A cohort of 113 women and their newborns from the coastal area of El Salvador were studied longitudinally to estimate
malaria incidence and indirect fluorescent antibody (IFA) response to
malaria infection. The district in which the study was conducted had an estimated annual parasite index of 600/1,000 inhabitants, and all
malaria infections were treated immediately with a
4-aminoquinoline. In the third trimester of pregnancy, the IFA response to Plasmodium falciparum was significantly depressed. As a result of
antimalarial therapy and depressed immune responsiveness, 49% (P. vivax) and 53% (P. falciparum) of the pregnant subjects had a
malaria IFA titer less than 1:20 at the time of delivery.
Malaria IFA crossed the placenta to the fetus with a step-down of approximately a 4-fold dilution, except for the step-up noted in the P. falciparum titer for 17 of 116 newborns. Due to the overall low prevalence and intensity of maternal IFA, a titer of at least 1:20 was passed to only 23% (P. vivax) and 45% (P. falciparum) of newborns. Passively-acquired
malaria IFA degraded with a half-life estimated between 43 and 52 days. During follow-up of infants to 6 months of age, no protection from
malaria resulting from passively-acquired antibody could be demonstrated. Because of the limited transplacental immunization of these newborns with
antimalarial antibody, it appears that passive immunity can exert little effect on the incidence of infant
malaria in coastal El Salvador.