Mosquito Control
p. 91 · Read in the full issue
Successful Cmpaign In Guiana
THE medical authorities in British Guiana have had remarkable suc cess with DDT in getting control of disease-carrying mosquitoes.
Control of malaria by drainage, and oil ing of stagnant water, were tried in 1934, but the task was too big in that territory. Early in 1945, two experts made experiments with a spray of 5 per cent.
DDT solution in kerosene, and had pheno menal success in removing the carrier of malaria, Anopheles darlingi, from houses.
Since then, the campaign has been so de veloped that the mosquito seems to be under control in most of the towns and settlements. The following are extracts from an article in the May issue of “Corona,” the new journal of the British Colonial Office.
In all, 325,000 people, representing over 90 per cent, of the population of British Guiana, are enjoying the benefits of DDT protection. Anopheles darlingi cannot now be found in districts which have been under treatment for two or three years.
Malaria parasite rates and spleen rates have been reduced almost to nil. On one typical estate parasite rates have been reduced from 40 per cent, to 3 per cent., and spleen rates from 70 per cent, to 4 per cent, in three years. In two of the sub urbs of Georgetown parasite rates have gone down from 55 per cent, to nil, and spleen rates from 44 per cent, to 1.5 per cent. Here the birthrate has increased to about twice its pre-DDT rate. The ratio of births to deaths has increased bv over 100 per cent., and the infant mor tality rate has fallen by about four-fifths.
Spraying is carried out by teams equipped with stirrup pumps fitted with special spray extensions and nozzles. The smallest unit is a three-man team, in which two men carry out the spraying while the third operates the pump. The teams work to a carefully organised plan and follow-up checks are made to see that the spraying operation has been thorough.
Under a regulation which became law in 1947, the spraying of houses is com pulsory in localities declared control areas by the Director of Medical Services.
Householders are given ample warning be fore spraying and are advised regarding the methods of protection of the contents of their houses. Spraying of an average sized cottage takes only fifteen minutes and it has been found that the nublic have co-operated admirably.
The interior of each building is sprayed at intervals of eight months with a 5 per cent, solution of DDT in kerosene, or a suspension of DDT wettable powder, ac cording to the type of surface to be treated, at a deposit rate of approximately 150 mgms. per square foot. The annual cost of spraying works out at approxi mately $4.39 per house, or $0,857 per head of the population. It will undoubtedly be possible to reduce this when refinements in organisation, improved equipment and cheaper insecticides become available.
IN the course of spraying, Aedes aegypti, the vector of yellow fever, has also been eradicated from all the bigger towns and other danger areas. The mos quito Culex fatigans, one of the trans mitters of filariasis, has proved somewhat more resistant to DDT, but nevertheless there has been a reduction in the num bers of this insect of about 80 per cent, in villages treated —more frequent spray ing may be necessary.
Another gratifying aspect of DDT treat ment has been the elimination of very large numbers of other domestic insect pests, which are extremely troublesome under tropical conditions.
It must not be assumed from this suc cess that exactly the same method will prove equally effective in other malarious countries. The malaria vector. Anopheles darlingi, in British Guiana is very domes ticated; that is, it remains in houses for long periods night and day, and therefore has ample opportunity of coming in con tact with the treated surfaces. Other species of Anopheles have different habits, and each eradication presents its own in dividual problem. Another point em phasised is that, although to date this method has been highly successful, the basic conditions for mosquito production that is, large breeding areas—remain un changed. There is, therefore, always the danger that, if protective measures are suspended, infiltration will take place from the hinterland.