Submitted to: Veterinary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/26/2006
Publication Date: 12/1/2006
Citation: Vincent, A.L., Lager, K.M., Ma, W., Lekcharoensuk, P., Gramer, M.R., Loiacono, C., Richt, J.A. 2006. Evaluation of hemagglutinin subtype 1 swine influenza viruses from the United States. Veterinary Microbiology. 118(3-4):212-222. Interpretive Summary: Swine influenza virus is a leading cause of viral pneumonia and financial losses to pork producers in the U.S. and around the world. In the last seven years, the swine influenza virus that was once stable and predictable has now become quite dynamic and difficult to control. This study compared swine influenza isolates of the H1N1 or H1N2 subtype from the last seventy-five years. These viruses showed considerable diversity both in the severity of disease that was induced and in their genetic make-up. The viruses isolated in recent years were demonstrated to have changed significantly from the older-type viruses. In addition, the traditional serologic laboratory tests to detect infected swine may be insufficient if not updated with the newly changed viruses. These results demonstrate the reason for the difficulty reported from swine veterinarians and producers in managing modern swine influenza.
Technical Abstract: Swine influenza viruses (SIV) of the hemagglutinin subtype 1 (H1) isolated from the United States (U.S.) have not been well characterized in the swine host. An increase in the rate of mutation and reassortment has occurred in SIV isolates from the U.S. since 1998, including viruses with the H1 subtype. Two independent animal studies were done to evaluate and compare the pathogenesis of ten SIV isolates dating from 1930 to currently circulating isolates. In addition, the hemagglutinin and neuraminidase genes of each isolate were sequenced for genetic comparison, and serological cross-reactivity was evaluated using all sera and virus combinations in hemagglutination inhibition (HI) and serum neutralization (SN) assays. Statistically significant differences in percentage of pneumonia and virus titers in the lung were detected between isolates, with modern isolates tending to produce more severe disease, have more virus shedding and higher viral titers. However, nasal shedding and virus titers in the lung were not always correlated with one another or lung lesions. Serologically, the classic historical H1N1 viruses tended to have better cross-reaction between historical sera and antigens, with moderate to good cross-reactivity with modern viral antigens. However, the modern sera were less reactive with historical viruses. Modern viruses also tended to have less consistent cross-reactivity within the modern group. Overall, H1 isolates collected over the last 75 years from the U.S. pig population exhibit considerable variability in pathogenicity. There also appears to be an increase in genetic and antigenic diversity coincident with the emergence of the swine triple reassortant H3N2 in 1998.