Larval Habitats of Malaria Vectors and Other Anopheles Mosquitoes Around a Transmission Focus in Northwestern Thailand

Journal of the American Mosquito Control Association, 1l(4):42g433, 1995

LARVAL HABITATS OF MALARIA VECTORS AND OTHER ANOPHELES MOSQUITOES AROUND A TRANSMISSION FOCUS IN NORTHWESTERN THAILAND

RAMPA RATTANARITHIKUL,I CHRISTOPHER A. GREEN,' SAKOL PANYIM,3 cHUMNoNc NOIGAMOL,' soMpoRN CHANAIMONGKOL, AND pRADrrH MAHAPIBUL,

ABSTRACT We sampled 199 bodies of water for Anopheles larvae around a malaria-endemic village near Mae Sot, Thailand, over 2 years. Anopheles dirus species A and D occurred in 54 small, well-shadel, stream and temporary ground pools. The larval habitat ol species D is reported for the first tim e. Anopheles minimus species A occurred in 8 samples from slow-moving streams, in 8 from rice fields, and in 2 from ground pools. Anopheles pseudowillmori occuned in one sample from a ground pool, in 3 from streams, and in 9 from rice fields.

INTRODUCTION into the Salween River. It is about 5 km due east of a second Karen village, Ban Pha De, which One method of malaria control is elimination lies on the eastern edge of the Moei River ero- of Anopheles larvae by application of chemical sion plain. Forest is confined to the watershed or biological toxicants or by source reduction itself, elsewhere having been eliminated for ag- through removal of their habitat. Although ricultural purposes zround the erosion plain. source reduction is often the least expensive and Much of the area between Ban Tham Seu and most environmentally acceptable control strate- Ban Pha De has been cleared and is used now gy, its feasibility is determined by accessibility for cash-crop production, with some reforesta- and consistency of habitats. tion with teak. These trees are now about 3 m The Anopheles (Cellia) dirus complex, which high and have developed a continuous canopy. in Thailand comprises 5 sibling species (Peyton The region has distinct wet (May-October) and 1990), includes the major vectors of malaria par- dry (November-April) seasons. The illustrations asites in much of Southeast Asia where forests (Figs. 1-3) are modified from the topographic exist (Scanlon and Sandinand 1965, Rosenberg map series L 7017, 4742 lll and aerial photo- 1982, Rosenberg et al. 1990). This is true for graphs LPR-PK, PK-120 No. 31168-81, scale 1: Ban Tham Seu, a Karen hill tribe village in 15,000, from the Geographical Survey of Thai- western Thailand where both species A and D land. of An. dirus are the major vectors (Green et al. Two larval surveys during August 1989 and 1991). We report here the larval habitat types October 1990 were made around Ban Tham Seu for these 2 species as well as for 2 secondary (Figs. l-3); a single survey was made at an area vectors and another 17 species of Anopheles of wet rice cultivation near Ban Pha De and Mae found in the area. Tao during 1990. Classification oflarval habitats follows the method used by Belkin (1962). All possible bodies of water were sampled MATERIALS AND METHODS even though Anopheles rarely colonize natural Ban Tham Seu is a Karen (Sgaw) village of containers, such as plant axils or stumps. Due to about 200 people, 12 km ESE of Mae Sot the morphological similarity of An. diras species (16%1'N, 98%l'E), near the Myanmar border A and D, identification was made using DNA with Thailand (Fig. 1). The village is compact probes specific for each species (Panyim et al. (Green and is located at about 200 m above sea level in 1988) and enzyme electrophoresis et al. (Cel.) the deciduous woodland of the eastern water- 1991). Species C of Anopheles minimus from A shed of the Moei River, which drains westward commonly can be distinguished species of An. minimus by the presence of a presector pale spot (Green et al. 1990). When specimens IDepartment of Entomology, U.S. Army Medical collected duing 3/z years of sampling were Component, Armed Forces Research Institute of Med- identified by electrophoresis (Green et al. 1990), ical Sciences (AFRIMS), 315/6 Rajvithi Road, Bang- species C had presector pale spots (n : 5) and kok 10400 Thailand. all species A lacked presector pale spots (n : 2 National Malaria Control Programme, Medical Re- 50). search Council, P O. Box 17120, Congella 4013 South Africa. RESULTS AND DISCUSSION 3 Department of Biochemistry, Faculty of Science, Mahidol University, Rama VI Road, Bangkok 10400 A total of 199 bodies of water were sampled: Thailand. 146 were ground waters and 53 were natural

