Conducted in Bangkok-Thonburi, Thailand, in 1969*

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27 OT One-Larva-per-ontainer Mosquito Surveys onduted in Bangkok-Thonburi, Thailand, in 1969* by ROBRT J. To 1 & Y. H. BAG2 One-larva-per-ntainer surveys have been nduted in Bangkok sine 1966.

1 27 OT One-Larva-per-ontainer Mosquito Surveys onduted in Bangkok-Thonburi, Thailand, in 1969* by ROBRT J. To 1 & Y. H. BAG2 One-larva-per-ntainer surveys have been nduted in Bangkok sine Tonn et al. (1969) reported the results of surveys made during the three limati seasons of 1967 at 14 sites in Bangkok-Thonburi. In January and February 1969, the surveys were repeated by the WHO Aedes Researh Unit at 13 of the original sites; in this report, the results of these surveys are mpared with those obtained in the surveys that were made in Deember 1966 and in January and February Methods and materials Study sites. In the original surveys, an attempt was made to selet a variety of sites so that the total results would be representative of all Greater Bangkok-Thonburi. Shop-house, slum, low-inme, middle-inme, and high-inme areas were hosen in northern, southern, and entral areas, and a suburban garden area in Thonburi was also inluded. The areas were desribed by Tonn et al. (1969) and a summary is given in Table 1. Makkasan was not inluded in the present study sine a pilot insetiide projet was being arried out there. olletion methods. The 1-larva-per-ntainer survey method used in these surveys was desribed in detail by Sheppard et al. (1969). This method entails the lletion of a single larva at random from eah ntainer in whih there are larvae. The larvae were plaed in lletion bottles numbered to rrespond with the prinipal breeding habitats (indoor water jars, misellaneous indoor ntainers, ant traps, outdoor water jars, misellaneous outdoor ntainers, and natural habitats. All the larvae were found to be Aedes aegypti. The information gathered from the surveys inluded a premises index, a positive ntainer index, * This study was supported by Publi Health Servie researh grant o. 261 from the enter for Disease ontrol, Atlanta, Georgia, USA. 1 Formerly Projet Leader, WHO Aedes Researh Unit, Bangkok, Thailand. Present address: Projet Leader, WHO ast Afria Aedes Researh Unit, Dar es Salaam, Tanzania. 2WHO ast Afria Aedes Researh Unit, Dar es Salaam, Tanzania Table 1. List of study areas in Bangkok and Thonburi Bangkok Sam Sen Tha Tian Area f Desription shop-house market near hao Phraya shop-house-river port market on hao Phraya River 22 July irle shop-house, not near river or klong Soi Aree high-inme, single-dwelling Soi high-inme, single-dwelling Huay Kwang government projet Klong Toei, Abattoir Soi King Phet Bang Pho slums near Port-Authority offie low-inme to slum northern low- to middle-inme Krung Thep Bridge southern low- to middle-inme Thonburi Bang Plat northern suburban residential area, inluding shop-houses Phran ok Road shop-house and slum Along Me Klong urban low- to upper-middle-inme and data on the kinds of ntainer and the proportion of eah kind present at the various sites. Sine all the mosquitos lleted were Ae. aegypti, the indies are an expression of the Ae. aegypti population. The results of the latest surveys were mpared statistially with those of similar surveys mpleted during the ol season of In both series of surveys, 1 houses were sampled. Results A summary of the results of the two series of surveys is given in Table 2. The information is mpared by means of a statistial analysis of data from

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272 OTS the 1966-67 and 1969 surveys nduted at eah site and of the average data for all surveys arried out during eah series of surveys. Premises index.

