Studies on the Plasmodium vivax relapse pattern in Delhi, India.

A five-year epidemiologic study of patients attending a malaria clinic in Delhi was conducted to find the relapse rate of infections with Plasmodium vivax, its seasonal correlation between the primary infection and subsequent relapses, the duration of the incubation period, and the patterns of relapse. By our definition, the relapse rate ranged from 23% to 44% depending on the duration of follow-up. The relapse pattern observed in the study clearly suggests the existence of both tropical and temperate zone types of P. vivax in the population characterized by distinct incubation periods and the possible existence of P. vivax subpopulations characterized by primary long incubation periods. The implication of different incubating forms of P. vivax on the epidemiology and control of malaria is also discussed.