Challenges in Understanding the Bionomics of Indian Malaria Vectors

ABSTRACT.

Many factors influence the success or failure of malaria vector control program such as political will, leadership, sustained funding, robustness of healthcare system and others. In addition, updated knowledge and information about the triad of host, parasite, and vector is of paramount importance. Vector bionomics studies that determine mosquito behavior in terms of feeding, resting, biting, mating, breeding, longevity, vectorial capacity, and response to different insecticides are a step towards enhancing our understanding. In the present work, we have compiled studies conducted in India over the past two decades (2000–2020) to identify gaps in our knowledge of malaria vector bionomics and the research that needs to be done in the future. We retrieved district-level data of India’s six primary malaria vector species. According to our findings, vector bionomics studies have been undertaken in ∼50% and ∼15% of the country’s high (annual parasite index > 1) and low (annual parasite index < 1) malaria-endemic districts respectively. Most of the research studies focused on mosquito density, insecticide susceptibility status, and parasite detection, whereas other vital bionomics parameters were neglected. Surveys conducted were incomplete, and vector bionomics data were not captured sufficiently. The absence of vector bionomics data can be a blind spot and the lack or inadequate understanding of vector bionomics can lead to use of inappropriate vector control tools. Thus, there is an urgent need to initiate comprehensive bionomics studies on India’s primary and secondary malaria vectors.

INTRODUCTION

Malaria is a major public health problem in India, which accounted for 83% of estimated cases and 82% of deaths in World Health Organization’s (WHO) South East Asia Region in 2020.¹ The disease is caused by a protozoan parasite transmitted to humans, the primary host, by Anopheles mosquitoes. Six Anopheles species, An. culicifacies, An. dirus, An. fluviatilis, An. minimus, An. stephensi, and An. sundaicus, are considered major malaria vectors among the 58 Anopheles species reported from India. An. annularis, An. philippinensis, An. subpictus, An. nivipes, and An. varuna act as secondary vectors in limited geographic areas.² Control of malaria primarily depends on parasite clearance and vector control tools. The former relies on drug therapy, whereas chemical, biological, and environmental tools are applied for vector control.

Anopheles mosquitoes are found worldwide as species complexes, members of which are referred to as sibling, isomorphic or cryptic species.^3,4 Sibling species are reproductively isolated with discrete gene pools and have unique biological characteristics. Each sibling species may have a distinct role in the transmission of malaria. Sibling species complexes are observed in India in five out of six malaria vectors, except for An. stephensi.⁵ Anopheles culicifacies has the most sibling species at five (A, B, C, D, and E) followed by An. fluviatilis (S, T, and U), An. dirus (An. baimaii and An. elegans), An. minimus (An. minimus s.s. or species A) and An. sundaicus (species D) in India.^2,5 Correct identification of sibling species helps in determining the differences between a vector and a non-vector. Behavior, distribution, and infection rate vector data will only be pertinent if they facilitate decision-making by national vector control programs in conjunction with the knowledge of sibling species. Hence, understanding sibling species complexes is essential for targeted vector control techniques because of their diverse biological characteristics and ecological preferences, including overlapping spatial distribution (sympatry).

Bionomics is the comprehensive study of the relationship between a species and its environment. In the case of mosquitoes, the bionomics study covers feeding, resting, biting, mating, breeding, longevity, vectorial capacity, and response to different insecticides. The selection of vector control measures and the assessment of their impacts are based on an understanding of vector bionomics.^6–8 The current malaria vector control program in India is mainly based on indoor residual spraying (IRS), long-lasting insecticide-treated nets (LLIN), and larval source management.⁸ The effectiveness of the IRS program depends solely on the resting behavior of mosquito vectors, whereas the LLIN impact is based on the biting time and feeding behavior of vectors. However, changes in vector biting and resting behavior have been observed from indoors to outdoors, where these two tools have been in use for years.^9,10 These possible behavior contribute to the outdoor transmission where vectors bite early and outdoors when LLIN or IRS does not protect a person. Hence, monitoring changes in the behavior is essential, especially in outdoor transmission.¹¹ Knowledge about the susceptibility status of vectors against the insecticides used in IRS and LLIN is also crucial for the success of a vector control program. Deployment of novel insecticides with different modes of action will necessitate a thorough understanding of vector bionomics.

