Vl. 10, N. 1. SINGAPORE MEDICAL JOURNAL 57 March, 1969. A STUDY ON ANOPHELES MACULATUS AND ANOPHELES SUNDAICUS IN SINGAPORE By K. L. Chan (Entmlgist, Vectr Cntrl Unit, Ministry f Health, Singapre.) INTRODUCTION There are abut 200 t 300 cases f malaria detected yearly in Singapre. Almst all these are imprted frm utside but because f the cntinued presence f the vectrs Anpheles maculatus and An. sundaicus, transmissin is still pssible, especially when there is a breakdwn in cntrl services in ptentially and entmlgically dangerus areas n the island. Singapre may be cnsidered as having reached the cnslidatin-maintenance phase in a malaria eradicatin prgramme althugh n such eradicatin prgramme has ever been carried ut. This is a reprt f sme entmlgical findings in a six mnths' study (10 March t 8 September 1966) at the Jalan Kayu area in Singapre. The bjective f the study was t develp a mre ecnmical and effective methd f cntrlling the vectrs. THE STUDY AREA The study area, Jalan Kayu (Map 1), is lcated in the nrth-east f Singapre and suth f the Ryal Air Frce Seletar Air Base. Histrically mre than half f the area t the nrth had never been cntrlled s that existing cnditins were natural and entmlgically pssibly dangerus. Gegraphically, it is situated near the cast where the tw vectrs were knwn t be present. Surveys made in the area frm 28 February t 3 March 1966, revealed 22 brackish fish pnds breeding An. sundaicus and fur seepages breeding An. maculatus. Of these, nly ne habitat f each species were lcated within the iling bundary (Map 1). METHODS Densities f bth species in the immature and adult stages were measured, immature by sampling f a fixed vlume and surface area f water in fur suitably chsen habitats, A,B,C and D (Map 1) and adult by adult trapping using human bait traps placed near the crrespnding breeding habitats (Map 1). Immature Stages Sampling was dne twice a week, n Tuesdays and Saturdays. The sample size used was arbitrarily fixed at 270 cc., the size f a standard ladle used fr larval cllectins. Sampling invlved cllecting 10 randm samples frm each f the fur statins. T reduce errrs due t sampling, the same cllectr, using the same cllecting technique, was used thrughut the study. All stages, eggs t pupae, were cllected and cunted in the labratry except frm 10 March t 10 May when nly larval and pupal stages were examined and cunted. Adult Human bait traps mdified frm the "Malayan type" as in Clless (1959) were used fr assessing density. Tw men per trap were used, ne acting as bait with bare bdy and legs and the ther acting as msquit catcher. Catching was dne nce a week at the same time (7.00-11.30 p.m.) and day (Wednesday) at all the statins. Meterlgical readings (temperature, relative humidity, wind) were taken cncurrently with adult catches. Rainfall readings were btained frm the R.A.F. at Seletar which is adjacent t the study area. RESULTS AND INTERPRETATION Anpheles maculatus Fig. 1 shws adult and larval densities during the study perid. Larval density was at a steady lw thrughut the six mnths while adult density shwed ne peak in April. N clse crrelatin between the tw was fund, due prbably t adult females shwing less preference fr human bld frm May nwards in the presence f mre preferred animal hsts during the succeeding mnths. N clear crrelatin with rainfall was fund. Anpheles sundaicus Fig. 2 shws An. sundaicus larval ppulatins in the three pnds A, B and C and adult density at C. Larval density was highest in July at pnds
M A P 1 STUDY AREA REFERENCE An. sundaicus X An. maculatus O Human bait traps C) Cllecting statins Scale: 1 : 30,000 (apprx.) MAP 1. LARVAL HABITATS AND COLLECTING STATIONS GO
