Background
Throughout the world, there was an estimated 247 million malaria cases among 3.3 billion people at risk in 2006, causing nearly a million deaths, mostly of children under five years of age. In 2008, 109 countries were reported to be endemic for malaria. Bangladesh had an estimated 2.9 million malaria cases and 15,000 deaths in 2006. Although 72% of the population are at some risk of malaria, the risk is greatest in the east and north-east of the country in areas bordering India and Myanmar. The majority of suspected cases are unconfirmed; among those that are identified as malaria, more than 70% are
Plasmodium falciparum [
1]. Malaria is a major health burden in this remote, mountainous south-eastern region of Bangladesh. Malaria transmission in Bangladesh is mostly seasonal and concentrated in the border regions with India and Myanmar. Out of the total 64 administrative districts, 13 are located along the border areas with India and Myanmar where about 98% of the total malaria morbidity and mortality reported from Bangladesh each year originate from these districts. [
2]. According to passive data collected by Directorate General of Health Services (DGHS) of Bangladesh last ten years (1999-2008), the country's malaria situation remains almost steady with an annual incidence of 4% in the endemic districts.
In a recent survey, it has been found that among these 13 malaria endemic districts, the overall malaria prevalence rate was 3.1% based on Rapid Diagnostic Test (RDT). The prevalence of
P. falciparum was 2.73% and the
Plasmodium vivax 0.16% and mixed infection with
P. falciparum and
P. vivax was 0.19%. The proportion of
P. falciparum was 88.6%, while
P. vivax and mixed infection with these two species were 5.2 and 6.25%, respectively. The overall malaria prevalence in Chittagong Hill Tracts was 11% [
2].
The forests of Bangladesh have remained an area of intense malaria transmission providing a focus for re infection for the plains. Bangladesh has 34 species of anopheline mosquitoes[
3]. Until 2009, only seven of these species were documented to be competent malaria vectors. Among these, four have been considered as the principal malaria vectors
i.e. Anopheles baimaii (= Anopheles dirus D),
Anopheles philippinensis,
Anopheles sundaicus and
Anopheles minimus s.l. [
4]. Other species, such as
Anopheles aconitus,
Anopheles annularis and
Anopheles vagus, were found to be capable of transmitting malaria during outbreak situations [
5‐
7]. Although
An. annularis and
An. vagus are considered to be zoophilic, exophilic and exophagic in nature, they have been considered to maintain malaria transmission in the plain land. These two species were incriminated during epidemic situation in the flood plain areas of Bangladesh. Possibly they were incriminated due to low density of mammalian host except human [
6,
7].
DDT was banned in Bangladesh since 1985 and the number of malaria cases began to increase. Since then due to lack of adequate funds and programs, no control efforts maintained in the malaria endemic areas of Bangladesh[
2]. Due to similar reason Malaria and Parasitic Disease Control Unit (M&PDC) of DGHS could not carry out regular entomological investigation in the endemic areas. However, they carried out sporadic entomological surveillance, but did not have the opportunity to work on incrimination of other anopheline species. Therefore, this study was designed to obtain some information regarding prevalent anopheline species and possible malaria vectors in the border belt areas where the malaria situation is endemic in Bangladesh, the results of which are presented here.
Results
A total of 634 female anopheline mosquitoes belonging to 17 species were collected by different methods (Table
1). 403 mosquitoes were collected by CDC light trap and 231 mosquitoes by other methods (HLC and resting). Majority of the mosquitoes were collected by other methods were found resting in the cattle shed, indoor or outdoor of human dwellings.
Anopheles vagus was the dominant species (18.6%) followed by
An. nigerrimus (14.5%) and
An. philippinensis (11.0%). Matiranga represented with the highest number of species (15) and mosquitoes (511). Although Deorgachh and Lengura has simialr number of species (8), but mosquito numebrs were higher in Lengura (73) than Deorgachh (41). Since the numbers of mosquitoes in HLC were few, calculation for human biting rate was not performed.
