The tail end of the epidemic and the exit to FMD-free status
by Dr Ruth Watkins


During the tail end the following situations pertain:

 

1.      Symptomatic acute infections in previously uninfected herds and flocks
are still occurring. These may be both on the outskirts of areas of infection
or occur within the hotspots, especially as animals are exposed to
contaminated grass.

 

2.      Asymptomatic acute infection is still spreading amongst sheep, Nsilent
infectionb. This is very much more frequent in sheep than other species.

 

3.      Asymptomatic acute infection may occur and still be spreading in cattle
(the calves that were responsible for bringing infection into Holland were
asymptomatic and are thought to have been exposed whilst waiting in a siding
at a station where infected sheep were taken in France: Source Simon
Barteling.) Just as with sheep not all infected herds of cattle can be
presumed to have declared themselves with clinically recognisable FMD.

 

4.      Persistent infections remain in some seropositive animals in herds or
flocks in which infection has not been recognised or has been asymptomatic,
but in which the chain of acute infections has ceased. These could be in
cattle as well as sheep. (There is some experimental evidence that pigs may
be persistently infected that contradicts the previously held view that this
was not so: source Fred Brown.) Persistent infections in some sheep may last
up to nine months, but in some cattle this may be as long as 3 years.

 


The reluctance of MAFF to send samples to Pirbright or to believe their
results:


As with human infectious disease clinicians (vets) dont like to believe that
they could be wrong about their clinical diagnoses. Neither do they want to
believe that infection could be more widespread than is clinically recognised
until it is demonstrated to be so by laboratory testing. There is reluctance
on the part of vets and MAFF to acknowledge the valuable contribution the
diagnostic laboratory at Pirbright can make to the accuracy of diagnosis and
extent of infection, especially in sheep (source: Prof Donaldson and Dr
Kitching).


The EEC directives clearly state that vets are responsible for seeing that
samples are collected from any holding in which infection is suspected. They
also state that samples should be taken from holdings in a surveillance zone
that extends 10km round each confirmed infected holding, confirmed by
laboratory testing. Particularly with this strain of Pan Asia serotype O FMDV
a large number of infected holdings can be expected to emerge when
serosurveillance is undertaken.

 

An infected holding is one in which there is evidence of infection, so together with the infected contiguous culled holdings the outbreak figure is currently over 3,000 (Charles Clover in the Telegraph based on the governments revelation of the laboratory results on about a 10th of those culled.)
Without the collection of samples for the laboratory the extent of this
epidemic will never be clearly known, nor the patterns and dynamics of the
infection.


The EEC acknowledges that the current epidemic in the UK is unlike any in
Europe in the last 10 years. We are on our own and can propose to the EEC how
we intend to manage it. We have already taken the unprecedented steps of
culling contiguous holdings and culling holdings within the 3Km protection
zone, one that is more usually used for ring vaccination.


Management proposal to exit to FMD-free status:

 

7        -Declare that we aim for reinstatement one year after the last outbreak has been identified. We may have to wait for at least 18 months by EEC and OIE rules.

7        -Use vaccination to halt further spread of FMD. It is notable that the last outbreak in Holland occurred 5 days after vaccination was completed. We could shorten our tail end of clinical outbreaks (1) above by several months. This could be done by ring vaccination around current outbreaks. If one year elapses before we regain FMD-free status we do not have to attempt to cull all infected sheep,(2) and (4) above, as persistently infected sheep will clear the virus at most after nine months. However we do have to stop infection spreading inthese flocks, or the rare case of a persistently infected animal infecting other susceptible animals so starting another outbreak.

 

Rather than search out and kill every flock with seropositive sheep, the best strategy would be to test all flocks and vaccinate all animals without antibodies
(seronegative) including lambs, leaving the seropositive sheep in the flock.
Uninfected flocks could also be vaccinated in the infected zones (surveillance zones). Where direct or indirect contact can occur as on upland or mountain commons all those animals that are released should also bevaccinated, before being released. The vaccine plan could be an initial course (before release) with a booster at 6 months to cover the whole 12 month period.


The problem of infected cattle remains particularly as their persistent
infection lasts longer than one year. There will be rather less of these in
(4) above and few in (3). Again by testing herds in the infected zone for
antibodies those seropositive cattle could be culled and the seronegative
cattle vaccinated within a herd including calves.

 

In the case of rare breeds seropositive cattle could be tested to see if they had a persistent infection, they might not. A rare breed animal with a persistent infection could be kept on the holding for at least one year, surrounded by vaccinated animals, until such time as eggs and semen had been collected (I need tocheck if persistently infected animals shed virus anywhere but from their
mouth and respiratory tract).

All Buffalo herds should be tested for antibodies and seropositive animals
culled and the rest vaccinated, as I believe buffalo can have a persistent
infection for the rest of their life and remain infectious (an exception in
domestic animals).


Animals infected subsequent to their vaccination are likely to shed virus at
a low titre, and are unlikely to infect other vaccinated animals that have
responded with a good level of antibody. Nor have these animals ever been
implicated in spread of infection when they subsequently become persistently
infected.

 

If 80% of animals are successfully immunised this will be sufficient to stop further acute infections. Vaccination against FMD is responsible for control and eradication of the virus. In Europe the 100,000s of outbreaks had been controlled by vaccination rather than slaughter and the FMD virus eradicated by 1991. Vaccination in Europe led to FMD-free status without vaccination, and not to endemic of infection.


Testing at Pirbright:


The test for antibody is the basic tool of serosurveillance. I understand
Pirbright and Weybridge laboratories can test 50,000 serum samples a day for
antibodies. Each positive on the competitive screening test for antibody is
confirmed by a neutralisation test.


PCR can be done when the presence of virus in acute or persistent infections
is sought. Samples to analyse for virus are essential as the EEC stipulates
that any changes in the infecting strain should be sought (This would be by
sequencing a PCR product directly from a sample). This would be of great
interest in our extensive outbreak. Has the virus adapted (evolved, acquired
new mutations), to sheep for instance, during our epidemic?