The burden of submicroscopic and asymptomatic malaria in India revealed from epidemiology studies at three varied transmission sites in India

Abstract

Malaria in India, while decreasing, remains a serious public health problem, and the contribution of submicroscopic and asymptomatic infections to its persistence is poorly understood. We conducted community surveys and clinic studies at three sites in India differing in their eco-epidemiologies: Chennai (Tamil Nadu), Nadiad (Gujarat), and Rourkela (Odisha), during 2012–2015. A total of 6,645 subject blood samples were collected for Plasmodium diagnosis by microscopy and PCR, and an extensive clinical questionnaire completed. Malaria prevalence ranged from 3–8% by PCR in community surveys (24 infections in Chennai, 56 in Nadiad, 101 in Rourkela), with Plasmodium vivax dominating in Chennai (70.8%) and Nadiad (67.9%), and Plasmodium falciparum in Rourkela (77.3%). A proportional high burden of asymptomatic and submicroscopic infections was detected in community surveys in Chennai (71% and 71%, respectively, 17 infections for both) and Rourkela (64% and 31%, 65 and 31 infections, respectively). In clinic studies, a proportional high burden of infections was identified as submicroscopic in Rourkela (45%, 42 infections) and Chennai (19%, 42 infections). In the community surveys, anemia and fever were significantly more common among microscopic than submicroscopic infections. Exploratory spatial analysis identified a number of potential malaria hotspots at all three sites. There is a considerable burden of submicroscopic and asymptomatic malaria in malarious regions in India, which may act as a reservoir with implications for malaria elimination strategies.

Spatial and temporal village-level prevalence of Plasmodium infection and associated risk factors in two districts of Meghalaya, India

“…Malaria diagnosis at the study PHCs, and throughout Meghalaya, is mainly performed using slide microscopy and RDTs. Due to declining Plasmodium transmission, microscopy and RDTs may not be sensitive enough to detect all cases of parasitaemia [22] resulting in underestimation of true prevalence, particularly if infection is increasingly subpatent and asymptomatic [23]. PCR detects more than twice the number of Plasmodium infections compared to microscopy and RDTs [24] and is very effective at detecting low-density infections [25].…”

Clinical and epidemiological characterization of severe Plasmodium vivax malaria in Gujarat, India

“…In a study recently conducted by our group, almost all infections in both clinic and community surveys at another site in Gujarat were detectable by microscopy. Because the area has a history of low malaria prevalence, residents can be expected to have low immunity, which may explain why most infections will develop parasitemia not only detectable by microscopy but also will lead to clinical episodes [ 26 ]. The proportion of Pv- positive cases in this other site was 85.9% (61/71) and 83.3% (55/66) by microscopy and PCR, respectively [ 26 ], which aligns with 78% (32/41) of Pv cases reported in the present study.…”

“…Because the area has a history of low malaria prevalence, residents can be expected to have low immunity, which may explain why most infections will develop parasitemia not only detectable by microscopy but also will lead to clinical episodes [ 26 ]. The proportion of Pv- positive cases in this other site was 85.9% (61/71) and 83.3% (55/66) by microscopy and PCR, respectively [ 26 ], which aligns with 78% (32/41) of Pv cases reported in the present study. Collectively, these findings suggest that the recent decline in malaria transmission in India has not affected the overall ratio of Pv: Pf in Gujarat, which was reported to be 80:20 in 2014 [ 12 ].…”

“…Thick and thin microscopy blood smears were air-dried, stained with Giemsa, and examined using a light microscope. Blood samples were subjected to DNA extraction using the Qiagen® QIAamp DNA Midi Kits followed by PCR amplification to assess the presence of Plasmodium species based on the protocol described elsewhere [ 24 – 26 ]. Briefly, 25 μl of PCR reactions were prepared including 5 μl of template DNA, primers, and 1x master mix (GoTaq Green, Promega), reaction volume was raised by PCR-grade water.…”

“…A BioRad T100 thermal cycler was used for amplification, and PCR products were run on 1.5% agarose gels (Agarose LE, Promega) in 1× TBE buffer (Thermo Scientific) to determine the presence of the amplified DNA, and PCR products were visualized using a UV trans-illuminator (BioRad Gel Doc EZ System). Different Plasmodium species were identified based on the amplified product sizes, as previously described [ 24 – 26 ].…”