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Keywords:

  • Anopheles minimus s.l.;
  • sibling species;
  • distribution;
  • bionomics;
  • malaria;
  • Southeast Asia
  • Anopheles minimus s.l.;
  • espèces jumelles;
  • distribution;
  • bionomie;
  • paludisme;
  • Asie du sud-est
  • Anopheles minimus s.l.;
  • especies hermanas;
  • distribución;
  • bionómica;
  • malaria;
  • Sureste Asiático

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Distribution
  5. Bionomics
  6. Role of An. minimus s.l. in malaria transmission
  7. Control strategies
  8. Conclusions
  9. Acknowledgements
  10. References

Background  The Minimus Complex of Anopheles subgenus Cellia is composed of two sibling species, A and C, on the Southeast Asian mainland, and a third allopatric species E that occurs in the Ryukyu Archipelago (Japan), a malaria-free region. Anopheles minimus s.l. is considered to be one of the main malaria vector in the hilly forested regions of Southeast Asia. Despite a large number of studies over its range of distribution, it is difficult to have a global view of the ecological and bionomical characteristics of the individual species as different identification methods were used, generally without specific identification of the sibling species.

Objectives  (1) To review the main malaria studies on An. minimus s.l.; (2) to discuss recently published data on the biology and ecology of each sibling species; and (3) to identify gaps in our understanding of the Minimus Complex.

Review Results  Major biological and ecological trends are addressed, such as the high plasticity of trophic behaviour and the sympatry of species A and C over the Southeast Asian mainland. Despite the availability of rapid molecular identification methods, we still lack important information concerning the biological characteristics of each sibling species. These gaps must be filled in the future because An. minimus species A and C may exhibit different abilities to transmit malaria.

Conclusion  We expect that entomological surveys will employ molecular methods to clearly identify these two species, and thus elucidate the biological characteristics of each species. As a consequence, current vector control strategies will be improved by targeting the most efficient vectors.

Données de base  Le Complexe d'anophèle Minimus (sous-genre Cellia) est composé de deux espèces jumelles, A et C, réparties sur le continent sud-est asiatique et d'une troisième espèce E allopatrique restreinte à l'Archipel des Ryukyu (Japon), une région exempte de paludisme. Anopheles minimus. s.l est considéré comme le vecteur majeur du paludisme dans les régions collinaires boisées d'Asie du sud-est. Malgré un grand nombre d’études menées sur l'ensemble de l'aire de distribution du complexe, il est difficile d'avoir une vue globale des caractéristiques écologiques et bionomiques de chaque espèce car différentes méthodes d'identification ont été utilisées, généralement sans identification spécifique des espèces jumelles.

Objectifs  (1) Faire la synthèse des principales études sur An. minimus. s.l.; (2) discuter les données récemment publiées sur la biologie et l’écologie de chaque espèce jumelle; et (3) identifier les lacunes dans notre connaissance du Complexe Minimus.

Revision des Resultats  Les tendances biologiques et écologiques majeures sont discutées telles que la forte plasticité du comportement trophique et la sympatrie des espèces A et C sur le continent sud-est asiatique. Malgré la disponibilité de méthodes d'identification moléculaire rapides, d'importantes informations concernant les caractéristiques biologiques de chaque espèce jumelle sont toujours manquantes. Ces lacunes doivent être comblées dans le futur car les espèces An. minimus A et C peuvent jouer des rôles différents dans la transmission du paludisme.

Conclusion  Il faut que dans le futur les enquêtes entomologiques emploient plus largement les méthodes moléculaires pour clairement identifier les espèces jumelles, et ainsi mieux appréhender les caractéristiques bio-écologiques de celles-ci, et par conséquent améliorer les stratégies actuelles de lutte antivectorielle en ciblant les vecteurs plus efficacement.

Antecedentes:  El complejo Minimus de Anopheles, subgénero Cellia está compuesto por dos especies hermanas, A y C en el Sureste del continente asiático y una tercera especie alopática E que se encuentra en el archipiélago Ryukyu (Japón), una región libre de malaria. Anopheles minimus s.l. es considerado el principal vector de la regiones forestales del Sureste Asiático. A pesar de un gran número de estudios sobre su rango y distribución, es difícil tener una visión global de las características ecológicas y bionómicas de las especies individuales, ya que se han utilizado diferentes métodos de clasificación, generalmente sin especificar la identificación de las especies hermanas.

Objetivos:  (1) Revisar Los estudios principales sobre An. minimus s.l.; (2) Discutir datos publicados recientemente sobre la biología y ecología de cada una de las especies hermanas; y (3) identificar huecos en nuestra compresión sobre el Complejo Minimus.

Resultados:  Se estudian las principales tendencias biológicas y ecológicas tales como la alta plasticidad del comportamiento trófico y la simpatría de las especies A y C en el sureste del continente asiático. A pesar de la disponibilidad de métodos rápidos de identificación molecular, aun no tenemos información importante sobre las características biológicas de cada especie. Estos vacíos deben llenarse en el futuro, porque las especies An. Minimus pueden exhibir diferentes capacidades para transmitir la malaria.