Table 1
List of anopheline species collected from three study areas by different methods
An. aconitus
| 1 | 2 | 3 | 1 | 0 | 1 | 0 | 0 | 2 | 2 | 4 (0.6) |
An. anularis
| 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 0 | 1 (0.2) |
An. barbirostris
| 14 | 5 | 19 | 3 | 0 | 3 | 0 | 0 | 17 | 5 | 22 (3.5) |
An. jamesii
| 35 | 0 | 35 | 0 | 0 | 0 | 6 | 6 | 41 | 0 | 41 (6.5) |
An. jeypurensis
| 27 | 0 | 27 | 0 | 0 | 0 | 0 | 0 | 27 | 0 | 27 (4.3) |
An. karwari
| 0 | 0 | 0 | 0 | 0 | 0 | 10 | 10 | 10 | 0 | 10 (1.6) |
An. kochi
| 35 | 0 | 35 | 0 | 0 | 0 | 0 | 0 | 35 | 0 | 35 (5.5) |
An. maculatus
| 8 | 0 | 8 | 0 | 0 | 0 | 0 | 0 | 8 | 0 | 8(1.3) |
An. minimus s.l.
| 1 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 1 (0.2) |
An. niggerimus
| 2 | 80 | 82 | 1 | 1 | 2 | 8 | 8 | 11 | 81 | 92 (14.5) |
An. philippinensis
| 54 | 0 | 54 | 6 | 3 | 9 | 7 | 7 | 67 | 3 | 70 (11.0) |
An. subpictus
| 1 | 58 | 59 | 0 | 6 | 6 | 1 | 1 | 2 | 64 | 66 (10.4) |
An. tessellatus
| 10 | 3 | 13 | 0 | 0 | 0 | 0 | 0 | 10 | 3 | 13 (2.1) |
An. umbrosus
| 56 | 1 | 57 | 5 | 0 | 5 | 0 | 0 | 61 | 1 | 62 (9.8) |
An. vagus
| 10 | 60 | 70 | 5 | 7 | 12 | 36 | 36 | 51 | 67 | 118 (18.6) |
An. varuna
| 42 | 2 | 44 | 0 | 0 | 0 | 0 | 0 | 42 | 2 | 44 (6.9) |
An. willmori
| 13 | 0 | 13 | 0 | 3 | 3 | 4 | 4 | 17 | 3 | 20 (3.2) |
N | 309 | 211 | 520 | 21 | 20 | 41 | 73 | 73 | 403 | 231 | 634 (100) |
CSP-ELISA was performed with 622 anopheline mosquitoes (remaining mosquitoes were kept as voucher specimen). 16 mosquitoes were positive in CSP-ELISA (Table
2). Thus, overall infection rate became 2.6% (16/622). Eight (1.3%) mosquitoes belonging to five species were positive for
P. falciparum, seven (1.1%) mosquitoes belonging to five species were positive for VK210 and a single mosquito belonging to
An. maculatus species was positive for VK 247. No mixed infection was found in this study.
P. falciparum-positive anopheles species included one
An. barbirostris, one
An. karwari, four
An. vagus, one
An. nigerrimus, and one
An. subpictus. VK 210 positive species included one
An. barbirostris, one
An. karwari, one
An. vagus, three
An. nigerrimus, and one
An. philippinensis. According to species, the highest infection rate (Table
2) was observed in
An. karwari (2/9, 22.2%) followed by
An. maculatus (14.3%),
An. barbirostris (9.5%),
An. nigerrimus (4.4%),
An. vagus (4.3%),
An. subpictus (1.5%) and
An. philippinensis (1.4%).
