An animal model for pneumococcal otitis media was studied histologically at varying intervals postinoculation. Histologic changes generally associated with acute inflammation (edema, diffuse infiltration with polymorphonuclear leukocytes, and hemorrhage) occurred early in the disease while the ears were culture positive. Changes generally associated with chronic otitis media (lymphocyte infiltration and osteoneogenesis) occurred later in the course of the disease when the ears were culture negative. Metaplasia of middle ear mucosa and middle ear granulation tissue were observed as early as five days following inoculation. Middle ear granulation tissue was identified in 82% of all cases; cholesteatoma was seen in only one ear. Tympanic membrane perforation did not occur. Labyrinthitis and endolymphatic hydrops were fairly common findings. The former was identified in 56% of ears examined, and the latter, in 45%. These two conditions coexisted in 34% of ears studied, while endolymphatic hydrops alone occurred in 11%.
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