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Title: Experimental infection of pigs with the 1918 pandemic influenza resembles infection with the classical H1N1 swine influenza

item Lager, Kelly
item Richt, Juergen

Submitted to: Pig Veterinary Society International Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 3/30/2008
Publication Date: 6/22/2008
Citation: Weingartl, H.M., Albrecht, R.A., Lager, K.M., Babiuk, S., Marszal, P., Neufeld, J., Embury-Hyatt, C., Garcia-Sastre, A., Richt, J.A. 2008. Experimental Infection of Pigs with the 1918 Pandemic Influenza Resembles Infection with the Classical H1N1 Swine Influenza. In: Proceedings of the International Pig Veterinary Society Congress, June 22-26, 2008, Durban, South Africa. Paper No. P01.021.

Interpretive Summary:

Technical Abstract: Introduction. Swine influenza was first recognized as a disease entity during the 1918 pandemic (1). It was proposed, based on phylogenetic analysis, that the virus causing the disease in pigs was the same as the virus causing the pandemic (2). The aim of this work was to determine whether the 1918 pandemic influenza virus can infect swine, and to determine development of clinical disease, and pathology in infected animals. Materials and Methods. The A/swine/ Iowa/15/1930 (H1N1) virus (1930v) was cloned into the eight plasmid system (3) using reverse genetics. The 1930v and the reconstructed pandemic human H1N1 influenza 1918 virus with the HA from the South Carolina isolate sequence (1918v) were rescued as described previously (3, 4). Their virulence was verified in mice, prior to inoculation of 18 piglets intratracheally or nasally with 10**5.4 PFU/animal of the 1918v, and 12 piglets intratracheally with the same dose of the 1930v. Six control piglets were mock inoculated with cell culture medium. Piglets were 4 – 5 weeks old at the time of inoculation. 1918v inoculated pigs were euthanized at 3, 5, 7dpi, 12 and 17 dpi; 1930v inoculated pigs were euthanized at 3, 5 and 7 dpi. Virus was detected in the animals using real time RT-PCR, virus isolation and immunohisto-chemistry. Commercial ELISA kits were used to detect selected cytokines. Serum antibodies were determined by virus neutralization test. Results. Only mild clinical signs (transient increase in temperature, mild respiratory signs) developed post inoculation in both groups.. Virus was detected mainly in the lower respiratory tract (lung, bronchoalveolar lavage fluid - BALF) at 3 and 5 dpi, targeting respiratory epithelial cells, and cells harvested from BALF. Titers of the virus re-isolated from the 1930v infected animals were generally higher than the titers of virus re-isolated from the 1918v inoculated animals (e.g. BALF titers at 3 dpi were 10**4.5 TCID50/ml compared to 10**2.6 TCID50/ml respectively). Some differences in virus distribution were noted between the two strains of influenza. Gross and microscopic lesions were comparable between the 1930v and 1918v inoculated animals at 3 and 5 dpi. At 7 dpi, the 1918v piglets did not seem to be able to recover as well from the infection as the 1930v pigs. Although no virus was detected in any of the infected animals past 7 dpi, the lung lesions in the piglets infected with the 1918v and euthanized at 7, 12 and 17 dpi were rather prominent; non-suppurative vasculitis with perivascular hemorrhages were observed on histopathology. Using ELISA, TNF-alpha, IFN-gamma, IL-6, IL-10, Il-8 were not detected in lungs of the infected animals. In the 1918 virus infected pigs, IFN-alpha was detected in the "healthy" areas of lungs but not the lesions. The 1918v was re-isolated from BALF of six piglets, and sequenced. No differences were detected in the nucleotide sequence between the inoculum and the re-isolated virus. Discussion. The clinical disease, pathology, virus replication, and cytokine induction in piglets infected with the 1918 influenza virus were not significantly different from the infection with the 1930v, representing classical swine H1N1 viruses. This would be in an agreement with the historical records (1). Somewhat better ability of the 1930 virus to replicate in the pigs and to cause less damage might reflect adaptation of the virus to swine. The lesions observed in the 1918v infected animals later post infection may be related to differences in immune response, but we were not able to determine the factors involved in the delayed pathological changes. Our experimental animal data support the hypothesis that the 1918 pandemic influenza is likely an ancestor of both, the human H1N1 influenza and the classical swine H1N1 influenza still present in pigs (5). References 1. Shope RE (1931) JEM 54: 349 -