Twenty three week duration of immunity following intradermal or intramuscular administration of PrimePac PRRS™ in 2 week old piglets

PrimePac PRRS™ is a newly introduced modified life PRRS Type II vaccine that can be administered either intradermally (ID) with the IDAL device or intramuscularly (IM). The objective of this study was to evaluate duration of immunity of this vaccine via both administration routes in SPF piglets vaccinated at 2 weeks of age and challenged with a virulent PRRSv strain 23 weeks later.

Two-week old piglets were vaccinated with a single dose of 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 non-vaccinated challenge controls. Piglets were challenged intranasally 23 weeks after vaccination with PRRSV Type II (strain VR-2332). Ten days after challenge half of the animals of each group were sacrificed for post-mortem investigation of lung lesions and remaining pigs were euthanized 28 days after challenge. Other variables measured included PRRSv serology and viremia, clinical signs, rectal temperature and weight gain.

Between 3 weeks after vaccination and the time of challenge, 100% and 80% of G1 and G2 piglets were seropositive (ELISA). At challenge, serum-neutralizing antibodies (range: 1 - 5 log2) were also measured in 100% and 80% of G1 and G2 piglets and a rapid 2-fold increase occurred postchallenge. G3 pigs remained seronegative until time of challenge.

None of the pigs developed any post-challenge clinical signs. Although post-challenge rectal temperatures were not different between treatment groups, a larger increase was measured in G3 pigs. Weight gain between Day 10 and 28 was numerically higher in vaccinates than controls (G1 +0.305 kg; G2 +0.336 kg).

Post-challenge PRRSv viremia was significantly lower in G1 and G2 compared to G3 during the first 10 days post-challenge. The percent of virus positive serum samples were G1-12%, G2-20% and G3-75%.

Lung lesion scores were low ranging from 0.14±0.31 to 2.86±3.35 depending on groups. Day 10 lung lesion scores were significantly lower in G1 and G2 than G3, with odds for lower lung lesions being approx. 9 and 43 times higher in G1 and G2 than G3.

Intramuscular or intradermal vaccination of 2-wk old piglets with PrimePac PRRS reduced the negative effect of challenge infection with VR2332 23 weeks after vaccination. Post-challenge viremia and gross lung lesions 10 days after challenge were reduced in vaccinated pigs irrespective of vaccination route. Although not statistically significant, weight gain was also improved in vaccinated pigs compared to the controls at all measured time points.

Disclosure of Interest: None Declared.