Bionomics of Anopheles maculatus complex and their role in malaria transmission in Thailand

The bionomics of Anopheles maculatus complex and its role in malaria transmission were conducted in Pakchong and Sadao districts, Nakhon Ratchasima and Songkhla provinces, respectively, from January 1984 to July 1985. In Pakchong, An. maculatus species A was the most dominant species, followed by species B form F and species C which was rare. The densities of species A and species B form F were high between July and November, with their peaks in October. Biting activities of both species occurred through out the night, with a major peak during the first quarter of the night on all seasons. In Sadao, only An. maculatus species B form E was detected with peak densities between February and June. Biting activities of this species varied according to seasons. The prevalence of mosquitoes was influenced by monthly rainfall, relative humidity and air-temperature. All species of female An. maculatus complex studied prefered to feed on animal rather than on human, and tended to bit human more outdoors than indoors, and thus exhibiting a zoophilic and exophagic behaviour. Life expectancies of An. maculatus species A ranged from 1.6 to 6.6 days, species B form F from 1.1 to 8.1 days, and species B form E from 0.7 to 21.2 days. The natural malaria infection rate was very low. Out of 4,430 guts dissected, only 0.23% were found infected with oocysts. There were no sporozoites detected in the 4,472 dissected salivary glands.     

Studies on malaria and responses of Anopheles balabacensis balabacensis and Anopheles minimus to DDT residual spraying in Thailand.

Studies on malaria and on A. b. balabacensis and A. minimus responses to DDT spraying were conducted in a forested hilly area in northern Thailand. In a first phase, base-line data were collected from July 1970 to March 1972. In a second phase, the study area received five round of DDT spraying over a period of two years and at the same time all malaria infections received radical treatment. During this two-year period of field operations, entomological and epidemiological observations were continued. The studies carried out in the second phase, showed that malaria transmission decreased under the applied optimum anti-malarial measures but was not interrupted. Human ecology and population movement inside the forest, especially during the dry season, contributed to a great extent to this result. The transmission occurring in the early part of the monsoon season clearly indicates the importance of the timing of DDT spraying. A. b. balabacensis appeared to be transmitting malaria all the year round in the deep forest but only in the monsoon season in the forest fringe. The vectorial capacity of both vectors was estimated separately for indoor and outdoor populations. The pre-spraying values obtained for A. b. balabacensis were much higher thaan for A. minimus. After DDT spraying A. b. balabacensis showed a decrease in vectorial capacity estimated at 31.5 times for the indoor population and 18 times for the outdoor population. A. minimus, on the other hand, showed a much smaller decrease, estimated at 6.8 and 1.9 times for the indoor and outdoor populations respectively.     

Behavioral responses to DDT and pyrethroids between Anopheles minimus species A and C, malaria vectors in Thailand

Behavioral responses of two field populations of Anopheles minimus complex species A and C for contact and non-contact actions of chemicals were compared during and after exposure to operational field concentrations of DDT (2 g/m2), deltamethrin (0.02 g/m2), and lambdacyhalothrin (0.03 g/m2) using an excito-repellency escape chamber. The two populations were collected from the Mae Sot District in Tak Province (species A) and the Tri Yok District in Kanchanaburi Province (species C) in western Thailand. Female mosquitoes of both populations rapidly escaped from chambers after direct contact with DDT, deltamethrin, and lambdacyhalothrin. The non-contact repellency response to DDT and the two synthetic pyrethroids was pronounced with An. minimus species A; however, non-contact repellency was relatively weak with An. minimus species C, but remained significantly greater than the paired controls (P < 0.05). We conclude that strong contact irritancy was present in both test populations, whereas non-contact repellency also played a significant role in the escape response of An. minimus species A.     

