The Effect of Total Body X Irradiation on the Susceptibility of Mice to Influenza A Virus Infection

Abstract

This study was undertaken to determine whether or not x-ray irradiation will lower the resistance of mice to infection with a respiratory virus. Mice were irradiated with x-ray doses ranging from 200 to 700 r and a probit analysis of mortality rates was carried out. Inoculation of mice with a virus aerosol was found to be more reliable than inoculation by intranasal instillation. The incidence of spontaneous pneumonitis in mice inoculated with an aerosol of sterile medium was sufficiently small to be ignored. Groups of mice were given x-ray doses ranging from 50 to 600 r. Three days later they were inoculated with an aerosol of mouse- or egg-adapted PR8 influenza A virus. When mouse-adapted virus was used there was no difference between morbidity rates of irradiated animals and non-irradiated controls but the extent of lesions was significantly greater in the irradiated mice. When egg-adapted virus was used both the morbidity rate and extent of lesions was greater in the irradiated mice than in non-irradiated controls. Grossly inapparent infection, as judged by development of antibody titres, never developed in mice infected with mouse-adapted virus but did occur when egg-adapted virus was used. Aerobic and anaerobic cultures of the lungs of mice revealed that secondary infection was not an important factor in the increased susceptibility of irradiated mice. Irradiation depressed development of antibodies. There appeared to be no difference between the histopathological course of infection in irradiated and non-irradiated mice. The possibility of using irradiation as a means of increasing susceptibility of animals to viruses to which they are ordinarily immune is discussed.