Effect of porcine circovirus type 2 (PCV2) load in serum on average daily weight gain during the postweaning period

Porcine circovirus type 2 (PCV2) is a ubiquitous virus that mainly affects nursery and fattening pigs causing systemic disease (PCV2-SD) or subclinical infection. A characteristic sign in both presentations is reduction of average daily weight gain (ADWG). The present study aimed to assess the relationship between PCV2 load in serum and ADWG from 3 (weaning) to 21 weeks of age (slaughter) (ADWG 3-21). Thus, three different boar lines were used to inseminate sows from two PCV2-SD affected farms. One or two pigs per sow were selected (60, 61 and 51 piglets from Pietrain, Pietrain×Large White and Duroc×Large White boar lines, respectively). Pigs were bled at 3, 9, 15 and 21 weeks of age and weighted at 3 and 21 weeks. Area under the curve of the viral load at all sampling times (AUCqPCR 3-21) was calculated for each animal according to standard and real time quantitative PCR results; this variable was categorized as "negative or low" (<10(4.3) PCV2 genome copies/ml of serum), "medium" (≥10(4.3) to ≤10(5.3)) and "high" (>10(5.3)). Data regarding sex, PCV2 antibody titre at weaning and sow parity was also collected. A generalized linear model was performed, obtaining that paternal genetic line and AUCqPCR 3-21 were related to ADWG 3-21. ADWG 3-21 (mean±typical error) for "negative or low", "medium" and "high" AUCqPCR 3-21 was 672±9, 650±12 and 603±16 g/day, respectively, showing significant differences among them. This study describes different ADWG performances in 3 pig populations that suffered from different degrees of PCV2 viraemia.

Comparison of different vaccination schedules for sustaining the immune response against porcine reproductive and respiratory syndrome virus

In order to better understand how immunization against porcine reproductive and respiratory syndrome virus (PRRSV) can be improved using commercial vaccines, different strategies of immunization were applied in the field using an inactivated vaccine (INV), a modified live vaccine (MLV) or a combination of the two and the responses compared. In experiment 1 (E1), 21 piglets were distributed in three groups. Group A was vaccinated with a commercial INV at 2.5, 3.5 and 6.5 months old; group B pigs received the INV at 1.5, 2.5, 5.5 and 6.5 months old, while pigs in group C were kept as unvaccinated controls. At 7.5 months of age all pigs were challenged with PRRSV and followed for 21 days. In experiment 2 (E2), 32 piglets were distributed evenly in four groups. Groups A, B and C were vaccinated with a commercial MLV at 1.5 months old, while group D pigs were kept as controls. At 4.5 months old, groups A and C received the INV while B received a second MLV, 1 month later group C pigs received a third INV. At 6.5 months old all pigs were challenged as in E1. In both experiments, total antibodies, neutralizing antibodies (NA) and cell-mediated immunity (CMI) were evaluated, and viraemia was determined after challenge. In E1, immunization with an INV induced high interferon-γ responses after the second and subsequent vaccinations. Development of NA after challenge was faster in INV vaccinated pigs compared to unvaccinated controls. In E2, re-vaccination with INV induced NA responses similar to re-vaccination with MLV; however, a significant increase in NA titres after challenge was only detected in group C pigs. The use of combined protocols (MLV+INV) was superior to the use of MLV alone in inducing cell mediated immunity. In conclusion, the highest immune responses against PRRSV after a single shot were achieved with MLV; after that, INV re-vaccination should be considered as the best strategy to induce significant boosters.

Protection of newborn animals through maternal immunization

Providing protective immunity to neonatal animals in early life is associated with numerous challenges regarding vaccine safety and efficacy. A much simpler approach is maternal vaccination, either before or during pregnancy, to provide the neonate with passively transferred immunity. In humans, the medical, societal and legal risks of immunizing pregnant women are important considerations in undertaking this approach. By contrast, maternal vaccination has been successfully employed in the animal health industry for decades. These veterinary vaccines have proven to be safe and efficient. Although only passively transferred antibodies have been extensively studied, other immunological mechanisms may be equally important in providing maternally derived immunity.

