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Within the framework of a sero-monitoring system, in operation since 1996. blood samples from wild boar shot during the hunting seasons 1999-2000 and 2000-2001 in The Netherlands were screened for the presence of antibodies against classical swine fever virus (CSFV), swine vesicular disease virus (SVDV), and Anjeszky's disease virus (ADV). The results indicate that CSFV, SVDV, and ADV are uncommon in the wild boar population in the Netherlands. Because of the recent foot-and-mouth disease (FMD) epidemic in the Netherlands in 2001, blood samples (approximately 200 samples) from wild boar shot in the Netherlands during the hunting season 2001/2002 were examined for antibodies against FMD. To date, antibodies against FMD have not been detected.

During the hunting season 1996-1999, blood samples were collected from wild boar shot in The Netherlands. Sera were screened for presence of antibodies against classical swine fever virus (CSFV), swine vesicular disease virus (SVDV), Aujeszky's disease virus (ADV), and Trichinella spiralis. The results indicate that CSFV, SVDV, and ADV are uncommon in the wild boar population. Therefore, it seems that CSFV, SVDV, and ADV infection in the wild boar population is not an important reservoir in The Netherlands. ADV and CSFV infections are endemic in the wild boar population in Germany. Since contact between the German and Dutch wild boar populations can not be excluded, continuation of the sero-surveillance system seems appropriate. In the decade before 1998, the wild boar population in The Netherlands seemed to be free of T. spiralis. Whether the finding, in the hunting season of 1998-1999, of a few wild boar with antibodies against T. spiralis is an artefact or not, should be investigated in further research.

A surveillance programme for swine vesicular disease (SVD) and Aujeszky disease was set up in 1993 in the Netherlands. Blood samples are taken from pigs by local veterinarians to enable testing for the presence of antibodies against these viruses. A programme to guarantee the identity of pigs tested for these diseases has been in operation since late 1995. In this programme, pigs are identified on the basis of the DNA profiles of blood and hair samples. The hair samples are collected by the department of Animal Health Control. Analysis proves whether blood and hair samples are from the same animal. A total of 655 animals have been tested from 94 farms. In 38.6% of these farms differences were found between blood and hair samples.

Research was undertaken to study the effect of a 'three times a year vaccination schedule in sows' against Aujeszky's disease (AD) on the maternal immunity of their offspring at a large pig farm. Only gI-negative sows and piglets were induced in the study. The maternal immunity was measured in the 8-week-old offspring of 6 gilts and 13 sows, vaccinated in early pregnancy and in 8-week-old piglets born from 12 gilts and 21 sows vaccinated 3 weeks prior to farrowing. At an age of 8 weeks only 5 percent of the piglets, born from sows vaccinated during early pregnancy showed antibody titres against AD of > 1:40 in their serum. In piglets born from sows vaccinated 3 weeks prior to farrowing more than 50 (54.4) percent of the piglets showed antibody titres > 1:40 in their serum. It was concluded that using a 'three times a year vaccination schedule' in a gI negative herd may lead to groups of seronegative piglets on breeding and multiplier farms. The implications are discussed.

Antibody titres against Aujeszky's disease virus (ADV) in the plasma of slaughter pigs were compared with respect to the site of sampling, namely on the farm (one day before delivery to the slaughterhouse) and in the slaughterhouse during slaughter. No significant differences were noted in the antibody titres against ADV with respect to the different sampling sites. Blood samples collected at the slaughterhouse can be used for monitoring and surveillance systems within the framework of organised pig health care.

Aujeszky's disease is caused by a herpesvirus. The pig is the natural host and sole source of dissemination of the virus. After a primary infection, during which the pig excretes virus for 10-14 days, the virus remains latent in the host. Eradication has been achieved in individual herds, areas and countries with a low prevalence. The crucial question is whether it is feasible to eradicate the virus in heavily infected areas. Using so-called marker vaccines in conjunction with companion diagnostic test kits the virus was eliminated in individual herds. The preliminary results of extensive field studies to investigate whether intensive vaccination curtails the circulation of virus are encouraging.

