Hypothermia in Mice Due to Influenza Virus Infection. ∗

Summary and Conclusions

1. Hypothermia rather than fever was the characteristic thermal response of mice to intranasal inoculation of the PR8 strain of type A influenza virus. 2. Mice showed marked hypothermia with fatal termination in response to inoculation of large amounts, e.g., 1,000 or more infectious doses (ID₅₀), of 2 sublines of the virus, one adapted to embryonated eggs and the other to mice. 3. Inoculation of mice with a small amount of virus, e.g., one to two ID₅₀, did not cause hypothermia. In mice inoculated with about 2 ID₅₀ of egg-line virus. the maximum level of virus in the lungs was lower than in mice inoculated with larger amounts of virus, and pulmonary lesions were present but failed to reach the level of 50% total lung-lesion score. 4. In mice inoculated with a moderate amount of egg-line virus. e.g., about 200 ID₅₀, the maximum virus level in the lungs was reached 3 to 4 days before the occurrence of hypothermia. Time of occurrence and degree of hypothermia appeared to be closely related to the extent of pulmonary lesions. 5. The mouse-passaged virus caused more rapid development of hypothermia than the egg-line virus. 6. It is concluded that hypothermia is related to the extent of pulmonary lesion rather than to the extent of virus increase per se.