3 272 OTS the and 1969 surveys nduted at eah site and of the average data for all surveys arried out during eah series of surveys. Premises index. The average proportion of houses with breeding populations of Ae. aegypti (premises index) was slightly lower in 1969 than in (64.8% against 76.6 %). Only two sites, Sam Sen market and Soi 49-55, showed a slight inrease. The premises index was lower at all the other sites, with Tha Tian market (45 % against 2%) and Soi Aree (92 % against 69 %) exhibiting the greatest redution. Total number of ntainers holding water. Despite the premises index being lower in 1969 than in 1967, the atual number of potential larval habitats (waterof ntainers inspeted. Among the 6 sub-divisions sampled, the greatest inrease was in the total number of ant traps, 11 of the 13 sites showing an inrease. ontainer index. In 1969, the average number of water-filled ntainers per house varied from 11. at Me Klong to 3.5 at Tha Tian market and the average number of Ae. aegypti-positive ntainers per house varied from 3.9 at Krung Thep bridge to.4 at Tha Tian. Of the ategories listed in Table 3, water jars were the most important habitat (3% of a total of 4 8 jars ntained larvae). There were rather more jars indoors (2 684) than outdoors (2 15), but a slightly higher perentage of those outdoors were positive. There were about half as many ant traps Table 3. Average number of different types of larval habitat per 1 premises season season ontainers I ~~~~~~~~~~~~Perentage ners Average no. % Average no. % hange Total no. of ntainers Water jars: indoor outdoor total Ant traps Misellaneous ntainers: indoor outdoor total filled ntainers) inreased during this period. Table 3 shows that the average number of suh ntainers inreased by 2.4% in the period between the first and send surveys. As an be seen in Table 2, this slight inrease in the total number of water-filled ntainers was not statistially signifiant. The number of ant traps inreased by about 31 %, and that of water jars dereased by about 11 %. However, water jars still represent about 6 % of the Ae. aegypti larval habitats. Misellaneous ntainers inreased in number by about 4 %; the more numerous, smaller misellaneous ntainers may reflet a greater degree of urbanization in Bangkok-Thonburi. Altogether, 8 of the 13 sites showed an inrease in the total number (2 636) as there were water jars, but the perentage oupied by Ae. aegypti was muh higher (about 4%). Out of a total of 8 21 ntainers, (92.6%) were water jars or ant traps; similarly, of ntainers positive for larvae, 2 47 (93.3 %) fell in the ategory of water jars or ant traps. Again, 33.3 % of water jars and ant traps mbined were positive for Ae. aegypti, i.e., virtually the same proportion as the ntainer index for all types of reeptale (32.8 %). Statistial analysis. Table 2 shows the results of a paired mparison of the two surveys. To derive t values for assessing the signifiane of differenes be-

MOSQUITO SURVYS I BAGKOK, THAILAD, 1969 273 tween the years, individual differenes for eah loality were mpared with the mean differene for all loalities and the varianes and standard errors were

