Foot-and-Mouth would have jumped fence
Vaccine model suggests a shot in the
rump is worth two in the head.
23 December 2002
TOM CLARKE
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| Six million farm animals
died in Britain's 2001 foot-and-mouth outbreak. |
| © GettyImages |
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Ring-fence vaccination would not have halted Britain's
devastating 2001 epidemic of foot-and-mouth disease (FMD), a new
analysis suggests1.
Blanket prophylactic vaccination or targeting high-risk farms
first are the best ways to avert or minimize future outbreaks of
FMD, say the report's authors, Matthew Keeling of the University
of Warwick, UK, and colleagues.
Similar tactics, they reckon, could work for other animal and
even human diseases, such as influenza and smallpox.
The heat from pyres of burning carcasses has died down, but
argument about the management of Britain's worst-ever FMD
outbreak has not. More than 6 million animals, mainly cows,
sheep and pigs, were slaughtered before the outbreak ended.
Vaccination wasn't used because of pressure from farmers.
Under current European law, vaccinated animals cannot be bought
or sold.
New tests for distinguishing vaccinated animals from infected
ones, and strategies such as those proposed in the new study,
show that it's "time for the rule book to catch up with the
science", comments veterinary immunologist Ian McConnell of the
University of Cambridge, UK.
Action replay
Based on data from the 2001 outbreak, Keeling and his
colleagues designed a model that predicted the infectious state
of every farm in Britain each day. They then replayed each day
of the seven-month outbreak with four different vaccination
strategies.
Nation-wide vaccination of all animals before an outbreak
offered the greatest protection. Localized culling on and around
infected farms was still required, as there are always gaps in
vaccine coverage.
Mass vaccination in response to an outbreak was the next best
strategy. The FMD vaccine takes seven days to confer immunity,
so extensive culling was again required. In the model, this
strategy knocked four months off the outbreak.
Ring-fence vaccination - where farms surrounding infected
hotspots are treated to contain the disease - failed because of
the vaccination-to-immunity lag and the rapid movement of
animals before, and for a time after, the outbreak was detected.
Without a complete movement ban on animals and people, says
Keeling, "sparks of infection jump out of the rings" and
propagate the epidemic.
The researchers also used their model to identify a
'predictive' vaccination strategy to be used in response to an
epidemic. This tactic targets the farms at greatest risk. In the
model it shortened the outbreak almost as much as mass
vaccination.
Assuming that the necessary information about farms and
animal movements is at hand before an outbreak, the predictive
approach is preferable, says McConnell. Mass vaccination
requires large amounts of vaccine and systems for delivering it.
In most parts of Europe, which suffer only occasional outbreaks,
it doesn't make economic sense. Predictive vaccination is "a
precise strategy", he says.
The poor performance of ring-fence vaccination in Keeling's
study suggests that it might fail for other diseases too.
Infectivity and immunization are different for different
diseases, but "all have this transmission pattern", says
Keeling. Indeed, recent research showed that ring-fence
vaccination probably wouldn't contain smallpox2.
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