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During field studies carried out in 1971, 1972 and 1973 in a highly malarious coastal area of El Salvador, it was possible to collect information on the patterns of Plasmodium vivax parasite occurrence in a large number of infected individuals. In most of the persons who had experienced a marlaria attack during the high transmission period in June, July, August and September, renewed activity occurred 5 to 8 months later, during the low transmission season the next year. Subsequent activity in these same cases occurred after intervals of about 4 to 9 weeks in duration. On epidemiologic grounds, and on the basis of the life patterns of the El Salvador P. vivax strains demonstrated in previous studies in volunteers, this renewal of activity probably represents the occurrence of relapses rather than new infections. This pattern is similar to the “temperate zone” strains of P. vivax, where initial relapses occur after prolonged periods, followed by subsequent relapses after much shorter intervals of inactivity. This type of relapse pattern in a tropical area may enhance the survival of the parasite through a prolonged period of vector inactivity, such as the long dry season experienced in El Salvador.
Present address: U.S. Department of Agriculture, Animal and Plant Health Inspection Service, Veterinary Services, Tres Picos #79, Mexico, D.F.