Combining quantitative and qualitative approaches to determine viability of a potential Salmonella Typhimurium vaccination program in pigs in Belgium.

Vaccinating pigs against Salmonella Typhimurium (ST) might be a way to control ST infections at farm level and reduce human infections. Two main issues have to be addressed before such a mandatory vaccination program can be implemented: the effective reduction of attributable human incidence has to be demonstrated and all socio-economic barriers impacting the attitude and motivation of the pig sector have to be lifted. The present research used a quantitative microbial risk assessment model to estimate the effect of different hypothetical Salmonella spp. and ST mitigation strategies on the annual prevalence of human salmonellosis along the minced pork production chain. In addition, a qualitative study aimed to list the potential concerns of the pig sector about the implementation of a hypothetical future vaccination program. The following themes were the most often mentioned: awareness, vaccine cost-benefit/effectiveness, legislation, monovalent vaccine, time and labour required to vaccinate, vaccine registration and trade restriction. Cost-effectiveness and cost-benefit of vaccination were cited by all the key interviewees (n = 12). However, based on the quantitative microbial risk assessment model, vaccination alone may not be sufficiently effective to reduce the annual human salmonellosis prevalence. A combination of different control measures along the food chain, with a special focus on interventions at the slaughterhouse, might be more effective in achieving the desired goal than vaccination alone.

Schmallenberg Virus in Belgium: Estimation of Impact in Cattle and Sheep Herds.

Schmallenberg virus (SBV) emerged during summer 2011. SBV induced an unspecific syndrome in cattle and congenital signs (abortions, stillbirths and malformations) in domestic ruminants. To study the impact of SBV in Belgium, a phone survey was conducted upon September 2012. Hereto two groups of cattle farmers (A and B) and two groups of sheep farmers (C and D) were randomly selected. Farms from groups A (n = 53) and C (n = 42) received SBV-positive result at RT-PCR in the Belgian National Reference Laboratory (NRL). Farms from groups B (n = 29) and D (n = 44) never sent suspected samples to NRL for SBV analysis but were however presumed seropositive for SBV after the survey. Questionnaires related to reproduction parameters and clinical signs observed in newborn and adult animals were designed and addressed to farmers. As calculated on a basis of farmers' observations, 4% of calves in group A and 0.5% in group B were reported aborted, stillborn or deformed due to SBV in 2011-2012. The impact as observed by sheep farmers was substantially higher with 19% of lambs in group C and 11% in group D that were reported aborted, stillborn or deformed due to SBV in 2011-2012. Interestingly, abortions or stillbirths were not clear consequences of SBV outbreak in cattle farms, and the birth of a deformed animal was an essential condition to suspect SBV presence in cattle and sheep farms. This study contributes to a better knowledge of the impact of the SBV epidemic. The results suggest that SBV impacted Belgian herds mostly by the birth of deformed calves, stillborn lambs and deformed lambs. This work also demonstrates that the birth of a deformed calf or lamb was a trigger for the farmer to suspect the presence of SBV and send samples to NRL for further analyses.

Delivery of DNA vaccines by agarose hydrogel implants facilitates genetic immunization in cattle.

The present study demonstrates the interest of two slow-release systems as vaccination tools in cattle. Two experiments show that a first intradermal administration of one DNA vaccine dose combined with the slow-release of a second dose conduct to a priming of the bovine herpesvirus 1-specific immune response similar to the one generated by two discrete administrations 4 weeks apart. The first experiment demonstrates the efficacy of the slow-release system with well-characterized Alzet osmotic pumps, whereas the second experiment extends the same concept with innovative agarose hydrogel implants. These latter implants are cheaper and more convenient than the osmotic pumps or repeated intradermal administrations since they contribute to an efficient priming of the immune response in a single manipulation of the animals.

Prime-boost strategies combining DNA and inactivated vaccines confer high immunity and protection in cattle against bovine herpesvirus-1.

DNA vaccines have frequently been associated with poor efficacy in large animals. In the present study, one administration of an inactivated marker vaccine to cattle considerably boosted both humoral and cellular arms of the immune response primed with Bovine herpesvirus-1 (BoHV-1) DNA vaccines encoding glycoprotein D (gD) or gC+gD. Calves vaccinated according to the DNA prime-inactivated boost also showed significantly enhanced virological protection as compared to controls. The 4-logarithms reduction of virus shedding observed in primed-boosted animals was comparable to the one previously reported in calves immunized twice with marker vaccines. Intradermal immunization of cattle with DNA vaccines promoted a Th2-biased immune response but also primed a cellular component that was further boosted by the inactivated vaccine. Individual IgG2 titers of vaccinated calves were significantly correlated to IFN-gamma production. The immunization protocol described in the present study demonstrates the complementarity between DNA and conventional marker vaccines.

Isolation of a glycoprotein E-deleted bovine herpesvirus type 1 strain in the field.

During a field trial to evaluate the efficacy of repeated vaccinations with bovine herpesvirus type 1 (BHV-1) marker vaccines, a glycoprotein E (gE)-negative BHV-1 strain was isolated from the nasal secretions of two cows, eight months after vaccination with a gE-negative live-attenuated vaccine, initially given intranasally, then intramuscularly. The strain isolated was characterised using immunofluorescence, restriction analysis and PCR. All the techniques used identified the isolated virus as a gE-negative BHV-1 phenotypically and genotypically identical to the Za strain used as a control.

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%.