Larval source management is dependent on the breeding behavior of mosquitoes. Also, vector-borne diseases are climate-sensitive diseases, and climate change influences mosquito’s longevity during the incubation period of parasites in vectors, altering the spatiotemporal distribution of diseases.¹² Five parasites cause malaria in India: Plasmodium vivax, Plasmodium falciparum, Plasmodium ovale, Plasmodium malariae, and Plasmodium knowlesi. Malaria due to P. knowlesi is zoonotic in origin as it is transmitted between long-tailed macaques and mosquito vector An. sundaicus in Andaman and Nicobar Islands, but its burden is understudied in India.¹³ Our work here is an attempt to compile all published studies conducted on the bionomics of malaria vectors in India during the past two decades (2000–2020). As India aims to eliminate malaria by 2030, our analysis will be crucial in defining vector control strategies. This work may provide insights to the national program and enable it to critically review the existing vector control strategies.

MATERIALS AND METHODS

Data sources and search strategy.

We searched the key terms Anopheles, bionomics, culicifacies, stephensi, minimus, dirus, fluviatilis, sundaicus, and malaria vectors of India indexed in PubMed and Google Scholar for publications on India’s bionomics of malaria vectors. We also looked at cross-references for selected relevant papers.

Inclusion and exclusion criteria.

Original research published in the most recent two decades (2000–2020) in the English language only were included in the current analysis. Studies conducted on six primary vectors of malaria in India were included. Publications having district-level information on different bionomics characterstics such as seasonal prevalence, vector density, breeding habitats, sporozoite rate, parity rate, human blood index, and insecticide susceptibility status were taken into consideration.

The present analysis did not include book chapters and annual reports. Studies carried out outside of India as well as published in a language other than that of English were excluded. Manuscripts not having information on primary malaria vectors were not included in the current study. Articles not having relevant bionomics information were also excluded.

Data extraction and analysis.

We extracted the information on seasonal prevalence, vector density, breeding habitats, sporozoite rate, parity rate, human blood index, and insecticide susceptibility status from all the selected articles. If any of these parameters were not mentioned in the publication, they were estimated using the information provided in the article.

Bionomics characteristics were arranged species-wise and district-wise and tabulated into a spreadsheet in MS Excel. The ranges of each bionomics feature were calculated, and the data were organized for use in a geographic information system (GIS). District-level maps indicating bionomics studies conducted in the country from 2000 to 2020 were created using Quantum geographic information system (QGIS), an open-source software.¹⁴

RESULTS AND DISCUSSION

We found 942 articles by manually searching PubMed and Google Scholar. Out of these articles, 165 articles were found to be duplicates and excluded. The remaining 777 articles were reviewed based on their titles, with the names of 385 articles being found to be irrelevant to the subject. Most of these dealt with larvicidal properties of plant extracts against different mosquito vectors, hence excluded from the study. Later, articles were assessed based on the abstract, and 240 articles were excluded from the study for the following reasons: 1) articles pertained only to molecular, genetic, or metabolomics studies; 2) studies that were carried out outside India; 3) articles that dealt with epidemiology of malaria, not with vectors; and 4) studies that were carried out on mosquitoes other than that of vector species. These inclusion and exclusion criteria are outlined in the Flow chart (Figure 1), wherein 108 articles were published between 2000 and 2020 and were included in the study. A distribution map was created for six primary species of Anopheles, and their bionomics features are described in Table 1. The following are descriptions of six Indian vector species.

Figure 1.

