Das et al. Malar J (2017) 16:181 DOI 10.1186/s12936-017-1833-9 Malaria Journal RESEARCH Open Access Malaria epidemiology in an area of stable transmission in tribal population of Jharkhand, India Manoj K. Das1, Brijesh K. Prajapati2, Régis W. Tiendrebeogo3,4,5, Kumud Ranjan2, Bright Adu6, Amit Srivastava7, Harvinder K. Khera7, Narendra Chauhan7, Sanjay Tevatiya2, Ikhlaq H. Kana3,4,5, Surya Kant Sharma2*, Subhash Singh7* and Michael Theisen3,4,5* Abstract Background: Malaria remains an important health problem in India with approximately 1 million cases in 2014. Of these, 7% occurred in the Jharkhand state mainly in the tribal population. Methods: This study was conducted in Dumargarhi, a tribal village about 42 km east of Ranchi city, Jharkhand, from May 2014 to September 2016. Four point prevalence surveys were carried out during consecutive high (October– December) and low (June–August) transmission seasons. Malaria cases were recorded from April 2015 to April 2016 through fortnightly visits to the village. Adult mosquito densities were monitored fortnightly by manual catching using suction tube method. Results: The study area consists of five hamlets inhabited by 945 individuals living in 164 households as recorded through a house-to-house census survey performed at enrollment. The study population consisted predominantly of the Munda (n = 425, 45%) and Oraon (n = 217, 23%) ethnic groups. Study participants were categorized as per their age 0–5, 6–10, 11–15 and >15 years. There were 99 cases of clinical malaria from April 2015 to April 2016 and all malaria cases confirmed by microscopy were attributed to Plasmodium falciparum (94 cases) and Plasmodium vivax (5 cases), respectively. During the high transmission season the mean density of P. falciparum parasitaemia per age group increased to a peak level of 23,601 parasites/μl in the 6–10 years age group and gradually declined in the adult population. Malaria attack rates, parasite prevalence and density levels in the study population showed a gradual decrease with increasing age. This finding is consistent with the phenomenon of naturally acquired immunity against malaria. Three vector species were detected: Anopheles fluviatilis, Anopheles annularis, and Anopheles culicifacies. The incoherence or complete out of phase pattern of the vector density peaks together with a high prevalence of parasite positive individuals in the study population explains the year-round malaria transmission in the study region. Conclusions: The collection of clinical data from a well-characterized tribal cohort from Jharkhand, India, has pro- vided evidence for naturally acquired immunity against malaria in this hyperendemic region. The study also suggests that enforcement of existing control programmes can reduce the malaria burden further. Keywords: Malaria, Plasmodium falciparum, Plasmodium vivax, Morbidity, Age, Jharkhand, India *Correspondence: suryaksharma@gmail.com; subhash@iiim.res.in; mth@ ssi.dk 2 National Institute of Malaria Research, Indian Council of Medical Research, New Delhi 110077, India 3 Department for Congenital Disorders, Statens Serum Institut, Artillerivej 5, 2300 Copenhagen, Denmark 7 Indian Institute of Integrative Medicine, Canal Road, Jammu 180001, India Full list of author information is available at the end of the article © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Das et al. Malar J (2017) 16:181 Page 2 of 10 Background Jharkhand from May 2014 to September 2016. The study Malaria is a vector borne disease caused by five different included four cross sectional surveys (CSS) to determine Plasmodium species. Of these, Plasmodium falciparum the point prevalence of P. falciparum and P. vivax during and Plasmodium vivax are the main causes of disease high and low transmission periods, respectively (Fig. 1d). and mortality worldwide [1]. Today, malaria is primarily A longitudinal cohort survey (LCS) was conducted confined to the poorest tropical areas of Africa, Asia and between April 2015 and April 2016 to determine the bur- Latin