Modifying procedures to assess immune competence in mature boars

Survival of progeny through to slaughter age is a key driver directly impacting on profitability and animal welfare within the Australian Pork Industry. Vaccinations against diseases causing mortality, such as Actinobacillus pleuropneumoniae (APP), are not always effective, suggesting a proportion of animals are responding poorly to vaccination. Immune responsiveness, the body’s ability to respond to foreign antigens and render it harmless, involves a complex network of factors (Mallard et al. 1992). Since it is not possible to identify all of the genes that contribute to enhanced immune competence, an alternative strategy is to consider immune competence as a quantitative trait with a measurable phenotype (Hine et al. 2012). Procedures using test antigens (Mallard et al. 1992; Wilkie and Mallard 1999) have been developed to assess immune competence phenotype in pigs, combining measures of an animal’s ability to mount both an antibody mediated immune response (AMIR) and cell mediated immune response (CMIR). This study tested the hypothesis that similar procedures, using commercial vaccines rather than test antigens to induce measurable responses, could be used to assess immune competence in mature boars. Use of commercial vaccines removes the requirement for test antigens to be registered for use in food-producing animals.

To assess AMIR, nine mature boars were bled on d 0 to establish base line levels of anti-tetanus toxoid specific immunoglobulin G1 serum antibody before being vaccinated with Ultravac 5-in-1 (Zoetis, Rhodes, NSW, Australia), containing tetanus toxoid antigen. Boars were vaccinated again at d 21 and bled on d 30 to measure secondary antibody responses. An in-house ELISA was developed (Miller et al. 2008), to measure antibody levels, represented as a sample to positive (S/P) ratio calculated for each boar at each time point. Delayed type hypersensitivity (DTH) reactions to vaccine preparations were measured to assess CMIR in the same nine boars. The DTH reactions were measured as the increase in skin fold thickness at the injection site 48 h after the vaccine was injected intradermally. Two injection sites were investigated: (1) the base of left ear and (2) the perineal area. On d 30 after vaccination, each boar received intradermal injections of either Ultravac 5-in-1 or Equivac T (Zoetis^®), both containing tetanus toxoid antigen. At each injection site the double skin fold thickness (mm), recorded in triplicate using a Harpenden Skinfold Caliper (Bowers Group, Burgess Hill, UK), was assessed both pre- (d 30) and 48 h post-injection (d 32). For CMIR, the phenotype analysed was the average value of the three replicates for skin thickness at each time point, fitting a mixed model where time (d 30 v. d 32), Site (Ear v. Perineum) and Antigen (Ultravac v. Equivac) were considered as fixed effect.

The antibody response was significant (P < 0.0001) between d 0 (S/P ratio: 0.05 ± 0.07) and d 30 (S/P ratio: 0.71 ± 0.07) demonstrating the vaccination induced a measurable antibody response. The DTH reaction was also significant (P < 0.0001), with average skin thickness increasing from d 30 (4.03 ± 0.25 mm) to d 32 (6.16 ± 0.25 mm). The DTH reaction measured at the base of ear (3.06 ± 0.43 mm) and perineal site (2.97 ± 0.43 mm) were not significantly different (P > 0.05), indicating that both sites are suitable for DTH testing. There was no significant difference in the magnitude of DTH reactions observed for Ultravac (5.31 ± 0.21 mm) and Equivac (5.63 ± 0.21 mm) antigens. In conclusion, a testing procedure based on the use of commercially available vaccines to induce measurable immune responses was developed to assess immune competence in mature boars.