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Year : 2021  |  Volume : 58  |  Issue : 3  |  Page : 281-284

Reappearance and disappearance of Anopheles minimus Theobald (Diptera: Culicidae) in West Singhbhum hills of Eastern India

Indian Council of Medical Research-Vector Control Research Centre, Medical Complex, Indira Nagar, Puducherry, India

Date of Submission20-Apr-2020
Date of Acceptance15-Aug-2020
Date of Web Publication15-Feb-2022

Correspondence Address:
Dr. Sudhansu Sekhar Sahu
ICMR-Vector Control Research Centre, Medical Complex, Indira Nagar, Puducherry, 605006
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0972-9062.321741

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Keywords: Anopheles minimus; disappearance; Eastern India; malaria; West Singhbhum hills; vector

How to cite this article:
Sahu SS, Patnaik MK, Dash S, Thankachy S, Kumar A. Reappearance and disappearance of Anopheles minimus Theobald (Diptera: Culicidae) in West Singhbhum hills of Eastern India. J Vector Borne Dis 2021;58:281-4

How to cite this URL:
Sahu SS, Patnaik MK, Dash S, Thankachy S, Kumar A. Reappearance and disappearance of Anopheles minimus Theobald (Diptera: Culicidae) in West Singhbhum hills of Eastern India. J Vector Borne Dis [serial online] 2021 [cited 2023 Mar 29];58:281-4. Available from: http://www.jvbd.org//text.asp?2021/58/3/281/321741

The Chhota Nagpur Plateau of eastern India covers most parts of Jharkhand State as well as some parts of Odisha, Chhattisgarh and West Bengal. Singhbhum hills are in the lowest hill range of the Chhota Nagpur plateau. Keonjhar, Mayurbhanj and Sundargarh districts of Odisha State are the part of West Singhbhum hills. Keonjhar and Sundargarh districts are highly endemic for Plasmodium falciparum and the mortality due to malaria is invariably recorded (149 reported deaths during 2010 to 2018)[1],[2],[3],[4].

The mosquitoes (Diptera: Culicidae) of West Singhbhum hills were initially surveyed by Watts in 1924 who reported the presence of Anopheles minimus in the region[5]. Subsequent comprehensive surveys in these areas were undertaken by White et al. in 1938, 1940, 1945 and 1947[6],[7],[8],[9]. They all reported that malaria was transmitted by An. minimus particularly in West Singhbhum hills during pre-DDT era. However, when Dash et al. (1982 and 1984), Collins et al. (1990)[10],[11],[12] conducted the entomological surveys in the West Singhbhum hill areas between 1980s and 1990s (post-DDT era), they did not encounter An. minimus. The intense residual spraying of DDT and ecological changes due to widespread deforestation over the latter half of the twentieth century affected the mosquito fauna and is considered an important factor in the elimination of An. minimus from West Singhbhum hills[1],[12]. Similar ecological phenomenon affecting the prevalence of mosquito species has been reported from other parts of Odisha state[12],[13] and elsewhere in India[14],[15],[16]. However, an entomological survey conducted after 45 years of the national malaria eradication programme has confirmed the re-appearance of An. minimus and its role in malaria transmission in West Singhbhum hills[1],[2].

Aligning with the aim of National Framework for Malaria Elimination (NFME), the National Strategic Plan (NSP) is in force in India to pave the way for malaria elimination in India by 2030. After launching of NSP, the incidence of malaria in India has been reduced by 70% from 2016 to 2019[4]. Despite the declining trend observed in the State, malaria is still persistent in inaccessible, hilly and forested villages of the State including Keonjhar and Sundargarh districts[4],[17]. Vector control is a key component for elimination of the vectors and the disease management[18]. Therefore, in the context of India’s commitment to malaria elimination by 2030, there is a need for monitoring the prevalence of malaria vectors to develop a suitable strategy to control the target vector. Since, An. minimus is an efficient vector of malaria and in the past has played a vital role in malaria transmission in these areas of West Singhbhum hills during pre-DDT era and from 2003-2007, a survey was conducted to ascertain the current status of An. minimus after a gap of 12 years.

The Keonjhar and Sundargarh districts of West Singhbhum hill region were selected for the current entomological survey [Figure 1]. Two districts cover an area of 18015 sq. km between 21° 51’ to 22° 05’ N latitudes and 84° 68’to 85° 68’ E longitudes and have a population of 44.8 million distributed in 3934 villages. The epidemiological data of malaria in Keonjhar and Sundargarh districts of Odisha State during 2003, 2004, 2005, 2007 and 2019 are presented in [Table 1] (Source: NVBDCP, 2020). P.falciparum was the predominant malaria parasite (>95%) in these districts (NVBDCP, 2020). There are three discrete seasons here i.e. summer (March–June), monsoon (July–October) and winter (November–February). From 1958 to 2000, yearly two rounds of indoor residual spraying (IRS) with DDTwas the main vector control intervention in these districts. Due to the development of resistance to DDT in An. culicifacies, another major malaria vector, synthetic pyrethroids (SPs) were introduced in IRS programme from 2001 onwards[1]. Later, during 2009, long-lasting insecticidal nets (LLINs) were distributed in some of the highly endemic villages of these districts. After launching the NSP in 2017, mass distribution of LLINs was carried out covering the total population of these two districts.
Figure 1: Map of Keonjhar and Sundargarh districts of Odisha State showing study villages

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Table 1: Epidemiological data of malaria in Keonjhar and Sundargarh districts of Odisha state, India

