Sero-surveillance of wild boar in the Netherlands, 1996-1999

Rev. sci. tech. Off. int. Epiz., 2000, 19 (3), 848-854
Sero-surveillance of wild boar in the
Netherlands, 1996-1999
A.R.W. Elbers L.J.M. Dekkers(2) & J.W.B. van der Giessen(3)
(1) Department of Pig Health, Animal Health Service, P.O. Box 4,5280 AA Boxtel, The Netherlands
(2) Department of Pig Health, Animal Health Service, P.O. Box 9,7400 AA Deventer, The Netherlands
(3) Microbiological Laboratory for Health Protection, National Institute of Public Health and the Environment
(RIVM), P.O. Box 1,3720 BA Bilthoven, The Netherlands
Submitted for publication: 2 September 1999
Accepted for publication: 9 December 1999
Summary
From 1996 to 1999, blood samples were collected from wild boar shot during the
hunting season in Crown properties, national parks and the free wildlife belt in the
Netherlands. Sera were screened for the presence of antibodies against classical
swine fever virus (CSFV), swine vesicular disease virus (SVDV), Aujeszky's
disease virus (ADV) and Trichinella spiralis. The results of the sero-surveillance
system indicate that CSFV, SVDV and ADV are uncommon within the wild boar
population. Hence, the wild boar population is not thought to be an important
reservoir of these viruses in the Netherlands. Infection with ADV and CSFV is
endemic in the wild boar population in Germany. Since contact between the wild
boar populations of Germany and the Netherlands cannot be excluded in the
southern part of the Netherlands, continuation of the sero-surveillance system
seems appropriate.
In the decade before 1998, no antibodies to Trichinella spp. were found in the wild
boar population of the Netherlands. The detection of some seropositive animals
during the hunting season of 1998-1999 corresponds to the previous findings in
wild boar before 1988. However, the recent data do not have consequences for
the pig industry of the Netherlands, since the country has been considered
Trichinella-free for many decades.
Keywords
Aujeszky's disease virus - Classical swine fever virus - Serological surveillance - Swine
vesicular disease virus - The Netherlands - Trichinella spiralis - Wild boar.
Introduction
Disease control among wild boar is of concern to those
interested in wildlife and domestic livestock because the wild
boar population is susceptible to, and may serve as a reservoir
for, diseases of domestic pigs.
Aujeszky's disease virus (ADV), classical swine fever virus
(CSFV) and Trichinella spiralis have been reported in wild
boar populations in Europe and the United States of America
(USA) (1, 2, 5, 7, 11, 15, 17, 22, 23, 24, 30). To the
knowledge of the authors, swine vesicular disease virus
(SVDV) has not yet been detected in wild boar, but another
vesicular disease, vesicular stomatitis virus, has been detected
in wild boar in the USA (25).
In the Netherlands, the wild boar (Sus scrofa) population lives
in a restricted number of areas: in completely fenced areas
such as Hoge Veluwe and the Crown dominions in the
central-eastern part of the Netherlands, or in the so-called
'free-wildlife belt', consisting of the national parks of De
Veluwe (in the central Netherlands) and De Roerstreek (in the
southern Netherlands) in which wild boar live in partly
fenced areas (Fig. 1). The estimated number of wild boar
living in these areas was approximately 100, 500, 2,860 and
170 respectively (in the spring of 1998), indicating that the
wild boar population is fairly small in the Netherlands, with a
total of approximately 3,600 animals.
Since 1994, a sero-surveillance system has been set up in
co-operation with associations and trusts that manage the
areas in which the wild boar live. A collection of blood

ffey.sc/. teck Off. int. Epiz.. 19(3)849
© 1998 Gezondheidsdienst voor Dieren, Kadaster, Tapografische Dienst
1 't Loo Crown dominions
2 Hoge Veluwe National Park
3 De Veluwe
4 De Roerstreek
Fig.1
Areas with wild boar in the Netherlands that contribute to the
sero-surveillance System
samples from wild boar shot during the hunting season is
examined annually with respect to antibodies against ADV,
SVDV, CSFV and T. spiralis (6). Between November 1996 and
Februaiy 1999, blood samples were collected from a total of
368 wild boar (Table I).
sampling and sent by mail or courier service to the laboratory
of the Animal Health Service, Deventer. Blood samples were
centrifuged at 2,500g, and stored at-20°C until examination.
An aliquot of the sample was sent to the référence laboratory
of the Institute for Animal Science and Health (ID-Lelystad) in
Lelystad for investigation of antibodies to CSFV.
