Asymptomatic Plasmodium parasitemia and the ecology of malaria transmission.

Abstract

Tip O'Neill, former speaker of the United States House of Representatives, famously pointed out that all politics is local. The same is true of malaria. Malaria transmission is not homogeneous through an endemic area but spotty, and depends on two primary factors: location of mosquito breeding sites, and clustering of human habitations where people serving as reservoirs of parasites for mosquito infection live.1 Whether malaria transmission in focal areas of a malaria-endemic region is stable or unstable is an important determinant of morbidity and mortality. Stable malaria implies that prevalence of infection is sufficiently high to engender a substantial level of clinical immunity within a population, i.e. the presence of parasitemia in the absence of malaria-related symptoms. In contrast, unstable malaria, characterized by spatial and temporal variability, is associated with lower levels of clinical immunity within a population, the propensity for epidemics, and a higher rate of severe disease in adults. Clinical immunity to malaria within a population occurs in the context of premunition, defined as the presence of an immune response that produces control of but not complete elimination of parasitemia and pathological sequelae.In the following pages of this issue of the Journal, Alves et al. describe a relatively high proportion of both asymptomatic Plasmodium vivax and P. falciparum infections among parasitemic individuals within populations of indigenous AmerIndians in the western Brazilian Amazon region. A high rate of asymptomatic P. falciparum prevalence has been well known to occur in areas of high malaria transmission in Africa and other parts of the world. This has been well demonstrated by work in Senegal and other malaria endemic areas.2 Using sensitive molecular techniques, these investigators have shown that while in places such as Dielmo, Senegal, many individuals in particular communities are parasitemic at any one point, only a small fraction of these individuals have malaria-related illness. Subpatent and asymptomatic parasitemias persist interseasonally in places with seasonal transmission 3 ; such patients likely represent a major mechanism for over-seasoning of malaria parasites. Similar findings have been reported regarding P. vivax infections in other regions, for example western Thailand , Sri Lanka , and in Brazilian western Amazonia as shown in previous studies by the group of Alves et al. 6,7 Using both cross-sectional and prospective approaches, Alves et al. examined the proportion of both P. falciparum and P. vivax parasitemic inhabitants of riverine communities in the Rondonia region of the Brazilian Amazon. Of substantial interest, in one community, they determined whether such parasitemic individuals were truly asymptomatic by following up microscopy or PCR-positive individuals for 60 days to ascertain whether they developed malaria parasite-related symptoms. The authors note, quite appropriately, that "the absence of symptoms in cross-sectional studies is not tantamount to asym...

Unstable Malaria Transmission in the Southern Peruvian Amazon and Its Association with Gold Mining, Madre de Dios, 2001–2012

“…38 These patterns of malaria transmission requires individualized public intervention approaches. 38,39 The reasons explaining the unstable transmission of P. vivax malaria in MdD remain unclear, but illegal gold mining volume was significantly correlated to malaria transmission in MdD and mining activities seem to have played a role in other regions in the Amazon. [40][41][42][43] Several mechanisms may underlie the association between gold mining and malaria.…”

“…Finally, the passive nature of the malaria surveillance system required that all malaria cases were symptomatic, and could not gather any information on asymptomatic malaria, although previous preliminary reports have noted that the frequency of asymptomatic cases is low in the region. 39 Despite these limitations, identification of an unstable and geographically heterogeneous pattern of malaria transmission in MdD, with high levels of transmission in areas of intense illegal gold mining, has important potential implications for malaria control and elimination. Another important issue for malaria elimination programs is the need for spatially oriented, targeted control measures, and our findings suggest that areas of illegal gold mining may constitute an important target for such interventions.…”

High Degree of Plasmodium vivax Diversity in the Peruvian Amazon Demonstrated by Tandem Repeat Polymorphism Analysis

“…4 Several factors combine to increase the priority for translating fundamental, benchbased research on P. vivax malaria into regional and global control efforts for control of this infection: relative neglect of the burden of disease caused by P. vivax infection; its globally widespread distribution; and increased cognizance of the possibility of severe vivax infections. [5][6][7][8][9][10] Infection by P. vivax is often assumed to result inevitably in symptomatic febrile illness, 11 being classically associated with 48-hour fever cycles. Yet, in malaria-endemic regions, asymptomatic malaria parasitemia, with degrees of prevalence that are geographically variable, is common.…”

Bednets, Information and Malaria in Orissa

“…Malaria prevalence (as well as hemoglobin levels) were recorded for all children under five (U5) and their mothers, all 8 These figures likely understate the actual burden of malaria in India, because they are largely based on active and passive case detection which monitor certain geographical areas and population groups disproportionately (Dutta 2000). Such monitoring systems often miss individuals who are parasitemic but asymptomatic because of acquired partial immunity (Vinetz and Gilman 2002). For a recent, broad overview of the malaria burden throughout India see also Kumar et al (2007).…”