Research letters
Volume 356, Number 9234 16 September 2000
Transmission of BSE by blood transfusion in
sheep
Lancet 2000; 356: 999 - 1000
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F Houston, J D Foster, Angela Chong, N Hunter, C J Bostock
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We have shown that it is possible to transmit bovine spongiform
encephalopathy (BSE) to a sheep by transfusion with whole blood taken from
another sheep during the symptom-free phase of an experimental BSE
infection. BSE and variant Creutzfeldt-Jakob disease (vCJD) in human beings
are caused by the same infectious agent, and the sheep-BSE experimental
model has a similar pathogenesis to that of human vCJD. Although UK blood
transfusions are leucodepleted--a possible protective measure against any
risk from blood transmission--this report suggests that blood donated by
symptom-free vCJD-infected human beings may represent a risk of spread of
vCJD infection among the human population of the UK.
The demonstration that the new variant of Creutzfeldt-Jakob disease (vCJD)
is caused by the same agent that causes bovine spongiform encephalopathy
(BSE) in cattle1 has raised concerns that blood from human beings in the
symptom-free stages of vCJD could transmit infection to recipients of blood
transfusions. There is no evidence that iatrogenic CJD has ever occurred as
a result of the use of blood or blood products, but vCJD has a different
pathogenesis and could present different risks. CJD is one of the
transmissible spongiform encephalopathies (TSEs) characterised by the
deposition of an abnormal form of a host protein, PrPSc; the normal isoform
(PrPC) is expressed in many body tissues.
Available evidence, based on detection of infectivity in blood in rodent
models, and absence of infectivity in naturally occurring TSEs, adds to the
uncertainty in risk assessments of the safety of human blood. PrPSc has been
reported in blood taken from preclinical TSE-infected sheep,2 but it does
not follow that blood is infectious.
Bioassays of human blood can only be carried out in non-human species,
limiting the sensitivity of the test. One way of avoiding such a species
barrier is to transfer blood by transfusion in an appropriate animal TSE
model. BSE-infected sheep harbour infection in peripheral tissues3 and are
thus similar to humans infected with vCJD.4 BSE infectivity in cattle does
not have widespread tissue distribution.
We report preliminary data from a study
involving blood taken from UK Cheviot sheep challenged orally with 5 g
BSE-affected cattle brain and transfused into Cheviot sheep from a
scrapie-free flock of New Zealand-derived animals (MAFF/SF flock). MAFF/SF
sheep do not develop spontaneous TSE and the transfused animals are housed
separately from other sheep.
All sheep in the study have the PrP genotype
AA136QQ171 which has the shortest incubation period of experimental BSE in
sheep.5 19 transfusions from BSE-challenged sheep have been done, mostly
with whole blood. Sheep have complex blood groups and only simple
cross-matching can be done by mixing recipient serum and donor erythrocytes
and vice versa.
Therefore single transfusions only were made between sedated cross-matched
animals to minimise the risk of severe reactions. Negative controls were
MAFF/SF sheep transfused with blood from uninfected UK Cheviot sheep. As a
positive control, MAFF/SF sheep were intravenously injected with homogenised
BSE-affected cattle brain.
We have seen BSE clinical signs and pathological changes in one recipient of
blood from a BSE-infected animal, and we regard this finding as sufficiently
important to report now rather than after the study is completed, several
years hence. The blood donation resulting in transmission of BSE to the
recipient was 400 mL of whole blood taken from a healthy sheep
318 days after oral challenge with BSE. BSE subsequently developed in this
donor animal 629 days after challenge, indicating that blood was taken
roughly half way through the incubation period. 610 days after transfusion,
the transfused sheep (D505) itself developed typical TSE
signs: weight loss, moderate pruritus, trembling and licking of the lips,
hind-limb ataxia, and proprioceptive abnormalities. This is the first
experimental transmission of BSE from sheep to sheep and so we have nothing
with which to compare this incubation period directly. In cross-species
transmissions, bovine BSE injected intracerebrally gives incubation periods
of about 450 days in these sheep,5 and the donor animal had an oral BSE
incubation period of 629 days (see above). There are no similar data
available on other infection routes.