Flow-chart depicting manuscript selection criteria.

Table 1.

Bionomics characteristics of the six primary malaria vectors as per 2000–2020 data

Bionomial characteristics	An. dirus	An. minimus	An. sundaicus	An. stephensi	An. fluviatilis	An. culicifacies
Resting	Outdoor	Indoor	NA	Indoor	Indoor	Indoor
Man-hour density*	NA	0.4–11.7	NA	1.0–13.8	0.6–31	0.1–200.0
Trap density	0.5–17.1	1.4–9.9	NA	NA	1.0–4.1	0.1–13.3
Human landing rate†	3.5–36.1	1.6–6.3	NA	0.3	1.5–6.4	0.2–0.95
Biting time	Whole night	Whole night	NA	Whole night	Whole night	Whole night
Peak biting time	21:00–03:00	00:00–04:00	NA	12 am–6 pm	9 pm–6 am	12 am–6 pm
Anthropophilic index (%)‡	92–100	>90	NA	0.4–39.9	1.9–100	0.9–74
Parity rate (%)§	43.9–61.5	62–76	NA	42–46	57–65	64–73
Sporozoite rate (%)‖	0.7–33.3	0.01–30.0	NA	0.26–4.5	0.06–13	0.12–34.2
Entomological inoculation rate¶	0.01–0.09	0.12–0.71	NA	2.35	0.39	0.01–4.8
Seasonal prevalence	Peak density: June to Aug	Peak density: July to Oct	NA	Peak density: July to Sept	Peak density: Sept to Feb	Peak density: May to Sept
Insecticide susceptibility status*	DDT (S)	DDT (PR), deltamethrin (S)	DDT, malathion, and deltamethrin (S)	DDT and malathion (R)	DDT, malathion, and deltamethrin (S)	DDT (R); deltamethrin (PR/R)
Breeding sites	Water stored in pool and elephant footprints	Slow-flowing streams with grassy banks, river, pond and paddy fields	Brackish water and observed in fresh water	Primarily cement tanks	Streams and rivers	Pools, seepages, pits, clean stagnant water, rice fields

An. = Anopheles; NA = not available.

^*Man-hour density: number of mosquitoes collected by one person for 1 hour.

^†Human landing rate: ratio of the total mosquitoes captured for a period to the total person-night used for the same period.

^‡Anthropophilic index: proportion of blood meals taken by mosquitoes from humans.

^§Parity rate: proportion of parous females.

^¶Sporozoite rate: proportion of sporozoite positive mosquitoes.

^‖Entomological inoculation rate: number of sporozoite positive bites received per individual per night.

^**≥ 98–100% mortality: susceptible (S); 90–97% mortality: possible resistance (PR); < 90% mortality: resistant (R).

Anopheles dirus.

Of eight sibling species of An. dirus s.l., only An. baimaii and An. elegans are prevalent in India.¹⁵ An. dirus is a highly proficient vector of malaria, mainly in deep forested areas of Northeast India. During 2000–2020, research studies on the bionomics of An. dirus were conducted in Dibrugarh (Assam), Dhalai (Tripura), and South Tripura (Tripura). Vectorial capacity is the ability of mosquitoes to spread Plasmodium species. It is defined as the average number of infective bites resulting from all mosquitoes blood-feeding on a single person per day. The number of sporozoite positive bites received per individual per night is defined as the Entomological Inoculation Rate (EIR), and it is an essential parameter for measuring vectorial capacity. Entomological inoculation rate was studied in only one study from Dibrugarh (Assam) which found it in the range of 0.014 to 0.096.¹⁶ The study also reported a mean biting rate of 36.1 bites/person/night, a sporozoite rate of 1.9%, and a parous rate of 58.7%. From 9 pm to 12 am, EIR was reported highest (i.e., 0.249 positive bites/person/night) and was the most likely period for malaria transmission by An. baimaii (Figure 2, Table 1).

Figure 2.

