Abstract
BackgroundThere is a low incidence of malaria in Iquitos, Peru, suburbs detected by passive case-detection. This low incidence might be attributable to infections clustered in some households/regions and/or undetected asymptomatic infections.MethodsPassive case-detection (PCD) during the malaria season (February-July) and an active case-detection (ACD) community-wide survey (March) surveyed 1,907 persons. Each month, April-July, 100-metre at-risk zones were defined by location of Plasmodium falciparum infections in the previous month. Longitudinal ACD and PCD (ACP+PCD) occurred within at-risk zones, where 137 houses (573 persons) were randomly selected as sentinels, each with one month of weekly active sampling. Entomological captures were conducted in the sentinel houses.ResultsThe PCD incidence was 0.03 P. falciparum and 0.22 Plasmodium vivax infections/person/malaria-season. However, the ACD+PCD prevalence was 0.13 and 0.39, respectively. One explanation for this 4.33 and 1.77-fold increase, respectively, was infection clustering within at-risk zones and contiguous households. Clustering makes PCD, generalized to the entire population, artificially low. Another attributable-factor was that only 41% and 24% of the P. falciparum and P. vivax infections were associated with fever and 80% of the asymptomatic infections had low-density or absent parasitaemias the following week. After accounting for asymptomatic infections, a 2.6-fold increase in ACD+PCD versus PCD was attributable to clustered transmission in at-risk zones.ConclusionEven in low transmission, there are frequent highly-clustered asymptomatic infections, making PCD an inadequate measure of incidence. These findings support a strategy of concentrating ACD and insecticide campaigns in houses adjacent to houses were malaria was detected one month prior.
Highlights
In Peru, the history and prevalence of the human-malaria causing Plasmodium species parasites are different from what is found in the widely studied African, Asian or Pacific countries
Between April, 2003 and July, 2003, 12,035 mosquitoes were captured, of which 87% were An. darlingi (Figure 3). These humanbait captures were on the porches of houses within our at-risk zones for Plasmodium falciparum transmission, so defined by having a P. falciparum infection centrally located within the 100-metre radius circle one month prior
The occurrence of clustered infections in at-risk zones and asymptomatic infections contributed to the higher prevalence by active case detection within at-risk zones versus passive case detection generalized to the community population
Summary
In Peru, the history and prevalence of the human-malaria causing Plasmodium species parasites are different from what is found in the widely studied African, Asian or Pacific countries. Plasmodium falciparum was first reported in Peru's Department of Loreto in 1988 [1]. In the region surrounding the capital city of Iquitos, there was a P. vivax and P. falciparum epidemic between 1995 and 1998 (Figure 1B). This is attributable to the abandonment of DDT campaigns leading to the increased geographic range or increased local abundance of the mosquito vector Anopheles darlingi [2,3]. There is a low incidence of malaria in Iquitos, Peru, suburbs detected by passive case-detection. This low incidence might be attributable to infections clustered in some households/regions and/or undetected asymptomatic infections
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