Conclusión:  Esperamos que estudios entomológicos utilicen métodos moleculares para identificar claramente estas dos especies, y así aclarar las características biológicas de cada una. Consequentemente, se mejorarán las estrategias de control vectorial actual, enfocándolas al vector más eficiente.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Distribution
  5. Bionomics
  6. Role of An. minimus s.l. in malaria transmission
  7. Control strategies
  8. Conclusions
  9. Acknowledgements
  10. References

A clear and comprehensive understanding of malaria transmission dynamics is crucially needed in the context of implementation and development of malaria control strategies. This is achievable only through a thorough knowledge of the anopheline vectors. Three main malaria vectors are recognized in Southeast Asia: Anopheles dirus sensu lato (Dirus Complex) occurring in forested areas, An. minimus s.l. (Minimus Complex), widespread in the hilly forested regions, and An. sundaicus s.l. (Sundaicus Complex), a brackish water mosquito present along coastal areas. The Minimus Complex belongs to the Myzomyia Series, Funestus Group,1 with 24 other species that are distributed in various areas across southern Asia and sub-Saharan Africa (Harbach 2004; Garros et al. 2005a,b). The Minimus Complex includes two sibling species on the Southeast Asian mainland, species A and C, and a third species, named E, in the Ryukyu Archipelago of Japan (Green et al. 1990; Somboon et al. 2001; Harbach 2004).

Current knowledge of the An. minimus s.l. vector system is incomplete. Although extensive work on the bionomics or ecology of the complex was carried out in different sites over the range of distribution, it was done at local scales using different identification methods. Moreover, misidentification of species closely related to this complex is not uncommon (Van Bortel et al. 2001). Thus, assessment of the biology of the species is imprecise or misleading because most of the earlier studies undoubtedly refer to other or a mixture of species. Since the discovery of species A and C (Green et al. 1990), and despite the development of simple molecular identification methods (Van Bortel et al. 2000; Phuc et al. 2003; Garros et al. 2004a,b), the recent bionomic or ecological studies refer mainly to An. minimus s.l. and are based on morphological identifications for which no molecular controls were done (Dev 1996; Overgaard et al. 2002; Chareonviriyaphap et al. 2003a,b). Few studies employed molecular techniques to distinguish the different sibling species and their biological characteristics (Trung et al. 2004; Van Bortel et al. 2004). As the sibling species may exhibit different trophic behaviour and vectorial capacities, it is important to know the biological characteristics of each species. In addition to improving our understanding of the complex, precise knowledge of the individual species may considerably improve current vector control approaches.

We reviewed the available information on the Minimus Complex, especially species A and C, which are important malaria vectors compared with species E, which occurs in Japan, a malaria-free region. This review presents (1) up-to-date information on the main malaria studies involving An. minimus s.l.; (2) a discussion of recently published data on the distribution, ecology, and bionomics of immature as well as adult stages, and on the vectorial capacity of each sibling species; and (3) it highlights the gaps in our understanding of the Minimus Complex.

Taxonomic information and species identification

The Minimus Complex belongs to the Minimus Subgroup, which is closely related to the Aconitus Subgroup within the Funestus Group (Harbach 2004; Garros et al. 2005a,b). Most species of these subgroups occur in sympatry in Southeast Asia, and are difficult to distinguish based on the morphology of the immature and adult stages. The key characters of adults are overlapping and polymorphic, such as ornamentation of the wings and maxillary palpi. Anopheles minimus s.l., An. aconitus (Harrison 1980), An. flavirostris (Harrison 1980; Kirnowardoyo 1985), An. fluviatilis s.l. (Harrison 1980), An. pampanai (Van Bortel et al. 2000), and An. varuna (Van Bortel et al. 2001) are often misidentified on the basis of morphology.

Three sibling species are recognized within the Minimus Complex: species A, C and E (Harbach 2004), and no morphological characters are currently known to reliably distinguish them. However, based on the observation of Sucharit et al. (1988), several authors used the presence or absence of pale spots on the wing costa to distinguish species A from C (Rwegoshora et al. 2002). Yet, these characters are unreliable because they vary over time and space (Van Bortel et al. 1999; Chen et al. 2002).