America, where it contributes as one of the world’s den of clinical malaria (Fig. 1d). The study area is situated greatest public health problems. in a forested area with terrain full of high and lowlands In India, about 1.0 million malaria cases were reported and a tropical humid climate. It receives high rainfall in 2014 of which approximately 7% originate from during Southwest monsoon (mid-May–September) and Jharkhand state (National Vector Borne Disease Con- low rainfall during Northeast monsoon (December–Feb- trol Programme of the Government of India). The for- ruary) with an average annual rainfall of 153  cm. The est, hilly terrain, favorable climate, inaccessible area, weather seasons are hot dry summer from April to mid- tribal culture, migration and social unrest are the main May, a monsoon from mid-May to September, autumn contributing factors to the malaria burden in Jharkhand from October to November, winter from December to [2]. An estimated 54 million tribals belonging to about 40 January, and spring from February to March. The maxi- ethnic communities (constituting around 28% of the total mum temperature during summer rises to around 41 °C population of Jharkhand) reside in the forest areas. This and the minimum temperature during winter falls to tribal population accounts for 8% of the total population about 5 °C. The relative humidity ranges between 35 and of India but contributes to 30% of all malaria cases [3]. 90%. Malaria transmission occurs year round but peaks Introduction of new epidemiological tools, bivalent rapid after the monsoon season from October to December. diagnostic test (RDT) kits for diagnosis, artemisinin- The study village consists of five hamlets located 1–3 km based combination therapy (ACT) for the treatment of from each other; these hamlets are connected by an all- P. falciparum malaria and long lasting insecticidal nets weather road. The nearest primary health center (PHC) (LLINs) has reduced the annual parasite incidence (API) is located 20 km from the study site. The study popula- of Jharkhand state from 6.20 in 2010 to 2.55 in 2015 tion belonged to different ethnic groups consisting of the (MK Das and SK Sharma, unpublished). Despite these Munda and the Oraon (the major ethnic groups) and the improvements, malaria remains a major public health other remaining ethnic groups were the Lohra, Bedia, problem causing morbidity and mortality in this region. Baraik and Kachhap. Each ethnic community has its own Individuals who are naturally exposed to malaria socio-cultural traditions but they live in harmony with develop a strong strain-transcending immunity [4, 5] other groups. Inter marriages between different ethnic which takes years of exposure to develop and is charac- communities are rare and not solicited by the community terized by a low grade parasitaemia in the presence of leaders. Most villagers work as subsistence farmers, but vigorous P. falciparum-specific immune responses [6]. some are engaged as daily wage laborers. Village children This immunity, termed premunition, has been exten- attend a local primary school where they get free mid-day sively characterized in Africa [7–10] primarily because meal under the Central Government sponsored scheme this is where the heaviest malaria related morbidity and to prevent malnutrition among school going children and mortality occurs [1]. However, with the current focus also as an incentive to prevent school drop outs. on malaria eradication, it has become more crucial than ever to investigate malaria epidemiology in other parts of Longitudinal cohort surveys (LCS) the world. Few such studies have been performed on the Malaria cases were recorded from April 2015 to April Indian sub-continent where malaria transmission rates 2016 in a longitudinal cohort study. An episode of clini- vary considerably over short geographical distances [11, cal malaria was defined as fever (auxiliary tempera- 12]. ture ≥36.5 °C, measured or reported) with slide positive The present study describes the demographic pro- for any asexual