Variant of mad cow disease may be transmitted by blood transfusions
6 September 2008
The risk of transmitting bovine spongiform encephalopathy (BSE,
commonly known as 'mad cow disease') by blood transfusion is
surprisingly high, according to a nine-year study in sheep conducted at
the University of Edinburgh and published online in the journal Blood.
The study could help pave the way for urgently needed diagnostic tests
for the disease.
BSE is one of a group of rare neurodegenerative disorders called
transmissible spongiform encephalopathies (TSEs), and there is no
reliable non-invasive test for detecting infection before the onset of
In addition to BSE, these diseases include scrapie, a closely related
disease in sheep, and Creutzfeld-Jakob disease (CJD) in humans, which
causes neurological symptoms such as unsteadiness and involuntary
movements that develop as the illness progresses, rendering late-stage
sufferers completely immobile at the time of death.
A new variant of CJD (termed vCJD) was recognized in the United
Kingdom in the mid-1990s, apparently as a result of the transmission of
BSE to humans. Because the symptoms of this disease can take many years
to appear, it was not known how many people might have been infected,
and without a reliable test for identifying these individuals,
clinicians were very concerned that the infection could be transmitted
between people by blood transfusion or contaminated surgical and dental
As a result, costly control measures were introduced as a
precautionary measure to reduce the risk of disease transmission,
although at the time it was unclear whether there really was a
significant risk or whether the control measures would be effective.
This sheep study sought to better understand how readily TSEs could be
transmitted by blood transfusion in order to help develop more targeted
"It is vitally important that we better understand the mechanisms of
disease transmission during blood transfusions so we can develop the
most effective control measures and minimize human-to-human infections,"
said Dr. Fiona Houston, now a Faculty of Veterinary Medicine, University
of Glasgow, UK, and lead author of the study.
The nine-year study conducted at the University of Edinburgh compared
rates of disease transmission by examining blood transfusions from sheep
infected with BSE or scrapie; the BSE donors were experimentally
infected, while the scrapie donors had naturally acquired the disease.
While scrapie is not thought to transmit to humans, it was included
as an infection acquired under field conditions, which could possibly
give different results than those obtained from experimentally infected
animals. Because of the similarity in size of sheep and humans, the team
was able to collect and transfuse volumes of blood equivalent to those
taken from human blood donors.
The outcome of the experiment showed that both BSE and scrapie could
be effectively transmitted between sheep by blood transfusion.
Importantly, the team noted that transmission could occur when blood was
collected from donors before they developed signs of disease, but was
more likely when they were in the later stages of infection.
Of the 22 sheep who received infected blood from the BSE donor group,
five showed signs of TSEs and three others showed evidence of infection
without clinical signs, yielding an overall transmission rate of 36%. Of
the 21 infected scrapie recipients, nine developed clinical scrapie,
yielding an overall transmission rate of 43%.
Investigators noted that the results were consistent with what is
known about the four recorded cases of vCJD acquired by blood
transfusion in humans. In addition to the stage of infection in the
donor, factors such as genetic variation in disease susceptibility and
the blood component transfused may influence the transmission rate by
transfusion in both sheep and humans.
"The study shows that, for sheep infected with BSE or scrapie,
transmission rates via blood transfusion can be high, particularly when
donors are in the later stages of infection. This suggests that blood
transfusion represents an efficient route of transmission for these
diseases," said Dr Houston.
"Since the results are consistent with what we know about human
transmission, the work helps justify the control measures put in place
to safeguard human blood supplies. It also shows that blood from BSE-
and scrapie-infected sheep could be used effectively in non-human
experiments to answer important questions, such as which blood
components are most heavily infected, and to develop much-needed
Blood is the official journal of the American Society of