Experimental inoculation of pigs with pandemic H1N1 2009 virus and HI cross-reactivity with contemporary swine influenza virus antisera

Background A novel A/H1N1 was identified in the human population in North America in April 2009. The gene constellation of the virus was a combination from swine influenza A viruses (SIV) of North American and Eurasian lineages that had never before been identified in swine or other species. Objectives The objectives were to (i) evaluate the clinical response of swine following experimental inoculation with pandemic H1N1 2009; (ii) assess serologic cross-reactivity between H1N1 2009 and contemporary SIV antisera; and (iii) develop a molecular assay to differentiate North American-lineage SIV from H1N1 2009. Methods Experiment 1: Weaned pigs were experimentally infected with A/California/04/2009 (H1N1). Experiment 2: The cross-reactivity of a panel of US SIV H1N1 or H1N2 antisera with three isolates of pandemic A/H1N1 was evaluated. Experiment 3: A polymerase chain reaction (PCR)-based diagnostic test was developed and validated on samples from experimentally infected pigs. Results and Conclusions In experiment 1, all inoculated pigs demonstrated clinical signs and lesions similar to those induced by endemic SIV. Viable virus and antigen were only detected in the respiratory tract. In experiment 2, serologic cross-reactivity was limited against H1N1 2009 isolates, notably among virus antisera from the same HA phylogenetic cluster. The limited cross-reactivity suggests North American pigs may not be fully protected against H1N1 2009 from previous exposure or vaccination and novel tests are needed to rapidly diagnose the introduction of H1N1 2009. In experiment 3, an RT-PCR test that discriminates between H1N1 2009 and endemic North American SIV was developed and validated on clinical samples.