P. falciparum and/or P. vivax parasitaemia file and provides an overview of the malaria transmis- and/or at least one other sign of malaria such as vomit- sion dynamics in a malaria endemic tribal population in ing, diarrhoea, or malaise. A trained field worker vis- Jharkhand, India. ited every house in all hamlets once in a fortnight on a fixed schedule for active surveillance which involved Methods recording auxiliary temperature and all febrile individu- Study design, area, and population als were examined using a rapid diagnostic test (RDT) This study was conducted in Dumargarhi village (Ranchi kit [SD Bioline Malaria Ag Pf/HRP-2/pLDH, 3 band kit district; Fig. 1; Table 1) about 42 km east of Ranchi city, Alere Inc.]. RDT positive cases were examined by thick Das et al. Malar J (2017) 16:181 Page 3 of 10 a c b d CSS1 CSS2 CSS3 CSS4 LCS Fig. 1 Map of the study region showing locations of a Jharkhand state in the Union of India. b Ranchi District. c Study site in Ranchi district includ- ing the hamlets Karam Tungri (1), Jarawadih (2), New Torang (3), Old Torang (4), and Dumargarhi (5). d Timing and duration of cross sectional (CSS) and longitudinal surveys (LCS) is shown and thin blood smears from finger prick blood samples. telephonic reporting of any febrile cases to the trained All slides collected from the study area were brought to field worker by family members of the patient. Confirma- a laboratory at the National Institute of Malaria Research tion of these reported cases was done by the trained field field unit, Itki, Ranchi. Slides were examined by trained worker through use of RDT kit and malaria positive cases microscopists under compound microscope at 100× were treated as described above. Presumptive treatment magnification after staining with Jaswant Singh Bhat- was not given to the patients. tacharjee (JSB) stain (Rankem, India). Asexual parasites were counted against 200 leukocytes and parasite density Cross‑sectional surveys (CSS) was calculated as number of asexual parasites per micro Four point prevalence surveys were carried out in all liter of blood assuming a mean normal leukocyte count hamlets twice during peak transmission (October– of 8000/µL. Slide-positive cases were provided anti- December) and twice during low malaria transmission malarial treatment as per the guidelines of the National seasons (June–August). All villagers who were available Vector Borne Disease Control Programme of the Gov- and willing to participate were included in these sur- ernment of India [13, 14]. A blood smear was classified veys. The study subjects were examined for malaria by negative if no parasite was found in 100 random micro- RDT kit and microscopy of blood smears was performed scopic fields. In between the fortnightly visits, passive irrespective of clinical symptoms. Malaria cases were surveillance of malaria cases was maintained through treated as per the national drug policy described above. Das et al. Malar J (2017) 16:181 Page 4 of 10 Table 1 Demographic and baseline characteristics Factor Dumargarhi Jarwadih New Torang Karam Tungri Old Torang Total P value* Number of individuals (%) 186 (19.7) 103 (10.9) 84 (8.9) 123 (13.0) 449 (47.5) 945 (100.0) Age group (years) 0–5 14 (1.5) 9 (0.9) 9 (0.9) 9 (0.9) 71 (7.5) 112 (11.9) 6–10 30 (3.2) 11 (1.2) 5 (0.5) 18 (1.9) 45 (4.8) 109 (11.5) 11–15 19 (2.0) 9 (1.0) 8 (0.8) 19 (2.0) 33 (3.5) 88 (9.3) ≥16 123 (13.0) 74 (7.8) 62 (6.6) 77 (8.1) 300 (31.7) 636 (67.3) 0.001 Sex Female 90 (9.5) 49 (5.2) 42 (4.4) 56 (5.9) 215 (22.8) 452 (47.8) Male 96 (10.2) 54 (5.7) 42 (4.4) 67 (7.1) 234 (24.8) 493 (52.2) 0.99 Education G 1 (0.1) 0 (0.0) 0 (0.0) 1 (0.1) 0 (0.0) 2 (0.2) H 15 (1.8) 7 (0.9) 26 (3.2) 33 (4.1) 43 (5.3) 124 (15.3) I 82 (10.1) 31 (3.8) 22 (2.7) 30 (3.7) 204 (25.1) 369 (45.4) Mi 31 (3.8) 4 (0.5) 22 (2.7) 29 (3.6) 48 (5.9) 134 (16.5) Pr 33 (4.1) 13 (1.6) 6 (0.7) 18 (2.2) 74 (9.1) 144 (17.7) U 12 (1.5) 1 (0.1) 0 (0.0) 0 (0.0) 26 (3.2) 39 (4.8) <0.005 Profession A 33 (20.5) 16 (9.9) 13 (8.1) 20 (12.4) 77 (47.8) 159 (98.8) NIL 0 (0.0) 0 (0.0) 1 (0.6) 0 (0.0) 0 (0.0) 1 (0.6) S 1 (0.6) 0 (0.0) 0 (0.0) 0 (0.0) 0 (0.0) 1 (0.6) 0.159 Income [0–50,000] 25 (15.3) 20 (12.3) 13 (8.0) 20 (12.3) 71 (43.6) 149 (91.4) [50,000–100,000] 8 (4.9) 0 (0.0) 0 (0.0) 0 (0.0) 6 (3.7) 14 (8.6) 0.002 Housing M 182 (20.1) 100 (11.0) 34 (3.7) 113 (12.5) 431 (47.5) 860 (94.8) <0.005 P 0 (0.0) 0 (0.0) 8 (0.9) 0 (0.0) 0 (0.0) 8 (0.9) T 0 (0.0) 0 (0.0) 39 (4.3) 0 (0.0) 0 (0.0) 39 (4.3) Bednet N 142 (15.7) 100 (11.0) 81 (8.9) 113 (12.5) 367 (40.5) 803 (88.5) Y 40 (4.4) 0 (0.0) 0 (0.0) 0 (0.0) 64 (7.1) 104 (11.5) <0.005 Education: G graduate, H high school education (intermediate), I illiterate, Mi middle, Pr primary, U undergraduate; Profession: A agriculture, NIL no profession, S service; Housing: M mud walls with thatched roof, P cement walls and roof, T brick walls with tin roof, Bednet: Y yes; N no * P value is based on Chi Square test Common ailments were treated by the health-care work- vectors [Anopheles culicifacies, Anopheles fluviatilis and ers, whereas individuals with serious life threatening Anopheles annularis (Diptera: Culicidae)] throughout diseases such as TB, hypertension, respiratory and liver the year. An. culicifacies breeds mainly in pools formed disorders were referred to Angara primary health center in streams and riverbeds; the most productive breeding or to a referral hospital located in Ranchi city. The rel- sites of An. fluviatilis are slow-moving streams while An. evant information for each study subject such as name, annularis breeds in margins of ponds, rivers and streams age, sex, resident status, fever, auxiliary temperature, and with abundant vegetation. The three vector species An. other clinical symptoms were recorded in a patient data culicifacies (sibling species B & C), An. fluviatilis (sib- sheet and a computerized data base for such information ling species T) and An. annularis (sibling species A) were was developed. identified based on their morphological characteristics and cytogentic analysis of their polytene chromosomes Vector species and entomological surveys [15]. The district is characterized by a large network of Entomological survey was conducted fortnightly from streams and other water bodies which provide innumera- May 2014 to March 2016 by manual catching using suc- ble and diverse breeding sites for species-specific malaria tion tube method [16, 17]. Indoor resting collections Das et al. Malar J (2017) 16:181 Page 5 of 10 were made in the morning between 06:00 and 08:00  h from four randomly selected human dwellings and four cattle sheds. Mosquitoes from each dwelling were kept separately in test tubes and species were identified based on morphology [18–20]. The sibling species identity of the captured vector species was determined through cytogenetic analysis of the polytene chromosomes [15]. Densities per person-hour searching (person-hour den- sity or PHD) of total anophelines and vector species were determined. Statistical analysis Statistical analysis was done with the R 3.3.0 and Graph- Pad Prism 5 software. Age was categorized in four levels Fig. 2 Monthly malaria incidence (cases per 945 population) in the (0–5, 6–10, 11–15 and >15 years) and demographic char- Ranchi study cohort from April 2015 to April 2016. During this period acteristics were compared between villages by using the there were 94 P. falciparum and 5 P. vivax cases diagnosed with febrile χ2-test. P values <0.05 were considered to indicate statis- malaria. Monthly incidence of P. falciparum (black bars), P. vivax (grey bars) is shown. The line plot is the monthly rainfall recorded during tical significance. the period Results Population characteristics The study area consists of five hamlets inhabited by mean density of P. falciparum parasitaemia was calcu- 945 individuals living in 164 households as recorded lated for the 0–5, 6–10, 11–15, and >15 year age groups, through a house-to-house census performed at enroll- respectively (Fig. 3a). It increased to a peak level of 23,601 ment. The population distribution within these ham- parasites/μl