Abstract
BackgroundThe hypotheses put forward to explain the malaria transmission cycle in extra-Amazonian Brazil, an area of very low malaria incidence, are based on either a zoonotic scenario involving simian malaria, or a scenario in which asymptomatic carriers play an important role.ObjectivesTo determine the incidence of asymptomatic infection by detecting Plasmodium spp. DNA and its role in residual malaria transmission in a non-Amazonian region of Brazil.MethodsUpon the report of the first malaria case in 2010 in the Atlantic Forest region of the state of Espírito Santo, inhabitants within a 2 km radius were invited to participate in a follow-up study. After providing signed informed consent forms, inhabitants filled out a questionnaire and gave blood samples for PCR, and thick and thin smears. Follow-up visits were performed every 3 months over a 21 month period, when new samples were collected and information was updated.ResultsNinety-two individuals were initially included for follow-up. At the first collection, all of them were clearly asymptomatic. One individual was positive for Plasmodium vivax, one for Plasmodium malariae and one for both P. vivax and P. malariae, corresponding to a prevalence of 3.4% (2.3% for each species). During follow-up, four new PCR-positive cases (two for each species) were recorded, corresponding to an incidence of 2.5 infections per 100 person-years or 1.25 infections per 100 person-years for each species. A mathematical transmission model was applied, using a low frequency of human carriers and the vector density in the region, and calculated based on previous studies in the same locality whose results were subjected to a linear regression. This analysis suggests that the transmission chain is unlikely to be based solely on human carriers, regardless of whether they are symptomatic or not.ConclusionThe low incidence of cases and the low frequency of asymptomatic malaria carriers investigated make it unlikely that the transmission chain in the region is based solely on human hosts, as cases are isolated one from another by hundreds of kilometers and frequently by long periods of time, reinforcing instead the hypothesis of zoonotic transmission.
Highlights
The hypotheses put forward to explain the malaria transmission cycle in extra-Amazonian Brazil, an area of very low malaria incidence, are based on either a zoonotic scenario involving simian malaria, or a scenario in which asymptomatic carriers play an important role
With the development of molecular biology techniques, the number of Plasmodium species known to be able to cause infections in humans has increased from four in the oldest references, to six in the most recent, with the recent split of Plasmodium ovale into two different species [6,7,8,9]. One of these new species, Plasmodium knowlesi, was originally identified as the species that caused simian malaria in Southeast Asia, but is known to play an important role in the disease in humans living in the same region [10,11,12]
Demographic data and information about participants’ occupations and leisure activities, as well as any habits potentially connected with malaria transmission were collected (Table 1)
Summary
The hypotheses put forward to explain the malaria transmission cycle in extra-Amazonian Brazil, an area of very low malaria incidence, are based on either a zoonotic scenario involving simian malaria, or a scenario in which asymptomatic carriers play an important role. With the development of molecular biology techniques, the number of Plasmodium species known to be able to cause infections in humans has increased from four in the oldest references, to six in the most recent, with the recent split of Plasmodium ovale into two different species [6,7,8,9] One of these new species, Plasmodium knowlesi, was originally identified as the species that caused simian malaria in Southeast Asia, but is known to play an important role in the disease in humans living in the same region [10,11,12]. This makes it necessary to develop more complex measures to either eliminate the disease or to at least impede its spread, as the elimination of diseases with a zoonotic cycle might only be attainable at an unacceptably high cost to the environment
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