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Mosquito collections were carried out in 6 villages of Keonjhar district viz. Mamulipusi, Puradihi, Dhankuniasahi, Mundasahi in the Banspal Community Health Centre (CHC) and Sapolanji and Pitanallain the Harichandanpur CHC, and three villages in K. Bolang CHC of Sundargarh district, viz. Sayamba, Chirodepa, and Rangada during winter season (February 2020). All the villages are of hilly ecotype and selected based on the earlier abundance of An. minimus (2003–2007)[1],[2]. Since, An. minimus density was recorded higher during winter seasons in earlier surveys conducted by Jambulingam et al (2005)[1], the current survey was carried out in February 2020. Indoor and outdoor resting mosquitoes were collected with oral aspirators and torch battery light between 05:00 and 08:00 h spending 3 person hours in each habitat (human dwelling, cattle shed and outdoors) in each selected village. The mosquitoes collected from the villages were brought to the laboratory and identified morphologically to species level using standard keys of Christophers (1933)[19].

A total of 351 mosquitoes comprising 16 species of Anopheles were collected from the current survey spending a total of 81 person-hours, 27 each in human dwellings, cattle sheds and outdoors [Table 2]. Among the anopheline species, An. splendidus (20.5%) was predominant followed by An. jeyporiensis (18.2%), An. maculatus (11.7%), An. nigerrimus (10.3%) and An. tessellatus (10.0%). The recognized major malaria vectors were An. fluviatilis (9.7%) and An. culicifacies (4.0%). While the surveys conducted during 2003–2007 showed the abundance of 19 anopheline species, the current survey showed the presence of 16 species. Notably, An. minimus, An. karwari and An. ramsayi which were recorded in cooler months in earlier surveys were absent during the current survey [Table 2]. The entomological collections conducted in the same villages during 2003–2004 and 2005–2007 showed that the per person-hour density (PHD) of An. minimus in human dwellings was 2.1 and 4.5 respectively. However, An. minimus mosquitoes were not seen to be prevalent in the current survey. The PHD of other major malaria vector, An. fluviatilis was 9.4 and 6.5 during earlier surveys conducted during 2003–2004 and 2005–2007 respectively, whereas the density of this species was greatly reduced to 0.1 in human dwellings in 2020.
Table 2: The relative prevalence of Anopheline mosquito species recorded during surveys conducted in 2003-2004, 2005-2007 and 2020 in West Singhbhum hills of Eastern India

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The current survey reported the disappearance of An. minimus in West Singhbum hills, which had resurfaced nearly 12 years ago. Several studies conducted during pre- DDT era showed that An. minimus were prevalent in West Singhbhum hills[5],[6],[7],[8],[9]. After introduction of the indoor residual spraying (IRS) with DDT in 1958 for the control of malaria vectors in India and environmental changes due to widespread deforestation, An. minimus disappeared from this area for four decades[10]. The entomological survey made in August 2001 in some of the current study villages of Koenjhar district had recorded An. minimus along with An. fluviatilis[1]. This perhaps prompted to carry out a series of entomological collections during 2003–2004 (one each in summer and rainy months and two in the winter months) which showed the seasonal prevalence of this species during winter months[1]. Further, the bionomics of this species was studied during 2005–2007[2]. The reappearance of this specieswas attributed to the decreased insecticide pressure of DDT[1],[2]. After the reappearance, the malaria incidence in the area also increased and the state had introduced new vector control tools like IRS with SPs followed by the distribution of insecticide-treated nets (ITNs) and long-lasting insecticidal nets (LLINs)[1]. In this region, An. minimus was ascertained to be susceptible to DDT, malathion and SPs[1]. In the current survey, a total of 81 person-hours were spent for mosquito collection in comparison to 24 and 159 spent during earlier surveys[1],[2]. However, the present collections were made during winter season, the favourable period for An. minimus spending substantial person hours in the villages where density of An. minimus was reported earlier.

The current survey yielded 16 species of Anopheles, but failed to locate three of the 19 species of Anopheles previously reported from the area[1],[2]. The disappearance of An. minimus and low prevalence of An. fluviatilis is corroborating with the declining trend of malaria incidences (37, 998 cases (API: 19.51) in 2016 to 453 cases (API: 0.22) in 2019) in Keonjhar district and (30,508 cases (API: 13.82) in 2016 to 1,755 cases (API: 0.72) in 2019) in Sundargarh district. The apparent disappearance of An. minimus and drastic reduction of An. fluviatilis population could be due to the high efficacy of SPs by LLINs. Earlier, during 1960s, after the introduction of DDT spraying in India, several changes were observed in the species composition and distribution of anopheline vectors country-wide[15]. Similar observation was found in South Africa in 1950, i.e., due to IRS with DDT, An. funestus, a morphologically similar species to An. minimus disappeared from the region[20]. Four years after replacement of DDT with SPs, An. funestus the species reappeared in 1990 and was found to be pyrethroid resistant[20].

The current vector control measures focussed towards the elimination of malaria vectors in the study region include yearly two rounds of residual spraying with DDT/SPs and community use of LLINs distributed gratis by the National Vector Borne Disease Control Programme (NVBDCP). The ecological changes occurred in West Singhbhum hills such as extensive excavation for mining activity, deforestation and wide use of insecticides, particularly SPs in LLINs[17] might be associated with the disappearance of An. minimus in this region. Since, An. minimus is considered as one of the primary malaria vectors in India[21] and the presence of this vector has been reported earlier in different areas of Singhbhum hills, extensive entomological surveys are warranted at regular intervals in different parts of Singhbhum hills to ascertain the existence of this species and its current role in malaria transmission.

Conflict of interest: None

  References Top

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  [Figure 1]

  [Table 1], [Table 2]


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