in the 6–10  years age group and gradually lets were Dumargarhi (n  =  186), Jarwadih (n  =  103), declined in the adult population to a level of 7066 para- New Torang (n = 84), Karam Tungri (n = 123) and Old sites/μl. A similar pattern was observed in the incidence Torang (n =  449) (Table  1). The mean age of the study rate of febrile P. falciparum malaria (Fig. 3b). population was 27.5 ± 18.8 years. Villagers were divided into 4 age groups: ≤5 years (n = 112, 11.9%), 6–10 years Malaria parasite prevalence during high and low (n =  109, 11.5%), 11–15 years (n =  88, 9.3%) and  >15 transmission periods years (n =  636, 67.3%). The study population consisted During the high transmission season (CSS1 and CSS3), predominantly of the Munda (n = 425, 45%) and Oraon 386 and 327 villagers respectively were examined for P. (n  =  217, 23%) ethnic groups. The remaining villagers falciparum and P. vivax infections by RDT kit (Table 2). belonged to Lohra, Bedia, Baraik and Kachhap ethnic Of these, 180 were positive for malaria parasites groups. Individuals who used bed net constituted 11.5% (Pf = 148, Pv = 10, mixed infection = 22) in CSS1 and 72 of the population. The gender distribution was not sig- were positive for malaria (Pf = 59, Pv = 9, mixed infec- nificantly different between villages (P < 0.99) while the tion = 4) in CSS3. The parasite rate during the first and distribution of age, bed net use, housing, income was sig- second surveys in the high transmission was 46.6 and nificantly different between villages (P < 0.005, χ2 analy- 22% respectively. During low transmission (CSS2 and sis) (Table 1). CSS4), 254 and 225 individuals were enrolled, 50% of whom had also participated in CSS1. Of these, 51 indi- Plasmodium falciparum infections recorded viduals were malaria positive by the RDT test in CSS2 through fortnightly surveillance (Pf = 46, Pv = 2, mixed infections = 3) and 39 were posi- Malaria transmission was perennial and P. falciparum tive for malaria in CSS4 (Pf = 35, Pv = 4). The parasite malaria was reported throughout the year (Fig. 2). There rate during the two surveys in the low transmission was were 99 cases of clinical malaria from April 2015 to April 20.1 and 17.3% respectively. The average parasite rate 2016 and all cases confirmed by microscopy were attrib- during the high (CSS1 + CSS3) and low (CSS2 + CSS4) uted to P. falciparum (94 cases) and P. vivax (5 cases), transmission season was 35.3 and 18.8%, respectively. A respectively. The monthly incidence of P. falciparum and significant variation in the seasonal malaria prevalence P. vivax is shown in Fig.  2. P. falciparum malaria inci- (P < 0.0007, χ2 analysis) was observed. dence was generally higher after the end of the monsoon Eighty-six (47.8%) and 13 (25.5%) of the RDT posi- season and lower in the hot dry summer months. The tive samples were also found to be slide positive for P. Das et al. Malar J (2017) 16:181 Page 6 of 10 Fig. 3 Mean P. falciparum density and febrile malaria incidence per age-group. a Mean parasite density of all slide positive P. falciparum malaria cases recorded during the longitudinal follow-up from April 2015 to April 2016 was calculated per age groups. b Incidence rate of febrile malaria in the same age groups falciparum from CSS1 and CSS2, respectively. The mean fluctuations in the month-wise person-hour density P. falciparum parasite density of these slide positive indi- (PHD) of An. culicifacies, An. fluviatilis and An. annu- viduals was 6113 (±1456) parasites/μl and 3480 (±1511) laris was observed (Fig.  5). The highest density of An. parasites/μl during high and low transmission season, fluviatilis was observed between October and February, respectively (Table  2). The prevalence of P. falciparum whereas the highest density of An. culicifacies and An. parasitaemia was highest in children up to 15  years of annularis was observed between May and September age and declined in the adult population during both and the lowest during October–April. high and low-transmission season (Fig.  4a). During