The existence of two or more morphologically undefined species within An. minimus s.l. was first suspected based on differences in egg morphology (Baba 1950) and ecological and behavioural heterogeneities (Suthas et al. 1986). Two forms, designated A and B, were described in China (Yu & Li 1984; Yu 1987) based on morphological features in larvae, pupae and adults (Sucharit et al. 1988), and form B was subsequently regarded as a species. However, genetic evidence for the existence of two species was first provided for populations in Thailand (Green et al. 1990), then confirmed in Vietnam (Van Bortel et al. 1999), where sympatric homozygotes at the Odh locus (Octanol deshydrogenase) occurred in the absence of heterozygotes. The species were informally named species A and C (Green et al. 1990), the latter designated to distinguish it from form B previously described in China (Yu & Li 1984). Recently, Chen et al. (2002) showed that forms A and B in China are morphological variants of An. minimus A. In Japan, Somboon et al. (2001, 2005) provided morphological, cytogenetic, molecular and hybridization evidence for the recognition of another sibling species, designated species E, on the Ishigaki island of the Ryukyu Archipelago. Crossing experiments between species E and either species A or C showed that F1 crosses were sterile (Somboon et al. 2001, 2005). Both studies showed hybrid male sterility, which is generally accepted as very clear evidence of specific status. There is now no reason to cast doubt on the specific status of An. minimus E (Van Bortel & Coosemans 2003; Walton & Somboon 2004). Therefore to date, three species, designated as A, C and E are formally recognized within the Minimus Complex (Harbach 2004).

However, the complex may include two other species, species D (Baimai 1989) and specimen no. 157 (Sharpe et al. 1999) in Thailand. The specific status of these two entities is uncertain and needs further study. It seems that species D is a chromosomal variant of An. minimus A (V. Baimai, personal communication). It is difficult to assess the status of specimen no. 157 because only a single individual of this molecular variant (variation of two nucleotides of 313 on D3 domain of the 28S subunit, ribosomal DNA) is known.

Morphological evidence (Harrison 1980; Chen et al. 2002) and recently the comparison of nucleotide sequences of An. fluviatilis S (AF437880 from India), a member of the Fluviatilis Complex, with sequences of An. minimus C from several locations in Vietnam and Thailand (Garros et al. 2005b,c) showed that these informally designated species are conspecific, the latter being the senior synonym (Harbach 2004). This finding has important implications for our knowledge on the distribution and bionomics of An. minimus C.

Therefore, the problem with the identification of An. minimus s.l. is twofold. First, the taxon on the Southeast Asian mainland is a complex of two isomorphic species, and secondly, it is closely related to the sympatric Aconitus Subgroup (Garros et al. 2005a,b) based on the morphology of both immature and adult stages. In recent years, several molecular methods were developed to accurately identify sympatric species of the Minimus Complex and related species (Van Bortel et al. 2000; Phuc et al. 2003; Garros et al. 2004a,b; Garros et al. 2005d). These reliable molecular techniques need to be used regularly during entomological surveys to avoid misidentifications. It is strongly recommended that future studies routinely use these methods to improve our knowledge of the individual species.

Distribution

  1. Top of page
  2. Summary
  3. Introduction
  4. Distribution
  5. Bionomics
  6. Role of An. minimus s.l. in malaria transmission
  7. Control strategies
  8. Conclusions
  9. Acknowledgements
  10. References

Anopheles minimus s.l. has a wide distribution over the Oriental Region (Harrison 1980), probably because of its ability to adapt to temperate climate. The range of An. minimus s.l. appears to extend from Uttar Pradesh southward to the north-eastern tip of Andhra Pradesh State in India across the Indochina–Malay peninsular countries and northward across southern China (Table 1). Harrison (1980) suspected that the published records of An. minimus s.l. in Sumatra and Java were An. flavirostris, which was confirmed in several recent surveys (Somboon et al. 2000; Barcus et al. 2002). According to Harrison (1980), it may occur to 30°N latitude in India and China and probably does not occur further south than 6°N latitude (Figure 1). The northern limit has been molecularly confirmed by Chen et al. (2002) who found species C in southern China up to 32.5°N.

Table 1.  Overview of the geographical distribution of the Minimus Complex based on literature
 BangladeshCambodiaSouthern ChinaIndiaJapanLaosMalaysiaMyanmarNepalTaiwanThailandVietnamReferences
  1. *Based on recent confirmed and published records (see the Distribution section).

An. minimus s.l.XXXXXXXXXXXXHarrison 1980
An. minimus s.l. XXXXX X XXX*
An. minimus A XX  X   XXXSee Table 2
An. minimus C  X  X    XXSee Table 2
An. minimus E    X       See Table 2
image

Figure 1. Distribution map of natural populations of species A and C of the Minimus Complex based on published data. Locations are detailed in Table 2.

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It seems that An. minimus s.l. is rare in Taiwan (Lien 1991; Chen et al. 2002) and has disappeared from Nepal and several parts of India (Parajuli et al. 1981). The species has been reported in the north-eastern states of South Bihar and Assam in India, but it seems to have currently disappeared (Dev 1996; Das et al. 2000). Because identifications were not based on molecular methods, these records in the extreme parts of the taxon's distribution are difficult to validate. There are no recent records of An. minimus s.l. in Bangladesh (Maheswary et al. 1992), north-western India (Gujarat, border with Pakistan) (Bhatt & Kohli 1996), the Malaysian peninsula (Rahman et al. 2002), Philippine islands (Wooster & Rivera 1985; Schultz 1989), or Sri Lanka (Amerasinghe et al. 1999).