Vector bionomics studies of primary malaria vectors conducted in India during the past 2 decades (2000–2020). State abbreviations: AP = Andhra Pradesh; AR = Arunachal Pradesh; AS = Assam; BR = Bihar; CG = Chhattisgarh; GA= Goa; GJ = Gujarat; HR = Haryana; HP = Himachal Pradesh; JH = Jharkhand; KA = Karnataka; KL = Kerala; MP = Madhya Pradesh; MH = Maharashtra; MN = Manipur; ML = Meghalaya; MZ = Mizoram; NL = Nagaland; OR = Odisha; PB = Punjab; RJ = Rajasthan; SK = Sikkim; TN = Tamil Nadu; TG = Telangana; TR = Tripura; UK = Uttarakhand; UP = Uttar Pradesh-UP; WB = West Bengal-WB. Union Territories: AN = Andaman and Nicobar Islands; CH = Chandigarh; DD: Daman & Diu; DH = Dadra and Nagar Haveli; DL = the Government of NCT of Delhi; JK = Jammu & Kashmir; LA = Ladakh; LD = Lakshadweep; PY Puducherry. This figure appears in color at www.ajtmh.org.

Human blood index, the proportion of blood meals taken by mosquitoes from humans, is determined by mosquitoes biting behavior. Anopheles dirus is a highly anthropophilic vector with a human blood index > 0.9 to 1.^17–19 Parity rate is the proportion of parous females, and it reflects the age of the mosquito population. In the case of An. dirus, it was found in the range of 43.9 to 61.5%.^16,17,20 The sporozoite positivity rate depicts the proportion of sporozoite positive mosquitoes, indicating that the mosquito can transmit malaria parasites to humans. Sporozoite positivity rate (SPR) was 0.7% to 3.2% in different studies undertaken in Assam, whereas, a high rate of sporozoite positivity was observed in studies from Meghalaya (33.3%) and Assam (9.9%).^16,17,20,21 Elephant footprints and temporary pools were the major breeding sites of An. dirus in deep forest, whereas ditches, wheel tracks, and animal hoof marks supported breeding in forest fringe areas.²² During the past two decades, only one study reported insecticide susceptibility status from South Tripura, where it was found susceptible to DDT.¹⁸ Due to the exophilic behavior and high anthropophagy of An. dirus, it is difficult to control the vector population. LLIN is effective in minimizing host–vector contact indoors, though new innovative tools and methods are required to prevent outdoor biting and outdoor resting of malaria vectors in northeast India.²³

Anopheles minimus.

Although it is an important malaria vector in the northeast region of India and the state of Odisha, it has not been studied extensively. The bionomics characteristics of An. minimus has been comprehensively identified in a few studies only. One study from Nagaland, Meghalaya, and Tripura was undertaken during the past two decades.^18,24,25 Since 2000, no published study on An. minimus bionomics could be found in the three Northeastern Indian states, Arunachal Pradesh, Mizoram, and Sikkim.

An. minimus is a species with perennial prevalence and high density from May to September. In Odisha, it has not been encountered during the summer months.²⁶ A few studies have reported the development of resistance against DDT in An. minimus from Keonjhar (Odisha), South Tripura (Tripura), Sonitpur, and Kamrup (Assam) districts.^26–28 However, it is generally considered susceptible to DDT and deltamethrin.^18,25,29,30 It is an anthropophilic species (anthropophilic index of > 90%) that feeds mainly on humans.^18,31–34 It has been found to bite throughout the night, with its peak biting time mainly after midnight (i.e., 12 am to 4 am).^17,23,28,29 Being an endophilic species, it prefers to rest indoors.^18,34

