A new method of measuring the relative prevalence of Aedes aegypti.

A NEW METHOD OF MEASURING THE RELATIVE PREVALENCE OF AEDES AEGYPTI 467 A New Method of Measuring the Relative Prevalence of Aedes aegypti by P. M. SHEPPARD,a Consultant, W. W. MACDONALD,b Consultant, and R. J. TONN, Project Leader, WHO Aedes Research Unit, Bangkok, Thailand For many years the measure of Aedes aegypti abundance has been the A. aegypti index, i.e., " the ratio, expressed as a percentage, between the num- ber of houses in a limited well-defined area on the premises of which actual breeding-places of Aedes aegypti are found, and the total number of houses examined in that area ".c This index enables com- parisons to be made between different areas, or between different seasons in a single area. To obtain an accurate index, however, large numbers of larvae are required from each container in and around the houses examined, and when the houses contain many larval habitats the time spent on such a con- ventional survey can be considerable; furthermore, the identification of the large number of larvae collected is time-consuming. (If only the A. aegypti index is required, the larvae from a house need only be examined until the first A. aegypti is iden- tified; at this point the remainder of the collections from that house can be discarded. If this is done, however information is lost on the species asso- ciated with A. aegypti and no data are gathered on the number of A. aegypti habitats per house.) The WHO Aedes Research Unit in Thailand recently decided that a survey of A. aegypti through- out the country was desirable, and the question of the most suitable method of larval sampling was carefully considered. It was finally decided to adopt the method in which only 1 larva is collected from each container. Preliminary surveys have shown that in Thailand the larval habitats of A. aegypti can be grouped into 6 main categories: 3 indoors, namely, water jars (1), ant-guards or anti-formicas (2), and miscellaneous containers (3); and 3 outdoors, namely, water jars (4), miscellaneous artificial containers (5), and natural containers, e.g., tree-holes (6). A collector's equipment, therefore, need comprise only 6 num- bered collection-bottles, 1 for each category of con- a Present address: Department of Genetics, University of Liverpool, Liverpool, England. b Present address: Sub-Department of Entomology, Liverpool School of Tropical Medicine, Liverpool, England. c World Health Organization (1966) International sani- tary regulations, 3rd annotated ed., Geneva. tainer. When the collector inspects a house and finds that breeding is taking place in containers, I larva from each container is transferred to the appropriate bottle. A simple record sheet is then completed for each house inspected, showing the number of positive and negative habitats. At the end of the day the number of larvae in the bottles should correspond with the number of positive habi- tats of each kind recorded; this check has proved most valuable where untrained staff have to be employed with limited supervision. In surveys up- country, when the collections cannot be identified on the day of collection, formalin is added to the bottles to kill and preserve the larvae. Larvae col- lected near a laboratory can be utilized for other purposes also, such as studies on insecticide resistance. The table shows the results of surveys in 2 areas in which the single-larva method was compared with the conventional method. The first area, near Nonthaburi, is a typical rural area where a prelimi- nary survey had shown that several species of mosquitos inhabited containers in and around houses. The second is a typical slum area, Soi King Phet, in Bangkok. In both areas, the same houses were surveyed by the 2 methods. The rural surveys were made 4 days apart and during the interval there was heavy rainfall; this may have been one of the reasons for the differences between the number of containers with water and larvae in the 2 surveys. The urban surveys were made on the same day and the slight differences may have been the fault of the collectors. The information provided by the 2 methods is not very different. The conventional method, in which a sample of larvae is collected from every habitat, gives the proportion of houses with A. aegypti breeding (the A. aegypti index) and a more accurate estimate of the number of A. aegypti habitats per house; the single-larva method shows more accurately the prevalence of A. aegypti relative to other species and the minimum number of A. aegypti habitats per house. The data could, of course, be analysed further to show the relative prevalence of each species in indoor and in outdoor 2314c 10 NOTES COMPARISON OF THE RESULTS FROM SINGLE-LARVA AND CONVENTIONAL AEDES AEGYPTI SURVEYS Average - No. of Total no. of containers no. of Relative S houses examined containers prevalenceE as per house Man-hours Are~~~~~~~ an-ehouioE< | lo<,iE||¢0lrsa:M 0-Survey 0 0 08 spent coi-Area CD 3 etn nmethod 0 &. (Nonthaburi) ~ ~~ :Covntoa 80 identifying 0 6532 . 077 003 019 9 U0 0 .0 0 10 o o o5 0 0 1.4 larvae :2 E : E0 0 ) 0 CL. .I C .- C) C ~ 0 -. 0(0 Rural Single-larva 80 68 - 881 291 256 5 30 3.6 3.2 0.88 0.02 0.10 33 (Nonthaburi) Conventional 80 64 62 926 257 211 20 65 3.2 2.6 0.77 0.03 0.19 92 Urban Single-larva 40 27 27 176 57 57 0 0 1.4 1.4 1.0 0 0 19 (Bangkok) Conventional 40 27 27 197 55 55 0 0 1.4 1.4 1.0 0 0 39 a These included -several species of Culex, Armigeres and Toxorhynchites. habitats and in different kinds of container. In rela- tion to the time spent on collecting and identifying larvae, the single-larva method is much more efficient. Surveys based on the single-larva method yield information that is sufficient for most purposes on the principal habitats available and the proportion that are occupied. The method, as compared with those in which samples of, say, 10 or 20 larvae are taken, is advantageous in that many more houses can be inspected and many more habitats can be sampled; a less biased estimate of the frequency of occurrence of each container-breeding species is obtained; the larvae can be used to measure the frequency of any character under study, e.g., insec- ticide resistance; more accurate standard errors can be attached to the data than are possible with arbitrary numbers of larvae; and the accuracy of the collectors can be checked from the data. Repli- cate surveys can be made at different seasons of the year, and the collections may reflect seasonal differences in population densities. On the other hand, with the single-larva method it is possible that a number of A. aegypti habitats may fail to be recorded as such when another species shares the habitat; and no information is gathered on the coexistence or association of different species, such as the association of A. aegypti and A. albo- pictus. In other words, the frequency of species would be shown but not the frequency of containers positive for a particular species. To obtain data on container frequency a more intensive survey would be required, involving large samples of larvae from each container. 468