The possibility of using sow's milk to detect antibodies to the gI protein of Aujeszky's virus was studied by the present authors. Antibody titres in serum (collected less than 30 days ante partum) and colostrum from sixty-three sows were determined using a commercially available ELISA; twenty-four of thirty-seven animals vaccinated with gI-positive vaccines showed an antibody response in both colostrum and serum. The titre in colostrum was significantly higher than it was in serum. Antibodies could not be detected in serum or in colostrum of the twenty-six animals which were only vaccinated with gI-negative vaccines. Subsequently, anti-gI titres were determined in milk collected during lactation. Antibodies were also detectable in milk of animals showing high colostrum titres (greater than 128) for at least seventeen days. This period decreased as the initial colostrum titre became lower. Antibodies could still be detected in milk of animals vaccinated with gI-positive vaccine, which were sero-negative at the time of the experiment. Milk therefore seems to be a useful alternative to serum in detecting anti-gI antibodies in sows.

On two farms in an area in which Aujeszky's disease is endemic, 192 pregnant sows were vaccinated with a gI-negative sub-unit vaccine against Aujeszky's disease. The rectal temperatures of the sows were recorded once daily for seven consecutive days, starting on the day of vaccination, and the vaccinated animals were observed for local and systemic reactions. The temperatures recorded did not exceed 39.8 degrees C in any case, and local or systemic reactions caused by vaccinations were not observed. One sow (0.6 per cent) aborted, but other symptoms of disease were not apparent, and there was no relationship with vaccination. In a second experiment, the carcasses of 120 pigs, vaccinated once or twice with the same vaccine, were carefully examined for local reactions. In eleven pigs (9.1 per cent) slight cicatricial reactions were visible within from 20 to 34 days after inoculation, the diameter not exceeding 1.5 cm in any of the cases. Traces of vaccination were no longer perceptible within 84 days after vaccination. The vaccine was found to be very safe for pregnant sows under field conditions.

Studies on the maternal state of immunity were done in 4-10 week-old piglets born of vaccinated sows free from Aujeszky's disease. The sows were inoculated with three different vaccines: two live vaccines (Aescovac Aujeszky) and the MK25 vaccine) and an inactivated vaccine (Geskyvac). The maternal state of immunity of the piglets was assessed from the titre of the virus-neutralizing (VN) antibodies in the blood. To do so, samples were taken from 300-400 4-10-week-old piglets per vaccine inoculated in the sow. At the same time, the VN titre of the blood of the sow was determined. The VN titres were highest in those sows which were vaccinated with Geskyvac. There was an obvious correlation between the VN titre of the sow and the maternal VN titres of the piglets. As a result, the maternal state of immunity in the sows vaccinated with Geskyvac also was highest. The value of the maternal VN titres in the assessment of maternal immunity was tested by exposing 28-37-day-old piglets showing various maternal VN titres to intracerebral challenge with 2000 TCID50 of Aujeszky disease virus (Van Doorn strain). Piglets showing VN titres higher than 1:8 were usually found to be immune against this challenge.

A standardized test model for Aujeszky disease (AD) vaccines in pigs is described. The first two vaccines tested, a modified live virus (MLV) vaccine and an inactivated vaccine, failed to produce satisfactory results. The inactivated vaccine induced a short-lived, but clear-cut increase in body temperature in some of the vaccinated pigs; the MLV-vaccine was shown to spread. Complete protection against intranasal (i.n.) challenge was not achieved with either vaccine in five-month-old pigs that had been vaccinated once or even twice two to three months previously. However, the vaccinated pigs were less severely affected, and for shorter periods, than were the control animals; losses from death did not occur. In the control groups, at least thirty per cent died, the average growth of the remainder was arrested for more than three weeks. The mean period of arrested growth after challenge of pigs vaccinated once with the MLV-vaccine was slightly shorter than that following a single vaccination with the inactivated vaccine: 5.8 and 9.5 days respectively. The average virus excretion of vaccinated pigs after challenge was less and was not as protracted as in unvaccinated controls. In pigs from sows hyperimmunized with the inactivated vaccine, maternal antibodies afforded partial protection against i.n. challenge at the age of two months. However, five-month-old pigs were fully susceptible, although most of them continued to show low levels of neutralizing antibody. The average virus excretion after challenge in the two age-groups was comparable with that in control pigs. Cattle were effectively protected by vaccination with the inactivated vaccine against i.n. challenge with AD-virus. It is recommended to vaccinate "threatened" cattle twice, for instance in the autumn when they are removed from the pastures for indoor housing. The possibility that temperature reactions may occur should be taken into account.

