It is ten years since the Netherlands was hit by an outbreak of foot and mouth disease (FMD). FMD is a highly contagious viral disease of animals which still occurs in many places in the world. When FMD is discovered in a country that used to be free of the disease, other countries will not accept animals and animal products from that country for a long time. For a country like the Netherlands which is very dependent on exports of animal products, an outbreak of FMD will have serious consequences.
A month before the outbreak in the Netherlands in 2001, FMD was discovered in the United Kingdom. The Netherlands was immediately put on alert and extra measures were taken to prevent the introduction and transmission of the disease. Despite all its efforts, it could not prevent FMD being discovered on a Dutch farm on 21 March 2001. The fact that the outbreak ultimately remained limited in the Netherlands to 26 infected farms and only lasted a month is largely thanks to the stringent measures taken to tackle the outbreak.
Unique to the Dutch approach was the use of emergency vaccination. Although this measure is permitted by the EU FMD directive, it had not previously been used since the European Union stopped preventative vaccination against FMD in 1992. Because emergency vaccination during an FMD outbreak had not previously been applied, there was great uncertainty surrounding it. Could FMD infected animals be sufficiently distinguished from FMD-vaccinated animals? Could the virus still spread unnoticed in a vaccinated population? What was the risk for neighbouring countries and trading partners? For these reasons, all the 186,645 vaccinated animals were later culled. On behalf of the Dutch government, the Central Veterinary Institute has conducted a great deal of research over the past ten years in order to answer these questions. As a result of this research, more insight has been acquired into the action and safety of emergency vaccination, and international regulations have consequently been amended. This means that in the event of another FMD outbreak, emergency vaccination may be used and there will be no need to remove and cull vaccinated animals.
In order to validate the FMD tests which could distinguish between infected and vaccinated animals, the so-called DIVA (Discrimination of Infected and Vaccinated Animals) tests, in 2004 an international workshop was organised. The Central Veterinary Institute (CVI), part of Wageningen UR, played an important role in this workshop by contributing test material. During this workshop, 6 tests were validated which could distinguish between infected and vaccinated animals, i.e. the DIVA tests. The comparison showed that the test sensitivity of the DIVA tests may have been lower than those of conventional tests, but high enough to detect the antibody response against the virus in a group of vaccinated animals.
A great many transmission experiments were also carried out in pigs, calves, dairy cattle and sheep, which showed that there was no transmission of FMD in a vaccinated population, or if there was, sufficient clinical symptoms were exhibited so that the disease did not remain unnoticed. It was thus proved that emergency vaccination can prevent the spread of the FMD virus and removed concerns that vaccination could mask its transmission.
Using these quantitative data about infection between animals and the data about transmission of the FMD virus between farms in the Netherlands, in 2001 an epidemiological simulation model was developed. This epidemiological simulation model enables the effectiveness of various tackling strategies to be compared. The main result is that in an area with a high livestock density, the use of emergency vaccination results in a shorter epidemic, fewer infected farms and fewer non-detected infected animals compared with the application of standard EU measures. The results of the epidemiological simulation model provided the input for an economic model developed by LEI, part of Wageningen UR, which showed that emergency vaccination was the preferred strategy, also in terms of costs. The model study also shows that after all the vaccinated animals had been tested in a final test round, emergency vaccination does not produce a higher risk for trading partners.
The results of the CVI study support Dutch policy to apply emergency vaccination in a future FMD outbreak. However, international legislation still penalises countries which do this, because in these international regulations trading restrictions are abolished later after use of the vaccination, creating the risk that many animals must be culled due to welfare problems. Our studies showed that there is no scientific basis for the assumption of a greater risk with the use of emergency vaccination. The CVI will therefore continue its efforts to gain acceptance for the use of emergency vaccination without extra trading restrictions in the international regulations of both the OIE (World Organisation for Animal Health) and the EU.
Aldo Dekker
Researcher at the Central Veterinary Institute, part of Wageningen UR