Based on recent application of molecular identification methods, members of the Minimus Complex are known to occur in the following countries (Table 1): Cambodia (Phuc et al. 2003); southern China (Chen et al. 2002); Japan (Somboon et al. 2001); Laos (Phuc et al. 2003); Taiwan (Chen et al. 2002); Thailand (Kengne et al. 2001); and Vietnam (Phuc et al. 2003; Trung et al. 2004). It has also been found based on morphological identification in India, Assam State (Jana-Kara et al. 1995; Dev 1996; Prakash et al. 2004), Bihar State (Das et al. 2000), and north-eastern states (West Bengal and Arunachal) (Nandi et al. 2000; Prakash et al. 2000), Hong Kong (Rueda et al. 2004), and Myanmar (Lin et al. 2000; Soe et al. 2001).

Since the majority of the available data on distribution concern An. minimus s.l., no complete and accurate information on the distribution of the individual members of the complex is available (Tables 1 and 2; Figure 1). Based on recent molecular studies, An. minimus species A is present in Cambodia, southern China, Laos, Thailand, Taiwan, and Vietnam (Van Bortel et al. 1999, 2000, 2003; Kengne et al. 2001; Chen et al. 2002; Phuc et al. 2003), and species C is known to occur in southern China (Chen et al. 2002), three provinces of Thailand (Sharpe et al. 2000; Kengne et al. 2001), Laos (Phuc et al. 2003), and northern and central Vietnam (Kengne et al. 2001; Phuc et al. 2003; Garros et al. 2005d) (Tables 1 and 2; Figure 1). At the present time, species A and C are known to occur largely in sympatry in a large area that includes northern Vietnam, southern China and northern Laos (Figure 1, shaded circles). Two local sympatric spots occur in central Vietnam and western Thailand (Figure 1, shaded circles). The assumption that species C has a restricted distribution compared with species A is refuted as more molecular studies are being done (Garros et al. 2005d). Moreover, synonymy of An. fluviatilis S with An. minimus C largely extends the distribution of this species to north-western India. On the contrary, An. minimus E is only known in the Ishigaki island, Ryukyu Archipelago, Japan (Somboon et al. 2001) (Figure 1, black triangle).

Table 2.  Details of sampling sites for Anopheles minimus species A, C and E
LocalityCollected speciesDate of collectionReference
  1. Pr., province; NA, not available.

Cambodia
 1 Rattanakiry Pr.An. minimus A1998–2000Kengne et al. (2001); Trung et al. (2004)
 2 Battambang Pr.An. minimus A06/1999Phuc et al. (2003)
China
 3 Taiwan Pr.Chen et al. (2002)
  Pingtung countyAn. minimus A08/2000
  Taichung countyAn. minimus A08/2000
 4 Guangdong Pr.
  Huidong countyAn. minimus A09/2000, 08/2001
  Taishan countyAn. minimus A08/2000
 5 Guangxi Pr.
  Lingzhan countyAn. minimus CNA
  Longlin countyAn. minimus C08/2000
 6 Guangxi Pr.
  Lonsheng countyAn. minimus C08/2001
 7 Guangxi Pr.
  Shangsi countyAn. minimus A08/2000, 08/2001
 8 Guizhou Pr.
  Guanling countyAn. minimus C08/2000
  Wangmuo countyAn. minimus C08/2000
 9 Sichuan Pr.
  Junlian countyAn. minimus C07/2000
  Qianwei countyAn. minimus C07/2000
10 Yunnan Pr.
  Daguan countyAn. minimus C09/2000
11 Yunnan Pr.
  Menglian countyAn. minimus C09/2000
12 Yunnan Pr.
  Jingdong countyAn. minimus A, C09/2001
13 Yunnan Pr.
  Mengla countyAn. minimus A, C07/2000, 09/2001
14 Yunnan Pr.
  Simao countyAn. minimus A, C08/2000, 09/2001
15 Yunnan Pr.
  Yuanjiang countyAn. minimus A, C07/2000
16 Hainan Pr.
  Baoting countyAn. minimus A08/2000, 08/2001
  Changjiang countyAn. minimus A08/2000, 08/2001
  Danzhou coutyAn. minimus A08/2001
Laos
17 Samneur Pr.An. minimus A, C12/1999Phuc et al. (2003)
18 Vientiane Pr.An. minimus A1998–2000Kengne et al. (2001); Trung et al. (2004)
19 Bolikhamxay Pr.An. minimus A02–04/1999Phuc et al. (2003)
Thailand
20 Kanchanaburi Pr.An. minimus A, C1998–2000Kengne et al. (2001)
21 Chiang Mai Pr.An. minimus CNASharpe et al. (2000)
22 Tak Pr.An. minimus ANA
Vietnam
23 Ha Giang Pr.An. minimus A, C03/1999Phuc et al. (2003)
24 Cao Bang Pr.An. minimus A, C11/1999
25 Lang Son Pr.An. minimus A, C04/2000
26 Son La Pr.An. minimus A, C10/1999
27 Hoa Binh Pr.
  Phu Cuong communeAn. minimus A, C1995Van Bortel et al. (1999)
  Phu Cuong communeAn. minimus A, C1999Phuc et al. (2003)
  Phu Cuong communeAn. minimus A, C1998–2000Kengne et al. (2001); Trung et al. (2004)
  Quyet Chien communeAn. minimus A, C1999Phuc et al. (2003)
28 Hanoi Pr.An. minimus ANAPhuc et al. (2003)
   Hanoi Pr.An. minimus A1998–2000Van Bortel et al. (1999)
29 Thanh Hoa Pr.An. minimus A, C11/1999Phuc et al. (2003)
30 Nghe An Pr.
  Quynh Thang communeAn. minimus A11/1999
  Cam Lam communeAn. minimus A, C11/1999
31 Ha Tinh Pr.
  Huong Khe communeAn. minimus A, C11/1999
  Ky Thinh communeAn. minimus A11/1999
32 Khanh Hoa Pr.
  Khanh Phu communeAn. minimus A1993–2002Garros et al. (2005c)
An. minimus A1998–2000Van Bortel et al. (2003)
An. minimus C2000Phuc et al. (2003)
An. minimus A, C2002Garros et al. (2005c)
33 Gia Lai Pr.An. minimus A06/1999Phuc et al. (2003)
34 Tay Ninh Pr.An. minimus A05/1999
35 Binh Phuoc Pr.An. minimus A1999
36 Dong Nai Pr.An. minimus A07/1999
37 Binh Thuan Pr.
   Binh Thanh communeAn. minimus A1999Phuc et al. (2003)
  Suoi Kiet communeAn. minimus A1998–2000Van Bortel et al. (2001)
Japan
38 Ryukyu ArchipelagoAn. minimus E08–10/1998Somboon et al. (2001)
  Ishigaki island09/1999