In the autumn of 1979, approximately, 400 animals on twenty farms were inoculated twice against Aujeszky's disease using an inactivated vaccine in a practice in the Achterhoek (province of Gelderland). The animals vaccinated were mainly yearlings and heifers on farms on which cattle and pigs could not be effectively separated. Adverse reactions to vaccination, other than a slight to moderate increase in body temperature, were not observed. Outbreaks did not occur in vaccinated cattle during the 1979-1980 winter season; on the other hand, Aujeszky's disease was observed in the pigs on a number of farms. The serological effects of vaccination were followed up in forty-one animals on sixteen farms. Very high antibody titres were attained, particularly after the second vaccination. Antibody titres continued to be recorded after twelve months in a number of the animals followed.

Chemicals are being used increasingly in the preparation of vaccines. Thus, an effective vaccine (MK25) against Aujeszky's Disease showing stable genetic characteristics was produced by having 5 iododesoxyuridine exert its action on a particular strain of virus. 5 Bromo-desoxyuridine also has an inhibitory effect on the synthesis of DNA viruses. The development of a new mutant of Aujeszky's Disease virus using this agent is reported. The resulting mutant, MK35, was found to be biologically stable, non-pathogenic for white mice, rabbits, sheep and piglets and, in addition, produced stable immunity in piglets, sheep and rabbits within four days after injection of this mutant.

A rather extensive outbreak of Aujeszky's disease in the area served by the above veterinarians is reported. Of 252 pig-breeding farms, eighty-two were contaminated. The other herds (170) were partly (81) vaccinated against the disease with the Bartha strain. Unlike de non-vaccinated group, the immunized herds showed few of any losses. In view of this fact it is concluded that Aujeszky's disease vaccination is a useful procedure (even in a contaminated environment).

Trials were made to study the extent to which 6-10-week-old piglets with and without virus-neutralizing (VN) antibodies are capable of a serological response to vaccination. The piglets were inoculated with three different vaccines: two live vaccines (Aescovac Aujeszky and MK25 vaccine) and an inactivated vaccine (Geskyvac). The titre of virus-neutralizing antibody in the blood was determined for each vaccine in approximately 200 piglets at the time of inoculation as well as within four and twelve weeks after inoculation. The maternal antibodies present at the time of inoculation were found to have an inhibitory action on the serological response following inoculation. Of those piglets in which a maternal VN titre was observed using the above technique, only 9 per cent showed an increase in titre after inoculation. This proportion was 74 per cent in the serologically negative piglets. Marked variations in serological response to the different vaccines were observed. the most positive response was that induced by Aescovac. There was an obvious relationship between the VN titre following inoculation and immunity against intracerebral challenge with 2,000 TCID50 of Aujeszky disease virus. Animals showing titres of 1:4 or over were immune to this challenge.

When Aujeszky's disease was detected on a large pig-breeding farm, all normal sows and boars were vaccinated with an ethyl-ethylene-imine (EEI) vaccine produced in the Bundesforschungsanstalt für Viruskrankheiten der Tiere (Federal Research Institute for Viruses Diseases in Animals) in Tübbingen, Western Germany. The vaccine contained DEAE dextran as an adjuvant. The vaccine was inoculated twice by intramuscular route at a three weeks' interval, 5 ml. being inoculated at one time. Hyperimmune serum was inoculated in young pigs. In addition, hygienic procedures were adopted, consisting in disinfection and isolation. The disease was successfully controlled within eight weeks. Sows which developed Aujeszky's disease showed hardly any symptoms suggesting lesions of the central nervous system. Pulmonary lesions were the outstanding clinical feature. The semen quality of the affected male swine had markedly deteriorated. This was resorted to normal, however, within from five to ten weeks. The incidence of abortion and piglet mortality were low. In view of the results of virological and serological studies, it was concluded that the persistence of this strain of virus (causing Aujeszky's disease) was very slight.