Sérum samples were tested for the présence of neutralising
antibodies against CSFV using the neutralisation
peroxidase-linked assay (27). The présence of antibodies
against SVDV were tested with a liquid-phase blocking
enzyme-linked immunosorbent assay (ELISA) (9). Sera were
tested for the présence of antibodies to glycoprotein E (gE) of
ADV by a commercially available ELISA, as described by Van
Oirschot et al. (34). Sera were examined for the présence of
antibodies against T. spiralis with an ELISA, as described by
Van Knapen et al. (33)...
Results
Sero-surveillance of wild boar between 1996 and 1999
revealed no évidence for CSFV, SVDV or ADV within the wild
boar population (Table II). During the hunting season of
1998-1999, antibodies against T. spiralis were detected in
nine out of 163 samples examined: five samples from area 1
('t Loo), three samples from area 3 (de Veluwe) and one
sample from area 4 (de Roerstreek).
Discussion
Materials and methods
Directly after the shooting of a wild boar, blood was collected
from the animal in two plastic, stérile tubes (plunger, gel,
9 ml). Tubes were identified by the unique licence number
that is given to the hunter. The tubes were cooled after
Table I
Number Bf blood samples from wild boar shot during the hunting season, collected in completely and partly fenced areas in the Netherlands
According to the classical epidemiological sampling theory
(3), if CSFV, SVDV or ADV infections were established in the
wild boar population, the probability of failing to detect
infection using the sampling scheme applied between 1996
and 1999 is very low. However, a continued sero-surveillance
System is required as an early waming System to detect
introduction and establishment of thèse viruses in the wild
boar population of the Netherlands in the future.
The occurrence of certain diseases in wild animal species in
the récent past has had a major influence on the existence and
nature of wild animal disease surveillance programmes in
many countries of Europe (14). Of particular importance in
this respect is classical swine fever (CSF). The pig is the only
animal which can perpetuate CSFV in nature, and hence the
YearFully fenced areas
(central-east région)Partly fenced areas
(central-east région)
65
99
79
243
Partly fenced areas
(southern région)
6
24
28
58
Total
1996-1997
1997-1998
1998-1999
Total
0
10
57
67
71
133
164
368

850
Rev. sci. tech. Off. int. Epiz., 19 (3)
Table II
Proportion of wild boar with antibodies against swine vesicular disease virus, classical swine fever virus, Aujeszky's disease virus and Trichinella
spiralis in a sample of wild boar shot during the hunting season in the Netherlands
Antigen Hunting season Total Antigen 1996/1997 1997/1998 1998/1999 1996-1999
List A diseases
Swine vesicular disease virus 0/71*
0/133 0/164
0/368
Classical swine fever virus 0/71 0/133 0/164
0/368
List B diseases
Aujeszky's disease virus 0/71 0/133 0/164
0/368
Trichinella spiralis 0/69 0 9/163
9/232
* Number of samples with antibodies/total number of samples
survival of the virus depends ultimately on domestic pigs and
wild boar. The importance of CSF for domestic pigs and
international trade in pork and pork products has caused
many national veterinary organisations to include wild boar in
sero-surveillance programmes (2, 16, 17, 19). Classical swine
fever has occurred in wild boar in many parts of the European
Union, including Germany, France, Italy and Austria (35),
and also recently in Switzerland (20). In the case of a sudden
occurrence of a CSF outbreak in these areas, the wild boar
population should always be considered as a possible virus
reservoir (1, 4, 13, 22). Since contact between the wild boar
populations of Germany and the Netherlands cannot be
excluded in the south of the Netherlands, continuation of the
sero-surveillance system for CSFV seems appropriate.
In the Netherlands, an ADV eradication campaign in domestic
pigs commenced in 1993, employing a nation-wide
compulsory vaccination programme with gene-deleted
marker vaccines (26). The programme has been very
successful, and is currently approaching the final stage of
ceasing vaccination (10). Identification of any lingering ADV
infections in the wild boar population is therefore important
for the eradication programme. The results of the
sero-surveillance system in wild boar between 1996 and 1999
indicate that, if present, ADV is uncommon within the wild
boar population. Thus, it seems that ADV infection in the wild
boar population is not an important wildlife reservoir in the
Netherlands. Therefore, the risk of reinfection of Aujeszky's
disease-free herds by the wild boar population is probably
negligible, especially as outdoor housing of pigs is uncommon
in the Netherlands. However, ADV is present in the wild boar
population in Germany. In the Federal States of Brandenburg,
Mecklenburg-Vorpommern, Niedersachsen, Sachsen-Anhalt
and Nordrhein-Westfalen, ADV-seroprevalence in wild boar
was found to be 9.8% (18), 0.3% (8), 1.7% (7), 0.9% (22) and
7% (16), respectively. In the Federal State of Brandenburg,
where the prevalence of ADV in wild boar is reasonably high,
ADV was eradicated from the domestic pig population in
1985. Infection with ADV in the wild boar population appears
to be endemic in this area and to persist completely separately
and, as yet, without affecting the domestic pig population.