Parity rate is an important parameter to determine longevity, but it has not been studied in Assam. Parity rate was determined only from Keonjhar (Odisha), Dimapur (Nagaland), and Santhal (previously in Bihar now in Jharkhand) with a range of 62% to 76%.^{27,29,31,33–35} Three studies each from Assam, Tripura, and Meghalaya reported EIR in the range of 0.045 to 0.71 (Figure 2, Table 1).^18,25,36 Human activities and deforestation in the northeastern region of India has disturbed the ecology leading to depletion of breeding habitats of An. minimus. These included perennial seepage water streams marked with grassy banks present in forest fringe areas, and their disappearance has lowered the density of An. minimus.^37–39 Due to the enhanced pressure of insecticides by virtue of use of LLINs and IRS in the region, there is a possibility of change in the resting and feeding behavior of the species. Therefore, comprehensive bionomics research studies might provide insights into this unexplored aspect of An. minimus.

Anopheles sundaicus.

An. sundaicus is predominantly a coastal vector species and has been reported from the Car Nicobar Island (Andaman and Nicobar archipelago) of India.³ It is a complex of four sibling species, of which only sibling species D is found in India. It was previously reported from mainland Odisha and West Bengal, but now it is confined only to Andaman and Nicobar Islands, making it the sole vector of the island. During the study period of this review, a total of nine studies from Andaman and Nicobar Island were published on An. sundaicus. The studies focused on insecticide susceptibility status and breeding habitats only—none of the studies researched bionomics comprehensively. Khan and Sunish (2017) demonstrated the susceptibility of this vector species to DDT, malathion, and deltamethrin.⁴⁰ Another study conducted by Krishnamoorthy et al. discussed the potential breeding sites for An. sundaicus in South Andaman, which revealed low-lying paddy fields and fallow land, with salinity in the range of 3,000 to 42,505 ppm supported profuse breeding of An. sundaicus.⁴¹ Recently, An. sundaicus mosquitoes collected from Katchal Island in the Andaman and Nicobar Islands were found to be positive for Plasmodium knowlesi parasite, which is of zoonotic origin transmitted from long-tailed macaques.⁴² Previously, 11.9% of archived blood samples from human malaria cases in the Andaman and Nicobar Islands were found positive for P. knowlesi.⁴³ Therefore, there is a possibility that An. sundaicus may be transmitting P. knowlesi to humans and more in-depth research into vector bionomics can provide helpful information (Figure 2, Table 1).

In addition to the islands of Andaman and the Nicobar Islands, vector bionomics surveys should be undertaken in Lakshadweep Islands and coastal areas of other states to explore its presence. Antilarval measures may not be useful to control An. sundaicus due to its adaptivity to breed in a wide variety of habitats. Therefore, LLIN may be useful to prevent malaria transmitted by An. sundaicus.

Anopheles stephensi.

An. stephensi is considered an urban vector that is responsible for the transmission of malaria. It is generally present in three biological forms, type, mysorensis, and intermediate forms. An. stephensi mysorensis is distributed mainly in rural areas, is zoophilic, and has the poor vectorial capacity.² The “type” form acts as a vector and the role of the intermediate form is uncertain. Anopheles stephensi is prevalent throughout the year, but its low density has been recorded during the winter.^44–46 It is distributed in most parts of the country, but only a few comprehensive bionomics studies have been done during the past two decades, mainly from Delhi, Chennai, Goa, and Kolkata.^7,45,47,48