Table 2
CSP-ELISA positive mosquitoes Infection rate according to species
An. aconitus
| 3 | 0 | 0 | 0 | 0 |
An. anularis
| 1 | 0 | 0 | 0 | 0 |
An. barbirostris
| 21 | 1 (4.8) | 1(4.8) | 0 | 2 (9.5) |
An. jamesii
| 41 | 0 | 0 | 0 | 0 |
An. jeypurensis
| 27 | 0 | 0 | 0 | 0 |
An. karwari
| 9 | 1 (11.1) | 1 (11.1) | 0 | 2 (22.22) |
An. kochi
| 35 | 0 | 0 | 0 | 0 |
An. maculatus
| 7 | 0 | 0 | 1 (14.3) | 1 (14.3) |
An. minimus s.l.
| 1 | 0 | 0 | 0 | 0 |
An. niggerimus
| 91 | 1 (1.1) | 3 (3.3) | 0 | 4 (4.4) |
An. philippinensis
| 69 | 0 | 1 (1.4) | 0 | 1 (1.4) |
An. subpictus
| 65 | 1 (1.5) | 0 | 0 | 1 (1.5) |
An. tessellatus
| 12 | 0 | 0 | 0 | 0 |
An. umbrosus
| 61 | 0 | 0 | 0 | 0 |
An. vagus
| 116 | 4(3.4) | 1 (0.9) | 0 | 5 (4.3) |
An. varuna
| 44 | 0 | 0 | 0 | 0 |
An. willmori
| 19 | 0 | 0 | 0 | 0 |
N | 622 | 8 (1.28) | 7 (1.1) | 1 (0.2) | 16 (2.6) |
According to place from Matiranga 11 CSP-positive (2.2%) mosquitoes had been identified in six species including
An. barbirostris, An. subpictus, An. vagus, An. nigerrimus, An. maculatus and
An. philippinensis (Table
3). In Lengura, five mosquitoes were identified CSP positive (6.9%) belonging to two species including
An.
karwari and
An.
vagus. In Deorgachh no mosquitoes were found CSP positive. Among 16 positive mosquitoes11 had blood on their abdomen, while seven had no visible blood meal (Table
4).
Table 3
Area wise CSP-ELISA positive rate
An. aconitus
| 1 | 1 | 2 | 0 | 0 | 1 | 0 | 1 | 0 | 0 | 0 | 0 | - |
An. anularis
| 0 | 0 | 0 | - | - | 0 | 0 | 0 | - | - | 1 | 0 | 0 |
An. barbirostris
| 13 | 5 | 18 | 2 | 11.1 | 3 | 0 | 3 | 0 | 0 | 0 | - | - |
An. jamesii
| 35 | 0 | 35 | 0 | 0 | 0 | 0 | 0 | - | - | 6 | 0 | 0 |
An. jeypurensis
| 27 | 0 | 27 | 0 | 0 | 0 | 0 | 0 | - | - | 0 | - | - |
An. karwari
| 0 | 0 | 0 | - | - | 0 | 0 | 0 | - | - | 9 | 2 | 22.2 |
An. kochi
| 35 | 0 | 35 | 0 | 0 | 0 | 0 | 0 | - | - | 0 | - | - |
An. maculatus
| 7 | 0 | 7 | 1 | 14.3 | 0 | 0 | 0 | - | - | 0 | - | - |
An. minimus s.l.
