Both the rate of nasopharyngeal carriage and the rate of pneumococcal infection are highest at the extremes of age. Invasive pneumococcal infection is relatively common in neonates and children below the age of 2 years (160 per 100000), the incidence falling sharply in adolescent and young adult years (5 per 100000), to rise again in populations over the age of 65 years (70 per 100000) (Breiman etal, 1990). Rates of both colonisation and invasive infection show marked seasonal variation, with considerable increase in the colder months (Balakrishnan etal, 2000).
Although pneumococci undoubtedly attack previously healthy people, several anatomical (dural tears and basal skull fracture) and physiological (mucociliary escalator dysfunction due to pollution, cigarette smoke or viral infection) defects predispose to pneumococcal infection. Defects of the reticuloendothelial system (hyposplenism due to any cause) and humoral immune system (hypogammaglobuli-naemia and complement defects) and AIDS also greatly increase host susceptibility. Other conditions that predispose to pneumococcal infection include alcoholism, diabetes mellitus and chronic cardiac, respiratory, liver and renal disease.
Host-to-host transmission occurs from extensive, close contact. Daycare centre attendance and crowded living conditions (prisons, nursing homes and homeless shelters) are associated with an increased transmission rate, but casual contact is usually not (Rauch etal, 1990; Hoge etal, 1994).
IgG2 has a pivotal role in defence against pneumococcal infection. Human FcyRIIa has two codominantly expressed allotypes, which greatly differ in their ability to ligate IgG2, FcyRIIa-R131 binding only weakly. Studies have shown that half of all patients with bacteraemic pneumococcal pneumonia were homozygous for FcyRIIa-R131, which was significantly higher than in other groups. Moreover, all bacteraemic patients who died within 1 week of admission were homozygous for this allele (Yee etal, 2000).
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