Oiling

In 1901 oiling was employed at Port Swettenham for a few months during the height of the epidemic, and until the new drainage system became effective; then its use was discontinued. On the flat land estates of the Coastal Plain, malaria was so easily controlled by good drainage that oiling was unnecessary; indeed it would have been impossible to oil the unopened jungle swamps.

The grave problem of persistent malaria in the hill land again raised the question of oiling; but in rapidly running water the prospect of success seemed remote indeed. “Drainage and eradication of breeding areas is the all-important work in the anti-malaria campaign” was the conclusion reached in Panama; and it confirmed my own views.

During a visit to Panama in 1913, I studied carefully the habits of the Anopheles, and the methods of mosquito control [24–26]. Excellent results were obtained from a mixture of crude mineral oil and a larvacide (the latter a poison made from resin, carbolic acid, and caustic soda), which was used both in pools and streams. The American species of Anopheles are, however, different from those of Asia; no Panama species lives in fast-running water like A. maculatus or A. aconitus. It was, therefore, impossible to be sure that the oiling, which gave good results in Panama would be equally effective in the hill streams of Malaya. Nevertheless, I decided to put aside all preconceived ideas, and investigate the subject de novo. If mixtures of mineral oil alone were found, after a careful and thorough trial, to be useless in controlling A. maculatus in our ravines, then it seemed to me worth going to the expense of manufacturing the larvacide and giving it a trial.

If success was to be obtained, the oil must be got to spread rapidly after spraying, and it must be applied not by drip barrels, but from sprayers which could be carried in the ravines, and from which the oil could be applied to each tiny seepage, spring, and side pool in the ravine, as well as along the edges of the main streams. Ordinary kerosene was too thin; the liquid fuel usually supplied too thick; but by trial, a mixture was obtained that appeared to have the necessary qualities. It now remained to try it on a ravine stream containing A. maculatus.

On Sungei Way Estate there is a ravine which I had known to contain A. maculatus for many years. It was examined, found to contain the larvae in large quantity, and then oiled thoroughly. A week later it was free from larvae; indeed there was a complete absence of all aquatic insect life, even Chironomus was dead or absent. On the surface of some of the side pools large numbers of adult insects were found dead; wherever touched by the oil, green vegetation was dead and brown in colour. Mr. W. S. Reeve-Tucker, the manager, carried out the oiling with great thoroughness; and on 25^th August 1914 I wrote to him:

It will be of interest to your Directors to learn that by oiling with the mixture in use on Sungei Way, one of the ravines in the West Division has been kept free from Anopheles for several months. Formerly they bred freely in it. I am encouraged to think that this is not merely due to the physical difficulty the larvae would have of getting air when oil is on the water, but that some change has actually taken place in the water as a consequence of the oil, and a change which would make it uncongenial to the stream-breeding mosquito. My reason for thinking this is that in the bottom of the ravine a green slime has developed, in the presence of which I have never found dangerous anophelines. It is common enough in flat land.

The practical point is that it is, in my opinion, additional proof of the value of oiling. And if your Directors decide to oil the ravines, instead of pipe-draining them, they can be satisfied that it will kill out the harmful mosquitoes.

The mixture had been more effective than we had anticipated; no larvacide or other poison had been mixed with the oil; we had got an alternative to subsoil drainage.

Mr. Reeve-Tucker was now satisfied of the value of oiling; and placed it under the immediate care of Mr. Muir, engineer of the estate, of whose work I cannot speak in too high praise. The result of the oiling was a rapid improvement in the health both of Indians and Europeans (see Table 17.1).

The oiling on an adjacent estate within the half-mile radius of the Sungei Way lines, has not, until recently, been as efficiently carried out as I desired; otherwise I believe the results would have been better than they are. Yet the health is better than the figures indicate; for not infrequently coolies, who have absconded to unhealthy estates, have returned to Sungei Way to die practically without doing a day’s work; for example, in 1919, no fewer than three of the eight deaths were among these “returned bolters.”

The manager and his wife have had one attack of malaria since 1913; it was contracted at Morib. His child, aged six and a half years, has not had malaria.

Formerly Mr. Muir, his wife, and his two elder children were frequently attacked; one of the children was seriously ill. During the past four years Mr. Muir has had one attack of malaria; his wife and three children have been completely free from it. An assistant has been on the estate over three years, another over two years, without being attacked.

The action of mineral oil on larvae is by no means fully understood. The original idea was that, forming a film on the surface of the water, it cut off the larvae from their air supply and killed them by suffocation. Another idea is that some oil penetrates their air-tubes, and poisons them directly; while some hold that the oil, or at least some poisonous substances in it, are dissolved in the water, and poison the tissues of the larvae. Yet another view is that the oil lowers the surface tension of the water, so that larvae can no longer support themselves at surface. However the oil acts, Anopheles larvae in this country are highly susceptible to its effects, and die more rapidly after contact with it than any other larvae.

Apart from any direct action the oil may have on the insect, there is probably an important indirect action. Previously, I mentioned the appearance of a “green slime” on the bottom of the ravine following on the application of the oil. It consists of an alga, the filaments of which are closely “felted” or “matted,” and attached to the stones or sand forming the bed, of the stream. The filaments are much finer than those of the ordinary alga that one finds in a pool of clear water in a ravine; and the “felting” differs from the “loose floating tangle” of the ordinary alga.

