Age-dependent acquired protection against Plasmodium falciparum in people having two years exposure to hyperendemic malaria.

An epidemiologic study of susceptibility to frequent and high-grade parasitemia by Plasmodium falciparum revealed that age-dependent acquired protection developed within a two-year period of exposure to hyperendemic infection pressure. The study was conducted in a single village in northeastern Irian Jaya, Indonesia, where half the residents were native to the province and the other half were transmigrants from areas of Java, where there is little or no malaria transmission. Five separate measures of susceptibility to the asexual parasitemia of falciparum malaria were derived from results of four months of biweekly surveillance of 240 volunteers. Increasing protection as a function of age among the Javanese was a consistent pattern among the five estimates of susceptibility. These age-dependent functions of protection were quantitatively parallel to those among life-long residents of Irian Jaya. When humoral immune responsiveness to ring-infected erythrocyte surface antigen (RESA) was measured by ELISA, a similar pattern emerged; the relative level of antibody to RESA increased as parallel functions of age among the two subpopulations. Acquired protective immunity against P. falciparum was not the cumulative product of many years of heavy exposure to antigen. Instead, the full benefit of protection appeared to develop quickly. The degree of protection was governed by recent exposure and age, independent of history of chronic heavy exposure.

Prevalence of malaria in native and transmigrant populations. Effects of age and history of exposure.

A malaria prevalence study was performed in a village in Irian Jaya, Indonesia, that contains a population of people who have been exposed lifelong to hyperendemic malaria and another population of people who had arrived 18 months previously from areas of very low endemicity. Mean spleen sizes correlated positively with prevalence of malaria, not resistance to it. Prevalence of sexual and asexual blood stage parasites was higher in transmigrants than in the natives. The data also show that clinical resistance to malaria in this part of the world includes resistance to Plasmodium falciparum gametocytemia and that this is not the passive byproduct of a reduction in asexual parasites. This indicates that the introduction of native people into a populated malarious area will increase the percent of gametocyte carriers and may, thereby, increase the entomologic inoculation rate.