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Transcript
Transmissible Spongiform Encephalopathy
Prion Protein Diseases
Lisa Kennedy, Dylan Bradford, Madi Hoagland Henefield, Anders Ohman
Advisor: Dr. Todd Livdahl
Transmission Routes
It is commonly held that prion diseases are mainly transmitted
between animals via ingestion or orally. A number of ingestion routes have
been identified that cause disease in humans as well as other animals
including deer, elk, and cattle.
The most common and surest way is to ingest tissue from an infected
animal, the brain tissue having the highest concentration of dangerous
prions. Omnivorous and carnivorous animals are susceptible to this mode
of infection, and exhibits a very real danger to human hunters of deer and
elk. A study of humans that were exposed to Chronic Wasting Disease (a
closely related prion disease) via hunted deer at a wild game feast suggested
two levels of risk, exposure and ingestion (Garruto et al. 2008). Those
subject that had actually ingested venison that could have come from an
infected deer were said to be of high-risk and are currently under close
watch in case CWD is found to transmit between deer and humans.
For herbivores it was found that prions, in the form of Scrapie Agent
263k, are able to stay dormant in soil for at least as long as 29 months and
cause infection (Seidel et al. 2007). The study was done using Syrian
hamsters and PrPSC were found to infect and proliferate within the hamsters
when fed soil samples. Aqueous extract was also shown to induce disease
in the reporter animals. These findings suggest a highly dangerous scenario
for cattle and sheep flocks. Prions can be deposited by decaying animals, as
well as infected organs including placenta, amniotic fluid, and possibly
urine (Gregori et al. 2008), making grazing animals highly susceptible.
There still remains much research to be done on prion diseases as a
whole including transmission routes. The way they are passed between
animals is very important to determine if only to protect humans from
possible cross-species infection. Given the amount of red meat consumed in
the United States, a large portion of infected but undetected meat due to a
lack of knowledge could cause a large loss of life.
Background
Prion diseases (formally, Transmissible Spongiform Encephalopathies) are neurodegenerative conditions in mammals that are
hypothesized to be caused by an infectious protein agent without the involvement of nucleic information or additional transport vectors. The
theorized prion agent is a modified form of the cell membrane protein PrPC, which is misfolded into the pathogenic form of the protein,
which is referred to as PrPSc. These prion proteins are able to convert normal protein molecules into the prion form, and the proteins are
extremely resistant to denaturation by temperature or chemical agents, making them both difficult to destroy once in a host as well as
capable of surviving in an exposed environment for an extended period of time.
Prion diseases have been documented for decades (centuries, in the case of Scrapie), but the discovery of the prion protein itself was
made as recently as 1997 by Stanley B. Prusiner, who won a Nobel Prize for his work. The mechanics of the disease are still largely
unknown. They can affect many lineages of mammals, from rodents to ruminants, as well as humans. Prion disease in livestock has serious
implications for human food source populations, as well as for potential infection in humans, as the Bovine Spongiform Encephalopathy
may potentially be linked to a variant of Creutzfeldt-Jakob Disease in humans. There is no current cure, inoculation, or treatment for prion
diseases, and livestock protection strategies largely include quarantine and destruction of infected organisms.
Symptoms:
TSEs cause the formation of holes in the host nervous tissue, causing a sponge-like appearance from which the term ‘spongiform’ is
derived. Damage to the nervous tissue causes a number of deteriorative cognitive and behavioral conditions in prion hosts of all susceptible
species, typically including dementia, acute mood changes, and issues with physical coordination and control.
Modeling the System
Tra nsmi ssio n Rate
Cattle p opul atio n
BSE Infecctions
BSE Cattle
Prop ortio n In fectivity
Types of TSE
In fectivity Pa ssed
Mate rial wi th BSE Infe cti vi ty
Prion disease occurs in a number of different mammals, and strains of prion
disease typically only affect peers in the species and closely-related lineages (Prusiner,
S.B. et al 1991). There are three primary categories of prion diseases: Sporadic,
where the cause is unknown ; Familial, where the disease is caused by the genetics of
the host; and Acquired, which is transmissible between organisms, either directly or
through the environment.
BSE i nfectivity in MBM
Cattle Feed wi th
In fected MBM
Redctio n of BSE Mate rial
Prop ortio n of MBM
i n Cattle Feed
Graph 1
Deer, Elks, and Moose are affected by Chronic Wasting Disease, an Acquired
condition that is transmissible directly or through contact with a shared environment
(Joly, D.O. et al 2003). CWD was first contained to the Central United States, but has
since spread to multiple areas of the US and Canada, including game parks. It causes
changes in host mood, diet, and behavior.