When a clear pool containing the ordinary floating alga is “oiled,” the alga dies; it becomes a dark green mass, in which the individual filaments are unrecognisable. In its place appears the “felted” alga; and whenever a ravine is thoroughly oiled this alga appears. It is a test by which to know if oiling is properly carried out. Long before I had used oil in ravines I had noticed that the presence of the “felted alga” meant the absence of A. maculatus, and I had noticed its association with pollution of the water. For example, on an estate there were four ravines, identical as far as the eye could see. At the head of one there was a well where clothes were washed; that ravine alone contained “felted alga.” On no occasion was A. maculatus taken in it. In the other three ravines, which were not polluted, and did not contain the “felted alga,” the insect flourished. Three bungalows, one on each of these three ravines, were so malarious that they were pulled down; a bungalow on the other ravine was much less malarious, and is still inhabited. This was observed before oiling was used in ravines.

Some ravines are to be found where the “felted alga” is growing freely, although no pollution apparent to the eye takes place, and no oiling has been done. On one occasion I found the “felted alga” in a ravine, and tracked it upwards. The ravine branched, and the alga followed only one branch, in which it could be found as far as a log of a newly-cut timber lying in the water. Beyond that the alga could not be found. I can give no explanation of its appearance; but took possession of a piece of the wood. In ravines where the pollution is extreme, where, for instance, the whole discharge from a rubber factory is poured into a small stream, the aquatic growth will be found to differ in different parts of the stream. Nearest the factory, the growth may consist of dense pendulous fawn-coloured masses, composed of colourless filaments containing no chlorophyll; lower in the ravine this is replaced by the “felted alga.”

A full study of the subject is necessary,35 and may well give us an entirely new method of controlling malaria. It is a part of the subject treated in Chapter 19, entitled “On the possibility of altering the composition of water and the anophelines breeding in it.”

The “Liquid Fuel” used in this country is, I understand, not “crude oil,” but a refuse after some of the more volatile oils have been distilled off. Its composition varies greatly. Some consignments, without any admixture with kerosene, can be used in a sprayer, and will give a film which spreads at once on striking the water; other consignments may require the addition of one part of kerosene to eight parts of “Liquid Fuel.”

It is best applied through a knapsack sprayer, of which there are several well-known makes, such as the “Four Oaks” and the “ Vermorel.” The “Four Oaks” is usually supplied with rubber valves, which perish in twenty-four hours after contact with oil; but leather valves are supplied if asked for. In Panama the “Meyer’s Sprayer” was considered best. The valves are metal, and give a minimum of trouble. This sprayer has an overhead lever action which is less fatiguing than the side action of some other sprayers; and it has the additional advantage that, by a simple adjustment, the lever may be turned to either side and used by either hand, which gives some relief in the day’s work.

The oil is sprayed particularly on to the edge of the streams, and for a foot or two up the banks. This destroys not only larvae, but all grass and ferns growing on the banks. It saves the costs of weeding these, and the cost is considerable; in addition the brown, discoloured bank shows the oiling has been properly done: the presence of living vegetation indicates lack of oil.

I do not encourage the use of barrels dripping oil on to a stream. Oil so applied fails to reach the side pools springs, and other small but important breeding places in a ravine. Drip barrels are, by themselves, insufficient, and may lead to a false sense of security; when spraying is properly done, they are unnecessary. I believe there are no drips on the estates under my care.

For each man spraying, two men are employed to carry oil. On a large estate there are usually several depots to which the drums of oil are carted; this reduces the distance that the coolies have to carry oil to the sprayer.

The cost of oiling an estate varies according to the number of acres of ravine or swamp in it, the cost of the oil, and the cost of labour. The cost of both oil and labour have increased during the last year; but from $200 to $600 a month is the cost at the present time on different estates.

Figure 17.1: Meyer’s sprayer

Figure 17.2: A well-oiled ravine, with the sides of the drain free from grass

Both oiling and subsoil drainage being effective against A. maculatus in hill streams, it may be asked which is the better of the two. A simple answer cannot be given; each has certain advantages over the other.

Oiling has two great advantages. The first is that it can be brought into action over a whole estate in a week or less, utterly destroying every Anopheles larva in the ravines; and many adult insects that return to their places in the ravines perish also when they alight on the oil. So it comes about that the complete control of mosquitoes on an estate, which is oiled, is rapid, and measured by a period of a few weeks.

The other great advantage is that there is no large capital outlay for oiling, such as is required for subsoil drainage. The cost of oiling is a permanent annual charge, and will not diminish materially, if at all, with the passage of time; but, generally speaking, the weekly, monthly, and annual cost is not heavy when compared with that of subsoil drainage. This is not unimportant, for malaria often causes heavy loss to an estate, and puts it in straightened circumstances.

Subsoil drainage, on the other hand, may involve a heavy capital expenditure. Another disadvantage is that it may take many months to drain the ravines, and during this time the people suffer from malaria, carried by the Anopheles breeding in the still undrained ravines. This disadvantage can be overcome by “oiling” the whole estate during the execution of the drainage work.

Drainage has a great advantage over oiling, in that it requires less European supervision. One can see at a glance in walking over a ravine whether or not it is dry; but one cannot check the work of the oiling coolie so easily.

In reality, however, there is no antagonism between drainage and oiling. Oiling should supplement drainage when necessary. In many very steep places on an estate it will be easier to oil than drain; such, for instance, as steep granite slopes. One would choose oil in preference to pipes in a wide ravine full of good rubber trees. In a narrow ravine one might oil, pipe, or allow jungle to grow up, depending on the particular zone of the country in which the ravine was situated, the amount of silt coming from the hills, and other local circumstances. In all cases, one would begin the control of malaria by oiling; and oil could be used as a “test solution” to determine the exact area to be drained.

Finally, I must emphasise, in the strongest possible manner, the absolute necessity of the manager, or assistant, in charge of the oiling, regarding this work as not second in importance to any other on the estate. Through failure to do so, oiling is sometimes neglected, with serious loss both of labour and of crop.