Abstract
BackgroundLow-density asymptomatic infections of Plasmodium spp. are common in low endemicity areas worldwide, but outside Africa, their contribution to malaria transmission is poorly understood. Community-based studies with highly sensitive molecular diagnostics are needed to quantify the asymptomatic reservoir of Plasmodium falciparum and P. vivax infections in Thai communities.MethodsA cross-sectional survey of 4309 participants was conducted in three endemic areas in Kanchanaburi and Ratchaburi provinces of Thailand in 2012. The presence of P. falciparum and P. vivax parasites was determined using 18S rRNA qPCR. Gametocytes were also detected by pfs25 / pvs25 qRT-PCRs.ResultsA total of 133 individuals were found infected with P. vivax (3.09%), 37 with P. falciparum (0.86%), and 11 with mixed P. vivax/ P. falciparum (0.26%). The clear majority of both P. vivax (91.7%) and P. falciparum (89.8%) infections were not accompanied by any febrile symptoms. Infections with either species were most common in adolescent and adult males. Recent travel to Myanmar was highly associated with P. falciparum (OR = 9.0, P = 0.001) but not P. vivax infections (P = 0.13). A large number of P. vivax (71.5%) and P. falciparum (72.0%) infections were gametocyte positive by pvs25/pfs25 qRT-PCR. Detection of gametocyte-specific pvs25 and pfs25 transcripts was strongly dependent on parasite density. pvs25 transcript numbers, a measure of gametocyte density, were also highly correlated with parasite density (r2 = 0.82, P < 0.001).ConclusionsAsymptomatic infections with Plasmodium spp. were common in western Thai communities in 2012. The high prevalence of gametocytes indicates that these infections may contribute substantially to the maintenance of local malaria transmission.
Highlights
Low-density asymptomatic infections of Plasmodium spp. are common in low endemicity areas worldwide, but outside Africa, their contribution to malaria transmission is poorly understood
The reductions have been more pronounced for P. falciparum than for P. vivax, which accounts for most local cases in Thailand [1]
The advent of artemisinin resistance on the Thai-Cambodian border [4] and more recently in western Thailand [5, 6], has emphasized the need to move from control to elimination of malaria in these areas [7, 8]
Summary
Low-density asymptomatic infections of Plasmodium spp. are common in low endemicity areas worldwide, but outside Africa, their contribution to malaria transmission is poorly understood. The advent of artemisinin resistance on the Thai-Cambodian border [4] and more recently in western Thailand [5, 6], has emphasized the need to move from control to elimination of malaria in these areas [7, 8]. Despite this renewed focus, there is still a substantial lack of understanding on how transmission in the remaining endemic areas is sustained. While Thailand has an excellent system that detects, treats and monitors clinical malaria cases even in relatively remote rural areas [9], there is no routine monitoring of the infectious reservoir
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