428 429 Decslreen 1995 ANopHELEs Lnnvnl Hnsttnrs N THnILeNo

F o

(no

bt) 430 Jounrtel- oF THE AMERTcnN Mosqurro CoNrnol AssocrerroN

Fig. 2. Map of Ban Tham Seu showing sites of malaria vectors larvae collected, including members of the Maculatus Group.

containers (e.g., bamboo stumps, tree holes and be a vector of human malaria parasites at our stumps, plant axils). All ground water sites con- study site (Green et al. l99l). Three other mem- tuned Anopheles larvae, as did one tree stump. bers of this complex, Anopheles (Cel.) macula- The numbers and types of water bodies in which /us Theobald, An. (Cel.) dravidicus Christo- important Anopheles species were found are phers, and An. (Cel.) sawadwongporni Rattan- shown in Thble 1. The geographic distributions arithikul and Green, were found biting man at of larval habitats of all vectors around Ban the same site but were consistently negative for Tham Seu, together with nonvectors taxonorni- malaria parasites. cally related to Anopheles (Cel.) pseudowillmori In this study An. minimus species A was (Theobald), are shown in Fig. 2. Tlrc distribu- found 18 times in flooded rice fields or stream tions of species A and D of An. dirus are shown margins; this species was collected 2 times from in Fig. 3. Anopheles pseudowillmori, a member ground pools in dry rice fields, next to the of the Maculatus complex was found earlier to streams. Species found together with An. mini- 431 Dscpvsen 1995 ANopHELEs Lenvlr Hegnnts n'l THAILAND

',.r'-.'\i._... - t', A/ I q:a-,--.,.t-.. TO BAN HUA FAI '.li\ ! i

MAE TAO NGAE SAI v-.

o s9.A o sp.D o sp.A+D 0 villoge corf frock :: river woter woy I foot triol I Pqlh, .-\---- 'O-t

TO PHADAENGMINING I * HIGHWAY TO MAE SOT

Fig. 3. Map of Ban Tham Seu showing the distribution of bodies of water where Anopheles diras species A and D were collected. rnas included Anopheles (Cel.) aconitus Doenitz, pools left by floods, pools in drying stream beds, An. (Cel.) annulnris Van der Wulp, An. (Cel.) and margins of streams where water was shallow kochi Doenitz, An. pseudowillmori, An. (Cel.) and stagnant or very slow running. Most of the tessellatus Theobald, An. (Cel.) vagus Doenitz, pools were less than 2 m in width and less than An. (Anopheles) barbirostris Van Der Wulp, An. 30 cm in depth and under partial to heavy shade, (Ano.) bengalensis Pui, and An. (Ano.) pedi- although some were exposed to the sun. A single taeniat us (Leicester). sample (9 larvae) of An. dirus A was collected The larval habitats of An. dirus D, which ap- from a cut tree stump, about 1 m in height, 50 pear to be similar or identical to those of An. cm in diameter, and 15 cm in depth containing dirus A, are reported here for the first time. The muddy water over leaves and plant debris. The major habitats were tracks made by domestic el- ratios of the number of sites positive for either ephants, wheels tracks and cart tracks along species A or D of An. dirus to the total number paths or road, animal wallows, seepage pools or of sites sampled around Ban Tham Seu (45:157) +32 JounNeI- on rgE AugnIceN Mosquno Colqrnol AssocrarroN Vol

Table 1. Larval habitats containing malaria vectors and closely related speciesaround the villages of Ban Tham Seu (TS) and Ban Pha De (PD) and rhe *it.i"" land at Mae Tao (MT), Thailand.

Localities Habitatsr Anopheles species TS PD MT sawadwongporni s00t2lt0 dirus sp. A 16 2 | s5801 dirus sp. D 2030 136400 dirusspp.A+D 930 s1600 minimus sp. A 1224 o2880 maculatus 2022 87540 pseudowillmori 61601390 Total water bodies r57 25 t7 42 3t 4l l7 53 rTypes of bodies of water: 1 : very temporary bodies of water likely to dry out in dry spells during the rainy season, footprints, wheel tracks; 2 : pools likely to remain as long as the rainy season, not associated with streams, wallows; 3 : bodies of water associated with streams, seepages, = pools; 4 rice fields; 5 : natural containers, bamboo stumps, leaf axils, puddles in rocks, tree stumps, etc.