Population genetic studies on the Anopheles minimus complex in Thailand

The population genetic studies on the Anopheles minimus complex revealed that An. minimus is a species complex evident by IK value were less than 0.0534 and 0.4163 for the morphotaxonomy and isozyme studies. There are two sibling species in the An. minimus species complex. The first is the typical minimus and its variations described by Harrison. Esterase-2 alleles 100 and 102 are predominant with a few other alleles of 96 and 104. It is exophagic and zoophilic. It is widely distributed in all provinces where minimus breeds. The second of the two sibling species is An. minimus species C, with 2 pale spots on the humeral dark band and Est-2 allele 98. It is found most frequently in Pu Toei District of Kanchanaburi Province, Thailand. An. minimus probably species B showing M1 + 2 wing vein entirely dark is found in China.     

Morphological variations of Anopheles minimus A in Tak Province, Thailand

Anopheles minimus Theobald is one of the major vectors of malaria throughout the Oriental Region, and it's complex consists of at least 2 sibling species (A and C) in Thailand. This study aimed to determine the morphological variations of wings of An. minimus A and to clarify the specific status of An. minimus in Ban Khun Huay, Ban Pa Dae, and Ban Tham Seau, Mae Sot district, Tak Province, Thailand. Anopheline larvae were collected from the fields between October 2002 and September 2003, allowed to emerge into adults in the laboratory and identified by morphological and molecular characterization. About 1,715 of female An. minimus A were separated into 8 groups based on their wing scale patterns. Polymerase Chain Reaction Restriction Fragment Length Polymorphism (PCR-RFLP) assay (ribosomal DNA ITS2) confirmed the identification of An. minimus A in all 8 groups.     

Well-breeding Anopheles dirus and their role in malaria transmission in Myanmar

Mosquitos were collected with human and animal baits from March 1996 to January 1998 in four villages located along the Yadana gas pipe line in Yepyu township, Dawae district, Tanintharyi Division, southern Myanmar. A total of 23 anopheline species were collected. Anopheles dirus were abundant in pre-monsoon (May/June) an post-monsoon (October) months. All An. dirus caught both humans and cattle were assayed with specific, sporozoite enzyme-linked immunosorbent assays (ELISAs). A total of 5/250 (2%) caught with human bait was found positive with Plasmodium vivax from Eindayaza, Ohnbinkwin and Thaechaung during rainy and cool-dry months. Larval surveys also showed An. dirus larvae/pupae were caught from domestic wells (6 to 46% found positive). Clinical surveys indicated that transmission is hyperendemic and occur all year round in all four villages.     

Bionomics and systematics of the oriental Anopheles sundaicus complex in relation to malaria transmission and vector control

The taxonomic history, distribution, bionomics, systematics, and vector control strategies for the Anopheles sundaicus complex are reviewed in relation to malaria epidemiology. The lack of data on the bionomics, insecticide resistance, and vector capacity, as well as the general lack of surveillance and monitoring of potential vector populations, make the development of targeted control measures problematic. It will be necessary to elucidate, characterize and identify all members of the complex to determine their distributions, disease relationships, ecologic relationships, and resistance to insecticides. This knowledge is essential for epidemiologic studies, the design and implementation of appropriate vector control measures, and the development of strategies for monitoring and assessing the potential risk of malaria outbreaks due to members of the complex.     

Confirmation of Anopheles varuna in vietnam, previously misidentified and mistargeted as the malaria vector Anopheles minimus.

