Synergistic effect in viral-bacterial infection. 4. The leukocyte response to parainfluenza virus infection in the respiratory tract of mice, and the influence of antibiotics on the development of infection.

The effect of infection with parainfluenza 1 virus or Haemophilus influenzae b on the mucosa of respiratory tract of mice was studied. A section of trachea just above the bifurcation was examined histologically for the presence of destructive changes and leukocyte accumulation in the epithelium and lamina propria. Polymorphonuclear (PMN) leukocyte accumulation was a prominent sign after both viral and bacterial inoculation. Viral inoculation produced more frequent and more pronounced inflammatory changes than bacterial inoculation. Disruption of the basement membrane was observed only following virus inoculation. Gross evidence of pneumonia was seen more frequently among virus inoculated animals. The histological changes were progressive with time of survival up to the third day after inoculation. The bacterial titer in homogenates of lung tissue correlated with the number of lobes affected with pneumonia and to some extent with the accumulation of PMN leukocytes in the tracheal epithelium. Bacteria, normally present in the upper respiratory tract, were isolated from a high proportion of virus inoculated lungs. The findings are compatible with the concept that parainfluenza 1 virus infection facilitates a secondary bacterial invasion because of functional impairment or destruction of the epithelium.