Immunocytochemistry with the antibody BG4 on tissues taken from sheep D505
showed widespread PrPSc deposition throughout the brain and periphery.
Western blot analysis of brain tissue with the antibody 6H4 showed that the
PrPSc protein had a glycoform pattern similar to that of experimental BSE in
sheep and unlike that of UK natural scrapie (figure), indicating that the
TSE signs resulted from transmission of the BSE agent. All other recipients
of transfusions and positive and negative controls are alive and healthy.
The positive controls, which involve a species barrier, are expected to have
lengthy incubation periods. With one exception, all transfused animals are
at earlier stages post-transfusion than was D505. The exception is a sheep
which is healthy 635 days after transfusion with BSE-blood donated at less
than 30% of the BSE incubation period of the donor sheep.
PrPSc (proteinase K treated) analysed by SDS-PAGE, immunoblotted with 6H4,
and visualised with a chemiluminescent substrate
All lanes are from the same gel with different exposure times. Size markers
are to the left of lane 1. Lane1: natural scrapie sheep brain,
3 min exposure. Lane 2: as lane 1, 10 min exposure.
Lane 3: sheep D505, blood-transfusion
recipient, 10 min exposure. Lane 4: experimental BSE-affected sheep brain,
30 s exposure.
Lane 5: as lane 4, 10 min exposure. Each lane loaded with amount of protein
extracted from
0·1 g wet weight of brain, except lane 3 which was extracted from 0·2 g
brain.
Although this result was in only one animal, it indicates that BSE can be
transmitted between individuals of the same species by whole-blood
transfusion. We have no data on blood fractions or on levels of infectivity
in blood of preclinical vCJD cases, but whole blood is not now used in UK
transfusions. The presence of BSE infectivity in sheep blood at an early
stage in the incubation period suggests that it should be possible to
identify which cells are infected, to test the effectiveness of
leucodepletion, and to develop a diagnostic test based on a blood sample.
We thank Karen Brown, Moira Bruce, Calum McKenzie, David Parnham, Diane
Ritchie, and the Scottish Blood Transfusion Service. The project is funded
by the Department of Health.
1 Bruce ME, Will RG, Ironside JW, et al.
Transmissions to mice indicate that 'new variant' CJD is caused by the BSE
agent. Nature 1997;
389: 488-501 [PubMed].
2 Schmerr MJ, Jenny A, Cutlip RC. Use of capillary sodium dodecyl sulfate
gel electrophoresis to detect the prion protein extracted from
scrapie-infected sheep. J Chromatogr B Biomed Appl 1997; 697: 223-29
[PubMed].
3 Foster JD, Bruce M, McConnell I, Chree A, Fraser H. Detection of BSE
infectivity in brain and spleen of experimentally infected sheep.
Vet Rec 1996; 138: 546-48 [PubMed].
4 Hill AF, Zeidler M, Ironside J, Collinge J.
Diagnosis of new variant Creutzfeldt-Jakob disease by tonsil biopsy. Lancet
1997; 349: 99-100.
5 Goldmann W, Hunter N, Smith G, Foster J, Hope J. PrP genotype and agent
effects in
scrapie: change in allelic interaction with different isolates of agent in
sheep, a natural host of scrapie. J Gen Virol 1994; 75: 989-95 [PubMed].
Institute for Animal Health, Compton, Newbury, UK (F Houston PhD, CJ Bostock
PhD); and Institute for Animal Health, Neuropathogenesis Unit, Edinburgh,
EH9 3JF, UK (N Hunter PhD, JD Foster BSc, Angela Chong BSc)
Correspondence to: Dr N Hunter