Batra et al. reported that An. stephensi played a major role in malaria transmission in non-reverine areas of Delhi.⁴⁷ However, even in riverine areas, it was an equally important malaria vector as An. culicifacies. The overall sporozoite rate was determined as 0.67, comparable with that of An. culicifacies (0.64), but the human blood index of An. stephensi was lower (0.45%–1.45%) compared with An. culicifacies (2.7%–3.7%).⁴⁷ Higher sporozoite positivity in An. stephensi may be due to better susceptibility of An. stephensi to Plasmodium infection.⁴⁹ An extensive study on bionomics of An. stephensi was conducted by Ghosh et al. (2010) in Kolkata in which seasonal distribution, parity, resting, host-seeking behavior, and association with malarial parasites was observed.⁴⁵ Monthly distributions of An. stephensi were highest during July (25.4%) and lowest in October (1.7%). This study confirmed the endophilic behavior of An. stephensi by human bait collection during the night. The biting was observed from 6 pm to 3 am, with its peak biting activity from 12 am to 3 am. The proportion of parous females, daily survival rate, daily mortality rate, and sporozoite rate were 46%, 82%, 18%, and 2.2%, respectively.⁴⁵ But in Goa, its peak biting was observed at 3 am to 6 am with an entomological inoculation rate of 2.35 and sporozoite rate of 3.6%.⁷ It was the only study in which EIR was determined. It was interesting to note that in Rajasthan, where An. stephensi plays an important role in malaria transmission, and no extensive bionomics study has been undertaken during the past two decades.

In India, the indoor residual spray has not been used to control An. stephensi, although in Gujarat, Karnataka, Rajasthan, and Uttar Pradesh, An. stephensi has developed resistance to DDT and malathion.^50,51 There are also reports of resistance against synthetic pyrethroids in this critical vector of urban malaria from other parts of the country^52,53 (Figure 2, Table 1). Chances of proliferation of An. stephensi breeding habitats are high due to the rapid and unplanned urbanization. This may lead to enhanced urban malaria transmission.

Moreover, it is also an invasive species that has established itself in new territories, as evident in Sri Lanka and African countries.^54,55 Therefore, bionomics studies will be important for applying targeted vector control activities. Since An. stephensi mysorensis is an established vector of malaria in Iran, hence monitoring of sporozoite positivity of An. stephensi mysorensis should be undertaken in India.^56,57

Anopheles fluviatilis.

An. fluviatilis is a sibling complex of three species, S, T, and U, of which S is an important vector. It is found in hilly and foothill regions of Odisha, Madhya Pradesh, Uttarakhand, and Chhattisgarh. However, the biology of this mosquito vector has been studied mainly in Odisha, from where its two sibling species S and T were reported. Species S was the main and anthropogenic vector with a human blood index of > 50% and a sporozoite rate of < 3% with a few exceptions.^11,58,59 In Odisha, it was prevalent throughout the year, but high density was observed in cooler months (October–January), and low levels were apparent in summertime (May–June).^58,60 Contradictory results were observed from two neighboring states of India, Rajasthan and Madhya Pradesh. An. fluviatilis was not detected in winter months (December–January), and a peak was observed in monsoon season (June–September) in Rajasthan, whereas in Madhya Pradesh, its density was lowest during the summer season (March–June) and highest during the winter season (October–December).^44,61 Parity rates ranged from 57% to 65% as per studies undertaken in the Keonjhar district (Odisha).^31,33,35 Similarly, EIR was determined in only one study conducted in Sundergarh district of Odisha, and its value was 0.395.⁶² An. fluviatilis bites from dusk to dawn. Its peak biting was 11 pm to 2 am in Keonjhar (Odisha), whereas in Goa, it was 3 am to 6 am.^7,31 In the Nainital district of Uttarakhand, peak biting was observed between 9 pm and midnight, and this timing varied greatly from east to west to northern parts of the country, possibly due to the climatic variations.⁶³

The vector was found to be susceptible to deltamethrin and malathion in most parts of the country except in Gadchiroli (Maharashtra), where possible resistance status has been reported.⁶⁴ In the Mayurbhanj district of Odisha, possible resistance was reported against malathion in An. fluviatilis. Similarly, it was found to be susceptible to DDT in Odisha but resistant in Jharkhand and Chhattisgarh (Figure 2, Table 1).⁵¹ Studies showed that An. fluviatilis had variable biting times in different ecological settings, and its resting behavior has changed from indoor to outdoors.¹¹ Therefore, regular studies on the bionomics of this malaria vector in various eco-epidemiological regions will provide further inputs for its control. Newer vector control tools such as attractive toxic sugar, endectocides application, or genetically modified mosquitoes may complement the existing LLIN for effective control of An. fluviatilis transmitted malaria.^65,66

Anopheles culicifacies.

