Bull. Org. mond. Sant} 1973, 48, 615-622 Bull. Wid Hith Org f Breeding places and seasonal incidence of Aedes aegypti, as assessed by the single-larva survey method* T. RAMACHANDRA RAO,1 M. TRPIS,2 J. D. GILLETT,' C. TEESDALE,' & R. J. TONN 3 The single-larva survey method was employed to study the breeding places and seasonal incidence of Aedes aegypti in Dar es Salaam, Tanzania. From May 1968 to May 1969, 28462 containers of water-located in approximately equal numbers indoors and out- doors-were investigated. The highest frequency of breeding (8.0 %) of A. aegypti was observed in tires and motor parts. Drums, barrels, water-pots, and other receptacles left outdoors showed a higher frequency (3.1 %) than those kept indoors (0.6 %). Metal containers were infested to a greater extent than those made of mud, wood, or other materials; 2.5 % of coconut shells, snail shells, etc. and 1.3 % of tree holes, plant axils, and cut bamboos were infested. The seasonal prevalence, expressed as a container index, closely followed andparalleled the fluctuations in rainfall. The value of this survey method for both ecological studies and practical control purposes is discussed. The city of Dar es Salaam (longitude: 390 12' E; are 35.20C and 12.80C. The relative humidity ranges latitude: 60 53' S), with a population of approxi- daily between a low of54-74% and a high of 94-97 %. mately 275 000 and a total area of about 830 ha, Except in the commercial and thickly populated is situated on the coast of Tanzania. Dar es Salaam central areas, the city is characterized by the pres- is an important seaport and has a busy international ence ofnumerous trees. Coconut and mango, cashew- airport. It is the seat of the Government of Tanzania nut, jackfruit, and other tropical trees are very com- and the commercial and industrial centre of the mon. The grass is lush and tall and has to be kept country. mown. Shrubs of various types flourish wherever they The hot, humid climate of Dar es Salaam is typi- are allowed to grow. Sisal is cultivated in one or cal of coastal areas in the tropics. The average two districts on the periphery of the city. annual rainfall is 1 096 mm in 108 days of rain There are three main types of housing: (1) large (Table 1), and there are two main seasons of rain- multi-storey buildings in the commercial and residen- fall: the so-called short rains during November and tial areas of central Dar es Salaam; (2) bungalows, December and the long rains during March, April, each located within its own compound. These vary and May. There is some rain in every month of the from small to fairly large buildings and are character- year, but the heaviest precipitation occurs in April. istic of the northern and north-western parts of the The mean maximum temperature ranges between city, including areas such as Oyster Bay; (3) a new 28.6°C (July) and 32.4°C (March), and the mean type of single-storey residential building in the more minimum between 18.1°C (August) and 23.5°C (Janu- recent extensions of the city. The population density ary). Extreme temperatures on any day of the year is highest in the first type of housing and lowest in the second type. Buildings of the last type are usually * From the East Africa Aedes Research Unit, Dar es occupied by more than one family. Salaam, Tanzania. This study was supported jointly by US Public Health Service research grant No. CC 00174 from Abundant and continuous piped water is supplied the National Communicable Disease Centre, Atlanta, Ga., by the city authorities. Most buildings have their USA, and the World Health Organization. own water taps. It is only in some of the most out- I Acting project leader. 