Very high carriage of gametocytes in asymptomatic low-density Plasmodium falciparum and P. vivax infections in western Thailand

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 (r 2 = 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.Electronic supplementary materialThe online version of this article (10.1186/s13071-017-2407-y) contains supplementary material, which is available to authorized users.

Utility of ultra-sensitive qPCR to detect Plasmodium falciparum and Plasmodium vivax infections under different transmission intensities

“…This study used 2252 archived DNA samples from cross-sectional blood collections in Thailand, Brazil and PNG between 2012 and 2014. A detailed description of these three study sites and results by standard qPCR were published previously [4,5,19] analysed. These cross-sectional surveys were conducted between May and July 2014 in Madang province.…”

“…Prior to performance of us-qPCR, good quality of the stored DNA was con rmed by repeating species-speci c Pf_ and Pv_18S rRNA qPCR assays in a minimum of 150 samples from each study site. Results in these repeated subsets were compared to the rst round of Pf_ and Pv_18S rRNA assays [4,5,19]. For Thailand and Brazil complete overlap for P. falciparum-positivity and almost complete overlap for P. vivax-positivity was found.…”

“…In epidemiological studies, quantitative PCR (qPCR) has increasingly been used to obtain more precise prevalence rates for Plasmodium spp. infections [2][3][4][5][6]. The most widely used qPCR assays for detection of Plasmodium species target genus-or species-speci c regions within the 18S rRNA genes [2,[7][8][9].…”

Immune effector mechanisms in malaria: An update focusing on human immunity

“…Many immunologic studies of malaria have revolved around understanding the mediators of the immunopathology associated with this disease and have identified the cell sources and responsible mediators. Yet, the global burden of P. falciparum today rests within those who are chronically infected and serve as reservoirs that perpetuate the transmission cycle . Therefore, more recent studies of malaria have interrogated the impact of chronic infections thereby increasing our understanding of immune regulators influenced by this parasite.…”

“…This phenotypic plasticity has been illustrated by CHMI studies 20 within those who are chronically infected 27,28 and serve as reservoirs that perpetuate the transmission cycle. 29,30 Therefore, more recent studies of malaria have interrogated the impact of chronic infections thereby increasing our understanding of immune regulators influenced by this parasite.…”