Bionomics

  1. Top of page
  2. Summary
  3. Introduction
  4. Distribution
  5. Bionomics
  6. Role of An. minimus s.l. in malaria transmission
  7. Control strategies
  8. Conclusions
  9. Acknowledgements
  10. References

Breeding site ecology

The most favourable breeding sites of An. minimus s.l. are streams or canals with clear, slow-running water partially shaded by grassy margins (Harrison 1980; Dev 1996) (Figures 2a,b). These habitats are commonly found in foothills. However, An. minimus s.l. larvae may also occur in rock pools, ground pools next to streams, seepage pools and rice-field terraces. It is, however, not known whether species A and C occupy different habitats or niches. Water flow is an important factor that decreases the larval density (Overgaard et al. 2002). Yet, in the suburbs of Hanoi along the Hong River delta (northern Vietnam), larvae of An. minimus s.l. are regularly collected in cisterns containing rainwater or water from wells (Phan 1973; Khai 1975) (Figure 2c). Recently, Van Bortel et al. (1999) identified this population as species A. Even though these cisterns may contain a high density of larvae, only a limited number of An. minimus A adults were captured during entomological surveys using cattle as bait (Van Bortel et al. 2003) and they apparently do not represent a nuisance for humans (H.D. Trung, unpublished data). More studies are needed to understand the low density of adult mosquitoes in this environment. This urban population reflects the ability of this species to adapt to environmental change induced by humans.

image

Figure 2. Habitats of the immature stages of An. minimus s.l. (a) Kanchanaburi Province (western Thailand); (b) Hoa Binh Province (northern Vietnam); (c) suburbs of Hanoi (northern Vietnam), unusual habitat of species A in cisterns (pictures taken by CG).

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Seasonal abundance and flight range

Over its distribution range, An. minimus s.l. exhibits peak densities during the dry season. This peak is temporally variable as the start of the rainy season fluctuates. Heavy rains, generally from June to the end of August, increase the turbidity and water flow of breeding sites, leading to a decrease in numbers of larvae. Two peaks were found in Thailand, the first before the rainy season (from March to May) and the second at the end of the rainy season (from July to November) (Harrison 1980; Ratanatham et al. 1988; Chareonviriyaphap et al. 2003a). It is unknown if these peaks are specific to either species A or C. In northern Vietnam (Hoa Binh Province), the distribution of An. minimus A and C based on molecular identifications was not identical from June 1995 until November 1995 (Van Bortel et al. 1999): An. minimus A showed a decline from June to October (mostly rainy season) and An. minimus C was particularly abundant in October (dry season) (W. Van Bortel, unpublished data).

