer Suffering from Chronic Wasting Disease

The case for spiroplasmas as the single cause of spongiform encephalopathies has not yet been made, although there are researchers who feel they may be on the correct path by exploring this line of inquiry. In one study, Novel Spiroplasma Spp. Cultured From Brains and Lymph Nodes From Ruminants Affected With Transmissible Spongiform Encephalopathy, researchers were able to isolate and culture spiroplasmas in tissue that had been harvested from animals that succumbed either to Scrapie or CWD. The authors of the study conclude that spiroplasma is a TSE pathogen and is " a candidate (for being a) causative agent of TSE". ⁴

Spiroplasma Disease in Corn

This blog really began with a plant, and an ant. When I wrote an article about ants in hospitals, I learned that leaf-cutting ants are very hard to eradicate. They have a special diet and are not attracted to traditional baits. Because of their diet, they harbor spiroplasmas in their gut. Is that bacteria harmless, I wondered? Then I found out that spiroplasmas have been associated with spongiform encephalopathies. Eventually I learned that leaf-cutting ants infect plants with spiroplasmas through saliva. The corn section shown in the picture below is taken from just such a plant. The corn is suffering from a disease called Spiroplasma kunkelii. My curiosity was piqued, and this blog was born.

Conclusion

Transmissible spongiform encephalopathies encompass a broad range of disorders. Do prions cause the diseases? If they do, what gives rise to the prions? Does the key to understanding TSEs reside with spiroplasmas? Maybe. Maybe not. Whatever the answer to that question turns out to be, there's no doubt that spiroplasmas will be the subject of much research in the future.

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Footnotes

¹ Mad Cow Disease Diagnosed in Florida Beef Cow
https://6abc.com/health/mad-cow-disease-diagnosed-in-florida-beef-cow/4088696/

² Florida cow tests positive for rare type of mad cow disease. State says no threat to public
https://www.miamiherald.com/news/local/environment/article217518390.html

³ Public Health Image Library
https://phil.cdc.gov/Details.aspx?pid=10131

⁴ Novel Spiroplasma Spp. Cultured From Brains and Lymph Nodes From Ruminants Affected With Transmissible Spongiform Encephalopathy
https://academic.oup.com/jnen/article-abstract/77/1/64/4633865

Some Sources Used in Writing This Blog

Novel Spiroplasma Spp. Cultured From Brains and Lymph Nodes From Ruminants Affected With Transmissible Spongiform Encephalopathy*,](https://academic.oup.com/jnen/article-abstract/77/1/64/4633865)

A Logical Causative Agent, The CWD Foundation Chronic Wasting Disease:A Review and working hypothesis, the Agent and its Transmission PART I: A Logical Causative Agent
http://www.stopcwd.org/library/cwd_paper.cfm

Spiroplasma spp. from transmissible spongiform encephalopathy brains or ticks induce spongiform encephalopathy in ruminants
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Characterization of Spiroplasma mirum and its role in transmissible
spongiform encephalopathies
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What are prions?
http://www.prionalliance.org/2013/11/26/what-are-prions/

Spiroplasma citri Movement into the Intestines and Salivary Glands of Its Leafhopper Vector, Circulifer tenellus
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Mycoplasma and Spiroplasma
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Learn more about Transmissible Mink Encephalopathy
https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/transmissible-mink-encephalopathy

Mad cow disease diagnosed in Florida beef cow
https://6abc.com/health/mad-cow-disease-diagnosed-in-florida-beef-cow/4088696/

Florida cow tests positive for rare type of mad cow disease. State says no threat to public.
https://www.miamiherald.com/news/local/environment/article217518390.html

Practical Neurology
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5520355/figure/F1/

Transmissible Spongiform Encephalopathies Information Page
https://www.ninds.nih.gov/Disorders/All-Disorders/Transmissible-Spongiform-Encephalopathies-Information-Page

United States Department of Agriculture Animal and Plant Health Inspection Service
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File:Atypical-scrapie-in-sheep-from-a-UK-research-flock-which-is-free-from-classical-scrapie-1746-6148-5-8-S1.ogv
https://commons.wikimedia.org/w/index.php?title=File%3AAtypical-scrapie-in-sheep-from-a-UK-research-flock-which-is-free-from-classical-scrapie-1746-6148-5-8-S1.ogv

Creutzfeldt-Jakob Disease Fact Sheet Scrapie
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Iatrogenic Creutzfeldt-Jakob Disease from Commercial Cadaveric Human Growth Hormone
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Public Health Image Library
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What Are Prions?
http://www.prionalliance.org/2013/11/26/what-are-prions/

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http://www.cfsph.iastate.edu/Factsheets/pdfs/bovine_spongiform_encephalopathy.pdf

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https://www.microbiologyresearch.org/docserver/fulltext/ijsem/61/4/703_ijs020529.pdf?expires=1546233318&id=id&accname=guest&checksum=43C926FDDBDBE2041DD36AFDBB70DCD8

High levels of co-infection with pathogens and symbionts in ticks from the Ardennes
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Spiroplasma - an emerging arthropod-borne pathogen?
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Chronic Wasting Disease (CWD)
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Identification and Characterization of Proteinase K-Resistant Proteins in Members of the Class Mollicutes
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Novel Spiroplasma Spp. Cultured From Brains and Lymph Nodes From Ruminants Affected With Transmissible Spongiform Encephalopathy
https://academic.oup.com/jnen/article-abstract/77/1/64/4633865