Vaccination against Foot and Mouth disease: Some Key Facts

by 5m Editor
1 April 2001, at 12:00am

This article, written by the Office of Science and Technology, UK Ministry 0f Agriculture, Fisheries and Food (MAFF) explains some key facts about vaccination against foot and mouth disease to aid understanding of how vaccination works and different possible approaches to applying it.

How Vaccination Works

1. Animals susceptible to foot and mouth disease can be immunised against the disease by vaccination.

2. Vaccines come in different forms and strengths. The UK Institute for Animal Health at Pirbright holds, on behalf of the International Vaccine Bank, stocks of an emergency vaccine which would protect against the O-type virus which is causing the present outbreak in the UK. 500,000 doses are ready to use in case they are needed over the next two months. The European Union Vaccine Bank holds 5 million doses of a suitable vaccine. Effective vaccines are also available commercially. Commercial vaccines need to be administered in two doses, three to four weeks apart, while the vaccine held at Pirbright needs one dose. Cattle require one whole dose of vaccine, sheep one third.

3. The impact of any vaccination varies between individual animals as well as between species. If the vaccine now available were given to a herd or flock, on average it would provide protection to 98% of them. The level of protection achieved will vary from animal to animal and will also depend on the degree of challenge to which they are exposed. The degree of challenge to a fully vaccinated herd of flock will be substantially reduced once the animals have developed immunity.

4. It takes a few days after vaccination for animals to develop immunity. Depending on the vaccine, this might be 3 to 4 days (for the IVB emergency vaccine) or 10 to 12 days (for commercial vaccines), during which the animals remain susceptible to infection. The level of immunity continues to increase over the following days, so the longer the time between vaccination and exposure to infection, the more effectively animals would be protected against the disease.

5. After a few months, the effects of vaccination begin to wear off. The single dose vaccine held at Pirbright would need to be boosted after 6 months. The commercial vaccines given with the booster dose would need to be boosted further after 12 months. The exact frequency of booster vaccinations depends on the state of the epidemic and the exposure of animals to live virus which is expected.

6. After vaccination, animals are unlikely to develop foot and mouth disease unless already infected at the time of vaccination. If exposed to the virus up to half of the animals may carry the virus, without becoming infectious or showing clinical symptoms of the disease: this is known as the carrier state.

7. Vaccine does not work if it is administered after an animal has caught the disease, though it may mask the clinical signs of infection. There is always a risk that this will happen, as there is an incubation period of about a week between catching the disease and showing the signs. An animal vaccinated during this period will still have foot and mouth disease and be capable of passing it on to others.

8. Females pass on immunity to their offspring through their milk. If young animals are vaccinated while they have maternal antibodies in their bloodstream, the vaccine immunity has to break through the maternal immunity, so protection is not certain. There is a period while the maternal antibodies are wearing off when vaccine may still not work, but the young animal is vulnerable to the disease, if exposed to infection.

Impact on Different Species

9. Across the world, many farmers use vaccination to protect their stock, particularly in areas where foot and mouth disease is endemic in the wildlife or among domesticated animals. It is most commonly used on cattle, because they are more valuable than sheep. The clinical signs of the disease are obvious in cattle, so the risk of vaccinating animals which already have the disease is lower than for sheep. Because of the way they are farmed, vaccination campaigns in sheep have been less successful.

10. Vaccinating pigs is common practice in some countries. Breeding pig herds are constantly producing progeny - around 20 piglets per sow per year. So, unless vaccine is administered at frequent intervals, there will be susceptible piglets in the herd, which could catch the disease. When pigs catch foot and mouth, they are very highly infectious: they can breathe out more than a thousand times as much virus as cattle or sheep. However strong the vaccination, this level of infectivity could overcome immunity, at least in some animals. So if one or two pigs in a herd catch the disease, they might pass it to others, even if they have been vaccinated.

11. There is some evidence that, left to itself, the disease can die out of a flock of sheep over a period of a few months: this seems to have happened in Greece and Italy, where surveillance following the end of an outbreak found some flocks where some animals had been infected and recovered. Uruguay successfully tackled the disease by vaccinating cattle but not sheep.

Different Approaches to Vaccination

12. Vaccination can be used in different ways. People talk of ring vaccination, protective vaccination, suppressive vaccination etc. Sometimes different people mean different things by the same term. The following paragraphs suggest one way to think about the different approaches.

13. Firebreak policy: vaccination can be used to create a firebreak by vaccinating in a zone around an infected area, or in a line across an area which is geographically contained, such as a peninsula. To be effective, the vaccination zone needs to start well away from the source of the infection, to avoid the risk that animals within it are already infected. Movements of susceptible animals and people who may have been in contact with them across the zone need to be carefully controlled for as long as the outbreak lasts, and for several weeks after the last outbreak in the infected area. In practice, however much care is taken, firebreak vaccination may not be completely effective in containing infection within an infected area, but, when it works well, the numbers of cases outbreaks occurring outside the zone are likely to be few, and therefore more easily controlled. Meanwhile, action against the disease would need to continue within the infected area at the centre of the firebreak.

14. Suppressive vaccination aims to control the spread of infection within and out of an infected area. It reduces the amount of virus circulating in the area, as the vaccinated animals will be less infectious. An approach along these lines has been agreed by the European Commission for use in the Netherlands. In the Dutch outbreak the EC approved vaccination of animals within 2 km of the infected premises, on condition that there was intensive surveillance for FMD within a further 10 km radius and that all vaccinated animals were eventually destroyed. To be effective as a means of disease control, suppressive vaccination needs very high coverage: usually at least 80% of the susceptible species are vaccinated. Because vaccination is not foolproof as a way of avoiding infection (see above), suppressive vaccination may not bring an outbreak under control as quickly or as effectively as fast cull of infected premises and the areas surrounding them.

15. Protective vaccination refers to vaccination of particular groups or species in an infected area in order to protect them against the spread of infection. It assumes that action continues in parallel to eradicate the epidemic in other groups or species.

16. When protective vaccination is used on a very large scale, over a whole region or country, it is sometimes described as mass vaccination. Tens of millions of doses would be required to do this for the UK. Countries which have mass vaccination policies generally have boundaries with countries where infection is widespread, or have other problems in containing the disease. It is more common to have a mass vaccination policy covering cattle and/or pigs than sheep, because of the large numbers in flocks of sheep, and their relatively low value.

17. These approaches can be combined. There has been discussion of whether cattle now in barns in Cumbria should be vaccinated, to protect them from infection when they go out into the fields for the Spring, as experience from the 1967 outbreak suggests that the risk of infection outside will be about twice as high as it is while they are inside. As they would be protected , they should not go on to spread the disease. So, as well as protecting the cattle it would reduce the potential level of infectivity in the area.

The End of the Outbreak : Resuming Trade

18. There is no known risk to human health from meat or other products derived from animals which have foot and mouth disease, or which have been vaccinated against it. However, countries free of FMD, including the EU, require controls on meat and meat products imported from countries which vaccinate against foot and mouth disease. The EU also requires that any emergency vaccination programme by a member state should be authorised by the Commission. An authorisation might include conditions for the resumption of trade

19. Once animals are vaccinated, they carry antibodies in their blood stream for some months which are not easy to distinguish from those carried for around 12 months by animals which have recovered from the disease itself. There are blood tests which can make the distinction, but on their own they do not give a complete reassurance that the virus does not persist in vaccinated animals.

Source: Ministry for Agriculture, Food and Fisheries