The prospects in Malaya in the immediate future may be briefly summarized as follows : For residual sprays, DDT and gamma BHC, unless some outstanding new compound appears, will continue to be the most widely used. Both kill the principal malaria carrier. Anopheles maculatus and gamma BHC for a short period after application also kills the common nuisance mosquito, Culex fatigans. Though DDT does not kill C fatigans it prevents them from resting in rooms and reduces the number of biting. Both insecticides should eliminate bed bugs Cimex hemipterus during the course of spraying and reduce cockroach infestation. For the elimination of cockroaches and ants from houses gamma BHC is superior to DDT but the effect is short lived. The other residual insecticides, Chlordane, Toxaphene, Methoxychlor and the newer products Aldrin and Dieldrin are not likely to be used extensively in Malaya. They cannot yet be purchased locally and on present indications have no outstanding advantages to recommend them instead of DDT or BHC. Personal protection against scrub typhus infection is now satisfactory. Either DBP or benzyl benzoate rubbed into clothing should prevent the attachment of mites for several days, and DMP is also effective if applied daily to the skin or clothing. The mite population in an infective area can probably be reduced effectively by spraying BHC on the vegetation, but this method of control is not likely to be attempted in many areas. DMP is a good mosquito repellent and forms the main ingredient of several commercial preparations; it is not as effective when incorpora ted in a cream as it is when pure. Of the several simple, efficient remedies now available for head louse and scabies control, gamma BHC in coconut oil is probably the most useful because of its cheapness, ease of application, and effect on both parasites.
This paper records some interesting findings of anophelines resting by day in vegetation In' Malaya. Anophëles maculatus was the commonest species and the majority of specimens were found to have had a recent blood meal, In general few Malayan anophelines are endophilic, so the author instituted systematic searches of vegetation in the vicinity of labourers' lines and cattle sheds on a rubber estate, where breeding sites of A. maculatus in the form of small streams and ravines abounded. Searching was carried out by trained assistants using hand catching with cyanide tubes, and catches on different occasions yielded results varying from 1 to 13 anophelines per man-hour. It was found that the densely shaded banks of streams had a relatively small population of anophelines. Rather they favoured the more open type of vegetation under trees, represented mostly by the common Malayan bracken fern. A. maculatus was by far the commonest anopheline found under these conditions, usually within a few inches of the ground where the bracken was 1-2 feet high and sheltered but reasonably free of access. Of the other 8 species of anophelines found, A. hyrcanus and A. barbirostris were usually found at a greater height from the ground, A. aconitus in more secluded spots and A. philippinensis in vegetation where ferns were replaced by sedges and small bushes. Analysis of the specimens captured showed that there were more than six times as many females as males of A. maculatus. Of these females approximately two-thirds contained fresh blood while the remainder were gravid or unfed indicating that adults in all stages of development make use of the same daytime harbourages. Dissections showed that maturation of the ovaries was complete 48 hours after a single blood meal and this same period appeared to hold good for several other of the species concerned. A. vagus, however, probably completes the whole cycle in 24 hours. Precipitin tests showed that 20 per cent. of A. maculatus had fed on man and nearly all of the remaining 80 per cent. on cattle. Of the other species only 7 per cent. of A. hyrcanus and a single specimen of A. karwari had fed at all on man.
This work, carried out on a rubber estate in Malaya during 1949, was a Continuation of the trials begun in 1948 previously recorded [this Bulletin, 1949, v 46, 1116]. Full details concerning the terrain and the nature of the experiment were given in the previous publication. In 1949 the malaria rate in the area approached the rates which were customary in pre-war years, for the first time since the reoccupation of the country. The Indian population which was chosen for the experiment contains the survivors of the Japanese occupation; many had been in Siam and almost all had suffered from malaria. Treatment had been entirely lacking or very inadequate, with the result that the survivors had developed a high degree of immunity by the end of the war. These facts probably explain the low incidence of malaria in post-war years in spite of high prevalence of A. maculatus. No anti-larval measures have been carried out since 1941. Neo-premaline completely suppressed malaria in one group, the control group showing a high incidence. In other groups chloroquine, or chloroquine and pentaquine combined, given once a week, promptly brought to an end primary waves of malaria which were rising rapidly.
1. Several experiments were carried out in the field and in the laboratory with various insecticides against A maculatus. 2. Almost all the mosquitoes that came into contact with surfaces sprayed with DDT preparations died in less than 30 hours but the length of life varied with the preparation and depended on various factors that are mentioned. Control mosquitoes lived almost twice as long. 3. Gammexane Dispersible Powder (P 520) killed in a much shorter time than the DDT preparations and holds out a greater hope of success in controlling ‘A maculatus’ malaria with insecticides. 4. It is difficult to assess the value of insecticides in any one year in areas where breeding of the vector is intense. The results of long term field experiments, under strict control, are awaited. 5. Meanwhile, Mass Suppression with drugs still remains the method of choice – in this area.
In the staining of leucocytes successful use has be en made in this Department of the method described by Field (1941) in the demonstration of malarial parasites in thick films. The methylene blue solution is made up according to the formula of Hitch (quoted by Field). In the staining of these films the procedure of Field is modified so that the red cells are left intact and stained. The method as carried out is as follows. The two solutions of stain are mixed as required in the proportion of 6 drops of the methylene blue solution to 2 drops of the eosin solution and with the addition of 10 drops of buffered water at about pH 6.4. The mixture is then thoroughly shaken. The blood film is fixed with pure, acetone free, methyl alcohol for 10 seconds. The alcohol is then run off and the stain mixture applied to the slide with a pipette. Staining is continued for 10 minutes after which time the film is washed by waving about in a beaker of the buffered water at pH 6.4 for a few seconds. This method stains the leukocytes very clearly and is not subject to the several difficulties in using alcoholic solutions especially in the tropics.