9 - Prion Diseases Flashcards
Why is BSE a one health issue? CWD?
BSE: agricultural, industry generation, distribution of infectious prions
CWD: transport of infectious animals, environmental reservoir of infectivity
How are prions unique pathogens?
They are acellular, non-living and are disease-specific protein structure (do not have their own genome)
The cellular prion protein is present in… What gene is it encoded by?
in all mammals
Encoded by prion protein gene (PRNP)
What happens when there is a prion infection? How (3)
Conversion of normal to misfolded (abnormal) prion protein
- presence of misfolded (prion mechanism)
- spontaneously
- PRNP gene alterations
How can the PRNP gene alteration affect likelihood of misfolding?
Changes in normal prion protein
Makes conversion to misfolded more or less likely depending
Healthy vs disease-causing prion protein
PRP^c (healthy): enriched in a-helices, glycoprotein cell surface, metal homeostasis, necessary for prion infection
PRP^TSE: same a.a. sequence, enriched in B-sheets, partially resistant to proteinase K digfestion
What is the difference between a prion and a prion protein?
Prion: infectious, transmissible protein agent
Prion protein: non-infectious form (PrP^C), infectious form (PrP^TSE, Sc, etc)
Slides 9-12
Look
What species can be affected by prion diseases? Which are not?
Sheep, cervids, mink, camel, cats, humans, cows, mice
Not: horses, dogs
Three categories of human prion diseases
- Acquired:
- kuru
- variant creutzfeldt jakob disease (vCJD)
- Iatrogenic CJD - Familial:
- familial CJD
- fatal familial insomnia
- gerstmann-straussler-scheinker syndrome - Idiopathic (sporadic)(85%):
- sporadic CJD
- sporadic fatal insomnia
Explain the basic of prion diseases
- spongiform degeneration
- transmissible (not all contagious)
- infection highest in brain and spinal cord. Also lymphatic
- accumulation of PrP^TSE, BSE, etc
- long incubation period
- extended preclinical stage
- fatal, no cure
- resistant to inactivation
Prion infectivity in hamsters, cattle, deer
Hamsters: 1-10 billion infectious units
Cattle: 1-10 million infectious units/gram of brain tissue
Deer: unknown
What are prions resistant to?
- chemical inactivation (acids, base, detergent)
- standard sterilization methods (autoclaving/medical sterilization: 120C 20 min)
- common UV and gamma irradiation levels
How do we inactivate prions?
Extreme conditions
- 1 N NaOH
- autoclaving at 134C for 1 hour
How do we detect prion diseases
- postmortem
- brain tissue
- distinguish abnormal protein from normal (infectious agent)
- clinical behaviour / characteristics (but occur late in disease)
Antibody response not useful
How long is the preclinical stage in deer? Cattle? Sheep? Humans
Deer (CWD) = years
Cattle (BSE) = years
Sheep (scrapie) = years
Humans (CJD) = decades
How do you distinguish normal from abnormal prions
Expose them to proteinase K, abnormal form is resistant
Slides 23-27**
Commercial detection kits for prions
Western blot or ELISA
government restrictions in place on purchase
What are PMCA and RT QuIC
PMCA: protein misfolding cyclic amplification
RT QuIC: real time quaking induced conversion
Highly sensitive technologies for detection of PrP^SC. Required expensive equipment and lab expertise.
Slide 30
Similarities and differences between PMCA and RT QuIC
Both can amplify detectable levels of abnormal prion proteins form small amounts of starting material
Both require PrP^c
PMCA uses PrP^c source from brain homogenate whereas RT-QuIC uses PrP^c from recombinant sources
Which prion diseases are transmissible? Contagious?
Transmissible: all
Contagious: scrapie & CWD
Characteristics that distinguish diff prion diseases
- contagious?
- length of incubation period
- clinical symptoms
- PK resistance
- size of PK resistant proteins
- distribution and size of spongiform lesions in brain
- interspecies transmission
Sheep scrapie vs BSE
Both transmissible from brain tissue
Sheep scrapie also contagious (sheep to sheep)
First recorded scrapie case in UK? Canada?
