SWINE VIRAL DISEASES PATHOGENESIS AND IMMUNOLOGY
Location: Virus and Prion Research Unit
Title: Experimental Challenge with Two Isolates of 2009 A/H1N1 in Weaned Pigs
| Ciacci-Zanella, Janice - |
| Lorusso, Alessio |
| Zanella, Eraldo - |
| Gauger, Philip - |
| Janke, Bruce - |
| Kehrli Jr, Marcus |
Submitted to: Pig Veterinary Society International Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: March 30, 2010
Publication Date: July 18, 2010
Citation: Vincent, A.L., Ciacci Zanella, J.R., Lorusso, A., Zanella, E.L., Lager, K.M., Gauger, P.C., Janke, B.H., Kehrli, Jr., M.E. 2010. Experimental Challenge with Two Isolates of 2009 A/H1N1 in Weaned Pigs. In: Proceedings of the International Pig Veterinary Society Congress, July 18-21, 2010, Vancouver, Canada. p. 74.
Introduction. The gene constellation of the 2009 pandemic H1N1 virus is a unique combination from swine influenza A viruses (SIV) of North American and Eurasian lineages, but prior to April 2009 had never before been identified in swine or other species (1). Although the hemagglutinin gene is related to North American H1 SIV, the infection potential in pigs was unknown immediately following the emergence of the virus in the human population. The objective of this study was to evaluate two isolates of 2009 pandemic H1N1 in a pathogenesis and transmission study in weaned pigs.
Materials and methods. Pathogenic and transmission characteristics between A/CA/04/2009 (CA/09) and A/Mexico/4108/2009 (MX/09) were compared in pigs. A total of 30 4-week old pigs were inoculated with A/CA/04/2009 H1N1 (n=15 pigs) or A/Mexico/4108/2009 H1N1 (n=15 pigs) with approximately 2 x 10**5 TCID50/ml of each respective virus. Pigs were sampled at 0, 2, 3, 5, 7, and 9 days post infection (dpi) by nasal swab. Direct (n=5) and indirect (n=5) contact pigs were placed with each of the primary infected pig groups on 2 days post infection (dpi) and nasal swab samples were collected at 0, 3, 5, 7, and 9 days post contact (dpc). Five primary challenged pigs were necropsied on days 3, 5, 7, and 21 pi. Contact pigs were necropsied on day 20 pc. Hemagglutination inhibition (HI) assays were done with 21 dpi and 20 dpc to evaluate sero-conversion using challenge virus as well as a panel of North American H1 SIV isolates representing each phylogenetic cluster (2).
Results. Pigs experimentally infected with pandemic H1N1 2009 influenza A viruses developed similar respiratory disease and demonstrated equivalent macroscopic lung lesions on each given necropsy day. Virus titers in the lung were not different between CA/09 and MX/09 infected pigs, with mean titers of 10**4.8 and 10**4.5 respectively on 3 dpi, 10**4.8 and 10**4.8 respectively on 5 dpi, and undetectable on 7 dpi. However, differences in viral replication in nasal secretions and transmission to contact pigs were detected between CA/09 and MX/09 (Figure 1). CA/09 inoculated pigs shed virus from the nose in a higher percentage of pigs as well as at higher viral titers. Additionally, direct and indirect contact pigs shed CA/09 virus at levels similar to primary inoculated pigs and for an extended time course. In contrast, MX/09 inoculated pigs shed virus in a lower percentage of pigs and at lower virus titers. A limited number of nasal swabs were positive from pigs in direct contact with MX/09 inoculated pigs and no samples from indirect contact pigs were found to be positive for MX/09 virus. Serologic cross-reactivity was demonstrated between the 2 pandemic isolates and with North American H1 isolates, although significant reductions in HI titers were detected.
Discussion. Understanding the shedding and transmission patterns of different genotypes of H1N1 2009 in swine is important as several countries are now reporting outbreaks in commercial pig herds. The difference in the patterns of shedding and transmission may partially explain why some swine outbreaks have been relatively self-limiting. Virus variants that replicate to higher titers in the nose of pigs and easily transmit from pig to pig could present a greater threat to animal and human health. Based on sequence analysis, the HA genes from CA/09 and Mexico/09 differ by 4 or 5 amino acids depending on the reference sequences compared. Further work must be done to correlate the changes in the HA gene to the reduced shedding and transmission of the MX/09 virus.
Acknowledgments. We thank Michelle Harland, Brian Pottebaum and Jason Huegel for assistance with laboratory techniques and animal studies. Funding provided by USDA-ARS and DHHS-CDC.
1. R. J. Garten et al., Science 325, 197 (Jul 10, 2009).
2. A. L. Vincent et al., Virus Genes 39, 176 (2009).