An. culicifacies is the most prevalent vector of malaria in India. It has been reported in almost all the states of India, and its presence is estimated in two-thirds of the country where it is responsible for up to70% of malaria cases in India.⁶⁷ It is a complex of five sibling species (named A, B, C, D, E), of which species B is not an efficient vector.² It is a perennial species with maximum density during the monsoon season and minimum density during the winter.^44,46,60 It prefers to breed in various habitats that include streams/rivers, river bed pools, rivulets, rice fields, artificial ponds, and seepage pools of the canal, and wells, for example.^68–70 This species rests indoors, both in human dwellings and cattle sheds. Anopheles culicifacies was found to bite the whole night, with its peak biting after midnight.^63,71 It is generally a zoophilic mosquito that prefers cattle feeding compared with humans, which is evident from its low anthropophilic index/human blood index. Therefore, the sporozoite rate of An. culicifacies is also very low (< 1%) with a few exceptions where it was found to be very high, ranging from 7% to 34%.^11,21,37 Its entomological inoculation rate was determined in two studies. EIR was 4.8 in the Sonitpur district of Assam, which was higher than Sundergarh district of Odisha, which had EIR < 0.02.^37,62 In a recent study on the bionomics of An. culicifacies, EIR ranged from 0.004 to 0.254 in Panchmahal and Kheda districts of Gujarat and Kalburgi district of Karnataka (ICMR unpublished report). Anopheles culicifacies is generally resistant to DDT and malathion, whereas susceptible to synthetic pyrethroids in India. However, recent reports indicates towards development of resistance against deltamethrin from Odisha, Maharashtra, Andhra Pradesh, Telangana and Chhattisgarh (Figure 2, Table 1).^51,53

Due to the growing resistance of An. culicifacies against all the insecticide classes used in India, there is an urgent need for alternative vector control strategies such as attractive toxic sugar. This has yet to be explored, owing to India’s rural setup, where there is no clear-cut segregation of human and animal dwellings, and control of An. culicifacies in animal sheds becomes essential from a human-disease perspective.⁶⁶

Research gaps and future studies.

Because India has set a goal of malaria elimination by 2030, understanding the bionomics of malaria vectors will be critical in supporting the national program. Incomplete understanding of vector bionomics has been flagged as a challenge to our elmination goals (Rahi and Sharma Lancet RH 2022). Here we highlight several research gaps.

• Endemic areas left unaddressed. Only a handful of studies on bionomics of malaria vectors have been undertaken in malaria-endemic districts of the country. As per the national strategic plan for malaria elimination (2017–2022), all districts of India have been categorized into four categories based on malaria endemicity (annual parasite index [API] defined as malaria cases per 1,000 population). In 2019, 32 districts were in category 3 (API ≥ 2), whereas 11 districts were in category 2 (API between 1 and 2), and the rest of the districts were in either category 1 (API < 1) or 0 (no case in past 3 years).⁷² Our study shows that bionomic studies have been undertaken in only ∼50% of the highly endemic districts falling under category 2 or 3. Studies have been conducted in only ∼15% of districts falling under category 1 or less from 2000 to 2020 (Figure 3). As India aims to eliminate malaria by 2030, knowledge of vector bionomics is crucial for assessing the effectiveness of currently used vector control strategies and tools and developing newer ones. Gadchiroli district of Maharashtra and Dantewada district of Chhattisgarh have been malaria-endemic regions for a long time; however, the bionomics of malaria vectors in this region is still unknown in the absence of any comprehensive bionomics study. Similarly, there are several malarious districts where malaria vector bionomics have not been documented.

