Symptomless Plasmodium vivax infections in native Amazonians

Abstract

Naturally acquired immunity to falciparum malaria has been recorded in holoendemic and hyperendemic Africa. , 2 Similar records do not exist for vivax malaria. Although individuals repeatedly infected with Plasmodium vivax m a y have attenuated symptoms, absence of symptoms has never been described in endemic areas of vivax malaria. , 3 W e provide evidence of symptomless vivax malaria in a riverine Amazonian population in the state of Rondonia, Brazil. Colonisers who came from malaria-free southern Brazil between 1960 and 1980 and their immediate descendants constitute more than 95% of the Rondonian population of 1 2 00 000 inhabitants. When those migrants arrived, they encountered a small native population that had been living for generations in settlements scattered along rivers. From the 1960s onwards, malaria spread to immigrants. It was often severe but remained endemic and less severe in the native people. The presence of blood parasites, either P falciparum or P vivax, was always accompanied by symptoms in migrants. In the riverine population a few people had blood parasites in absence of symptoms. W e considered that some of the native population might be immune to malaria and also serve as reservoirs of the d i s e a s e . To test this hypothesis we studied a native population, adding PCR-amplification of ribosomal DNA to clinical examination and blood-smear microscopy for the diagnosis of vivax malaria. After giving informed consent, 183 inhabitants (92% of a village population) were followed up from September, 1998. 14 cases of vivax malaria were diagnosed up to January, 1999. All cases had malaria symptoms with positive blood smears and positive PCR. The majority of the patients (82%) had lived for more than 1 year in the village and were less than 16 years old. We also detected 25 individuals (68% of them older than 16 years) who were PCR-positive for P vivax, but without symptoms. 16 were put under medical supervision for 30 days. Eight had positive blood smears with very low