January 10, 2002
Researchers Assess Risk of Mad Cow Disease in Sheep
Although a great deal of uncertainty exists about whether bovine
spongiform encephalopathy (BSE), also known as mad cow disease, can be
transmitted from cattle to sheep, researchers have developed a
mathematical model to assess the health risks of humans contracting BSE
from sheep. The model also takes into consideration the effects
protective measures would have on transmission of BSE from cattle to
sheep.
The newly developed model indicates that if BSE infects sheep in
Great Britain, the current health risk to humans, although low, is
likely to be greater than that of contracting BSE from cattle –
largely reflecting the more stringent control measures in place that
protect against cattle BSE. The researchers said, however, that any
risk could be greatly reduced by regulations that limit the age at
which sheep are slaughtered and further restrictions on the use of
animal products in feed.
“Much research needs to be done on how infectivity develops through time and how BSE or scrapie might transmit from animal to animal. We know it happens, but we know very little about the detailed mechanics of transmission.”
Neil M. Ferguson
Howard Hughes Medical Institute international research scholar Neil
M. Ferguson and colleagues at the Imperial College of Science,
Technology and Medicine in London published the model and discussed
their interpretation of the initial results in an online article in the
January 10, 2002, Nature.
According to Ferguson, sheep have been infected with BSE in the
laboratory by feeding them brain material from infected cattle.
However, a recently completed attempt to detect the disease in the
sheep flock failed due to contamination. Nevertheless, given the
possibility that sheep might transmit the disease, he and his
colleagues had been commissioned by the British government Food
Standards Agency to develop a model to guide research and
policy-making.
“I should emphasize that we weren’t asked to evaluate
the probability that BSE has gotten into the sheep flock — although
with the current basis of knowledge there’s no reason to rule out
that possibility,” said Ferguson. “Rather, we were
commissioned to develop a risk assessment of the potential consequences
should infectivity of the sheep flock exist.”
The scientists’ model aimed to explore three major elements
– how many sheep might have been infected through consumption of
tissue from infected cattle; the likelihood of spread of the disease
through sheep flocks; and how the age of the infected sheep might
affect the risk to humans of contracting the human version of BSE,
called variant Creutzfeldt-Jakob disease (vCJD).
Central to the model, said Ferguson, is an infectiousness profile of
BSE in sheep that was developed based on data from other researchers
who studied scrapie, a disease similar to BSE that affects sheep. The
infectiousness profile postulates that infectivity depends on the age
of the sheep and the tissue involved.
“A critical issue is that infectivity really begins to grow to
significant levels in the animals about twelve months after
infection,” said Ferguson. “Thus, if animals are under
twelve months old when they are slaughtered – which is the case
for most sheep – then they pose less of a risk to
humans.”
Ferguson and his colleagues tested their model by running it using
three representative scenarios ranging from optimistic to worst-case.
In the most optimistic scenario, the disease would not spread beyond a
flock, and in the worst-case scenario, the disease would spread to
become a self-sustaining epidemic.
“Our analysis indicates that, on the basis of even the very
limited current data, we can just about rule out a very large epidemic
of BSE in sheep,” said Ferguson. “We believe that this
means that any historical exposure of the human population to BSE
infectivity from sheep was perhaps a hundred times lower than the scale
of the epidemic in the cattle population.”
Although the risk of infection from sheep might be low, Ferguson
said it would be higher than the current risk of infection from cattle,
because of the stringent measures taken to protect the population from
infection from cattle. He emphasized that two protective measures could
greatly reduce even that low risk.
“Since younger sheep would have only recently been infected,
they would have a much lower burden of infectious material in their
tissues than older animals,” said Ferguson. “Thus,
regulations requiring slaughter of animals when young would reduce the
risk of infection,” said Ferguson.
“Secondly, a total ban on the use of all internal organs in
feed products would reduce the risk even further, with a total
reduction from both steps of up to 90 percent,” he said.
Ferguson cautioned, however, that a cost-benefit analysis of these
protective measures would have to be taken into account before they
were implemented. He emphasized that for the model to be useful in the
future, the researchers will have to depend heavily on accurate,
large-scale screening of sheep flocks to determine whether BSE
infection does indeed exist.
“Also, much research needs to be done on how infectivity
develops through time and how BSE or scrapie might transmit from animal
to animal,” he said. “We know it happens, but we know very
little about the detailed mechanics of transmission.”
Regarding the estimated health risk to humans of contracting vCJD
from cattle, Ferguson emphasized that the uncertainty remains quite
broad. Human deaths over the next 80 years caused by BSE from cattle
could range from 50 deaths to 50,000, concluded the scientists. Adding
the potential deaths from sheep under the worst-case scenario would
increase mortality to 150,000 over that period.
“However, this worst-case scenario is the only one in which
including sheep has more than a negligible effect,” said
Ferguson. “And that scenario is unlikely and very pessimistic,
although it can’t be excluded.”
The current model represents only a foundation for future efforts,
Ferguson said. “This model represents a first pass, and the aim
of this effort was largely to put the methodological framework in
place,” he said. “We hope to refine these measures
considerably and make the risk estimates more precise as we feed in
results from screening studies and new experimental findings about the
disease.”
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