Porcine reproductive and respiratory syndrome (PRRS) virus-specific interferon-gamma(+) T-cell responses after PRRS virus infection or vaccination with an inactivated PRRS vaccine

Although field studies have found porcine reproductive and respiratory syndrome (PRRSV) inactivated vaccines to be beneficial in reducing losses linked to PRRSV infection, immune mechanisms induced by these vaccines need better understanding. In the study reported here, we examined the interferon-gamma(+) (IFNgamma(+)) PRRS-specific T cell responses induced after infection and vaccination with an inactivated PRRS vaccine. Autologous monocyte-derived dendritic cells loaded with the PRRSV P120 strain were used to re-stimulate ex vivo T cells that had been primed in vivo by either the virus or the vaccine, or both. Virus-specific IFNgamma(+) T cells were quantified by using a porcine IFNgamma- ELISpot assay. A specific but low live virus-induced response was observed between days 35 and 70 for most of the pigs tested, while a significant inactivated vaccine-induced PRRSV-specific IFNgamma(+) T-cell response was measured soon after vaccination. Moreover, we observed that vaccination of pre-challenged pigs clearly favoured the PRRSV-specific cell-mediated immunity primed by the live virus. To characterize further the nature of the PRRSV-specific T cells, the different T-cell subsets involved in PRRSV immunity were analyzed by flow cytometry. We showed that the inactivated vaccine was able to prime both CD4(+)CD8(int+) and CD8(high) virus-specific T cells and that CD4(+)CD8(int+) were preferentially recalled by the live virus.

PMWS: experimental model and co-infections

Porcine circovirus type 2 (PCV2) is now recognised as the causal agent of porcine multisystemic wasting syndrome (PMWS), an economically important wasting disease of young pigs [J. Vet. Diagn. Invest. 12 (2000) 3]. Gross lesions of PMWS include generalised lymphadenopathy, hepatitis, nephritis and pneumonia and typical histological lesions include lymphocytic depletion and multinucleated giant cell formation in lymph nodes, degeneration and necrosis of hepatocytes, and multifocal lymphohistocytic interstitial pneumonia. This communication will review the results of experimental infections of gnotobiotic (GN), colostrum-deprived (CD) and colostrum-fed (CF) pigs within our group, and elsewhere, with PCV2 and the conclusions that can be drawn from this work.

Differential reactivity of a monoclonal antibody directed to the membrane protein of porcine reproductive and respiratory syndrome virus

A monoclonal antibody (2C12) against the 19 kDa membrane (M) protein of a Canadian isolate of porcine reproductive and respiratory syndrome (PRRS) virus was produced. By indirect immunofluorescence (IIF) cytoplasmic fluorescence was observed in infected cells, but the pattern of fluorescence was generally different and intensity was weaker than that observed using the nucleocapsid protein-directed monoclonal antibody SDOW17. When tested by IIF towards a total of 26 PRRS virus isolates from Canada, 122 isolates from the US and 13 isolates from Europe the 2C12 MAb reacted with all the North American isolates tested including the VR-2332 isolate and the vaccine (RespPRRS) isolate. However no reactivity was observed towards the European isolates tested including the Lelystad virus. This reactivity pattern suggests that the epitope recognized by this MAb on the M protein of PRRS virus appears highly conserved among North American isolates but absent or weakly expressed on European isolates of PRRS virus.

Study of feline leukemia virus immunity

Fifteen specific-pathogen-free cats were experimentally infected with FeLV; 8 cats recovered after transient or nondetectable viremia, and 7 cats became persistently viremic. Four additional cats served as noninfected controls. Antibodies to whole FeLV (ELISA and immunoblot [western] analysis), antibodies to fixed FeLV-infected cells, and virus-neutralizing antibodies were monitored for as long as 3 years after infection. As a group, cats that recovered after acute infection developed higher titer of these various antibodies than did cats that became persistently viremic. However, specific combination or titer of antibodies was not always found in recovered cats or in persistently viremic cats. Six cats that had recovered from acute FeLV infection nearly 3 years earlier were reinfected with the same virus. Three of the cats appeared to be resistant to reinfection, 2 cats became transiently viremic, and 1 cat became persistently viremic. Slight and transient anamnestic ELISA-detectable antibody response to whole virus was seen after reinfection; immunofluorescence- and western blot-detectable responses were not greatly enhanced. Five FeLV-recovered cats were monitored for 2 years; FeLV infection spontaneously recurred in 1 cat.