Submitted to: United States Japan Natural Resources Animal and Avian Health Panel
Publication Type: Proceedings
Publication Acceptance Date: 11/17/2005
Publication Date: 11/17/2005
Citation: Vincent, A.L., Lekcharoensuk, P., Ma, W., Gramer, M., Richt, J.A., Lager, K.M. 2005. Evaluation of North American hemagglutinin subtype 1 swine influenza isolates. 40th United States-Japan Cooperative Program in Natural Resources Panel of Animal and Avian Health Meeting. p. 7.
Technical Abstract: Swine influenza viruses (SIV) of the hemagglutinin subtype 1 (H1) isolated in North America have not been well characterized in the swine host. A shift in the rate of mutation and reassortment has occurred in SIV during the latter part of the twentieth century, including viruses with the H1 subtype. Two independent animal studies were done to evaluate and compare the pathogenesis of 10 SIV isolates dating from 1930 to currently active isolates. In addition, serological cross-reactivity was evaluated using all sera and virus combinations in hemagglutination inhibition (HI) and serum neutralization (SN) assays. Pigs were challenged intratracheally at 4 weeks of age and euthanized at 5 days post infection (dpi) in two independent replicates. Isolates included in replicate 1 were A/Swine/Iowa/15/30 H1N1, A/Swine/MN/1192/2001 H1N2, A/Swine/NC36883/02 rH1N1, and field isolates from the Minnesota Veterinary Diagnostic Lab, including a 2003 H1N2, a 2004 H1N2, and a 2004 rH1N1. Isolates included in replicate 2 were A/Swine/Iowa/15/30 H1N1, IA 1945 H1N1, A/Swine/WIS/1/1968 H1N1, IA 1973 H1N1, and A/Swine/MN/37866/1999 H1N1. Rectal temperatures were taken daily from -2 dpi to 5 dpi. Nasal swabs were taken on 0, 3, and 5 dpi for virus isolation. At necropsy, lungs were evaluated for pneumonia and lavaged to obtain bronchioalveolar lavage fluid (BAL) for virus titration. The HA and NA genes of each isolate were sequenced for genetic comparison. Two pigs per isolate were hyper-immunized and serum was collected for HI and SN assays. Statistically significant differences in percentage of lung with pneumonic lesions and virus titers were detected between isolates, with recent isolates tending to produce more severe disease, have more virus shedding and higher viral titers. Genetic analysis revealed high variability among the H1 isolates, although there was little apparent correlation between genetic relatedness and pathogenicity. Serologically, the classic "historical" viruses tended to have better cross-reaction between historical sera and antigens, with moderate to good cross-reactivity with modern viral antigens. The modern sera were less reactive to historical viruses and tended to be less consistent in cross-reactivity within the modern group. The sequences of the 10 isolates demonstrated amino acid substitutions in predicted antigenic regions of the HA1 region that may play a role in serological cross-reactivity. Examination of antigenic sites in the HA1 protein in addition to sequence analysis may be a better indicator of cross-reactivity than sequence homology alone.