Submitted to: Pig Veterinary Society International Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: March 30, 2010
Publication Date: July 18, 2010
Citation: Gauger, P.C., Vincent, A.L., Loving, C.L., Lager, K.M. 2010. Enhanced Pneumonia With Pandemic 2009 A/H1N1 Swine Influenza Virus in Pigs. In: Proceedings of the International Pig Veterinary Society Congress, July 18-21, 2010, Vancouver, Canada. p. 255. Technical Abstract: Introduction. Swine influenza A viruses (SIV) in the major swine producing regions of North America consist of multiple subtypes of endemic H1N1, H1N2, and H3N2 derived from swine, avian and human influenza viruses with a triple reassortant internal gene (TRIG) constellation (1). Genetic drift and reassortment has resulted in four H1 phylogenetic clusters designated as alpha, beta, gamma, and delta that co-circulate in U.S. swine (2). Control of SIV has relied on the use of commercially available or autogenous influenza vaccines in the U.S. swine industry. However, inactivated vaccines have demonstrated poor cross-protection against heterologous homosubtypic, or heterosubtypic SIV. The emergence of diverse genetic subtypes has increased the need for effective SIV vaccines with adequate cross-protection. In this report, we describe a vaccine-heterologous virus challenge model demonstrating enhanced lung lesions in pigs given inactivated delta-cluster H1N2 vaccine (human seasonal H1-lineage HA) challenged with heterologous pandemic 2009 A/H1N1compared to non-vaccinated challenged pigs. Materials and Methods. Twenty, three-week old conventional pigs free of SIV, PRRSV and M. hyopneumoniae were randomly divided into two groups. The vaccinated and challenged (V/C) group was vaccinated with 10**50% tissue culture infectious dose (TCID50) units of A/SW/MN/02011/08 (MN08) inactivated by UV irradiation with the addition of a commercial adjuvant. Vaccine was administered intramuscularly at six and three weeks prior to challenge. V/C and non-vaccinated/challenged (NV/C) pigs were inoculated intratracheally with 2ml of 1x10** TCID50/ml A/California/04/2009 H1N1 (CA/09). Five pigs remained non-vaccinated, non-challenged controls (NV/NC). Pigs were observed daily for signs of clinical disease and fever. Serum samples were obtained at each vaccination, challenge and 5 days post-infection (dpi). Nasal swabs were taken on 0, 3, and 5 dpi to evaluate nasal virus shedding. All pigs were necropsied at 5 dpi. Lungs were removed and evaluated for the percentage of lesions typical of influenza virus infection in pigs. Each lung was lavaged with 50ml MEM to obtain bronchoalveolar lavage fluid (BALF). Results. All pigs were free of influenza A virus and antibodies prior to vaccination. At 24 hours post-challenge, V/C and NV/C pigs had significant (P<0.05) febrile responses compared to controls. At 72 hours post challenge, V/C pigs had a significantly elevated (P<0.05) febrile response to infection that persisted to 4 dpi compared to NV/C pigs. Febrile responses in NV/C pigs returned to normal at 72 hours. V/C pigs had significantly higher (P<0.001) macroscopic pneumonia scores averaging 20.5 percent of the lung at 5 dpi compared to an average of 8.9 percent of the lung in NV/C pigs. Macroscopic lesions were absent in negative controls. At 3 dpi, 9/10 and 5/10 pigs in the V/C and NV/C groups, respectively, were positive for nasal shedding. By 5 dpi, both groups had 10/10 pigs positive for virus in the nasal cavity. Virus levels at 5 dpi in nasal secretions were significantly less (P<0.001) in the V/C group compared to NV/C pigs. Virus levels in the lung were also significantly higher in the NV/C pigs at 5 dpi. Isolate specific hemagglutination inhibition (HI) titers developed only in pigs primed with the delta-cluster inactivated vaccine and no cross-reactivity between sera from vaccinated pigs and CA/09 antigens were detected prior to challenge. Neutralizing antibodies (NA) against the delta-cluster (MN08) virus were detected prior to challenge in the sera from vaccinated pigs although NA responses were lacking against the CA/09 virus at the same time. Additional antibody assays are in progress. Discussion. This study demonstrates an enhancement of pneumonia in vaccinated pigs challenged with a heterologous, homosubtypic virus, consistent with previous reports by our group (3). The enhanced pneumonia was significantly greater compared to pigs challenged with the same virus without previous vaccination or prior SIV specific antibodies. Therefore, it is speculated that the vaccine-induced immune response in the lung contributes to the enhanced pneumonia. In addition, virus replication in the nasal cavity and lung were lower in V/C pigs in spite of the enhanced lung lesions. To date, this phenotype has been experimentally reproduced when pigs are administered an inactivated SIV H1 vaccine followed by heterologous homosubtypic challenge. This phenomenon has the potential to be realized in the U.S. swine population due to the exposure of genetically diverse H1 SIV currently circulating among swine vaccinated with inactivated heterologous virus vaccines. References 1. A. L. Vincent et al., Adv Virus Res 72, 127 ( 2008). 2. A. L. Vincent et al., Virus Genes 39, 176 (2009). 3. A. L. Vincent et al., Vet Microbiol 126, 310 (2008).