Since contact can occur between the wild boar population of
Germany and the Netherlands, continued sero-surveillance
for ADV is recommended.
Eradication of SVDV in the Netherlands was achieved in
1975, although minor outbreaks of SVDV occurred in 1992,
1993 and 1994 (28). The isolate responsible for the outbreak
in 1992 was identical to a strain isolated in 1987 in Eastern
Europe, and the infection is assumed to have been introduced
from that area. Regular transport of weaners and slaughter
pigs occurs from the Netherlands to Italy. In 1997, 80,000
weaners and 570,000 slaughter pigs were transported by
truck to Italy (Productboard for Livestock and Meat, Rijswijk,
the Netherlands). Swine vesicular disease is endemic in Italy
(21), and therefore a risk of introduction of SVDV into the
Netherlands is presented by trucks returning from Italy.
Swine vesicular disease is an important Office International
des Epizooties List A disease and inclusion of SVDV in the
sero-surveillance system for wild boar is therefore warranted
(2).
Results of sero-surveillance of the wild boar population
between 1988 and 1998 indicated that Trichinella infections
were uncommon within the wild boar population of the
Netherlands (12, 31). However, low infection rates were
found occasionally. In the period from 1976 to 1980,
sero-surveillance of 993 wild boar revealed 1.8% positives
and detected a low rate of infection with Trichinella in two
animals, using artificial digestion of diaphragm tissue (32).
The detection of nine wild boar with antibodies to Trichinella
during the hunting season of 1998-1999 again indicates that
low infection rates are occasionally present in wild boar.
Whether these positive reactions to the ELISA represent wild
boar infected with T. spiralis is not certain because of the
serological cross reactivity between the different species of
Trichinella. Trichinella britovi has also been identified in
wildlife in the Netherlands (29). Future studies should
therefore also attempt to isolate Trichinella larvae from wild
boar, by peptic digestion of Trichinella-related muscles, and
eventually identify these to species level. However, the low

Rev. sci. tech. Off. int. Epiz., 19 (3)
851
infection rates of Trichinella in wild boar have not had any
consequences on commercially raised pigs in the Netherlands
(12).
Conclusion
No antibodies against CSFV, SVDV or ADV were detected in
the wild boar population of the Netherlands between 1996
and 1999, and the authors conclude that wild boar in the
Netherlands are not infected with these viruses at present.
Currently, the domestic pig population of the Netherlands is
also free of SVD and CSF. Therefore, the wild boar population
does not appear to be an important wildlife reservoir of CSFV,
SVDV or ADV in the Netherlands and constitutes a negligible
threat for the commercial pig industry. The threat is
diminished to an even greater degree because outdoor
housing of pigs is uncommon in the Netherlands.
Between 1988 and 1998, no antibodies to Trichinella were
detected in the wild boar population of the Netherlands. The
identification of some seropositive boar during the hunting
season of 1998-1999 corresponds to the previous findings in
wild boar before 1988. However, the recent data have no
consequences for the pig industry of the Netherlands which
has been considered Trichinella-free. for many decades.
The continuation of this sero-surveillance system in the wild
boar population is important for the pig industry of the
Netherlands and the international trade of pork and pork
products. The presence of CSFV and ADV in the wild boar
populations in neighbouring Germany increases the
importance of this sero-surveillance, since the possibility of
contact between the wild boar populations of Germany and
the Netherlands cannot be excluded.
Surveillance sérologique des sangliers aux Pays-Bas, 1996-1999
A.R.W. Elbers, L.J.M. Dekkers & J.W.B. van der Giessen
Résumé
Entre 1996 et 1999, des prélèvements de sang ont été effectués sur des sangliers
abattus pendant la période de chasse dans les domaines de la Couronne, les
parcs nationaux et les régions réservées à la faune sauvage aux Pays-Bas. Les
anticorps contre le virus de la peste porcine classique, le virus de la maladie
vésiculeuse porcine, le virus de la maladie d'Aujeszky et Trichinella spiralis ont
été recherchés. Les résultats de ce programme de surveillance sérologique
montrent que les virus de la peste porcine classique, de la maladie vésiculeuse
porcine et de la maladie d'Aujeszky ne sont pas répandus chez les sangliers et
que cette population ne constitue donc pas un réservoir important pour ces virus
aux Pays-Bas. En revanche, en Allemagne, la maladie d'Aujeszky et la peste
porcine classique sont enzootiques chez les sangliers. Comme des contacts entre
les sangliers des deux pays ne sauraient être exclus dans le sud des Pays-Bas, le
maintien du système de surveillance sérologique semble justifié.