CSS1, there were 44 cases of P. falciparum malaria. The Discussion remaining slide positive individuals (n = 42) who did not Malaria is a focal disease in India, influenced by sev- have fever or any other symptom of malaria were classi- eral local ecological and social factors [21]. The study fied as asymptomatic P. falciparum carriers. The number team collected samples and clinical data from 945 indi- of malaria cases and asymptomatic carriers were 4, 15, viduals of tribal origin living in closely placed hamlets 10, and 15 and 3, 7, 5, and 27 in the 0–5, 6–10, 11–15, in the forested hills of Jharkhand in the Eastern parts and >15 years age groups, respectively. The mean parasite of India. This study involved four cross-sectional sur- density of each age group was plotted for malaria cases veys two each during the high (October–December) and asymptomatic carriers, respectively (Fig.  4b). Para- and the low (June–August) transmission seasons and site densities of acute malaria cases were 66-fold higher fortnightly surveillance during the periods intervening in the 0–5 age group than those of asymptomatic carri- the cross-sectional surveys. In summary: (1) malaria ers (Fig.  4b). During CSS3 and CSS4 lower prevalence transmission was hyper-endemic, and (2) there was evi- of malaria observed in this study population could be dence suggesting that the population develops naturally attributed to the ongoing case surveillance during the acquired immunity (NAI) as a function of age and expo- preceding study period and effective treatment of the sure. The observations that children had (i) a high inci- malaria positive cases, thereby reducing the parasite dence and prevalence rate of P. falciparum infections, transmission. (ii) a high malarial attack rate, and (iii) higher parasite densities compared to adults are indications of hyper- Malaria vectors and seasonal prevalence endemicity in this study area. It is generally observed Average annual vector density determination in the that individuals living in hyper-endemic areas gradu- study area established An. fluviatilis as the most preva- ally develop an anti-parasite immunity which protects lent vector constituting 49% of all the vector species. them from febrile malaria. The age-dependent variation An. annularis and An. culicifacies constituted 34 and of malaria attack rates, parasite prevalence and density 17%, respectively. The cumulative annual average of all levels in the present study was characterized by a grad- the vectors captured in the study area during fortnightly ual decrease with increasing age. This relationship is surveys was 21 and 79% for vectors resting in human typical of NAI against malaria and is similar to findings dwellings and cattle sheds, respectively. Some seasonal in highly endemic areas of Africa [22–24] and Asia [25]. Das et al. Malar J (2017) 16:181 Page 7 of 10 Table 2 Malaria prevalence and mean parasite density during high and low transmission seasons in the study population Transmission season Study RDT performed Total positive Pv Pf Mix Pf parasite rate (%) Pv parasite rate (%) Mean Pf parasite d ensitya Pf malaria prevalence (%) High (Oct–Dec 2014) CSS1 386 180 10 148 22 44 8.3 6113 (±1456) 11.4 Low (Jun–Aug 2015) CSS2 254 51 2 46 3 19.3 1.9 3480 (±1511) 2.8 High (Oct–Dec 2015) CSS3 327 72 9 59 4 19.3 3.9 17,195 (±7854) 8.6 Low (Jun–Aug 2016) CSS4 225 39 4 35 0 15.6 1.8 1689 (±2483) 0.0 RDT Rapid diagnostic test. Pf Plasmodium falciparum, Pv Plasmodium vivax. Mix; Pf and Pv a Parasitemia was determined by microscopy Das et al. Malar J (2017) 16:181 Page 8 of 10 Fig. 4 a Age-dependent prevalence of P. falciparum positive individuals during the high (circles) and the low (squares) transmission seasons. b Mean P. falciparum density per age group in acute malaria cases (triangles) and asymptomatic carriers (diamonds) during high transmission season in 2014 (CSS1). Parasite densities of acute malaria cases were approximately 66-fold higher than those of asymptomatic carriers in the 0–5 years age group and this threshold decreased