2 Ecologist. lying areas that water has to be brought from distant 3Project leader. public taps. Wells are extremely rare. 3057 --615- 616 T. RAMACHANDRA RAO ET AL. co 0 As the water supply is excellent, the practice of E .tco water for domestic purposes -C wE storing for long periods 0) n is not prevalent. Because of the satisfactory pressure C- 0E of the water, even overhead tanks are not as common 2 NNN_-__-_NNNNN 0 as in other large cities. Among the poorer members 10 Ur-N-000! ILO N N' r-0 0 of the community, who are without their own water 0 00 0 0 >. co C (-010( 4- co C') co) 1co0 C'w ( 0 taps, water is stored for short periods in pots or c 0 V)C') v-r-CD0000)0L IOCDC') (%t 0)0LOCON- v D000W metal drums rather than in the large earthenware 'nN 00000NNN___NN'-m"ON (010(0 0 0 jars that are commonly used in other parts of Africa. t, CT) 0 0'- W Nu- 001NU) N N O 0 CD to C9e0 0b0c .CO a O 0)0 METHODS 0) (O 0o C, Single-larva collections according to the technique 0-N0CO)00- CN100v0LOC) r- 0 C) ID developed by Sheppard et al. (1969) were made *0o - C w- 1% 0) C') N)01(000- in 15 districts of the city (Fig. 1). Houses C0 co 00- (0N 0 ., periodically CM N 'M~C')N N N NNNN C and surrounding premises were examined. The sur- N W vey consisted essentially in (a) searching for water 0f 0 00 N C') N NO N N- N NO C') NO CO) C)C' 0 C'n N .Ca 0 b V) C, C')LoLN 0NOLOc4O - - N1- L101O I*00 cC' o,0, NT- LU E 0 00m Z4 co 0) CL 0.- _ C'f) C')m 0- 0 0 N(C')0N N0L(0'- it 0 0 E . to 0 I._ ._ 0) co -J0 leNLO W N- 0)11-co0P- P% LO 0)C, E 0) 0 v014t0- (0Iteq00)1 000 '-0v co 0 0 N co N 2 0(0( (0(0r-L01010(0(0 C con I- 1% 0) Ni 61C')C')4 C')4Ni(000 0 0) 0 0, 0 0' E 0 C, C, I.0 S 00 C._ 0 0 ._ N' 00 0 C. U10(0DeON00"t(0U-0)-r%1-(-qt0 1% 1%(0(14 r CO 10 0))0 0)0)ir:a0) 0)00 0)0)i0):- 0 0 0 -( S_ E x 0-<6orvoo,6o.->xO N(00 NO C')-_%1-w-C 1 C 0 0 S CD C ItCOC' C' C C' r'C')C' C x 0 0 0 cn c. wco N E 10U C-' 0CN N a-0)00 00000) N 0 C, 0 0 ED E 0 cx 0e ~0Q C') C'C)C' C)NCN NC')C')C')C' c )@ co as Co _J (D E LOCDLOLOM____MMM 00 0 SE w- 0) % ON - C)- 0N 1000 "-0 (A cr ni06 a000 CD0, 0 10 0 0. +-x C E w r C4 0O a 040404CoCo N 04 00a.- i- .5 1- D >C1 D (0a- c > C, 0 0D 0 0- 2 M's 0 ED E Fig. 1. Districts of Dar es Salaam searched for breeding places of A. aegypti by the single-larva survey method. SINGLE-LARVA SURVEY METHOD 617 Table 2. Surveys of water-filled containers for A. aegypti larvae, Dar es Salaam, 1968-1971 Period containers Automobile parts Natural cavities Indoor containers Total No. % No. % No. % No. % No. % May 1968 54/439 12.3 5/139 3.6 17/359 4.8 45/542 7.6 121/1 479 7.4 Aug.-Oct. 1968 37/1 818 2.0 8/163 4.9 1/64 0.2 7/3 993 0.2 53/6 038 0.8 Nov. 1 968-Jan. 1969 135/5 440 2.5 55/707 7.8 12/771 1.6 20/7 659 0.3 222/1 457 1.5 April 1969 47/1 130 4.2 16/113 14.2 11/266 4.1 4/1 150 0.3 78/2 656 2.9 May 1969 35/1 038 3.4 18/117 15.4 18/362 5.0 15/1 360 1.1 86/2 877 3.0 June 1969 17/783 2.2 10/109 9.2 2/117 1.7 4/993 0.4 33/2 002 1.6 August 1969 12/681 1.8 5/95 5.5 3/206 1.5 2/658 0.3 22/1 640 1.3 December 1969 6/243 2.5 3/44 6.8 1/12 8.3 0/447 0 10/746 1.3 December 1970 5/762 0.7 0/42 0 0/48 0 0/507 0 5/1 653 0.3 Feb.-Mar. 1971 45/13 233 0.3 3/243 1.2 4/484 0.8 3/6 664 0.05 55/20 614 0.3 May 1971 92/3 066 3.0 4/42 9.5 4/453 0.9 31/4 441 0.7 131/8 002 1.6 July 1971 15/2 556 0.6 5/127 3.9 0/292 0 2/3 923 0.1 29/6 898 0.4 containers, counting them, and classifying them When assessments were commenced in May 1968, according to their type and location (indoors or they served only as a means of rapid reconnaissance. outdoors); (b) examining the containers for the pres- As experience was gained, the survey techniques were ence of A.