Pathogenesis of bloodstream invasion with Haemophilus influenzae type b

Possible route(s) by which encapsulated bacteria invade the blood from the nasopharynx include (i) the direct invasion of submucosal blood vessels and (ii) clearance via lymphatics to regional nodes followed by bloodstream invasion. These possibilities were investigated in rats after intranasal inoculation with 10(5) Haemophilus influenzae type b. Within 24 h of inoculation, 10 of 42 rats with sterile blood cultures had similar numbers of H. influenzae b recovered from both cervical (local) and periiliac (distant) lymph nodes, which suggested early bacteremic spread. When virtually continuous blood cultures were obtained for 30 min after inoculation with 10(8) H. influenzae b, early transient bacteremia was documented in four of eight rats. Also, we found no significant difference in bacteremia among rats whose cervical lymph nodes had been removed surgically compared with sham-operated rats. These findings favor the hypothesis of a rapid, perhaps direct invasion of pharyngeal blood vessels as an initial determinant of the systemic spread of H. influenzae b.