The flight range of Anopheles varies generally from 1 to 3 km, with an estimation up to 1 km for species E in the Ryukyu Archipelago (Tsuda et al. 1999) and up to 2 km for An. minimus s.l. in Thailand (Meek 1995).

Factors influencing the occurrence of the members of the Minimus Complex

It is noteworthy that the species of the Minimus Complex exhibit population fluctuations in relation to human actions on ecosystems. An. minimus s.l. became rare or disappeared from Taiwan, Nepal and north-eastern parts of India after the use of insecticides (Parajuli et al. 1981; Lien 1991; Chen et al. 2002). In central Vietnam (Khanh Hoa Province), An. minimus A occurred in high densities (17 bites/man/night) in Lang Nhot village until April 1998, then decreased drastically after November 1998 (Trung et al. 2004; Van Bortel et al. 2004), whereas an increase of the An. minimus C population has been noted in the same village since 2000 (Garros et al. 2005d). In Thailand, ecological changes to develop agriculture and tourism decreased the forested areas, as well as breeding sites of the species, leading to reductions in populations of An. minimus s.l. Indeed, the density of An. minimus s.l. is negatively affected by increased landscape diversity and forest fragmentation (Overgaard et al. 2002). Moreover, higher water flow, due to deforestation, along with the absence of vegetation, also have a negative impact on suitable breeding sites (Overgaard et al. 2002). The effects of land use on the species have not yet been investigated, but in areas were both species are sympatric the relative abundance may vary from one village to the other (Van Bortel et al. 1999) indicating that micro-environmental factors probably influence the occurrence of each species. Moreover, it seems that species C may be linked to altered environments, such as recent agro-ecosystems located in deforested areas, whereas An. minimus A occurs in less disturbed forested areas and traditional rice agrosystems (C. Garros and S. Manguin, unpublished data). No environmental data is available for species E.

Behaviour of the members of the species complex

Anopheles minimus s.l. bites throughout the night, but exhibits geographical and seasonal variation in peak biting activity. A large peak from 22.00 hours to 03.00 hours was recorded in Laos (Vythilingam et al. 2001), Myanmar (Myo Paing et al. 1998), and Vietnam (Phung & Quang 1997). Peak activity was earlier in Thailand from 21.00 hours to 22.00 hours (Harbach et al. 1987). Several authors also reported two feeding peaks, regardless of season, one after the sunset and a second before dawn (Ratanatham et al. 1988; Rattanarithikul et al. 1996; Chareonviriyaphap et al. 2003a). Relative risk (RR) of being bitten hourly by An. minimus A before 22.00 hours compared with after 22.00 hours ranged from 0.29 (central Vietnam) to 1.00 (Cambodia) (Trung et al. 2005). The RR was higher for An. minimus C (1.03) than for An. minimus A (0.79) in Hoa Binh Province of Vietnam (Trung et al. 2005).

Anopheles minimus s.l. is characterized as being principally an anthropophilic mosquito (Harrison 1980). Many data are available on the trophic behaviour of An. minimus s.l., but it refers to populations with uncertain species identity. In Bangladesh, India, and Nepal, the taxon was reported to be anthropophilic (with 5.82 bite/person/night vs. 0.12 bite/cow/night in India) (Covell 1944; Parajuli et al. 1981; Jana-Kara et al. 1995; Dev 1996; Dev et al. 2003). However, it was observed to be zoophilic in Bihar State (north-eastern India) and in Thailand (Das & Baruah 1985; Ratanatham et al. 1988; Das et al. 2000), with a ratio of females feeding on humans and cattle of 1.0:2.6 in Thailand. In India, Jana-Kara et al. (1995) reported that An. minimus s.l. was the main species collected indoors on humans (endophagic behaviour), but in western and upper central Thailand An. minimus s.l. was more attracted to humans outdoors (exophagic behaviour, 3.27 mosquitoes/man/night) than indoors (1.15 mosquitoes/man/night) (Ratanatham et al. 1988). In Laos, An. minimus s.l. was found to bite both indoors and outdoors (densities not provided) (Vythilingam et al. 2001). As we do not know precisely which species of the Minimus Complex were studied, it is difficult to infer whether these heterogeneities are due to a mixture of species or to behavioural polymorphism and plasticity of the individual species.

To date, one research programme in Southeast Asia2 has assessed the specific behaviour of species A and C. Significant intraspecific behavioural differences were observed among populations of An. minimus A from Cambodia (one site), Laos (one site) and Vietnam (two sites in northern and central areas). A wide range of anthropophilic behaviour was observed, with a ratio of outdoor human (OH) landing vs. cattle (OC) collections ranging from 0.10 (northern Vietnam) to 10.11 (Laos); endophagic behaviour, with a ratio of indoor (IH) vs. OH landing collections ranging from 0.62 (Cambodia) to 7.95 (central Vietnam); and indoor resting behaviour (endophilic behaviour), with a ratio of indoor morning collections (IMR) vs. combined IH, OH, and cattle collections at night ranging from 0.04 (Laos) to 0.67 (northern Vietnam) (Trung et al. 2005). Unfortunately, only one sympatric population of An. minimus A and C in northern Vietnam (Hoa Binh Province) was studied, and intraspecific behavioural variation could not be assessed. Zoophilic behaviour was pronounced for both species A and C (ratio OH vs. OC of 0.10 and 0.09, respectively), but species C was more exophagic than species A (ratio IH vs. OH of 0.35 and 0.93, respectively), as well as more exophilic than species A (ratio IMR vs. IH–OH–OC of 0.15 and 0.67, respectively) (Trung et al. 2005).

