Detection of polychlorinated biphenyls and dioxins in Belgian cattle and estimation of the maximal potential exposure in humans through diets of bovine origin.

The methodology used to detect a polychlorinated biphenyl (PCB)/dioxin contamination in a Belgian cattle population that was not exposed to the PCB/dioxin incident in 1999 is presented. This population is directly or indirectly destined for human consumption. The methodology consisted in the systematic sampling of all calf-fattening stations and groups of cattle destined for export, and in the random sampling of slaughter cattle. This approach is compared to the method described in directive 96/23/CE from the European Council. When PCB concentrations exceeded the tolerance level of 0.2 micro g/g body fat (seven congeners with numbers 28, 52, 101, 118, 138, 153, and 180), dioxins (seventeen 2,3,7,8-substituted congeners of PCDD and PCDF) were also determined. The prevalence of Belgian slaughter cattle with PCB concentrations above this cutoff was 0.3% (95% confidence interval: 0.01-1.50%). Results indicate that the incidence of contamination was minimal, with environmental origin and common in all industrial countries. The maximal potential exposure of an adult human consumer to dioxins through diet of bovine origin is estimated in two worst-case scenarios. The first one corresponds to the consumption of contaminated food products by a small number of consumers during a long period (local consumption) and the second simulates the consumption of contaminated products by a large number of consumers during a short period (supermarket purchase). The theoretical maximum daily intake of dioxins in adults was respectively 374 and 123 pg TEQ/d. The estimated maximum increase of dioxin body burden corresponds to 7 pg TEQ/g fat in the local consumption scheme and 0.07 pg TEQ/g fat in the supermarket consumption scheme.

Prevalence of paratuberculosis (Johne's disease) in the Belgian cattle population.

The national bovine paratuberculosis (PTB) seroprevalence (apparent prevalence) in the Belgian cattle population was determined by a serological survey that was conducted from December 1997 to March 1998. In a random sample of herds (N=556, 9.5%), all adult cattle of 24 months of age or older (N=13,317, 0.4%) were tested for the presence of antibodies using a commercially available absorbed ELISA test kit. The PTB median within-herd seroprevalence (proportion of detected animals within the seropositive herds) and the PTB individual-animal seroprevalence (proportion of detected animals) were, respectively, 2.9% (quartiles=1.6-5.6) and 0.87% (95% confidence interval (CI)=0.71-1.03). The PTB herd seroprevalence (proportion of detected herds) was 18% (95% CI=14-21). Assuming a test sensitivity and specificity of 45 and 99% [Sweeney et al., 1995. J. Vet. Diagn. Invest. 7 (4), 488; Sockett et al., 1992. J. Clin. Microbiol. 30 (5), 1134], respectively, the median true within-herd prevalence and the true individual-animal were estimated to be 7 and 2%, respectively. The true herd prevalence of Mycobacterium paratuberculosis infection was first estimated according to currently accepted methodology. This calculation revealed that the specificity of the used test has a dramatic effect on the estimation; assuming a test sensitivity of 45% and a true within-herd prevalence of 7%, the true herd prevalence estimation decreased from 36 to 0.8% if the test specificity decreased from 99. 9 to 99%, respectively. This sensitivity analysis showed that the practical limits of the accuracy of the used screening test jeopardize the estimation of the true herd prevalence within reasonable confidence limits, because the within-herd PTB true prevalence was low. For this reason we augmented the herd specificity for herds with larger adult herd size (>5). This was done by increasing the cut-off number of positive cattle required (>/=2) to classify a herd truly positive and including herds with one positive test result if there was historical evidence of PTB (previous diagnosis and/or clinical signs). This approach resulted in an estimated true herd prevalence of M. paratuberculosis infection of 6%. The true herd prevalence for dairy, mixed and beef herds was, respectively, 10, 11 and 3%.

Prevalence of bovine herpesvirus-1 in the Belgian cattle population.

The national bovine herpesvirus 1 (BHV-1) seroprevalence (apparent prevalence) in the Belgian cattle population was determined by a serological survey that was conducted from December 1997 to March 1998. In a random sample of herds (N=556), all cattle (N=28478) were tested for the presence of antibodies to glycoprotein B of BHV-1. No differentiation could be made between vaccinated and infected animals, because the exclusive use of marker vaccines was imposed by law only in 1997 by the Belgian Veterinary Authorities. Twenty-one percent of the farmers vaccinated continuously against BHV-1. In the unvaccinated group, the overall herd, individual-animal and median within-herd seroprevalences were estimated to be 67% (95% confidence interval (CI)=62-72), 35.9% (95% CI=35.0-36.8) and 33% (quartiles=14-62), respectively. Assuming a test sensitivity and specificity of 99 and 99.7%, respectively, the true herd, individual-animal and median within-herd prevalence for the unvaccinated group of herds were estimated to be 65, 36 and 34%, respectively. The true herd prevalence for dairy, mixed and beef herds were respectively, 84, 89 and 53%; the true individual-animal prevalence for those types of herds were, respectively, 35, 43 and 31%; whereas, the true median within-herd prevalences were 36, 29 and 38%.

Evaluation of parenteral vaccination methods with glycoproteins against Aujeszky's disease in pigs.

