Potentiation of experimental meningitis due to Haemophilus influenzae by influenza A virus.

When Haemophilus influenzae type b was given intranasally to infant rats, a very large dose was required to produce histologic evidence of meningitis in even half of the animals tested; meningitis developed in 16 of 31 rats that received 10(7) viable bacteria at the age of five days. However, when the animals first received influenza virus, the dose of bacteria required to produce meningitis was reduced 100-fold; meningitis occurred in 10 of 21 rats given virus at two days and 10(5) viable bacteria at five days. These results suggest that prior viral infection of the upper respiratory tract may be a factor in the pathogenesis of haemophilus meningitis.