Title: Chronic%20Subclinical%20Transmissible%20Spongiform%20Encephalopathy%20Infections
1Chronic Subclinical Transmissible Spongiform
Encephalopathy Infections
2- Transmissible Spongiform Encephalopathies
- Despite defining nosologic feature, lt 1 of all
human cases are a consequence of recognisable
horizontal transmission events 85 sporadic
15 genetic. - Although infectivity very low compared to
viruses/bacteria, epidemics have occurred given
unique circumstances e.g. natural scrapie -gt
recycled through cattle -gt BSE epidemic-gt vCJD - Incidence steady at 1-2 cases per million
population/year, independent of country. - Sporadic cases thought to arise spontaneously
as a consequence of failure of cellular clearance
of misfolded PrPc, possibly related to somatic
neuronal mutations or more likely decline in
cellular protein quality control-gt gradual
accumulation of PrPres -gt invariably and quite
quickly leads to clinical disease. - Concept of natural reservoirs of infection
serving as sources of point source or
case-to-case transmission, probably through
low-level, largely covert, contamination episodes
-gt ??helps to sustain low-level endemicity of the
1-2 cases per million/year no clear
epidemiological support despite detailed
investigation of human CJD clusters, but very
difficult to identify if host is asymptomatic at
the time and for lengthy period thereafter and
the contamination event not dramatic. - Interest in natural reservoirs and possible
under/un-appreciated case-to-case transmissions,
especially during provision of health care,
rekindled by recent mouse models of chronic
subclinical TSE infection concerns supported by
two large epidemiological case-control studies.
3- Probably first description of animal of model of
chronic subclinical TSE infection - Zlotnik 1965
- Symptomatic scrapie in sheep mice
intragastricaly inoculated sheep brain mice not
develop scrapie after 13 months but brains showed
vacuolation mice brain/spleen pools IC and IP
inoculated into further mice with all developing
clinical scrapie. - Current understanding would consider 13 months
was not long enough an observation period for
enteral inoculation route
4- Dickinson 1975
- 22A scrapie strain in C57BL and VM mice
inoculated log dilution series IC and IP mice
observed gt600 days mice IP at 10-1 developed
clinical scrapie but higher dilutions not develop
clinical disease and brains negative histol but
spleens of 10-2 and 10-3 mice had low titres of
infectivity on bioassay gt 300 days post-IP
inoculation - Collis 1985
- 87V scrapie strain in IM mice IP inoculation
3/53 mice developed clinical disease after 600
days however, spleens of these mice showed
ubiquitous infectivity by bioassay from soon
after inoculation which continued at similar
levels until asymptomatic mice culled at 80
weeks. - Implies low-level peripheral inoculation may
never cause overt neurological disease but
peripheral (especially lymphoreticular) organs
are infectious over lengthy period of hosts life
and in the incubation period may in fact exceed
the life span of the host - Also underscores that ID/LD 50 titres determined
by clinical disease in recipients may be
underestimated by 1000-fold.
5- Bueler 1994
- RML strain in PrP/ and PrP/- mice IC
inoculated PrP/- mice express half PrPc levels
-gt prolonged incubation period (290 days cf 140
days) and disease duration (125 days cf 13
days) - however - PrPres detectable day 84, peaked 168
days, plateaued thereafter, - - vacuolation and astrogliosis evident from
140 days, most prominent after 290 days,
plateaued thereafter, - - titre 8log10 at 20 weeks and not
significantly increase thereafter.
6- Frigg 1999
- RAG-2-/- and uMT immunodeficient mice IP
inoculated -gt asymptomatic for 665 days with
randomly selected mice culled during the time
course in 5/14 randomly selected mice brains
positive for PrPres on western and/or transmitted
on IC bioassay to Tga20 mice four mice further
IC inoculated into Tga20, RAG-1-/-, and uMT mice. - - titres (by time interval assay) frequently
comparable to those seen in terminally sick
mice. - - only symptomatic mice had brain vacuolation.
- Immunodeficient mice (except SCID mice) all
develop scrapie similar to wild-type mice after
IC inoculation suggests peripheral inoculation
may be more relevant than immunodeficiency to
creation of subclinical infection state.
7- Race 1998, 2001, and 2002.
