Experimental transmission of avian-like swine H1N1 influenza virus between immunologically naive and vaccinated pigs

Background Infection of pigs with swine influenza has been studied experimentally and in the field; however, little information is available on the natural transmission of this virus in pigs. Two studies in an experimental transmission model are presented here, one in immunologically naive and one in a combination of vaccinated and naive pigs. Objectives To investigate the transmission of a recent 'avian-like' swine H1N1 influenza virus in naive piglets, to assess the antibody response to a commercially available vaccine and to determine the efficiency of transmission in pigs after vaccination. Methods Transmission chains were initiated by intranasal challenge of two immunologically naive pigs. Animals were monitored daily for clinical signs and virus shedding. Pairs of pigs were sequentially co-housed, and once virus was detected in recipients, prior donors were removed. In the vaccination study, piglets were vaccinated and circulating antibody levels were monitored by haemagglutination inhibition assay. To study transmission in vaccinates, a pair of infected immunologically naive animals was co-housed with vaccinated recipient pigs and further pairs of vaccinates were added sequentially as above. The chain was completed by the addition of naive pigs. Results and conclusions Transmission of the H1N1 virus was achieved through a chain of six pairs of naive piglets and through four pairs of vaccinated animals. Transmission occurred with minimal clinical signs and, in vaccinates, at antibody levels higher than previously reported to protect against infection.