Sheep and Goats are affected by Scrapie, the oldest-known prion disease first named
in 1732 (www.defra.gov.uk). Scrapie affects both the nervous system as well as the
tonsils and the distal ileum, and causes behavioral abnormalities such as the
compulsive scraping of the host on objects in the environment from which the
diseases’ name is derived. Although found in Europe and in America, successful
quarantine has kept the disease out of Oceanian populations (Parsonson, I.M. 1996).
Cows are affected by Bovine Spongiform Encephalopathy. While BSE (a.k.a. Mad
Cow Disease) is a strictly Familial disease, domestic animal practices for feeding
livestock resulted in direct consumption of infected brain tissue as animal feed, and
therefore spread the disease from individual to individual (Prusiner, S.B. 1997). BSE
leads to the formation of protein fibers in the brain, leading to the presence of holes in
the tissue. Symptoms include nervous or aggressive behavior, weight loss, and
coordination problems, but can take years to actually appear.
Cattle population
12.7 million*
Transmission rate
.001
Proportion infectivity
.85
Reduction rate of infectivity
.1
Proportion of MBM in Cattle
Feed
New cases of BSE
.2
Redu cti on Rate
of Infectvi vi ty
New Case s of BSE fro m MBM
There are four primary prion diseases that affect humans: Creutzfeldt-Jakob Disease,
Kuru, Fatal Familial Insomnia, and Gerstmann-Straussler-Scheinker Syndrome. CJD
has variant strains in all three categories, while Kuru is an Acquired disease and both
FFI and GSSS are Familial (Soldevila, M. et al 2006). Symptoms of CJD, the most
common human prion disease with 1-2 instances in a million people annually, has the
symptoms of dementia, issues with coordination, and visual hallucinations. GSSS
shares the symptoms of dementia and coordination problems, but also results in
difficulty speaking. Sufferers of Kuru have laughing fits and trembling fits. FFI causes
insomnia, hallucinations, and also dementia (Monari, L. et al 1994).
The model below calculates the number of new cases of
BSE that result from the use of infected BSE cattle in cattle
feed. The practice of “recycling” cattle was relatively
common before the outbreaks of BSE in the UK. This
STELLA model (fig. 1) utilizes data used in a published
model used to calculate the hypothetical reproductive ratio
of BSE, as well as actual data on the cattle population of
the UK in 1986.
372
342
*source: BSEinquiry.gov.uk
1947:
1967: Chronic
Transmissible
1921:
Wasting
mink
Creutzfeldt-Jakob
Disease CWD:
encephalopathy
disease (CJD)
captivity
1981: CWD
wild
populations
1996: Variant
CreutzfeldtJakob Disease
TIMELINE
1732*:
Scrapie
1936: Gerstmann
Straussler-Scheinker
Syndrome (GSSS)
Kuru:
1959
1974: Fatal
Familial
Insomnia
1986:
BSE
1997: Prion
protein
discovered***
2003: Feline
spongiform
encephalopathy
*Timeline not to scale
**The dates indicate descriptions and identifications of the respective diseases.
At the earlier dates, cause of disease by the prion protein was not known
***By Stanley B. Prusiner winner of the Nobel Prize in Physiology or
Medicine 1997
Molecular Biology of the
Pathogenesis of TSE
Prion disease are unique in that they are
transmissible particles that lack any form of nucleic acid,
and yet are still infectious (Prusiner, 1998). While prion
diseases are not completely understood, it is clear the
disease is caused by an accumulation in the brain of a
misfolded cellular protein known as the prion protein
(PrPc) (Campana et al., 2008).
The normally occurring prion protein is converted
into the modified infectious protein PrPSc
posttranslationally. During this process PrPc misfolds into
PrPSc which contains a high number of β-sheets. The
process during which the normal protein changes to the
abnormal takes place when some of the α-helices and
coils of its structure refold into β-sheets. This structural
alteration causes less understood but still profound
changes in the chemical properties of the PrP. The
changes that result in the PrPSc have been shown to act
as a template upon which PrPc is refolded into PrPSc
(Prusiner, 1998). This is how the misfolded protein is able
to become infectious to other prion proteins around it.
Furthermore, PrPSc has shown the ability to resist
degradation even after the death of its host for up to
months or even years at a time, and still be infectious to a
new host if picked up.