Pneumococcal pneumonia in a rat model of cirrhosis: effects of cirrhosis on pulmonary defense mechanisms against Streptococcus pneumoniae. To study alterations in host defense mechanisms that enhance pneumococcal virulence, a model of Streptococcus pneumoniae pneumonia was developed in cirrhotic rats. Cirrhosis, with or without ascites, was produced in rats by intragastric administration of carbon tetrachloride (CCl4). Histopathologic and laboratory studies demonstrated that CCl4-induced cirrhosis was similar to alcoholic cirrhosis in humans. Cirrhotic rats were more susceptible to type 3 pneumococcal pneumonia induced by intratracheal challenge than controls, and the presence of ascites was associated with the lowest LD50. More cirrhotic rats with ascites had bacteremia and elevated levels of circulating capsular antigen after challenge compared with cirrhotic rats without ascites or controls. Pulmonary clearance of pneumococci was markedly reduced in rats with cirrhosis and ascites and was associated with reduced serum complement levels. This model may be useful in further studies of the pathogenesis and therapy of pneumococcal infections in the compromised host. 