Severity of fever in influenza: differential pyrogenicity in ferrets exhibited by H1N1 and H3N2 strains of differing virulence.

Intracardial inoculation of large quantities (200 micrograms viral protein/kg body weight) of infectious or u.v.-inactivated purified influenza viruses into ferrets resulted in a rapid febrile response which was significantly lower for two recently isolated H1N1 viruses, A/USSR/90/77 and A/Fiji/15899/83, than for two virulent clones, 7a and 64c, of the A/Puerto Rico/8/34-A/England/939/69 (H3N2) reassortant virus system. These results, which are in accord with the severity of fever produced by these strains in intranasally infected ferrets, show that influenza virus strains can differ in their capacity to induce fever (probably reflecting a differential capacity to induce endogenous pyrogen from phagocytes) and indicate, since u.v.-inactivated strains are pyrogenic, that this may be due to differences between strains in the nature or amount of certain virion components.