Experimental otitis media in chinchillas.

Otitis media was induced in chinchillas by intrabullar injection of type 3 Streptococcus pneumoniae. It was found that the severity of a primary infection was related to the dose of bacteria injected, and animals that had recovered from a primary infection were resistant to reinfection with type 3 S pneumoniae. Middle ear effusions from infected animals contained antibodies to type 3 pneumococcal polysaccharide, whereas the sera of these animals lacked this antibody, suggesting that resistance to reinfectin was due to local immune mechanisms. This idea was tested by immunizing chinchillas by various routes. Animals immunized with formalin-killed type 3 S pneumoniae by intramuscular injection (to stimulate the systemic immune system) or by intranasal inoculation (to stimulate the secretory immune system) were found to be susceptible to infection with type 3 pneumonococcus, while animals immunized by inoculation of bacterial suspension directly into the middle ear were resistant to infection. In addition, we found that ampicillin treatment of a primary infection blocks the development of resistance since antibiotic-treated animals were susceptible to reinfection with the type 3 pneumococcus. These results indicate that chinchillas develop an immunity to pneumococcal otitis media following recovery from primary infection and that this immunity is mediated primarily by local immune mechanisms rather than systemic immune responses.