Effectiveness and costs of different strategies to eradicate Aujeszky's disease virusA. Stegeman
Institute for Animal Science and Health (ID-DLO), Department of Immunology, Pathobiology and Epidemiology, P.O. Box 65, 8200 AB Lelystad, The Netherlands
Abstract - In several regions of the world, either Aujeszky's disease virus (ADV) has been eradicated, an eradication programme has been implemented, or plans to eradicate the virus are being made. There is, however, a considerable difference between eradication campaigns with respect to the measures applied. Most frequently they include either stamping out infected herds, test-and-removal of infected pigs, vaccination, or a combination of these measures. In this paper, I will discuss the effectiveness and relative costs of these different eradication measures. Stamping out ADV-infected herds has been successful in the eradication of ADV from Great Britain, Denmark and parts of Germany and France. This strategy is successful if infected herds can be depopulated before they transmit the virus to on average more than 1 other herd. Consequently, a good surveillance system, a restricted contact rate between herds and sufficient killing capacity are prerequisites for the successful implementation of this strategy. Furthermore, movement restrictions have to be implemented in the area around infected herds. In addition, great efforts have to be made in preventing the virus from re-entering the country after it has been eradicated. Economic analyses have revealed that a stamping out strategy in endemically infected regions with a moderate to high pig density, even if technically possible, would be far too costly. Also, in ADV-free regions with a very high pig density, it is unlikely that a stamping out policy will result in a quick fade out of the infection once the virus has re-entered the region. Test and removal includes serological testing of all pigs in a herd and removal of those animals who have antibodies against ADV. Obviously, this strategy cannot eliminate actively circulating ADV. It can eliminate latent ADV from herds after active virus circulation has stopped either spontaneously, or by vaccination. Thus, in eradication programmes, test and removal is usually complementary to either stamping out, or area-wide vaccination. Because we do not know the rate of reactivation of seropositive pigs, we cannot say how quickly they must be culled. However, even if test and removal is not important for the eventual eradication of ADV, it shortens the time in which a herd or a region reaches the ADV free status. Because of the trade benefits accompanying such a status, test and removal is economically justified. In recent years, area-wide vaccination using marker vaccines, has been implemented to eradicate ADV from infected regions or entire countries. It has been reported that vaccination can sufficiently reduce ADV transmission among individually housed breeding stock. However, although vaccination also reduces transmission among finishing pigs, major (sub-clinical) outbreaks of AD may occur, even if they have been vaccinated twice. This indicates that area-wide vaccination has to be supported by additional measures that further reduce virus transmission, such as the housing of finishing pigs separated from breeding and replacement stock and applying all-in all-out of finishing pigs housed in the same room. Observations in the Netherlands indicate that the combination of vaccination and these management measures stops the transmission of ADV within a herd. As a result, the spread of the virus within the region (assuming homogeneous herd immunity of all herds) will also stop. This reasoning is supported by the results of vaccination campaigns in both the Netherlands and Germany. These countries now face the challenge of how and when to stop vaccination. Economic analyses have shown that in high density regions, a strategy in which first the transmission of the virus is strongly reduced and after a few years vaccination is accompanied by test and removal is preferred. Vaccinating the sows three times a year and finishing pigs once is best in regions with a moderate pig density, whereas double vaccination of the finishers is preferred in regions with a high density ( > 1000 pigs per km 2). If subsequently, a region has a high probability of being re-infected, continued vaccination is to be preferred economically. Stamping out infected herds seems the method of choice for eradicating ADV from regions with a relatively low incidence of infections. However, in endemically infected regions with moderate to high pig density, area-wide vaccination seems the best, if not the only way to eradicate the virus. For economic reasons, both strategies are best combined with test and removal of infected pigs in herds without actively circulating ADV.
Corresponding author: A. Stegeman Tel.: (31) 320 238 238; fax: (31) 320 238 050;
© INRA, EDP Sciences 2000