| 1 | 0 | 1 | 0 | 0.0 | 0 | 0 | 0 | - | - | 0 | - | - |
An. niggerimus
| 2 | 79 | 81 | 4 | 0.0 | 1 | 1 | 2 | 0 | 0 | 8 | 0 | 0 |
An. philippinensis
| 54 | 0 | 54 | 1 | 1.9 | 5 | 3 | 8 | 0 | 0 | 7 | 0 | 0 |
An. subpictus
| 0 | 58 | 58 | 1 | 1.7 | 0 | 6 | 6 | - | - | 1 | 0 | 0 |
An. tessellatus
| 9 | 3 | 12 | 0 | 0 | 0 | 0 | 0 | - | - | 0 | - | - |
An. umbrosus
| 55 | 1 | 56 | 0 | 0 | 5 | 0 | 5 | 0 | 0 | 0 | - | - |
An. vagus
| 9 | 59 | 68 | 2 | 2.9 | 5 | 7 | 12 | 0 | 0 | 36 | 3 | 8.3 |
An. varuna
| 42 | 2 | 44 | 0 | 0 | 0 | 0 | 0 | - | - | 0 | - | - |
An. willmori
| 13 | | 13 | 0 | 0 | 0 | 2 | 2 | - | - | 4 | 0 | 0 |
N | 303 | 208 | 511 | 11 | 2.2 | 20 | 19 | 39 | 0 | 0 | 72 | 5 | 6.9 |
Table 4
Summary table for positive anopheles female mosquitoes in CSP-ELISA from border belt areas of Bangladesh
38 |
An. nigerrimus
| Pv-210 | 0 | Matiranga | 14.05.09 | Others |
58 |
An. nigerrimus
| Pv-210 | 1 | Matiranga | 14.05.09 | Others |
67 |
An. nigerrimus
| Pv-210 | 1 | Matiranga | 14.05.09 | Others |
70 |
An. nigerrimus
| Pf | 1 | Matiranga | 14.05.09 | Others |
134 |
An. subpictus
| Pf | 1 | Matiranga | 14.05.09 | Others |
185 |
An. karwari
| Pf | 0 | Lengura | 12.08.09 | LT |
187 |
An. karwari
| Pv-210 | 1 | Lengura | 12.08.09 | LT |
201 |
An. vagus
| Pf | 1 | Lengura | 12.08.09 | LT |
209 |
An. vagus
| Pf | 0 | Lengura | 12.08.09 | LT |
228 |
An. vagus
| Pf | 1 | Lengura | 12.08.09 | LT |
264 |
An. vagus
| Pv-210 | 1 | Matiranga | 14.05.09 | Others |
272 |
An. vagus
| Pf | 1 | Matiranga | 14.05.09 | Others |
314 |
An. maculatus
| Pv-247 | 0 | Matiranga | 19.06.09 | LT |
406 |
An. barbirostris
| Pf | 1 | Matiranga | 19.06.09 | LT |
414 |
An. barbirostris
| Pv-210 | 1 | Matiranga | 19.06.09 | LT |
438 |
An. philippinensis
| Pv-210 | 0 | Matiranga | 19.06.09 | LT |
Discussion
Malaria transmission pattern in Bangladesh is still poorly understood. The on-going Malaria Control Programme in Bangladesh, stresses the fact of up-to-date information on malaria vectors. As a result the current vector control programmes are being implemented on little reliable report involved in malaria transmission. Successful implementation of a vector control programme in Bangladesh, the prevalence of infection with malaria sporozoites among the local anopheline mosquitoes is important, which will help to pinpoint the main vectors and other new vectors and to develop knowledge on the bionomics of the species involved in the disease transmission.
Anopheline mosquitoes were collected from the three study sites representing three geographically different endemic regions in Bangladesh. Anopheles vagus and An. philippinensis were previously incriminated as malaria vector in Bangladesh. There was, however, no previous report in favour of infections in An. karwari, An. maculatus, An. barbirostris, An. nigerrimus and An. subpictus in Bangladesh.
This study was conducted within a short period of time and mosquitoes were not collected on a seasonal basis. Although it was planned to collect by similar number of trapping in all three areas but failed to do so in Deorgachh. Thus, there might be a chance to miss some of existing anopheline species there. In Lengura, the highest prevalence rate (6.9%) of CSP in Anopheles mosquitoes was found whereas in Matiranga the CSP prevalence rate was found 2.2%. No sporozoite-positive mosquito was found in Deorgachh. In Derogachh, most traps were set up or conducted HLC in Chaklapunji tea garden, a famous entomological site where bionomics of
An. baimaii was studied in 70s [
13,
14] where 15 anopheline species were recorded [
14].