Malaria control programs in Southeast Asia are faced with several questions concerning vector behavior and species identification, which need to be answered to consolidate and further improve the results of control practices. The vector system in Southeast Asia is complex because of the number of species potentially involved in malaria transmission. Additionally, the follow-up and evaluation of preventive control measures are hampered by the misidentification of vectors due to overlapping morphological characters of the female mosquitoes. In central Vietnam, control practices are aimed at 2 main species, Anopheles dirus s.l. and Anopheles minimus s.l. These reputed vectors were studied in an area of Binh Thuan Province of south-central Vietnam. Different collection methods were used to capture mosquitoes quarterly during a 1-year period. Mosquitoes were identified in the field and later subjected to detailed morphological examination and polymerase chain reaction-restriction fragment length polymorphism analysis. What was thought to be an unusual morphotype of An. minimus was shown to be Anopheles varuna, and most specimens identified as the former species in the field proved to be the latter species. Very few An. minimus individuals were found during the study period. The population of An. varuna was found to be highly zoophilic, and based on this behavior, it cannot be considered a vector in Vietnam. Because this species was previously being misidentified as An. minimus, a nonvector was mistargeted as a malaria vector in Binh Thuan Province. Anopheles dirus, which was found positive for Plasmodium falciparum circumsporozoite via enzyme-linked immunosorbent assay, is clearly the main vector in this area. Despite the fact that several potential secondary vectors were found during the study, the primary target for vector control in the region should be An. dirus.     

Intraspecific hybridization of Anopheles minimus (Diptera: Culicidae) species A and C in Thailand

Hybridization tests of laboratory-raised, isolines of Anopheles minimus, species A and C were conducted by induced copulation. The three isolines were established based on three morphological variants of wild-caught, fully engorged females and two distinct types of metaphase chromosomes. They were An. minimus species A: V form (X1,Y1), M form (X2,Y1); species C: P form (X3,Y2). The results of reciprocal and back crosses indicated that the two morphologically variant forms of species A were genetically compatible, providing viable progeny and completely synaptic salivary gland polytene chromosomes, whereas they were genetically incompatible with species C and/or the P form. Hybrid progeny was only obtained from both forms of species A females x species C males, but asynaptic salivary gland polytene chromosomes on 3L and partial development of ovarian follicles in females were seen. Back crosses of F1 hybrid males with parental species A females provided viable progeny, while back crosses of F1 hybrid females with parental species C males provided progeny of low viability and adult males with abnormal spermatozoa, suggesting the partial reproductive isolation of An. minimus species A and C.     

Biting behavior and seasonal variation in the abundance of Anopheles minimus species A and C in Thailand

We measured the seasonal abundance and bloodfeeding behavior of species A and C of the mosquito Anopheles minimus Theobald 1901 in an endemic malarious area of western Thailand. An. minimus s.l. is a major vector of human malarial and filarial parasites in Southeast Asia. Mosquitos were collected once a month for one year using four collection methods: human-baiting indoors, human-baiting outdoors, human-baiting in the forest, and cow-baiting. We found that both species A and C tend to feed from cows rather than humans; we did not find any preference for indoor, outdoor or forest-biting in either species. Both species had a peak biting density in October/November, at the end of the rainy season, and species C showed a second, smaller peak at the end of the cool season. These findings are discussed in relation to previous reports of the behavior of An. minimus s.l., particularly in light of suggestions that An. minimus s.l. has changed its feeding behavior in response to DDT spraying.     

Ikonos-derived malaria transmission risk in northwestern Thailand

We mapped overall malaria cases and located each field observed major malaria vector breeding habitat using Global Positioning System (GPS) instruments from September 2000 to October 2003 around the three malaria-endemic villages of Ban Khun Huay, Ban Pa Dae, and Ban Tham Seau, Mae Sod district, Tak Province, Thailand. The land-use/land-cover classifications of the three villages and surrounding areas were performed on IKONOS satellite images acquired on 12 November 2001 with a spatial resolution of 1 x 1 m. Stream network was delineated and displayed. Proximity analysis was performed on the locations of the houses with and without malaria cases within a 1.5 km buffer from An. minimus immature mosquito breeding habitats, mainly stream margins. The 1.5 km used in our proximity analysis was arbitrarily estimated based on the An. minimus flight range. A statistical t-test at 5% significance level was performed to evaluate whether houses with malaria cases have higher proximities to streams than houses without malaria cases. The result shows no significant difference between proximity to streams between houses with malaria cases and houses without malaria cases. We suspect that the actual flight range of An. minimus may be greater than 1.5 km. The An. minimus larval habitat deserves more detailed investigation. Further studies on human behavior contrary to that required for adequate malaria control among these three villages are also recommended.     

