The "in vivo" production of "new" influenza viruses. 3. Isolation of recombinant influenza viruses under simulated conditions of natural transmission

Recombination between influenza A viruses from man and pigs and from chickens and turkeys was demonstrated under experimental conditions of natural transmission. The viruses were allowed to spread naturally from infected to contact animals, and recombinant viruses present in the contact animals were studied. In all experiments recombinant viruses were detected in the contact animals. The percentage of contact animals with recombinant viruses varied; it was as high as 50% in one experiment, and in another, the recombinants were the predominant type of virus present.Recombinants isolated from turkeys infected with fowl plague virus and turkey/Wisconsin/66 influenza virus or from turkeys infected with turkey/Ontario/7732/66 and turkey/Wisconsin/66 influenza virus always possessed the hemagglutinin subunit of the virulent parent. Thus, recombinant viruses possessing T/Ont_(H)-T/Wis_(N) or FPV_(H)-T/Wis_(N) were isolated and were highly virulent and genetically stable. The reciprocal recombinants were never isolated. In experiments with Hong Kong influenza virus and swine influenza virus in pigs, both kinds of recombinants were isolated HK_(H)-SW_(N); SW_(H)-HK_(N)] and both of the recombinant viruses were genetically stable and caused mild infections in pigs.These studies provide experimental evidence that recombination can occur between influenza viruses from man and lower animals under simulated conditions of natural transmission in vivo and may offer an explanation for the origin of new strains of pandemic influenza viruses.