Submitted to: Pig Veterinary Society International Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/21/2008
Publication Date: 6/22/2008
Citation: Vincent, A.L., Swenson, S.L., Lager, K.M., Gauger, P.C., Loiacono, C.M., Beach, T., Landgraf, J., Zhang, Y. 2008. Experimental Evaluation of a Swine Influenza Virus Isolated from a County Fair Outbreak in the United States. In: Proceedings of the 20th International Pig Veterinary Society Congress, June 22-26, 2008, Durban, South Africa. p. 7.
Technical Abstract: Since 1998, 3 predominant swine influenza virus (SIV) subtypes have circulated in US swine, H1N1, H1N2, and H3N2. Distinct antigenic and genetic clusters have been demonstrated within the H1 and H3 SIV subtypes (1, 2). In August 2007, pigs and people became clinically affected by an influenza-like illness during attendance at an Ohio county fair. Influenza A virus was identified from pigs and people and the virus isolates were characterized as swine triple reassortant H1N1 (rH1N1) similar to the swine H1N1 viruses currently circulating in the U.S. pig population. The swine isolate, A/SW/OH/511445/2007, was evaluated in this experimental challenge and transmission study. Materials and methods. Thirty-nine 4-week-old cross-bred pigs from a herd free of SIV and PRRSV were divided into three groups. Twenty pigs were inoculated intratracheally with 2 x 10**5 TCID50/pig of A/SW/OH/511445/2007 (OH07) rH1N1 prepared in MDCK cells. Ten contact pigs were commingled with inoculated pigs on day 2 post-inoculation (dpi) to study transmission efficiency. Nine control pigs were inoculated with noninfectious cell culture supernatant. Five inoculated pigs and 3 control pigs were euthanized on 3, 5, and 7 dpi. Nasal swabs were taken on 0, 3, 5, and 7 dpi or days post contact. Blood was collected from all pigs prior to challenge for pathogen screening. Blood was additionally collected from inoculated and contact pigs on approximately 14, 21 and 35 dpi. At necropsy, each lung was lavaged to obtain bronchoalveolar fluid (BALF). Virus titers in BALF were determined by serial dilutions added to MDCK cells in 96-well plates, and virus from nasal swab samples were isolated on MDCK cells in 48-well plates. Wells were evaluated for cytopathic effects after 48 to 72 hours. Lungs were removed in toto and the percentage of gross lesions of lobes showing the purple-red consolidation typical of SIV infection was recorded. A weighted mean value was determined for the seven pulmonary lobes of each animal. Hemagglutination inhibition (HI) assays were done to evaluate sero-conversion using homologous virus (3). In addition to homologous HI assays, the OH07 virus was evaluated against a panel of previously described H1 antisera (2). Results. All pigs were seronegative for antibody against SIV by HI assay prior to the start of the study. Principal infected pigs and contact pigs became clinically ill, demonstrating lethargy, decreased appetite and dyspnea. Necropsy revealed severe macroscopic lung lesions typical of SIV (plum-colored, consolidated areas) in inoculated pigs but none in control pigs Macroscopic lung lesions averaged 23.5% on 3 dpi, 25.1% on 5 dpi, and 21.0% on 7 dpi. Virus was titrated in BALF and virus was isolated from nasal swab samples. Virus titers in BALF averaged 106.4 TCID50/ml on 3 dpi and 10**5.0 on 5 dpi and were negative by 7 dpi in the primary challenged group. In the primary challenge group, 85% of the nasal swab samples were positive on 3 dpi, 100% were positive on 5 dpi, and none were positive on 7 dpi. In the contact group, 100% of the nasal swab samples were positive on 3 days post contact, 100% were positive on 5 days post contact, and 90% were positive on 7 days post contact. All of the contact and the remaining primary challenge pigs were seropositive at 21 dpi. The OH07 virus was weak-to-moderately cross-reactive with H1 SIV anti-sera from the same genetic cluster, but failed to cross-react with most of the other H1 SIV anti-sera in our panel. Discussion. The OH07 isolate was evaluated in an in vivo challenge and transmission model. Our results indicate that the OH07 virus was pathogenic in pigs, was transmissible among pigs, and failed to cross-react with many swine H1N1 anti-sera. The OH07 induced macroscopic and histopathologic lesions that are indistinguishable from that induced by contemporary SIV; however, the