and Ban Pha De (8:25) were similar (X' = 0.06; the 2 species share resources. Whether or not df : l; P > 0.05). These ratios do not differ differences in the microhabitats of these species significantly from those at Mae Tao (1:17; 1, = can be identified, prospects for vector control by 1.95; df = l; P > 0.05). The greater number of means of source reduction of their larval habitats bodies of water positive for species D was un- seem to be extremely poor because both species expected based on the relative abundance of the use small, widely dispersed bodies of water (Fig. 2 species in human bait collections where a ratio 3). of species A to species D of 2:l occurred con- In contrast to An. dirus and An. maculatus, sistently each month over 3 rainy seasons (Rat- which prefer to lay their eggs in temporary hab- tanarithikul et al., unpublished data; Green et al. itats, the strategy of An. minimus and An. pseu- 1990). Two possible explanations for this differ- dowillmori seems to be to oviposit in more sta- ence in larval and adult abundance are: l) spe- ble (semipermanent) habitats in rice fields and cies A concentrates eggs in fewer bodies of wa- streams. Prospects for control of immature ter, or 2) species A is more anthropophilic than stages of An. minimus and An. pseudowillmori species D. would seem to be greater than that for species During the dry season, An. dirus was found that utilize many varied and temporary habitats. in streams, along the banks of pools left after streams flood, or in other seepage areas. When ACI(NOWLEDGMENTS the rainy season started, larval habitats in- creased, particularly in small pools in the soil of We are grateful to Kenneth Linthicum, Chief, paths and roads trodden by men and animals Department of Entomology, U.S. Army Medical [Wilkinson et al. (1978) as Anopheles balaba- Component, AFRIMS; Ronald Rosenberg and censis Baisasl. The occurrence of both species Ronald Ward, Department of Entomology; and in temporary habitats, the survivability of their Daniel Strickman, Department of Rickettsial eggs in soil, and their tendency to oviposit on Diseases, Division of Communicable Diseases damp soil in the wet season (Wilkinson et al. and Immunology, Walter Reed Army Institute of 1978) suggest that An. dirus could be expected Research, for support, encouragement, and crit- to occur around Ban Tham Seu throughout the ically reviewing the manuscript. We are partic- year. The relationships between species A and ularly grateful to the following individuals who D of An. dirus are particularly interesting. Al- assisted in all aspects of the field and laboratory though both species can occur together, species work, Suda Ratanawong and Prasertsri Rohitar- D seems more likely to occur in very temporary atana, AFRIMS, and Porn Sawadwongporn and bodies of water (Habitat l, Table 1) than spe- Udom Kijchalao, Department of Biology, Mah- cies A. idol University. Somprathana Kiewlongya is ac- Further studies of the microhabitat differences knowledged for typing the manuscript and Pra- of species A and D of An. dirus would provide chong Panthusiri for preparing illustrations and useful information on how the larval stases of the final copy of the manuscript. 433 Decerragrn1995 ANqqHELES LARVAL Haetrnrs IN THALAND

REFERENCES CITED Peyton, E. L. 1990. A new classification for the Leu- cosphyrus Group of Anopheles (Cellia). Mosq. Syst. Pa- Belkin, J. N. 1962. The mosquitoes of the South (1989) 2l:r9'l-205. Press, Berkeley and cific, Volume 1. Univ. Calif. Rosenberg, R. 1982. Forest malaria in Bangladesh.- Los Angeles. IIL Breeding habitats of Anopheles dirus. Am- I. R. E Gass, L. E. Munstermann and V. Green. C. A., T!op. Med. Hyg 3l:192-2O1. Baimai. 1990. Population genetic evidence for two Rosenberg, R., R. G. Andre and L. Somchit. 1990. species in Anopheles minimus in Thailand. Med. Highly efficient dry season transmission of malaria Vet. Entomol. 4:25-34. in Thailand. Trans. R. Soc. Trop. Med. Hyg.89:22- A., R. Rattanarithikul, S. Pongparit, P. Sa- Green, C. 28. wadwongporn and V. Baimai. 1991. A newly-rec- Scanlon, J. E. and U. Sandinand. 1965. The distri- ognized vector of human malarial parasites in the bution and biology of Anopheles balabacensis in Oriental Region, Anophele s (Cellia) p seudow illmori (Diptera: J. Med. Entomol. 2: (Theobald, 1910). Trans. R. Soc. Trop. Med. Hyg. Thailand Culicidae). 85:35-36. 6149. Boonyakanist and H. Panyim, S., S. Yasothornsrikul, V. Baimai, R. Rosen- Wilkinson, R. N., D. J. Gould, P berg, R. G. Andre and C. A. Green. 1988. Identi- E. Segal. 1978. Observations on Anopheles bala' fication of isomorphic malaria vectors using a DNA bacensis (Diptera: Culicidae) in Thailand. J. Med. probe. Am. J. Thop. Med. Hyg. 38:4749. Entomol. 14:666-471.