De 1988 à 1998, aucun anticorps anti-Trichinella spp. n'avait été décelé chez les
sangliers aux Pays-Bas. La détection de quelques animaux possédant des
anticorps pendant la saison de chasse 1998-1999 confirme les observations
antérieures à 1988. Cependant, les données récentes n'ont aucune conséquence
sur la production de porcs domestiques aux Pays-Bas, dans la mesure où le pays
est considéré indemne de trichinellose depuis plusieurs décennies.
Mots-clés
Pays-Bas-Sanglier-Surveillance sérologique — Trichinella spiralis-Virus de la maladie
d'Aujeszky-Virus de la maladie vésiculeuse porcine-Virus de la peste porcine classique.

852
Rev. sci. tech. Off. int. Epiz., 19 (3)
Serovigilancia del jabalí en los Países Bajos, 1996-1999
A.R.W. Elbers, U.M. Dekkers & J.W.B. van der Giessen
Resumen
Entre 1996 y 1999 se recogieron muestras sanguíneas de jabalíes abatidos
durante la temporada de caza en propiedades de la Corona, parques nacionales o
zonas de protección de la fauna silvestre de los Países Bajos. Una vez extraídas
las muestras, el suero se sometía a pruebas de detección de anticuerpos contra
Trichinella spiralis y contra los virus de la peste porcina clásica, la enfermedad
vesicular porcina y la enfermedad de Aujeszky. Los resultados de este programa
de serovigilancia ponen de manifiesto que ninguno de los tres virus citados es
frecuente en la población de jabalíes salvajes, lo que da pie a pensar que este
animal no es un reservorio importante de esas enfermedades en los Países Bajos.
Las infecciones por los virus de la enfermedad de Aujeszky y la peste porcina
clásica son endémicas entre las poblaciones de jabalíes de Alemania. Dado que
no cabe excluir la posibilidad de contacto entre poblaciones de jabalíes de
Alemania y de los Países Bajos en la zona meridional del país, parece
conveniente seguir aplicando el sistema de serovigilancia.
Durante el decenio 1988-1998 no se detectaron anticuerpos contra Trichinella
spp. entre la población de jabalíes de los Países Bajos. La detección de algunos
ejemplares seropositivos durante la temporada de caza de 1998-1999
corresponde a la situación observada antes de 1988. Aun así, los nuevos datos no
tienen por qué afectar a la industria porcina neerlandesa, pues el país goza desde
hace muchas décadas del estatuto de libre de la enfermedad.
Palabras clave
Jabalíes - Países Bajos - Trichinella spiralis - Vigilancia serológica - Virus de la
enfermedad de Aujeszky - Virus de la enfermedad vesicular porcina - Virus de la peste
porcina clásica.
References
1. Aubert M., Picard M., Fouquet E., Conde J., Cruciere C.,
Ferry R., Albina E., Barrat J. & Vedeau F. (1994). - La peste
porcine classique du sanglier en Europe. Ann. Méd. vét., 138,
239-247.
2. Baradel J.M., Barrat J., Blancou J., Boutin J.M., Chastel C,
Dannacher G., Delorme D., Gerard Y., Gourreau J.M.,
Kihm U., Larenaudie B., Le Goff C., Pastoret P.-P., Perrau P.,
Schwers A., Thiry E., Trap D., Uilenberg G. & Vannier P.
(1988). - Results of a serological survey of wild mammals in
France. Rev. sci. tech. Off. int.. Epiz., 7 (4), 873-883.
3. Cannon R.M. & Roe R.T. (1982). - Livestock disease
surveys: a field manual for veterinarians. Australian Bureau
of Animal Health, Department of Primary Industry.
Australian Government Publishing Service, Canberra, 35 pp.
4. Caporale V., Rutili D., Nannini D., Di Francesco C. &
Ghinato C. (1988). - Epidemiology of classical swine fever in
Italy from 1970 to 1985. Rev. sci. tech. Off. int. Epiz., 7 (3),
599-617.
5. Corn J.L., Swiderek P.K., Blackburn B.O., Erickson G.A.,
Thiermann A.B. & Nettles V.F. (1986). - Survey of selected
diseases in wild swine in Texas. J. Am. vet med. Assoc., 189,
1029-1032.
6. Cromwijk W.A.J. (1995). - Serological investigation on wild
boar in the Veluwe region in the Netherlands. Tijdschr.
Diergeneeskd., 120, 364-365.
7. Dahle J., Patzelt T., Schageman G. & Liess B. (1993). -
Antibody prevalence of hog cholera, bovine virus diarrhoea
and Aujeszky's disease virus in wild boars in Northern
Germany. Dtsch. tierärztl. Wochenschr., 100, 330-333.