An outbreak of Plasmodium falciparum malaria due to Anopheles minimus in central Assam, India

Epidemiological investigations were conducted in Nellie subcentre, PHC Jhargaon, under Morigaon district (Assam). The results of fever cases revealed 68 per cent slide positivity rate (SPR) and 40 per cent slide falciparum rate (SfR). The Pf proportion was > 87 per cent and remaining cases were P. vivax infections. An. minimus was incriminated as a malaria vector during the study period. The sporozoite rate was 3.08 per cent. The indoor man mosquito contact was 35 per bait/night as against 23 in outdoors. Results of susceptibility test revealed that the vector was still susceptible to both DDT and malathion at discriminating dosages. The study revealed that inadequate surveillance and vector control measures were contributing factors for malaria outbreak. In view of this, insecticide treated nets may be introduced to provide cost-effective control of malaria.     

Seasonal and spatial variation in natural densovirus infection in Anopheles minimus S.L. in Thailand

We report the first detection of a mosquito densovirus in anophelines, An. minimus species A and species C, and describe temporal and spatial variation in natural densovirus infection. A total of 814 (136 species A; 678 species C) adult mosquitos, obtained over a one year period from human biting catches at three locations in a village in western Thailand, were PCR tested for densovirus infection. Overall infection prevalence did not differ between species (15.4% species A; 14.5% species C). Infection prevalence showed significant seasonal variation. Some spatial heterogeneity in infection was also noted, with timing of peak infection prevalence varying between sites. PCR-screening of An. minimus s.l. larvae found an overall infection prevalence of 18.8%. Larval infection showed a significant positive association with rainfall recorded two months previous to larval collection. Infection in adult mosquitos showed a moderate relationship to environmental variables, but a significant negative correlation with larval infection in the previous month.     

云南微小按蚊A和C栖性的比较研究

目的 对我国云南地区微小按蚊A、C栖息习性进行初步探讨。 方法 在云南省勐腊县和元江县各选取一个自然村的人房和牛房，采用紫外灯通宵悬挂诱捕法采集蚊虫样本，将经形态学鉴定的微小按蚊的样本，分别取单蚊蚊腿，经多重PCR方法鉴别为微小按蚊A或C，统计其在人房和牛房的分布差异。对疑为微小按蚊A/C杂合子的，采用等位基因特异扩增法、PCR产物酶切片段长度多态性分析和测定D3序列进行鉴定。 结果 在人房中，微小按蚊A所占比例为21.4%，微小按蚊C为78.6%；牛房中，微小按蚊A所占比例为18.5%，微小按蚊C为81.5%，分布差异无统计学意义（χ²=0.4157，P>0.05）。疑为微小按蚊A/C杂合子的样本，等位基因特异扩增法（PCR-ASA）、PCR产物酶切片段长度多态性分析（PCR-ASA）结果同时出现微小按蚊A、C的特征条带(PCR-ASA: 376、294和112 bp, PCR-RFLP: 108、268和376 bp)，其D3序列在微小按蚊A与C的所有5个变异位点均出现杂合峰信号，即同时具有微小按蚊A和C相应碱基的峰信号。 结论 尚未发现我国云南微小按蚊A、C在人房和牛房的栖息比例存在差异。在我国云南勐腊县首次发现微小按蚊A/C杂合子。     

微小按蚊抗药性的历史及现状

微小按蚊是我国疟疾传播的主要媒介,为控制疟疾的流行,人们研制并使用了大量的杀虫剂.与此同时,蚊虫也对杀虫剂产生了抗药性.目前,蚊虫的抗药性问题已成为世界性难题,该文从微小按蚊抗性发展、影响蚊虫抗性的遗传基因两方面作了综述,以期为微小按蚊控制提供基础资料.