SWINE INFLUENZA : III. FILTRATION EXPERIMENTS AND ETIOLOGY

1. It has been possible to demonstrate, in Berkefeld filtrates of infectious material from experimental cases of swine influenza, a virus which when administered intranasally to susceptible swine induced a mild, usually afebrile illness of short duration. The changes in the respiratory tract resembled those in swine influenza but were usually much less extensive. When the filtrable virus was mixed with pure cultures of H. influenzae suis and administered to swine a disease identical clinically and pathologically with swine influenza was induced. The data presented indicate that the filtrable virus of swine influenza and H. influenzae suis act in concert to produce swine influenza and that neither alone is capable of inducing the disease. 2. One attack of swine influenza usually renders an animal immune to reinfection. Blood serum from an animal made immune in this way neutralizes infectious material from swine influenza in vitro, as shown by the failure of the mixture to produce disease in a susceptible animal. 3. The virus can be stored in a dried state or in glycerol for several weeks at least. In one instance dried material apparently retained both the virus and H. influenzas suis in viable form for a period of 54 days. 4. Fatal cases of experimental swine influenza have been observed in which H. influenzae suis was the only organism that could be cultivated from the respiratory tract. 5. Attention has been called to some features of marked similarity between epizootic swine influenzae and epidemic influenzae in man.