Foot and Mouth Disease: Why not Vaccinate?
Authors: John Beale
Journal: Journal of the Royal Society of Medicine (2001)

At the time of writing (May 4), the Government believes the massive outbreak of foot and mouth disease in Britain to be under control. The option of a limited vaccination programme, in severely affected areas, has therefore been shelved. The aim is to reinforce the slaughter programme, to ensure that all infected animals are destroyed within 24 hours of detection. The main reason for the Government's earlier hesitancy about selective vaccination was opposition from farmers. What are the facts on vaccination, for control or prevention?

Foot and mouth disease (FMD) virus comes in seven major types (O, A, C, Asia 1, and SAT 1, 2 and 3). Unfortunately, variants within a type arise which are not cross protective, so the variant of the type in the vaccine must be matched as closely as possible with the strain circulating in nature. The Pan Asian O strain, the cause of the present outbreak, differs from the previously prevalent strains in Europe (e.g. OBFS, OLausanne, OKaufbeuren) but is closely similar to OManisa, a widely used vaccine strain first isolated in 1969. The latest FMD vaccines are purer and more potent than their forerunners and give at least partial protection even if the match is not perfect. Cross-neutralization studies indicate that the vaccine in the strategic reserve (OManisa) would give good protection against the outbreak strain. Another aspect of modern vaccines is the speed at which protection is generated1. Thus in cattle a single dose begins to protect by 4 days—although in pigs it takes longer, about 21 days.

One often-mentioned drawback of vaccination strategies is the difficulty of establishing whether antibodies found are due to infection or vaccination. In fact, these two sorts of response can now be distinguished: a method that seems convenient and reliable2 is to use an enzyme-linked immunosorbent assay (ELISA) for antibodies to a long B peptide from one of the nonstructural proteins of FMD virus that are present only in infected animals. A second worry is that infected vaccinated animals can become carriers. In experiments such as those described by Doel et al.1 about half the animals became carriers of virus in the pharynx, but these were exposed to massive challenge from contact with infected pigs soon after immunization. Under natural conditions carrier rates would be lower because of less intense and later challenge. Moreover, carrier animals in most experiments did not transmit infection to controls3, though clearly there is a risk. A third concern is food safety. However, there is plenty of evidence that products from vaccinated (and even infected) animals are safe. In Continental Europe vaccination was for many decades the general policy to control the disease, before the slaughter policy was introduced in 1992; no human-health issues arose. In the wake of the BSE fiasco, assurances that there is no danger might not be believed by a disillusioned public, but the level of anxiety does not seem high—an opinion poll in April suggested that most people would buy meat that came from a vaccinated animal.

If vaccine is used to control an epizootic, what happens to vaccinated animals once the outbreak has been dealt with? It would seem logical to slaughter all animals shown by a test such as that proposed by Shen et al.2 to be infected. Before the idea of vaccination was effectively shelved, Government policy (according to its website) was to vaccinate cattle in infected areas before they were turned out to grass and then let them live out their commercial lives whether they had been infected or not. Clearly this policy would have postponed the country's return to an FMD-free state; but one can understand the opposition of farmers to vaccination of healthy animals if this merely offered a stay of execution when without vaccination an uninfected animal might be allowed to survive. Another control strategy in infected areas would be to supplement existing measures by vaccination within 3 km of an infected farm, thus buying time for the hard-pressed authorities to slaughter animals and safely dispose of their carcasses. But if the country wished to regain its FMD-free status, these animals would ultimately have to be slaughtered.

When the British epizootic comes to an end, policies on FMD will need to be reconsidered. In particular, the review should include cost-benefit analyses such as were done before the European Commission adopted its import control and slaughter policy in 1992. The picture may have changed. With the continuing increase in international exchange of people and goods, more importations of FMD into disease-free areas seem inevitable. These trends, coupled with advances in vaccine technology, particularly the possible development of synthetic peptide vaccines that allow very rapid production in response to new strains, have strengthened the case for vaccination as the bedrock of control. But vaccination is not a trivial exercise. Though a single dose is recommended for outbreak control, routine immunization requires a primary course of two doses 3-4 weeks apart, a booster at 4-6 months and thereafter, for cattle, an annual booster; sheep have the same schedule but lower doses, and for pigs the question of boosters depends on the husbandry and the time to slaughter. The next consideration is the type and strain to use. In Europe, with O virtually the only type likely to arise, vaccination could safely be confined to this single type, but global monitoring of strains and types is clearly essential, as already done by the World Reference Laboratory at the Institute for Animal Health, Pirbright. A vaccination programme will be most effective if it incorporates modern techniques of diagnosis— differential serology to distinguish vaccinated animals from those infected, and a rapid PCR-based test on the farm for current infection. The decision whether to vaccinate routinely must take account of commercial factors; but if we simply consider animal welfare and the previous successes of vaccination as a component of programmes to eradicate the disease from large geographical areas, the choice is clear.