Vaccination against Aujeszky's disease (AD) does not lead to eradication of the disease. Nevertheless, to reduce the economic losses, vaccination against AD is widely practised in many countries. With regard to control of AD, a method to distinguish infected from vaccinated pigs is a prerequisite in those countries. This paper deals with such a method. The assay (gI-ELISA) makes use of 2 monoclonal antibodies directed against 2 different epitopes on glycoprotein I (gI) of AD virus (ADV), and detects antibodies against those 2 epitopes. The gI-ELISA is sensitive, specific and suitable for serological screening on a large scale. Five of the 14 tested ADV vaccines did not elicit antibodies to gI in pigs, in other words they were gI-negative. On the other hand, all field strains of ADV tested so far did induce antibodies to gI. Pigs that were first given a gI-negative vaccine and subsequently were infected with a field strain of ADV produced antibodies to gI. These antibodies appear into the blood approximately 2 weeks after infection, and probably persist for over a year. Field experiments have demonstrated that it is possible to identify infected pigs in vaccinated herds. There is strong evidence, that ADV has been eliminated in a large multiplyer herd by removal of gI-seropositive pigs. The results described above provide a sound basis to implement a combined vaccination-control programme for AD in the Netherlands.

On seven fattening farms and seven breeding farms, investigations were carried out to examine whether Aujeszky virus was capable of persisting in particular animal groups or units of the farm as a productive infection following a virologically verified outbreak. After an outbreak on fattening farms, virus persisted temporarily as newly arrived piglets became systematically infected. On the farms on which this could be accurately followed, virus persistence continued for at least 1 1/2 to 2 months. However, this cycle of infection was found to be interrupted on each farm within approximately three months after the outbreak, which resulted in a gradual return to a totally sensitive animal population, so that new outbreaks could occur. On the seven swine breeding farms, virus persistence in the farrowing house was examined by placing seronegative sentinel piglets with sows during the immediate post-partum period. Twelve sentinel piglets were placed in the various farrowing houses for a total period of sixty-seven weeks, during which time 172 sows farrowed. None of these piglets became seropositive during their stay on the breeding farms. The results of this study show that Aujeszky's disease virus did not persist on breeding farms and that the productive infection on fattening farms in only transient following an outbreak. Therefore, it is very likely that new outbreaks of the disease are due to reintroduction of Aujeszky's disease virus on the farm. Though the role of latent carrier animals cannot be totally disregarded, natural reactivation of latent virus is believed an exception rather than the general rule.

A microneutralisation test and an indirect fluorescent antibody technique for the detection of antibodies to Aujeszky-virus in pig sera are described. All the sera were tested with both techniques and the results were compared. Serum dilutions of less than 1:32 which produced a positive reaction in the indirect FAT were considered nonspecific. Bij contrast, all neutralising antibody titers of less than 1:8 gave nonspecific reactions in the indirect FAT.

The serological studies reported in the present paper were done on forty-one pig-breeding farms contaminated with Aujeszky's disease and four farms that were free from this disease and on which fifty per cent of the animals had been inoculated with the Bartha vaccine. The serological studies were based on the serum neutralization test (SN test). On contaminated farms, the majority of the animals present at the time of outbreak of the disease were serologically positive. Comparison of the serological findings on farms on which there had been outbreaks of the disease from two to seven and from seven to twelve months previously, showed that there was hardly any difference in titre levels between the two groups. Of twenty-eight breeding boars used for service at the time of the outbreak, twenty-one per cent were serologically negative. Serological studies in breeding gilts showed that virulent virus was only transmitted up to two months after the outbreak of the disease.