Onset of immunity is four weeks following intradermal or intramuscular vaccination of 2 week old piglets with PrimePac PRRS™

The objective of this study was to evaluate the onset of immunity of PrimePac PRRS, a modified live PRRSv Type II vaccine, in piglets vaccinated intramuscularly (IM) or intradermally (ID) with IDAL, followed by challenge with PRRS Type II strain VR2332 4 weeks later.

Two-week old piglets were vaccinated with one dose PrimePac PRRS vaccine (produced by MSD-AH, The Netherlands) dissolved in Diluvac Forte. Vaccinations were done in the neck with either 1 ml IM (G1; n=11) or 0.2 ml ID (G2; n=10). Piglets in a third group (G3, n=10) were nonvaccinated challenge controls. Piglets were challenged intranasally 4 wks after vaccination with PRRSV Type II strain VR-2332. Ten days after challenge half of the animals in each group were sacrificed for post-mortem investigation of lung lesions with a weighted lesion score, and remaining pigs were examined 28 days after challenge. Other variables measured included PRRSv serology (IDEXX PRRS X3; S/P ratio); viremia and nasal shedding measured on porcine alveolar macrophages (AUC – area under curve); clinical signs; rectal temperature and weight gain between 10 and 27 days postchallenge.

After vaccination or challenge, no PRRSV related clinical signs were observed in any of the piglets. Post-challenge rectal temperature was significantly lower in G1 (-0.7°C) and G2 (-0.7°C) than in G3.

Weight gain between Day 10 and 27 was significantly higher in vaccinates than control pigs (G1 +0.245 kg; G2 +0.171 kg).

At challenge, G1 and G2 pigs were PRRSv seropositive while G3 pigs were seronegative. By 10 dpc, all G1, G2 and G3 pigs seroconverted with the highest S/P ratios in G1 and G2. Post-challenge PRRSv viremia was significantly lower (p< 0.05) in G1 (AUC: 17.4) and G2 (AUC: 15.2) compared to G3 (AUC: 22.2) pigs during the first 10 days of challenge and PRRSv could not be isolated from nasal fluids in any of the animals. The percentage of all collected serum samples that were found virus positive was 68% (G1), 67% G2) and 79% (G3).

Lung lesion scores were low, ranging from 0.99±2.15 to 4.97±3.34 for all three groups and tended to be lower in G1 and G2 than G3. These differences were only significantly different between G1 and G3. Although the odds for lower lung lesions on Day 28 were 13.7 and 6.07 times higher in G1 and G2 pigs, respectively, than in G3.

Two week old piglets vaccinated intradermally or intramuscularly were protected from a virulent PRRSv challenge 4 weeks later as demonstrated by the absence of increased rectal temperature, reduced level of viremia, weight loss and lung lesions.

Disclosure of Interest: None Declared.