The control of Aedes aegypti during the yellow fever epidemic in Luanda, Angola, in 1971

Bull. Org. mond. Sante 1973, 48, 504-505 Bull. Wld Hlth Org. 1 The control of Aedes aegypti during the yellow fever epidemic in Luanda, Angola, in 1971 H. RuBuRo 1 Abstract Vector control was carried out in three cycles with ULV technical malathion sprayed from aircraft at the application rate of 500 ml/ha. Successive reduc- tions of 84%, 93.6%, and 96% in the pretreatment density of A. aegypti were attained, a range of 77- 98% being observed over a 24-day period. In view of the promising results obtained by Kilpatrick et al. (1970), Lofgren et al. (1970), and Brooks et al. (1970) in the control of yellow fever vectors by ULV aerial spraying of malathion, the same technique was adopted for controlling A. aegypti during the yellow fever epidemic in Luanda in March 1971. Material and methods A single-engined Piper Pawnee aircraft, equipped with 4 Micronair AV 3000 rotary atomizers, was used for the aerial spraying of technical malathion at a rate of 500 ml/ha. Three successive treatments were carried out, beginning on 17 March, 23 March, and 1 April, respectively. As the area to be sprayed was large, each treatment cycle took several days, the spraying being carried out from 06.00 to 08.00 and from 16.00 to 18.00 hours. During the spraying operations, the aircraft was flown at a height of 6-15 m and at a speed of about 150 km/h. The wind velocity was 0-8 km/h, the air temperature 22-250C, and the relative humidity 75-90%. From the examination of a limited number of oil- sensitive cards, it seemed that a satisfactory droplet distribution had been obtained. The size of the droplets was seen to vary greatly, with a mean diameter of about 80 ,um. Bioassays with larvae, pupae, and adults of A. aegypti were performed indoors and outdoors. The adults were exposed in cages made of nylon tulle with about 40 holes per cm2 and the larvae in open dishes with a capacity of 250 ml. Readings 'Assistant Lecturer, Department of Entomology, Insti- tute of Hygiene and Tropical Medicine, Lisbon, Portugal. were made 1 h and 24 h after spraying. The density of A. aegypti was measured only through indoor counts of adults, performed daily from 15 March to 30 April. Readings, expressed in females per man-hour, were based on the numbers of females caught indoors by 3 couples of collectors equipped with aspirator tubes and working from 07.00 to 09.00 hours. During that period, any mosquito land- ing on a human being and seen at rest was caught. About 70% of all A. aegypti females caught in houses from 31 March to 17 April were dissected in order to determine whether they were nulliparous or parous, as well as the proportions at each of Christopher's stages. Results The adult mosquitos in the outdoor cages were dead after 1 h, whereas mortality among the outdoor larvae and among the adults exposed indoors attained 100% only after 24 h. No significant mor- tality was observed among the larvae exposed indoors or among pupae, whether indoors or outdoors. The pretreatment indoor density (5.2 A. aegypti females caught per man-hour) was significantly reduced after each treatment cycle (Fig. 1). Immedi- ately after the first treatment, it fell to 0.83-a reduction of 84%. Following the second and third spraying cycles, respectively, reductions of 93.6% and 96% were attained, corresponding to vector densities of 0.33 and 0.2. The indoor density of A. aegypti was investigated until the end of April-i.e., 24 days after the last treatment cycle (see Fig. 1). During this 24-day observation period, satisfactory levels were main- tained, the fairly low figure of 0.08-corresponding to a 98.4% reduction-being found as late as 29 April. Until 11 days after the completion of the last treatment cycle all the females dissected were nulli- parous. Their age distribution, expressed in percent- ages, according to Christopher's stages, was as follows: (I) 27.1; (II) 24.3; (III) 23.4; (IV) 20.5; and (V) 4.7. 3039 -504 BRIEF COMMUNICATIONS 505 [1f1 [~~~~~~~ TRtEATMENT CYCLES l E 15 17 19 2123 25 2729 31 24 6 81012 1416 1820 2224 26 2830 l ~~~~~March __AprilI 4-- 3 Fig. 1. Aedes aegypti densities. Conclusions This is believed to be the first report on the ULV application of malathion from aircraft for vector control during a yellow fever epidemic. The evolu- tion of the A. aegypti density curve and of the age composition of the local vector population following three such applications suggest that the transmission of yellow fever can be interrupted in a short time by means of this technique. The technique is thus invaluable in situations where the disease has reached epidemic proportions. ACKNOWLEDGEMENTS The author thanks Dr F. L. Sousa Pereira, Public Health Secretary of Angola; the public health authorities of Luanda; and Dr V. M. R. Casaca, Director of the Institute of Public Health, Angola (ISPA), for their co- operation in this study. He is indebted also to the entomology staff of the ISPA, headed by Mr R. Antunes Capela, without whose assistance the necessary field work would not have been possible, and to Mr Pimenta Franca and Mr Jaime Lopes for their valuable advice on spraying equipment and techniques. REFERENCES Brooks, G. D. et al. (1970) Bull. Wld Hlth Org., 42, 37-54 Kilpatrick, J. W. et al. (1970) Bull. Wid Hlth Org., 42, 1-14 Lofgren, C. S. et al. (1970) Bull. Wld Hlth Org., 42, 15-25