with age Fig. 5 Monthly density of An. culicifacies (black bars) An. Annularis (green bars), and An. fluviatilis (red bars) in the study area based on indoor resting hand catch collections once every month. The line plot is the monthly rainfall recorded during the period The development of NAI is further underscored by the adults but not in children below 3 years of age [28]. Col- very high difference in parasite densities between acute lectively these findings support the notion that individ- malaria cases and asymptomatic carriers. The authors uals who live in highly malaria endemic areas of India observed that parasite densities in acute malaria cases develop NAI. were much higher than those in asymptomatic carriers. Another important observation of the present study This difference was particularly high in young children was the apparent decrease in the malaria parasite bur- and decreased with age i.e. the parasite density causing den between 2014 and 2016. In the first cross-sectional fever decreases with age and exposure. This finding is survey during high transmission season of 2014 the rate related to the phenomenon of the pyrogenic threshold of P. falciparum-positive samples was around 44% with which has been described in Africa [26]. Similar age- a clinical malaria prevalence of 11.4%. This was reduced dependent decrease in the prevalence of parasitaemia to 19.3% for P. falciparum-positive samples and clinical and febrile malaria has also been observed in Odisha, malaria prevalence of 8.6% during the next high trans- which is located in a neighboring state of India [27]. mission season. This reduction in the parasite burden in Another study in the Sundargarh district in Odisha, also the study population is likely due to the combination of demonstrated high levels of agglutinating antibodies active case detection and ACT of all identified positive against P. falciparum infected erythrocytes in healthy cases. Das et al. Malar J (2017) 16:181 Page 9 of 10 The entomological studies revealed that the study poor surveillance, inadequate vector control and gen- area is under the influence of three vector species An. eral lack of motivation and health awareness all facilitate fluviatilis and An. culicifacies as the primary vectors persistent transmission. Therefore, focus should be on and An. annularis as the secondary vector for malaria strengthening of health infrastructure at the periphery to transmission. The climatic conditions in the forest ensure early case detection and prompt treatment. This ecotype are more conducive for higher survival of vec- should be supported by quality vector control measures, tors that are associated with a predominance of P. falci- creating health awareness in the community and ensur- parum [3]. An. fluviatilis is widely distributed in India ing economic growth of the tribal areas. These efforts although its role in malaria transmission varies from along with the development of an efficacious malaria vac- place to place depending on the local prevalence of dif- cine will lead to a sustained reduction in malaria morbid- ferent sibling species [29]. An. fluviatilis (species S) is ity and mortality. among the most efficient vectors of malaria particu- larly in hills and foothills of India and it has previously Conclusion been incriminated as a vector in the tribal districts of In conclusion, the collection of clinical data from a well- Koraput, Orissa [28] and Bastar, Madhya Pradesh [30]. characterized tribal cohort from the state of Jharkhand in In contrast, other studies have found that An. fluvia- Eastern India has provided evidence for the presence of tilis (species T), though prevalent in high densities in NAI against malaria in this hyperendemic region. the foothills of Shiwalik range was not playing role in malaria transmission [31]. However, An. fluviatilis (spe- Authors’ contributionsSKS, SS and MT designed the study. MKD, BKP, KR, ST and SKS collected the cies T) has been found to be susceptible to P. vivax [32] data. AS, HKK and NC processed the samples. RWT, BA, SKS, IHK and MT and P. falciparum infections (MK Das, unpublished) and analysed the data. SKS, SS and MT wrote the manuscript. All authors read and has been incriminated as an efficient vector of malaria approved the final manuscript. for transmission in the mountainous areas of the Hor- Author details mozgan province, south Iran [33]. The role of An. flu- 1 Field Unit, National Institute of Malaria Research, Ranchi, Jharkhand, India. 