No abstract

We have described the existence of asymptomatic carriers of Plasmodium vivax and Plasmodium falciparum infections in native Amazon populations. Most of them had low parasitemias, detected only by polymerase chain reaction (PCR). Because they remain symptomless and untreated, we wanted to determine whether they could infect Anopheles darlingi Root, the main Brazilian vector, and act as disease reservoirs. Fifteen adult asymptomatic patients (PCR positive only) were selected, and experimental infections of mosquitoes were performed by direct feeding and by a membrane-feeding system. Seventeen adult symptomatic patients with high parasitemias were used as controls. We found an infection rate in An. darlingi of 1.2% for the asymptomatic carriers and 22% for the symptomatic carriers. Although the asymptomatic group infected mosquitoes at a much lower rate, these patients remain infective longer than treated, symptomatic patients. Also, the prevalence of asymptomatic infections is 4 to 5 times higher than symptomatic infections among natives. These results have implications for the malaria control program in Brazil, which focuses essentially on the treatment of symptomatic patients.

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Progress towards the development of a malaria vaccine against Plasmodium vivax, the most widely distributed human malaria parasite, will require a better understanding of the immune responses that confer clinical protection to patients in regions where malaria is endemic. The occurrence of clinical protection in P. vivax malaria in Brazil was first reported among residents of the riverine community of Portuchuelo, in Rondônia, western Amazon. We thus analyzed immune sera from this same human population to determine if naturally acquired humoral immune responses against the merozoite surface protein 1 of P. vivax, PvMSP1, could be associated with reduced risk of infection and/or clinical protection. Our results demonstrated that this association could be established with anti-PvMSP1 antibodies predominantly of the immunoglobulin G3 subclass directed against the N terminus but not against the C terminus, in spite of the latter being more immunogenic and capable of natural boosting. This is the first report of a prospective study of P. vivax malaria demonstrating an association of reduced risk of infection and clinical protection with antibodies against an antigen of this parasite.Malaria remains one of the major public health problems in South and Central America, where Plasmodium vivax is the main species responsible for infections. Although malaria caused by this human parasite is not lethal, the clinical manifestations represent a major socioeconomical burden in the major cities and rural areas of the Brazilian Amazon, which contains 99.7% of all malaria cases. Although a few studies indicate that some cross-protection between Plasmodium vivax, the most widely distributed human malaria parasite, and P. falciparum, the most virulent species, does occur (33), immunity in malaria is mostly species specific; thus, it is likely that a vaccine against P. falciparum will not cross-protect against P. vivax. Progress is being made to develop P. vivax vaccines per se (for a review, see reference 2); however, to date there are no prospective studies associating human immune responses to any P. vivax antigen with clinical protection.The history of malaria in Brazil has been punctuated with epidemics associated with migration movements of nonimmune population to areas where malaria is endemic (20,30,36). Previous studies done in these epidemiological settings showed that Plasmodium infection was always associated with symptoms, and clinical protection was not observed (27). However, through a cross-sectional and longitudinal study among native Amazon residents of the riverine community of Portuchuelo in Rondônia, located in the western Brazilian Amazon, the occurrence of symptomless Plasmodium vivax-infected individuals was reported (1, 6). Symptomless Plasmodium infections were defined as individuals who contain parasites in their peripheral blood, as detected by Giemsa blood smears and/or PCR, and who were not drug treated yet did not develop clinical symptoms during a 2-month individual follow-up. These rigorous criteria...

Corpora non agunt nisi fixata (Paul Ehrlich, 1909) The