MARCH, 1969 59 1.0 0.0 300 N. Immature stages per 10 0 Samples t 10 3.50 3.39 Mar Apr May Jun Jul Aug Sep STUDY PERIOD 1966 Fig. 1. An maculatus adult and larval densities at statin D A and B and in May at C, thus shwing that there was n real seasnal peridicity in the ppulatin as a whle in the area except that higher densities in all the pnds ccurred between the perid end f April and end f July. Larval ppulatins in each pnd were fund t be related t pnd cnditins rather than rainfall althugh ppulatin peaks in all three pnds cincided with heavier rainfall in May and July (Fig. 2). This is s as all the pnds were never dry during the study perid and rainfall culd nt have affected the availability f water in the pnds except t lwer their salinity. Observatins n pnd cnditins accrding t the manner f fish and prawn rearing by farmers in the area shwed that all pnds fllwed the same cycle pattern f changes. Disused, neglected pnds after fish rearing usually cntained little water and sme vegeta - tin typically Entrrnrpha species. These usually bred prfuse numbers f msquites, typically Culex sitiens and Anpheles sundaicus, the frmer usually ccurring in higher numbers and usually preceding the latter, with sme verlap. When the pnds were again utilized fr fish rearing usually planned t cincide their harvesting with Chinese New Year, they wuld be dug t the desired depth and filled with brackish water thrugh sleuce gates which cnnect the pnds t rivers. At this time, the msquit ppulatins wuld fall smewhat. Fishes, and smetimes prawns, were then intrduced and tgether with them the vegetatin and fd they feed n. When this happened, the msquit ppulatins wuld fall ff t a lw minimum r disappeared cmpletely. The cnditins f the pnds became rather stable during the fish rearing perid and cntinued t be s until the fish and prawns were harvested
60 SINGAPORE MEDICAL JOURNAL Adult 2 2000 62 0 1000 C 3.50 3.39 2014 2481 4000 m mm +1a), `n 3000 a >~ai O c 0 2000 r1 1165 P G 646 H. 2825 6 B H t+. P4 1000 0 L 520 168 2 1538 26 0 A Mar Apr May Jun Jul Aug Sep STUDY PERIOD 1966 Fig. 2. Añ sundaicus adult and larval densities at Jalan Kayu, Singapre. T STATI ONS (PONDS)
MARCH, 1969 61.i. -1 6 8 zyti gág O O í 07 aj Q) 0) + E E ri N P 0 4{ 8 N a 1, ri 1 ó Om r0. 1.p A Ó uew/ N) I.,1ISN21 6 ZIIIQtl (V O (sui) ZTtl3NItl2i
TABLE I PAROUS RATE OF AN. SUNDAICUS AT STATION C BASED ON OVARY DISSECTIONS Date Week N. parns N. with Ttal stmach cysts N. with sprzites March 10 1 1 1 17 & 18 2 1 2 26 & 27 3 1 2 April 2 4 4 10 7 5 7 15 6 3 14 22 7 1 1 28 8 5 9 May 5 9 3 9 12 10 2 19 11 2 5 26 12 6 21 June 2 13 8 14 15 15 2 2 June 22- - July 27 16-21 - August 3 22-1 August 10- Sept. 7 23-27 TOTAL 29 86 Overall parus rate = 29/86 = 0.337 'p' (3 -day cycle) = 31/ 0.337 = 0.696 Prbability f survival thrugh ne day = 69.6% Daily mrtality = 30.4% TABLE II PAROUS RATE OF AN. MACULATUS AT STATION D BASED ON OVARY DISSECTIONS Date Week N.N. parus N. with wi Ttal stmach sprzites cysts March 10 17 & 18 26 & 27 1 2 3 5 April 2 4 2 7 5-2 15 6 22 7 28 8 3 3 May 5 9 1 May 12- July 27 10-21 - - August 3 22 1 1 - August 10- Sept. 7 23-27 - TOTAL 4 14 Overall parus rate = 4/14 = 0.286 'p' (3 -day cycle) = 31/ 0.286 = 0.6588 Prbability f survival thrugh ne day = 65.9% Daily mrtality = 34.1%
MARCH, 1969 63 when, nce again, the cycle f changes wuld be repeated. Adult density at statins A and B was very lw due likely t deviatin f adults frm the human baits t a herd f cattle stabled clse t the trapping sites. Breakdwn f sundaicus larval and adult densities at statin C (Fig. 3) shws clse relatinship between the tw with larval density preceding the adult as expected. There was n relatinship between adult density and rainfall and als between adult density and relative humidity. There was hwever a crrelatin between adult density and daily mean temperature (crrelatin