- 263K hamster scrapie strain in C57BL mice
- Hill 2000
- Sc237 strain (263K) in CD-1 mice
- Hamster scrapie brain (at high titre) IC
inoculated into mice no clinical disease in any
mice up to 800 days up to 400-600 days mice
brains negative for mouse PrPres, but after this
period some mice contained murine PrPres in their
brains (likelihood increased with the length of
post-inoculation period) hamster PrPres never
seen in mouse brains. - If mouse brain negative for PrPres, only but very
commonly transmitted to hamsters with IC
inoculation (with incubation periods inversely
related to length of observation period in mice). - If mouse brain positive for PrPres, generally
transmitted to mice but always to hamsters.
8- Thackray 2002
- ME7 and RML scrapie strains in Tga20 mice
undertook IC inoculations with log dilution
series. - higher dilutions (10-6 and 10-7 in ME7 10-8 and
10-9 in RML) associated with fluctuating/reversibl
e neurological signs that did not progress to
terminal disease still classed as subclinical. - length of subclinical mice survival not
clearly stated but culled and found to have - - abundant PrPres (more than in terminally sick
mice) - - high brain titres (by time interval assay in
Tga20 mice) 100,000 ID 50/g brain in ME7
1000-100,000 ID 50/g brain in RML. - no brain vacuolation unless developed terminal
scrapie.
9Box Final inoculum dilution Sick mice/total inoculated Incubation period range (days post-ICI) Mean incubation period (days)
2028 10-1 6/6 151-175 161.8
2029 10-2 6/6 158-184 170.6
2030 10-3 6/6 158-193 176.5
2031 10-4 5/5 193-399 290.8
2032 10-5 2/6 283 283
2033 10-6 0/6
2034 10-7 1/6 240 240
2035 10-8 0/5
2036 10-9 0/3
Two mice became ill much later and died at 391
and 399 days. Mean incubation period for the
other three mice was 221.3 days.
10Original Box Original Inoculum Dilution Mouse from original Box Western blot For brain PrPres Histol for vacuolation IHC for PrPres plaques Recipient or Indicator Tga/20 miceBox Successful transmission/ Total mice inoculated Positive western blot for PrPres of selected mice/total assessed Mean incubation period days (range)
2032 10-5 1 Pos 2085 4/4 ND 62.3 (61-65)
2 neg 0 0 2086 2/3 2/3 113.5 (107-120)
3 pos / 2087 4/4 1/1 67.5 (65-72)
4 pos / / 2088 4/4 1/1 65.3 (63-68)
2033 10-6 1 neg 0 0 2089 3/4 3/4 133.3 (98-202)
2 neg 0 0 2090 3/4 3/4 139.6 (123-154)
3 neg 0 0 2091 4/4 2/2 82.5 (79-86)
4 pos / 2092A B 3/3 ND 63.7 (62-65)
2034 10-7 1 neg 0 0 2093 0/5
2 neg 0 0 2094 0/5
3 neg 0 0 2095 0/5
4 neg 0 0 2096 0/5
2035 10-8 1 neg 0 0 2097 1/5 1/1 142
2 neg 0 0 2098 0/4
3 neg 0 0 2099 0/5
4 neg 0 0 2100 1/4 1/1 129
2036 10-9 1 neg 0 0 ND
2 neg 0 0 ND
3 neg 0 0 ND
NDnot determined. Single sick mouse culled
214 days post-inoculation with brain negative for
PrPres on western immunoblot. There was a
single remaining mouse, which was healthy at
culling 218 days post-inoculation brain negative
for PrPres on western immunoblot
11- Conclusions
- inefficient inoculation peripheral route,
decreased PrPc expression, low titre innoculum,
cross-species associated with prolonged
incubation periods, which can approximate or
exceed life-span of host. During this prolonged
incubation period, and possibly from relatively
early, high titres of infectivity may exist in
peripheral lymphoreticular organs (and perhaps
lower titres in other organs and blood) as well
as brain, allowing potential unrecognised
transmission to occur. - Great concerns regarding potentially large
numbers of subclinical vCJD carriers. - Titres underestimated by 2-3 logs if only
clinically apparent transmissions used for
calculations. - Circumstantial support for relevance to human
TSE - Kuru incubation periods now exceed 50 years in
most recent victims. - two large epidemiological case-control studies
linking surgery to increased risk of sporadic
CJD. - Priorities
- To minimise secondary transmission risk from
subclinical infection in health care setting,
screening biochemical tests will need to be
highly sensitive (eg CDI or high performance
western blots for PrPres) and approximate
bioassay results seen in sensitive recipients
like Tga20 mice.