Experimental infection of weanling pigs with A/Swine influenza virus * 1. Epidemiology and serological response

Naturally occurring swine influenza is caused by a strain of virus closely related to influenza strains isolated from man in 1918 and later. Information is lacking on certain aspects of the epidemiology of swine influenza that, if obtained, might shed some light on the epidemiology of human influenza, particularly with respect to inter-epidemic reservoirs and shedders of the virus. In a first series of experiments undertaken by the authors pigs were experimentally infected intranasally with swine influenza virus and the course of clinical infection, spread by contact, and the serological response of infected animals were studied. Observations were also made on persons in contact with the infected swine to determine whether cross-transmission occurred. Respiratory and feverish clinical signs of the disease were observed in infected animals. Contact infection of several animals in the same piggery occurred, as revealed by serological tests, although the contact-infected pigs showed no clinical signs. There was some but not highly significant serological evidence of human infection in laboratory workers and animal handlers exposed to swine influenza virus. Some pigs infected as weanlings developed specific antibodies that lasted for 1½ years of observation. Antibody titres decreased markedly towards the end of 1 year in animals of a lower age at the time of infection (42 days old), as compared with older animals (52-77 days) in which there were lesser decreases in titre.