cefficient 0.399) during April and May when the ppulatin was highest. This culd be due t a keener chemreceptive sense f the msquites at higher temperatures when the human hst perspired and attracted them mre. As the air was still n almst all the nights f catch, it was nt pssible t determine whether wind affected adult biting density. Parns Rate and Prbability f Survival Tables I and 2 shw the parus rate and prbability f survival thrugh ne day respectively f Anpheles sundaicus and An. maculatus. The verall parus rate fr the whle study perid was 0.337 fr sundaicus and 0.286 fr maculatus, giving respectively 0.696 and 0.659 fr the value f 'p' (prbability f survival thrugh ne day) based n a 3 -day gntrphic cycle nrmal fr trpical climatic cnditins (Detinva, 1965). Dissectins f the gut and salivary glands f females revealed the absence f stmach cysts and sprzites, thus indicating lw r absence f malaria parasite reservir in the area. DISCUSSION AND CONCLUSION Density -wise, Anpheles maculatus can be eliminated as an imprtant malaria vectr in the Jalan Kayu area since there were nly fur small larval habitats and the ttal ppulatin was lw as indicated by bth adult and larval densities. Anpheles sundaicus, n the ther hand, pses a number f questins, but n basis f the lack f cysts and sprzites, can be cnsidered as nt imprtant in the Jalan Kayu area. Unlike An. maculatus which is a mre efficient vectr under natural cnditins (Sandsham, 1965), An. sundaicus requires high densities t cause epidemics (Clless, 1952). Its lw sprzite rate in nature and its lw attractin t human bld (Reid, 1961) makes it epidemilgically unimprtant at lw ppulatins. In the Jalan Kayu area, its ppulatin numbers were significantly high but the numbers caught at human bait were cnsiderably lw by prprtin, indicating that it was nt much attracted t man fr bld. Its capacity as a vectr wuld therefre be lw. The absence f sprzites and cysts indicates that either few adult females reached the critical age f 10 days r mre fr malaria transmissin t ccur, r mre likely that there was insufficient parasite reservir in the area althugh frm time t time malaria cases were reprted in the area. This study shws that t cntrl An. sundaicus breeding in brackish fish pnds in cstal areas f Singapre, it is necessary t knw the histry and cnditin f such pnds. Cntrl shuld be carried ut nly when the pnds are breeding the msquit. This wuld nt nly be ecnmical but culd als be dne easily and effectively. Infrmatin n the stage f develpment f pnds culd be btained frm the farmers thrugh sme kind f reprting system. ACKNOWLEDGEMENTS I wish t thank Prf. M.J. Clburne f the Department f Scial Medicine and Public Health, University f Singapre, fr prviding labratry and transprt facilities at the Institute f Health, Dr. D.H. Clless f C.S.I.R.O., Canberra, Australia, and Dr. J.A. Reid f the British Museum, Lndn, fr the supply f valuable literature. I am grateful t Dr. K. Kanagaratnam, Deputy Directr f Medical Services, Ministry f Health, fr permissin t publish. REFERENCES 1. Clless, D.H. (1952): "Observatins n the peridicity f natural infectins in the Anpheline msquites f Brne", Med. J. Malaya, 6: 234-240. 2. Clless, D.H. (1959): "Ntes n the Culicine Msquites f Singapre. VI. Observatins n catches made with baited and unbaited trap -nets", Ann. trp. Med. Parasit., 53:251-258. 3. Detinva, T.S. (1965): "Age -gruping methds in Diptera f medical imprtance with special reference t sme vectrs f malaria", Wld Hlth Org. Mngraph Series, 47:216 pp. 4. Reid, J.A. (1961): "The attractin f msquites by human r animal baits in relatin t the transmissin f disease", Bull. ent. Res., 52:43-62. 5. Sandsham, A.A. (1965): "Malarilgy with special reference t Malaya", Univ. Malaya Press, xvi + 349 pp.