• Incomplete capturing of bionomics information. In the limited bionomics studies undertaken in India, the primary aim was to determine the density, insecticide susceptibility status, and parasite detection in mosquitoes. Other bionomics parameters, however, such as feeding, resting, parity rate, biting behavior, mating behavior, and estimation of vectorial capacity, which are essential aspects of vector bionomics, have been overlooked.

• Change in vector behavior contributing to outdoor malaria transmission. A primary cause of residual malaria transmission (i.e., outdoor transmission) has been reported from the African continent with just a few reports from India.^9,10,73 Outdoor malaria is mainly caused by the change in vector species behavior due to increased pressure of insecticides used in LLIN and/or IRS. Such change in resting behavior of An. fluviatilis has been reported from Odisha, where its resting shifted from cattle shed to mixed dwellings.¹¹ The change in behavior of vector mosquitoes may only be observed by conducting comprehensive bionomics studies rather than reporting only vector incrimination and insecticide susceptibility studies. Current vector tools such as IRS and LLIN are targeted toward indoor resting and indoor feeding mosquitoes. Any significant change in vector behavior would necessitate deploying effective novel vector control methods targeting early and outdoor biting mosquitoes.^74,75

• Emerging threat of climate change. Climate change is affecting all major domains of life.⁷⁶ There are also variations in mosquito diversity with altitude and landscape.^36,77 Growing urbanization leads to change in the breeding habitats of mosquitoes.^78,79 Ecological succession has been evident in a study conducted by Saxena et al.³⁷ These abiotic malaria determinants should be the focus of future research studies as recent modeling studies have shown a change in transmission windows with prospects of creating new malaria foci.¹² Prior knowledge of vector bionomics will help carry out vector control activities in these potential new regions of malaria.

• Insecticide susceptibility status. Regular monitoring of insecticide susceptibility status is imperative for the sustainability of the insecticides in use. Entomological surveillance, including insecticide susceptibility, is an integral component of the national malaria control program, which needs to be strengthened. This will facilitate evidence-backed decisions on switching to newer insecticides.

• The incursion of malaria vectors and their expansion in newer areas. There is evidence that malaria vectors are invading newer regions and expanding their geographic range. Anopheles stephensi, an urban malaria vector of India, was recently reported from Sri Lanka and some African countries like Djibouti, Ethiopia, and Sudan.^54,55 Similarly, An. culicifacies has spread to India’s northeastern states and has established itself as a malaria vector.³⁷ Such incursion of malaria vectors to newer areas can only be observed by carrying out bionomics studies regularly in different ecological settings.⁸⁰

Figure 3.

Categorization of districts overlaid with bionomics studies undertaken in India during 2000–2020. This figure appears in color at www.ajtmh.org.

Neglected plasmodium species.

Malaria is both vector-borne anthroponoses and vector-borne zoonoses. India primarily reports P. vivax and P. falciparum and cases by other species P. malariae, P. ovale, and P. knowlesi are not tracked, and hence data on non–P. falciparum and P. vivax infections in India are available only in published case reports and other studies.¹³ There are few reports from India pointing out at transmission of P. knowlesi infection in the human population in the Andaman and Nicobar Islands and other parts of the country.^43,81 Plasmodium knowlesi is transmitted between long-tailed macaques and vector An. sundaicus in Andaman and Nicobar Island.⁴² Thus malaria caused by P. knowlesi is a zoonotic infection, and its burden is understudied in India.

CONCLUSIONS

Our analysis of vector studies over the past two decades indicates limitations in bionomics studies within India. Because malaria is heterogeneous in India, the vector control strategy of an area must rely on the vector bionomics of that particular area. Access to this information will be vital in the coming years as India targets malaria elimination, given the last-mile challenges. The absence of vector bionomics data can be a blind spot and leave behind malaria residuals in pockets where usual vector control tools may not suffice. Thus, there is an urgent need to initiate comprehensive studies on all primary and secondary malaria vectors in India in the context of their bionomics profiles.