From the above mentioned studies, it is clear that An. minimus s.l. exhibits behavioural heterogeneities relevant for disease transmission and vector control. However, the recognition of the two species cannot explain all behavioural heterogeneities within this taxon (Green et al. 1990). It appears that both species exhibit variable trophic behaviour depending on the environment. Some behavioural differences are also linked to the abundance of cattle in localities in northern Vietnam and Cambodia (Van Bortel et al. 2004) as well as differences in housing construction. For instance, An. minimus A in central Vietnam (Khanh Hoa Province) exhibits a high anthropophilic (ratio of 8.10) and endophagic (ratio of 7.95) behaviour most likely influenced by the largely open houses with incomplete walls that allow mosquitoes to easily detect human stimuli and enter into houses. On the contrary, in northern Vietnam (Hoa Binh Province) where cattle are quite abundant and are kept under the stilt houses, An. minimus A exhibits a low degree of anthropophily (0.10) and endophagy (0.93), and a high degree of endophily (0.67) (Van Bortel et al. 2004; Trung et al. 2005). Therefore, it is clear that intraspecific behavioural differences in members of the Minimus Complex occur due to the high behavioural plasticity. Unfortunately, to date, no information on the trophic behaviour of An. minimus E is available.

Role of An. minimus s.l. in malaria transmission

  1. Top of page
  2. Summary
  3. Introduction
  4. Distribution
  5. Bionomics
  6. Role of An. minimus s.l. in malaria transmission
  7. Control strategies
  8. Conclusions
  9. Acknowledgements
  10. References

Anopheles minimus s.l. has been incriminated as a major malaria vector in all areas where it occurs (Harrison 1980; Chareonviriyaphap et al. 2000; Chen et al. 2002). In 1990, sporozoite infections were recorded all year round for An. minimus s.l. in India (Assam), except during August and September. The infection rate was lowest in March (0.7%) and highest in October (8.5%) (Dev 1996). In southern China, Yunnan Province, An. minimus s.l. was regarded as the main vector with an average sporozoite rate of 7.05% (Dong 1997). In Gia Lay Province of central Vietnam, the sporozoite rate was 2.58% in 1994 (Phan 1998). Anopheles minimus s.l. also has been incriminated as the main malaria vector in Myanmar (Oo et al. 2004).

In Cambodia and central Vietnam, An. minimus A was found positive in ELISA tests for the detection of circumsporozoite protein (CS) of Plasmodium falciparum and P. vivax 210 and 247 (Trung et al. 2004). Anopheles minimus C has been regarded as a less competent malaria vector than species A based only on its more exophagic behaviour (Van Bortel et al. 1999). However, the vectorial status of species C is still questionable and needs to be clearly determined, especially as it is conspecific with An. fluviatilis S recognized as an efficient malaria vector in India (Dev 1996; Dev et al. 2003). Anopheles minimus C is suspected to be responsible for malaria transmission in areas of southern China (Chen et al. 2002) and in Thailand (T. Chareonviriyaphap, personal communication), where it is the main species of the complex. Anopheles minimus specie E has not been incriminated as malaria vector because the Ryukyu Archipelago is malaria-free.

Control strategies

  1. Top of page
  2. Summary
  3. Introduction
  4. Distribution
  5. Bionomics
  6. Role of An. minimus s.l. in malaria transmission
  7. Control strategies
  8. Conclusions
  9. Acknowledgements
  10. References

Vector control is largely based on the use of insecticides in residual house spraying and bed net impregnation. Residual indoor spraying programmes were organized throughout the Indochina peninsula during the 1960s with good results for the control of An. minimus s.l. (Meek 1995). The use of insecticides in Nepal eliminated An. minimus s.l. (Parajuli et al. 1981). In Thailand, after years of DDT application, populations of An. minimus s.l. were reduced in the peninsula and central plains (Harrison 1980), but remained abundant in hilly forested areas. An. minimus s.l. is still reported as susceptible to DDT and permethrin in Thailand, except in some areas where a slight increase of tolerance was observed (Somboon et al. 2003). In Vietnam, longitudinal surveys from 1960 to 1980 in 12 northern sites and four central ones did not report resistance to DDT or any other insecticides (Phan 1998). Then, from 1993 to 1997 insecticide resistance tests indicated possible resistance to permethrin (at a discriminating concentration of 0.25%) in populations from Hanoi (northern Vietnam) and Khanh Hoa (central Vietnam) Provinces (Phan 1998). Moreover, possible differences in the resistance status between An. minimus A and C are currently under investigation (MALVECASIA project3). Furthermore, the Resistant Pest Management database (http://www.pesticideresistance.org) cited Indonesia, Philippines and Thailand as countries where resistance to organochlorides (DDT and cyclodienes) was reported but without further information. These data are doubtful for the first two countries because An. minimus s.l. does not occur there. The data may concern An. flavirostris or another closely related species.