A comparative evaluation of vaccination methods with glycoproteins for the induction of immune responses and protection of the pig against Aujeszky's disease virus (ADV) was performed. Different vaccination routes (intradermal (i.d.) versus intramuscular (i.m.)), inoculation sites (the neck versus the back) and number of inoculation points (2 versus 6) per site were compared. Body weight (BW) changes and viral excretion after challenge were compared with virus-neutralizing titers, antigen-specific IgG and IgA responses in serum and virus-specific lymphoproliferative responses in peripheral blood during the immunisation period. According to BW changes better protection was obtained with six-point than two-point i.d. injections. i.d. vaccination in the back at six points gave similar results as i.m. vaccination in the neck but appeared inferior in the reduction of virus excretion. Regarding the immunological parameters, the virus-specific IgA response in serum gave the best indication for protection. It can be concluded that according to BW changes, six-point i.d. immunisation in the back and i.m. immunisation in the neck provided the best protection and that six-point i.d. injections resulted in a better vaccination than two-point i.d. injections.

Inhibition of virus replication in the tonsils of pigs previously vaccinated with a Chinese strain vaccine and challenged oronasally with a virulent strain of classical swine fever virus.

After an oronasal (O.N.) infection with classical swine fever (C.S.F.) virus, virus multiplication can be detected in the tonsils from Day 2 post infection (p.i.) till death. The course of viral replication during the first 10 days after O.N. challenge exposure of pigs, previously vaccinated with a Chinese strain vaccine in the presence or absence of maternal antibodies, was studied using direct immunofluorescence techniques on cryostat sections and virus isolations. When piglets were challenged O.N. in the presence of maternal antibodies, virus replication in the tonsils still occurred. The multiplication period and the localization of the virus, however, were directly correlated to the maternal antibody levels. The maternal antibody level also seems responsible for the efficacy of the vaccination to prevent challenge virus replication in the tonsils: vaccination in the presence of low maternal antibody titers completely inhibited virus replication; vaccination in the presence of high maternal antibody titers only reduced the multiplication period of the O.N.-administered virulent virus. In both cases, animals were challenged 1 week post vaccination. Vaccination of seronegative animals resulted in an almost complete inhibition of the virus replication in the tonsils during a full fattening period: cryostat sections revealed a limited virus replication in three out of 20 animals. In one of these animals, virus replication was probably so negligible that virus isolation remained negative.

An epizootic of African swine fever in Belgium and its eradication.

In March 1985 an outbreak of African swine fever was diagnosed in Belgium, in the province of West Flanders. The source of the infection was probably pork imported from Spain which was fed to only one boar. A total of 12 farms were infected in the epizootic out of 185 farms which were in contact. Severe control measures were imposed and the pigs on 60 farms were slaughtered (34,041 animals). In September 1985, it was concluded that African swine fever had been eradicated. This conclusion was based mainly on a serological survey of 3008 farms (116,308 blood samples) which remained negative.

Epizootics of respiratory tract disease in swine in Belgium due to H3N2 influenza virus and experimental reproduction of disease.

In Belgium, influenza virus was isolated from swine in 22 epizootics of respiratory tract disease in swine during 1984. In 8 of the epizootics, H3N2 influenza virus, related to the A/Port Chalmers/1/73 strain, was isolated. Intratracheal inoculation of the isolates induced clinical signs. It was concluded that the A/Port Chalmers/1/73 strain was established in the Belgian swine population and was responsible for the epizootics of respiratory tract disease in swine.

Comparison of serological tests used in three state veterinary laboratories to identify maedi-visna virus infection.

Three immunoassays used in Dutch, Belgian and British state veterinary laboratories for the detection of antibodies to maedi-visna virus were compared. These were an indirect enzyme-linked immunosorbent assay (ELISA), an indirect immunofluorescence test (IIFT) and an agar gel immunodiffusion test (AGIDT). The results indicated that all three assays were of similar sensitivity and specificity with identical results being recorded for 91 of 100 samples. The results of the ELISA and the AGIDT produced the closest correlation with agreement in 97 of 100 samples. With the IIFT and the AGIDT there was agreement in 94 of 100 samples and with the ELISA and the IIFT in 91 of 100 samples.

Vaccination of cattle against pseudorabies (Aujeszky's disease) with homologous virus (herpes suis) and heterologous virus (herpes bovis 1).

Study was made to determine whether vaccination of cattle against pseudorabies (PR; Aujeszky's disease) affords protection upon subsequent intranasal challenge exposure with virulent virus. Vaccinations were performed with a commercially available oil-adjuvant PR virus vaccine, in some cases supplemented with A1(OH)3 (given subcutaneously), with the attenuated NIA4 strain of PR virus (given intranasally), and with a commercially available temperature-sensitive mutant of bovine herpesvirus-1 (infectious bovine rhinotracheitis [IBR] virus) given intranasally. Challenge exposure was performed intranasally with 10(3) median lethal doses (LD50) of the virulent PR virus. In earlier experiments, it was found that LD50 median tissue culture infective doses of this virulent PR virus in primary pig kidney cells represented about 1 LD50 for cattle. This LD50 was practically no different for cattle whether or not they had antibodies against PR virus before exposure to virulent PR virus. In the seropositive animals, antibodies were present presumably as a consequence of a previous IBR infection in the field. Vaccination intranasally with NIA4 or IBR virus did not result in serologic response or protection against challenge exposure. The inactivated-virus vaccine induced a good serologic reaction, but there was poor protection against challenge exposure. Consequently, vaccination of cattle at risk of exposure to virulent PR virus cannot be advised. In the present experiments, initiation of infection with virulent PR virus in cattle always resulted in disease and death.