Issue |
Vet. Res.
Volume 31, Number 1, January-February 2000
|
|
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Page(s) | 160 - 161 | |
DOI | https://doi.org/10.1051/vetres:2000025 | |
How to cite this article | Vet. Res. (2000) 160-161 |
Terminating Aujeszky vaccination in the Netherlands?
A. De Koeijer and A. StegemanInstitute for Animal Science and Health (ID-DLO), P.O. Box 65, 8200AB Lelystad, the Netherlands
Abstract -
Since 1993, the Netherlands have been using obligatory vaccination against Aujeszky's
disease to eradicate this virus (ADV) from the porcine population. The disease incidence
has become rather low, but can we stop vaccinating? This leads to the following questions.
When is the incidence low enough to try a non-vaccination policy? What is the expected size
of an outbreak when the virus is introduced? How often do we expect outbreaks? What is the
best way to stop an outbreak? To answer such questions, we developed a model describing the
spread of the virus in Dutch pig husbandry. It contains a submodel describing the dynamics
of the infection within one farm. The results of this submodel were used in an overall model
that describes the transmission of the infection between farms. We distinguished several
different types of farms and different transmission routes. The basic reproduction ratio
of ADV between different herds
could be calculated from the model. When this
reproduction ratio is below one, the epidemic will fade out to extinction, when above
one, a major part of the dutch herds may become infected during a major outbreak. Data
from a large survey for animal transport were used to quantify transmission by that route.
Data from a recent outbreak of classical swine fever were used to quantify transmission
by other routes than animal transport. Finally data from an Aujeszky outbreak in Denmark
were used to calibrate the model for ADV in an unvaccinated population. The model shows
that presently in the Netherlands
ranges from 0.6 in areas with low pig density to 0.8
in areas with high pig density. This was used to validate the model for the decreasing
prevalence over the last 5 years. If vaccination is stopped only amongst finishing pigs,
will remain below 1 in areas with rather low pig density, but in very dense areas,
will be slightly above 1. Thus, in areas with low pig density, major outbreaks are not
expected when vaccination of finishing pigs is discontinued. We found that eradication
is difficult at this moment because (1) the prevalence of the infection is still rather
high in some areas and (2) the risk of introducing the virus from abroad is substantial.
However, in a northern part of the Netherlands, both pig density and Aujeszky's disease
prevalence are very low. In this areas, the risk of new introduction of the virus is also
rather low and eradication is feasible if re-introduction of the virus can be minimised.
The vaccination program could be stopped after an intensive survey and final stamping out
of the last sources of infection. We calculated the expected size of such outbreaks and
methods to stop the outbreak. For the northern part of the Netherlands we found that the
basic reproduction ratio between herds,
was 3.2, whereas
was 6.8 in the area with
the highest pig density, so in all areas an outbreak may occur when the virus is
reintroduced. In areas with high pig density, an outbreak cannot be stopped by a ban on
animal transport, supported by an extra clinical survey
.
Corresponding author: A. De Koeijer Tel.: (31) 320 238321; fax: (31) 320 238050;
e-mail: A.A.deKoeijer@id.dlo.nl
© INRA, EDP Sciences 2000