Foot and mouth policy conundrums.

Abstract

Britain's outbreak of foot and mouth disease highlights how effective control measures have been to prevent a major outbreak for more than 30 years given the distribution and highly infectious nature of the virus, reports Nigel WilliamsAlthough Europe and North and Central America are generally free of foot and mouth disease, it remains endemic and occurs frequently in many countries in Africa, the Middle East, Asia and South America. Animals can become infected by inhalation, ingestion or reproduction. Under the right conditions long-distance spread over several kilometres by wind-borne virus can occur.But the real challenge to maintaining disease-free status in certain parts of the world is the ability of the virus to persist in meat and other food products as they are carried or traded around the world. FMD virus can survive for long periods in meat if the pH does not fall below 6.2. It can also survive in frozen lymph nodes, bone marrow and viscera. FMD virus can also survive well in salted and cured meats and in non-pasteurised dairy products. So the virus can spread by the import of infected products or by passengers on aircraft and ships, through the mail or on fishing vessels or other boats.Although vaccines made from inactivated virus are used in many parts of the world, some vaccinated animals may not show disease but still become infected and shed small amounts of virus. Resistance also falls fairly quickly and animals must be re-vaccinated at regular half-year intervals. At present the presence of antibodies to the virus cannot be used to distinguish between animals that have been vaccinated and those that are infected.The alternative ‘stamping out’ policy in normally disease-free areas such as Europe involves the slaughter of all infected animals, their contacts and others in close proximity. Animal movements are also restricted and protection and surveillance zones of 3km and 10km respectively are established around each outbreak site. This hugely expensive, difficult and disruptive policy is seen as essential to maintain disease-free regions and to gain economic advantage by producing certified disease-free products. But the strains of such a policy have led to growing demands to use vaccination in an attempt, at least, to stem the spread of the disease by creating a ‘ring-fence’ of vaccinated animals around an outbreak.Research is also under way to refine the analysis of vaccinated and infected animals. During infection, antibodies to both structural and non-structural proteins are produced in the animal whereas after vaccination generally only antibodies to structural proteins are induced. One non-structural protein, 3ABC, is the focus of a number of studies aiming to distinguish between vaccinated and infected animals. Researchers at the World Reference Laboratory for Foot and Mouth Disease at Pirbright, UK, together with colleagues elsewhere, have been testing an assay based on antibodies to this protein in sera from animals in affected areas including the Balkans, North Africa and South America. Tests are continuing and the European Commission has been organising workshops to examine whether the test may help in the analysis of animals in any program of ring vaccination in the region of a confirmed outbreak. Success of this new test or any other that can distinguish infected animals could see the control of foot and mouth disease transformed.