4 MOSQUITO SURVYS I BAGKOK, THAILAD, tween the years, individual differenes for eah loality were mpared with the mean differene for all loalities and the varianes and standard errors were determined; the t value is the mean differene divided by its standard error. This proedure, whih is the rret one when surveys have been arried out loality by loality, indiates signifiant differenes between the 1967 and 1969 surveys for 6 of the 1 quantitative measurements listed in Table 2. If unpaired mparisons are made, thereby adding the variation from one loality to another, these differenes disappear. For example, the standard error (of the differene between the means) for the total number of positive water jars bemes 3.19 and thus the t value (for 12 degrees of freedom) is This value is below signifiane at the.5 probability level. If an analysis of variane is performed on the total number of positive water jars, it is found that whereas between years the F1,12 is 9.66, whih is just signifiant at the.1 level, the F12,12 between loalities is 7.3, signifiant at the.1 level. The analysis therefore indiates that the differenes in indies for the 13 sites (owing to environment) were greater than the total differene between the 1967 and the 1969 surveys. In other words, even though there were signifiant differenes between the results of the two surveys, these differenes were not as great as those between the individual areas surveyed. Disussion The 1-larva-per-ntainer method developed for larval mosquito surveys provided an exellent mparison of kinds of habitat oupied and amount of breeding between different survey areas, or the same area surveyed at different times. The two series of surveys desribed here were nduted to illustrate that suh mparisons an be made. In the 2-year period between the two series of surveys, many hanges ourred; entire setions of slum areas disappeared and others appeared, arding to population movements. This trend applies also, though to a smaller extent, to the dwellings of the various inme groups. However, the general features noted in the send survey were idential with those found in the first survey. For example, the hinese shop-house areas of Tha Tian and 22 July irle have fewer ntainers and, overall, fewer mosquito breeding plaes than do other areas. This is a result of the nature of the markets, i.e., they nsist, basially, of rowded nrete slabs on whih the inhabitants do not have enough room to store large amounts of water. Another example is the high-inme areas of Soi Aree and Soi These areas are inhabited mainly by foreigners, and a surprising amount of mosquito breeding ours there. The surveys reveal that the servants, who maintain water jars and ant traps in their own quarters, are largely responsible for this situation. Both the premises index and the number of positive ntainers dereased signifiantly in the period between and At the same time, the number of water-filled ntainers found and examined in 1969 was slightly, but not signifiantly, higher. Of the various types of ntainer, the inrease was in the average number of ant traps (47.7), while there was an almost equally large derease (45.8) in the average number of water jars. This means that the majority of new ntainers in the areas surveyed in Bangkok were small misellaneous ntainers. It must be remembered that hanges in the premises index (whih an result from the presene of a single positive ntainer in formerly negative premises) and hanges in the number of potential larval habitats are unlikely to have muh bearing on the amount of mosquito breeding or the size of the mosquito population. Breeding within the esystem is determined, in part, by the number of ntainers atually serving as larval habitats and by the potential of eah positive ntainer to produe adult mosquitos. For example, a positive water jar produes, as a rule, more adults than a positive tin an, espeially over a long period. Thus tin ans, broken jars, et., that hold water temporarily will inrease the ntainer unt of an area, but may ause a derease in the proportion of positive ntainers, i.e., the ntainer index. At the same time, this hange may have little influene on the atual mosquito population within the survey area. Other studies made by the WHO Aedes Researh Unit do not substantiate the ourrene of a general redution in the Ae. aegypti population in Bangkok in the 2 years between the surveys. Therefore, muh emphasis should not be plaed on the premises index or on the number of positive ntainers when populations are mpared, unless the type of ntainer and its larval produtive apaity are known. There are a number of fators that uld aunt for mosquito population hanges in the survey areas. The elogy of speifi premises, and to some extent that of the entire area, hanges eah time premises are oupied by a new resident. ew ntainers replae old ones and different nepts of leanliness may influene the number of misellaneous ntainers in the environment. The same houses in a

274 OTS survey area were not neessarily visited by the same lletor during eah survey, and differenes our both in the type of premises and in the effiieny of the lletor.

5 274 OTS survey area were not neessarily visited by the same lletor during eah survey, and differenes our both in the type of premises and in the effiieny of the lletor. Finally, the 1969 survey was nduted during the ol dry season, a limati period when a redution of Ae. aegypti ativity is most likely to our, whereas the survey vered all three seasons. Although these fators annot lead to the nlusion that there was a real redution in the numbers of Ae. aegypti from 1966 to 1969, they do illustrate that nditions playing no atual role in determining the size of a mosquito population are apable of produing a signifiant differene in survey results. This should not, however, reflet adversely on the value of 1-larva-per-ntainer surveys as a means of assessing Ae. aegypti populations in a speifi area, even when the surveys are ntinued over a long period. The differene between the results of the two surveys was less signifiant than the differenes in individual areas during eah survey. Signifiant differenes between the two surveys were apparent in the premises index, the total number of positive ntainers, and the numbers of ant traps and positive water jars. H6wever, these differenes were smaller than those between a shop-house and a slum area examined during the same survey. Thus surveys an perhaps be used to measure small hanges in an esystem over a period of years but any suh measurements must take into nsideration fators of human population suh as hanges in enomi status and demands for mosquito ntrol within the area, as well as limati and other non-human environmental fators that might influene the numbers of mosquitos. RFRS Sheppard, P. M., Madonald, W. W. & Tonn, R. J. (1969) Bull. Wid Hlth Org., 4, Tonn, R. J., Sheppard, P. M., Madonald, W. W. & Bang, Y. H. (1969) Bull. Wid Hlth Org., 4,

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