Biological control attempts have also been made to control larvae of An. minimus s.l. in Thailand using larvivorous fish (Tang Am 1993; Meek 1995). The effectiveness of this strategy has not been demonstrated. The typical breeding sites of An. minimus s.l. in small streams with slow-running water may not be suitable for the larvivorous fish to develop. Recently in Japan, copepod species, commonly found in rice fields during the summer season, were tested as predators of An. minimus s.l. larvae (Dieng et al. 2003). The results supported the contention that these copepods have the potential to be used as biological control agents against mosquito immature stages. More recently, a root extract from Stemona curtisii (Stemonaceae) showed significant larvicidal activity against An. minimus s.l. (Mungkornasawakul et al. 2004). However, biological control may be efficient only if included within an integrated approach.

Because of the high behavioural plasticity of An. minimus A, and marked differences between species A and C with regard to local environment (housing construction, cattle abundance, landscape), preliminary entomological studies focusing on each species of the Minimus Complex need to be implemented before choosing and applying an appropriate vector control strategy. This is necessary in order to apply vector control strategies targeting specifically malaria vectors species.

Conclusions

  1. Top of page
  2. Summary
  3. Introduction
  4. Distribution
  5. Bionomics
  6. Role of An. minimus s.l. in malaria transmission
  7. Control strategies
  8. Conclusions
  9. Acknowledgements
  10. References

For a better and more precise knowledge about the distribution of members of the Minimus Complex, it is essential to use molecular tests in order to avoid morphological confusion with other species of the Funestus Group. The two members of the Minimus Complex on the Southeast Asia mainland seem to show bionomical differences that may be epidemiologically relevant when they occur in sympatry. However, available information indicates that both species exhibit considerable behavioural and ecological plasticity and this high biological heterogeneity is apparent depending on the geographic location, abundance of cattle, and the construction of houses. However, it is not clear to what extend the stated biological and bionomical variation over the range of distribution are explained by inter- or intraspecific differences. Therefore, the relative role of the individual species in malaria transmission, as well as the specific biological variability of their bionomics, should be further analysed over their ranges of distribution. Several questions of importance need to be addressed: (1) How does immature and adult ecology influence the distribution of the members of this species complex? (2) What are the specific differences in seasonal abundance and nocturnal biting patterns? (3) What role does species C play in malaria transmission? (4) What is the status to insecticide resistance of each species in different regions of Southeast Asia? (5) Is there further evidence to support the conspecificity of An. minimus C and An. fluviatilis S?

Asian countries are in constant development which leads to deforestation and modification of the original environment. Studies are needed on how the different sibling species will adapt and respond to anthropogenic changes and how these adaptations will influence population densities, distributions, and trophic behaviours.

Footnotes
  • 1

    Funestus Group: An. jeyporiensis; Aconitus Subgroup: An. aconitus, An. filipinae, An. mangyanus, An. pampanai, An. varuna; Culicifacies Subgroup: An. culicifacies A, B, C, D and E; Funestus Subgroup: An. aruni, An. confusus, An. funestus, An. parensis, An. vaneedeni; Minimus Subgroup: An. flavirostris, An. fluviatilis T and U, An. leesoni, An. minimus A, C and E; Rivulorum Subgroup: An. brucei, An. fuscivenosus, An. rivulorum, An. rivulorum-like sp.

  • 2

    INCO-project n°ERBIC18.CT.970211.

  • 3

    MALVECASIA (Malaria Vectors in Asia), INCO-project n°IC4-CT-2002-10041.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Distribution
  5. Bionomics
  6. Role of An. minimus s.l. in malaria transmission
  7. Control strategies
  8. Conclusions
  9. Acknowledgements
  10. References

We are grateful to our Asian partners from NIMPE (Hanoi, Vietnam), NCMC (Phnom Penh, Cambodia), CMPE (Vientiane, Laos), Mahidol and Kasetsart Universities (Bangkok, Thailand) for their involvement in a fruitful collaboration. Financial support was partially provided by two European INCO-projects.

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  2. Summary
  3. Introduction
  4. Distribution
  5. Bionomics
  6. Role of An. minimus s.l. in malaria transmission
  7. Control strategies
  8. Conclusions
  9. Acknowledgements
  10. References
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