Attack rate of poststreptococcal nephritis in families. A prospective study.

Twenty-two families comprising 141 first-degree relatives (42 parents and 99 siblings) were studied prospectively after the appearance in the family of an index case with acute poststreptococcal glomerulonephritis (APSGN). There was no epidemic when the studies were conducted. Families were observed weekly for 4-6 weeks after the appearance of the index case, and at least every 6 months thereafter for 1-3 years. At each examination all members of the family gave a history and had a physical examination, urine analysis, and appropriate cultures; antistreptococcal antibody titres and serum C3 were determined. 33 parents and 95 siblings showed changes in antistreptococcal antibody titres suggestive of infection during or shortly before the period of weekly observation. Group A streptococci of various M and T types were isolated from 17 individuals in 14 families. 25 cases of clincal APSGN (including 22 index cases) and 12 cases of asymptomatic APSGN were detected. The results indicate that within families the streptococci responsible for nonepidemic APSGN are very infectious. The subclinical/clinical disease ratio (excluding index cases) was 4.0. The high attack rate (37.8%) in 77 sibling contacts at risk may indictae that susceptibility to nephritis after streptococcal infection is a familial trait.