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سلنا

PUBLIC RECORD OFFICE

Reference :---

TTL

C.O.885

7

PUBLIC RECORD OFFICE, LONDONİ

ALLY WITHOUT PERMISSION OF THE BE REPRODUCED PHOTOGRAPHIC- COPYRIGHT PHOTOGRAPH—NOT TO

(Letter. Undated.)

DRAR ROSS,

42

I HAVE carefully examined the various works which have been undertaken with a view to the serious diminution in the number of mosquitoes in Freetown. Sierra Leone.

The common mosquitoes found are:-Anopheles costalis, the carrier of malaria, and also Filaria nocturna, Stegomyia fasciata (Culex taeniatus, brindled mosquito, Calcutta), the mosquito supposed to carry yellow fever.

Two Culices (I think "fatigans, which carries Filaria nocturna, and another which is known both on the East Coast and Shire and Uganda Highlands, but does not attack men) were found, but not commonly.

A Funestus was found near, but not in, Freetown.

In my opinion already your efforts have been crowned with a large degree of success, as there has been a noteworthy diminution in the number of the first two genera found in the houses. The number of breeding grounds has been enormously diminished.

The operations, having been only recently begun, are of course as yet far from complete. A considerable part of the town, perhaps half, has not been touched. Even in the parts longest under treatment, in the yards adjoining the streets, there are still numerous breeding grounds, and in the streets themselves occasional places have either been overlooked or the works undertaken have not been effective as yet.

The breeding places dealt with could be only of importance in the peculiar circumstances of Freetown, i.e., where the soil is impervious and the rainfall excessive (110 inches to 200 inches). Even in Freetown most of them would be destroyed by a week's dry weather, and some less. As, however, the wet season in Sierra Leone is a prolonged one, for this place these breeding grounds are of great importance, and in dealing with them an excellent beginning has been made.

A great part of the work will not be permanent. The rock cuttings are too narrow, many of them being blocked after cach shower. The earth cuttings are also very liable to fall in. This results in much extra work and supervision, as con- siderable supervision and labour is required constantly to keep the work already done in order.

When

I suggest that during the dry season the rock cuttings should be broadened so as to be at least three inches at the bottom, the sides being inclined at about 60. the rush of water is greater a broader cutting will be requisite.*

The earth cuttings should in all cases have sloping sides, where possible, as this minimises the liability to formation of pools, and ensures, even with a small amount of water, a persistent current. There are few things more suitable for Anopheles breeding grounds than a drainage system in which the water supply is insufficient to flush the drains.

The plan adopted of placing large stones at the edge of the channel, blocked behind by smaller ones, will, I think, suffice if the work is strengthened with cement; but brick drains would be preferable, in my opinion, as they are easier to clear.

A large amount of work has been done by filling up rock pools with small broken stones, and even where the traffic is great this, when strengthened with cement, will prove to be permanent.

The work is so far incomplete that it is essential that at least one other complete wet season should be spent here. Constant European supervision is necessary, and one man is not sufficient for the purpose. There should be at least two Europeans engaged in supervising, and a larger staff of workmen (quite twice the present) would, I think, be required, as so much of the work will require redoing, and there are other places to deal with.

Towards the foot of Mount Aureole there are in places numerous springs from which the water is constantly running. Pits, usually shallow, have been dug in this district, and in these Anopheles larvæ are constantly found. Some of them will be difficult to deal with by cuttings alone, and the more permanent should, I think, be converted into covered wells with an outflow underground, say two feet below the surface, leading into a drain to the nearest stream. None of these places have as yet been dealt with.

• Exact uniformity is not essential, but an approximation to it will save a large amount of labour in clearing the channels; and as this requires to be done very frequently the point is of importance.

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There is one similar place in the Grassfields District, and I feel sure that there are others both near the Wilberforce Barracks and near Kissy. The constant rains and the general waterlogged condition of the ground prevent more definite information being obtainable till there is some continued fine weather. Such places are common, and are the important ones in the hilly districts of Central Africa.

In a few of the wells which are so numerous in many districts of Sierra Leone, Anopheles (Costalis) larvæ were found in numbers. Though I do not think from previous experience that these will at any season in the year be of very great import- ance, still they are an additional source. In none of the broad public wells which contain fish were larvæ found, and on placing a few fish in one of the infested wells the larvæ speedily disappeared, but many of the fish died.

Covered wells in any case are safe, but to repair the numerous broken down wells and provide them with covers would be costly and uncertain, as the covers would not be used in many cases.

I am informed that there would be serious difficulty in closing these private wells and in substituting for them a smaller number of public ones, but that when a good town water supply is obtained much could be done in this direction. Such a water supply, it is expected, will be shortly sanctioned.

Equally dangerous are the numerous pits remaining from disused latrines. Those in use (in many cases overflowing) are dangerous for other reasons, but not as breeding grounds for Anopheles.

There are a few deep pools which probably contain water during the greater part of the dry season, and which harbour Anopheles larvæ; these require to be filled up.

I notice that some pits are being filled with the mixed assortment of tins and bottles removed from houses. Broken bottles, or others, are well adapted for filling in pits, but the use of tins is to be avoided, as the ground will certainly fall in. On these, as on other points, the practical experience of the details gained by Dr. Taylor will be invaluable in the next wet season. Opposite houses I think some bridging of gutters should be done, as otherwise the edges of the gutters are bound to fall in.

As regards the proceedings that will be requisite in the dry season, I can, of course, only theorise from experience elsewhere, combined with my observations here of the character and lie of the ground.

The evidence seems to be clear that mosquitoes may be expected to be more numerous in the dry season, because, though of the present breeding grounds few will remain in the dry weather, still there will be other breeding places, and these will not be so constantly disturbed or flushed by heavy rains, and therefore a large proportion of larvæ will reach maturity.

Of the present breeding grounds there will only remain in the dry season some of the springs, and perhaps some of the wells and pools; the rest of the water will then have dried up.

The new places will be mainly the streams, small and large, which remain; possibly some of the other wells, and artificial collections of water in tubs, &c. Any scheme for destroying these breeding places must take into account (1) that abundant places must be left for the people to get drinking water; (2) that places must be left in which people can wash clothes, &c. These two conditions will prevent any extended application of kerosine or any odorous or poisonous larvicide, and particularly their application to streams, as it is mainly in expanded pools in the course of streams, and not in isolated adjoining pools, that larvæ live and proceed to maturity.

The streams are said to contain fish, but, even if they do not, much could not be hoped from stocking them, as in such situations abundant larvæ are often present in the presence of the fish.

Two possible methods which are most obvious are the formation of a central channel in the bed of the stream, with larger collections of water in sufficient numbers of places for drinking purposes and, lower down the stream, other places for washing,

&c.

The second which might be cheaper but less certainly effective would be to dam up the streams so as to obtain a sufficient head of water to flush out the whole channel at intervals.

Either scheme would be expensive and would have to be strong, as a rainfall of 50 inches or more in a month of the wet season will destroy any but strong works in the bed of these rocky streams.

• When the new water supply is obtained these points will not be essential.

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