Anopheles baimaii, the major vector in tea garden area was not found in this investigation. Also the numbers of anopheline species were few in the tea garden area. Three reasons could be contributing such as effects of organic pesticide (deltamethrin) for the controlling of tea plant pests, deforestation and a delay in monsoon rains in Bangladesh in 2009. Due to delay in monsoon rain and prolonged dry season natural breeding places of
An. baimaii and other anopheline species might have disappeared [
15].
The presence of CSP in some anopheline species has been reported for the first time in Bangladesh, which is an imperative finding of this study. A total of seven species was found CSP-positive in the present study. The result of this study was compared with a recent study conducted in Assam state of north-eastern India, where there was evidence of CSP infection in
An. karwari,
An. maculatus,
An. nigerrimus,
An. barbirostris and
An. subpictus [
16].
Although
An. barbirostris and
An. subpictus were found positive in CSP ELISA in Sri Lanka [
17], they had never been incriminated as malaria vector in Bangladesh.
Anopheles vagus was highest in this study collection and also in CSP infection (5/116, 4.3%): this species has been incriminated as malaria vector in Bangladesh [
7]. Although
An. aconitus,
An. annularis,
An. jeyporiensis and
An. varuna also appeared to have vector potential [
5,
6,
18], but CSP was not detected in these species in the present study.
CSP-ELISA has emerged as a useful tool for vector detection, indicating that several species once considered un-important in the epidemiology of malaria, such as
An. subpictus and
An. vagus in Sri Lanka [
17]. In this study,
An. nigerrimus was found CSP-positive, which is probably a first-time report in this region, while the species remains the principal malaria vector in the Indo-Chinese Hills and the Malaysian Zones(Varma MG: Geographical distribution of arthropod borne disease and their principal vectors, unpublished document WHO/VBC/89967). Similarly,
An. karwari is considered a secondary vector in the Australian region [
19], but its vectorial status in South-East Asia was unknown. There is still remaining controversy for CSP-ELISA particularly due to its false positive results in previous studies. Thus, positivity in a CSP-ELISA should not be taken as the only criterion in confirming the vector status of an
Anopheles species [
20‐
22].
The present result does not report any infection evidence in
An. minimus s.l. This might be due to only one mosquito of this species was tested. In a recent study conducted in Chakaria of Bangladesh, which is geographically similar to Matiranga, a higher percentage of
An. minimus s.l. was caught (97.3%; 651/669), of which 19 were positive for
Plasmodium infection by the microtiter plate hybridization (MPH) method[
23]. Before 1950,
An. minimus s.l. was the principal vector recognized in Bangladesh, but its population declined due to the routine spraying of DDT, to which it remains susceptible[
14]. In recent past, the density of
An. minimus s.l. was negligible, as observed in a few sporadic entomological investigations carried out by M&PDC (personal communication with NP Maheswary, a veteran entomologist). Hence, the higher number of
An. minimus s.l. reported in Chakaria might be due to misidentification. A similar situation occurred in Vietnam where formally identified
An. minimus s.l. was found to be
Anopheles varuna [
24]. A reasonable number of
An. varuna in the present study is also supportive to this fact.
Acknowledgements
This investigation received financial assistance from WHO/SEARO in collaboration with UNICEF/UNDP/World Bank/WHO Special Programme for Research and Training in Tropical Diseases (TDR).
We are grateful to the staffs of M&PDC for their active participation during entomological investigation. We are also grateful to Sumit Chakma, Milka Patracia Podder, AEM Rubayet Elahi and SM Khaled Abdullah for their contribution in this study. We would also like to acknowledge the contributions of Touhid Uddin Ahmed and Rajib Chowdhury for critically reviewing this manuscript.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
MSA conceptualized and designed the study collected and identified sample, analyzed data, drafted the manuscript and made final revisions. MGMK, SD, FN did sample analysis and made critical revision of the manuscript. NC organized the field activities, analysed data and helped in the revision of the manuscript. AMB and RH participated in study design, critical analysis of data and helped in drafting the manuscript. All authors read the final manuscript and approved.