UK: ~1700
Canada: 1938
Scrapie transmission
Contagious
Infectivity present in fluid and tissue from placentas of infected ewes
Environmentally persistent
Variations in PRNP Gene leading to susceptibility or resistance to scrapie
ARR = resistance to scrapie
VRQ = susceptible
ARR/ARR = highly resistant
VRQ/VRQ = highly susceptible
How are we trying to control scrapie
- reportable disease (suspect cases must be reported to CFIA vet immediately)
- sheep documented with scrapie and all animals exposed to same birthing environment must be destroyed (producers compensated)
- selective breeding for genetic resistance to minimize risk
What is atypical scrapie
Abnormal protein more sensitive to PK than classical
Gets digested away = hard to identify
More abundant in cerebellum & cortex than brain stem
Identified in “scrapie resistant” sheep
How many BSE+ cattle in GB? North America? Cost of BSE to canadian cattle industry?
~200,000 in GB
26 in NA
~$7 billion dollar cost to industry
How does BSE propagate in cow populations
Animal carcasses, slaughter facility waste and unused meat products are sent to rendering facilities, cooked at high temps into meat and bone meal, then fed back to cattle
If prions are in the carcasses, they are resistant to rendering temperatures
How much animal carcass/animal by-products are processed in rendering plants yearly in NA
50 billion pounds converted into 18 billion pounds of product
Value $3 billion
How much of every cow/pig is not consumed by humans
Half of every cow, a third of every pig
When did BSE spike in the UK? What was done to control it? What about vCJD deaths?
Spiked from 1990-1995
Feed ban implemented in 1988 (no m&b meal to cattle)
Enhanced ban in 1996
vCJD cases spiked from 1995-2000 (BSE is zoonotic = variant CJD)
What codon is important in CJD? How?
Codon 129 of prion protein
met/val (50% of pop) and val/val (10% of pop) are safe
met/met (40%) of pop died
When were feed bans implemented in Canada for BSE?
1997: Ruminant feed ban
- prohibit feeding of most mammalian derived proteins to ruminants
- prohibit feeding of poultry litter and restaurant waste to ruminants
2007: Feed ban
- SRM banned from all animal feeds, pet foods, fertilizers
Political pressures were coming from who when the feed bans were implemented in canada
From the rendering industry, arguing we had not yet seen BSE in Canada
What is SRM? Banned…
Specified risk material
- skull, brain, eyes, spinal cord, etc
SRM from cattle 30 months or older banned from human food supply
SRM banned from all animal feeds, fertilizers and pet foods
Distribution of the 26 NA cases of BSE
1 imported from GB
5 US
20 Canada
Classical vs atypical BSE
Classical: caused by feeding cattle MBM, solution = stop feeding cattle infected material
Atypical: heavy (H) and light (L) migration of abnormal protein, NOT linked to feed, very old cattle (>10 years). Spontaneous?
Which of scrapie, BSE and CWD are zoonotic?
Scrapie = no
BSE = yes
CWD = ?
CWD facts (8)
- contagious brain disease
- always fatal (no treatment or cure)
- complicated detection
- many strains
- environmental contamination
- long preclinical
- slow spread
- zoonotic?
Where did CWD begin in NA in 2000? How
Colorado, Nebraska, Wyoming
Animal to animal, animal to environmental (transport trucks: game farms) to animal
Slides 64-67
Spread of CWD
How did CWD get to Saskatchewan
in 1996, preclinical farmed elk moved from south dakota to sask
% of mule deer, white tailed deer, elk and moose positive for CWD in AB
23.4% mule
8.3% white
1.6% elk
2.9% moose
How did CWD get to South Korea?
In 2000, captive elk moved from Sask
Is CWD in Europe?
Norway 2016, Finland 2018, Sweden 2019
How did Norway try to stop CWD spread
Culled thousands of reindeer
When do infected deer become contagious?
Lymph tissue positive for PrP^CWD as early as 3 months post oral infection (shedding)
What tissues are low-moderately infective during early preclinical? Clinical?
Lymphatic, tonsil, saliva
Brain & spinal cord
Slide 77***
How do you control CWD?
- no drugs/vaccines/cures
- depopulation?
- reduce population?
White tailed deer and CWD
- very susceptible
- numerous strains
- free-ranging and captive
Mule deer and CWD
- very susceptible
- AB and Sask more prevalent than white-tailed
Elk and CWD
- susceptible
- free ranging and game farms
Moose and CWD
- relatively rare in NA (Colorado, Wyoming, AB)
- environmental transmission likely route
- more common in europe
Caribou and CWD
- no cases to date in NA
- present in Norway
- considered very susceptible
- range of disease in deer is starting to grow into areas with caribou