2 viatilis (species T) as the primary vector in malaria National Institute of Malaria Research, Indian Council of Medical Research, New Delhi 110077, India. 3 Department for Congenital Disorders, Statens transmission in this study has been established by way Serum Institut, Artillerivej 5, 2300 Copenhagen, Denmark. 4 Centre for Medical of its high prevalence period coinciding with the peak Parasitology at Department of International Health, Immunology and Micro- transmission period, more preference for human biting, biology, University of Copenhagen, Copenhagen, Denmark. 5 Department of Infectious Diseases, Copenhagen University Hospital, Rigshospitalet, high human blood index and susceptibility to plasmo- Copenhagen, Denmark. 6 Noguchi Memorial Institute for Medical Research, dial infections (MK Das, unpublished). The other pri- University of Ghana, Legon, Ghana. 7 Indian Institute of Integrative Medicine, mary vector, An. culicifacies (species C), is responsible Canal Road, Jammu 180001, India. for 60–70% of all malaria cases in India despite being Acknowledgements predominantly a zoophagic species and being prone to The authors are thankful to the Council of Scientific and Industrial Research environmental factors with epidemiological implica- (CSIR), Indian Council of Medical Research (ICMR) and the Director, National Institute of Malaria Research (ICMR–NIMR) for permitting us to conduct this tions in different ecotypes [12, 34]. An. culicifacies may study. Technical help provided by the field and laboratory staff, Sh. Vijay Om be regarded as playing a complementary role in main- Gupta, LT and Sh. Ramkesh Pandey, LA for their assistance during the course taining perennial transmission during pre-monsoon of study is acknowledged. BKP, KR, AS, NC and ST acknowledge DBT for fellow-ships. HKK acknowledges ICMR for fellowship. and monsoon months where transmission is low. The secondary vector An. annularis though primarily zoo- Competing interests phagic, exophilic and exophagic, is associated with low The authors declare that they have no competing interests. human biting rate and sporozoite rate, and has a mar- Availability of data and materials ginal role in malaria transmission in this study area. The datasets used and analysed in the current study are available from the While An. annularis plays a limited role for malaria corresponding author on reasonable request. transmission in the present study area, it is responsible Ethics statements for year-round transmission as observed in inlands of The study was approved by the Institutional Ethics Committee of the National Odisha [32] and forests of Assam [35]. The presence of Institute of Malaria Research (NIMR), Indian Council of Medical Research, New Delhi. the three vector species and their sibling species com- Villagers were informed about the purpose of the study and an informed plexes reported here together with a high prevalence consent was obtained from study participants or their guardians before enroll- of parasite positive individuals in the study population ment in the study. explains the year-round malaria transmission. Besides technical inadequacies, the problem posed by Funding malaria in tribal areas of India is multifaceted. Opera- This work was supported by the Department of Biotechnology (DBT), Gov-ernment of India (BT/IN/Denmark/13/SS/2013) and the Danish Council for tional constraints such as inaccessible remote areas, Strategic research (Grant 13127). Das et al. Malar J (2017) 16:181 Page 10 of 10 Publisher’s Note 20. Knight KL, Stone A. 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