Foot-and-mouth disease - the vaccine
As the United
Kingdom comes to terms with its current foot-and-mouth disease (FMD)
outbreak, many questions are raised, not least the current control
measure of mass slaughter of animals. While slaughter is the only
assured method of eradicating the virus, and therefore the
resumption of export trade, is it defensible? Media comment has
revealed some misconceptions about this and the alternative policy
of vaccination. This is an opportunity for clarification.
FMD
is an acute, highly contagious disease that affects cloven-hoofed
wild and domesticated animals. It is characterised by the formation
of vesicles in and around the mouth and on the feet, It reduces
feeding and often causes lameness. Abortion, sterility, permanent
decline in milk yield, decrease in meat production, and reduction in
breeding ability are common sequelae. Mortality can result and
although low for adult animals, can be higher than 50% in the young.
The virus needs to be eliminated to re-establish disease-free
status; failure would have serious economic consequences.
FMD can spread rapidly by several mechanisms and so it is a
very difficult disease to control. The cull of all infected and
in-contact livestock, the so-called 'stamping-out' policy (along
with associated zoo-sanitary measures and movement restrictions) is
aimed at eliminating the virus as quickly as possible. The reality
of mass slaughter has raised public concern, notably as to why such
actions are required when vaccines are available.
Vaccination, in whatever form, is always an option, and one
that could be implemented in three major ways. Routine immunisation
of animals (pan-vaccination) and/ or temporary mass vaccination
utilising 'conventional' vaccine. The third method is ring
vaccination around a discrete focus of infection, not adequately
controlled by slaughter, using high potency 'emergency' vaccine. To
understand the reluctance of the UK to initiate any vaccination
measures we must reflect on the arguments that led to such a policy
in this and other countries which are normally free of FMD.
Arguments for a vaccination policy
Assuming the most appropriate vaccine strain was used, the
amount of virus in the environment and associated with livestock
should be reduced.
Mass vaccination, particularly under circumstances of wide
geographical dissemination, could be used if 'stamping-out' and
associated zoo-sanitary measures failed. Where a single epicentral
outbreak occurred, 'emergency' ring-vaccination could be
implemented.
Vaccination would eliminate the need for
'stamping-out'.
Arguments for a non-vaccination
policy
Control of disease without resorting to vaccination should
lead to a quicker return to disease free status and the removal of
export embargoes.
L'Office International des Epizootics (World organisation for
Animal Health) requires countries that suffer an outbreak of FMD
and which do NOT vaccinate to wait for a period of at least 3
months after the last case before they can regain their FMD free
status, provided stamping out and serological surveillance have
been applied. By contrast, if stamping out is not carried out and
if vaccination is employed, the period until FMD free status can
be regained is 24 months.1
If it were decided to vaccinate there would be a delay until
immunity develops. Vaccination should then protect against
clinical signs but not against local virus replication in the
oropharynx of ruminants. If vaccinated animals are challenged by
virus they can become carriers' irrespective of their immune state
and can therefore be a potential source for further transmission.
Surveillance for the disease is easier under a non-vaccination
regime, particularly where clinical signs are often obscure
e.g., sheep.
Routine vaccination is costly, logistically difficult and
labour intensive. Wild life would still be susceptible and they
could harbour the virus.
For effective pan-vaccination, antigenic matching of the
vaccine and field strains is essential.
Continued vaccination campaigns are considered by many
countries to be an indication of uncertainty about the presence of
the virus.
A non-vaccination policy was considered to be more
economical.2
Non-vaccination would allow free movement of livestock and
animal products.2
With pan-vaccination, the export trade in bloodstock to many
parts of the world would be lost.
Although arguments
for non-vaccination outweigh those for vaccination, the UK situation
must be monitored constantly before further action is considered.
The decision to vaccinate would he taken at Ministerial level with
advice from the Chief Veterinary Officer and others. It remains an
option if other control measures are not working. But because
vaccination would isolate us from other EU members, and countries
like the United States that do not vaccinate, there is an
understandable reluctance to use
vaccine.
References
- Office International des Epizooties (OIE), International
Animal Health Code for Mammals, Birds and Bees, Edition 1999.
- Commission of the European Communities (1989) Report from
the Commission to the Council on a study carried out by the
Commission on policies currently applied by Member States in the
control of foot-and-mouth disease. SEC (89) 1731 final, 61
pages.
Paul V Barnett CBiol, MIBiol, PhD
Intemational FMD
Vaccine Bank
Institute for Animal Health
Pirbright |