CO129-303 - Public Offices & Others - 1900 — Page 376

>

The Malarial

C. J.

26653

Mosquito in Hongkong,

FROM THE

44 CHINA OVERLAND TRADE REPORT."

The accompanying notes concerning the existence of the Malarial mosquito in Hongkong were read by Mr. C. V. Ladds nt a meeting of the South China Branch of the British Medical Association on Monday evening :-

In a Circular Despatch from the Right Honourable the Secretary of State for the Colonies, dated December 6th, 1898, His Excellency the Governor was requested to forward to the Colonial Office a tellection of mosquitos from the colony with a view to investi- gating the possible connection of Malaria with this genus of insect. Two collections were forwarded as requested upon August 18th and November 24th respectively, and none of the species Anopheles having been included in these collections it was thought at that time that they did not exist in Hongkong. Having always taken a great interest in the Malaria question, and not being satisfied with the result obtained during the above investigation, I determined to take up the subject privately, and commenced work in December, 1899.

After going thoroughly into the matter, and after having cultiva- ted the larva of many varieties of mosquito, I at length found that the Anopheles undoubtedly did exist in the colony and was to be lenud in most of its known malarial districts. I first discovered them close by a Government Bungalow at Kennedy Town. The different occupants of this house having been subject to repeated attacks of fever ever since its erection in 1895, 1 determined to make this place my starting point, and after a very careful exam- ination, I found a small pool swarming with Anopheles larvæ within fifty yards of the building.

Extending my search 1 found the same species near the Kennedy Town Fever Hospital, at Magazine Gap, at Wong-nei-cheong village, near a house on Bonham Road, at Quarry Bay, close by the Taikoo Sugar Refinery, all round the camp of the Weich Fusilier detachment at Kowloon, and in many other places.

This being sufficient evidence of their existence over a con- siderable area of the colony, and having now no doubt that they were to be found in all the malarial districts, I next proceeded to examine the particular kind of pool which the Anopheles appeared to select as most suitable for the propagation of its species,

A small shallow pool containing algae, or green flocculent water- weed, and fungoid growths, abounding in all kinds of aquatic insert life, and which is not quite stagnant (or if stagnant at the time of discovery is not likely long to remain so, but is renewed pcasionally by small flushes of rain water), appears to be their favourite habitat.

In early every instance I found them only in such pools, which were in most cases ted by minute streams issuing from small springs in the hill side, and those not being of sufficient volume to acour ont a channel of their own, just spread themselves over the surrounding lower ground, and so help to form and feed the typical Anopheles pool.

They do not seem to inhabit pools which are entirely stagnant (or likely to remain so long) or which contain no water weed; and appear to be particularly careful in their selection, as, although there may be dozens of pools in the locality, only a few of them may coutain Anopheles.

Having repeatedly found the mosquito hovering round a pool which contained the same species of larva, it is probable that the adult insect returns to its original home to deposit its eggs, and this theory may, if correct, account in a measure for their not being found scattered indiscriminately about everywhere. After once seeing the particular kind of pool which these mosquitos affect it is easily recognized, but it is not quite so easy to describe it correctly; yet I trust that I have given a fair general idea of what to look for.

&

REG 16 AUG 371

I find by experimenting that the Anopheles larvæ appear to feed upon the algae or water weed which is found in their particular pools, and also possibly upon some of the animalcula which they contain. This I proved by placing some larvæ in clear tap water and others in water taken from an Anopheles pool which contained water weed; those in the former dying quickly, while those in the laster hatched out in due course. I should not say that the larval stage of the insect is a long one, probably about a week to ten days, this depending a good deal upon the nature of the pool; after which time they emerge therefrom as adult mosquitos. The life of the mosquito itself probably extends over a period of several weeks, perhaps longor.

I have so far not succeeded in finding the Anopheles larvæ in any of the healthy districts of the onlony, or far distant from human habitations, nor have I yet found them in rice fields, in large pools of water, in rapidly running streams, in pools with sandy bottoms, in wells, old broken pottery, tubs, pans, dc,, as in the care of Culex, which may be found in almost anything which contains water.

In contradistinction to the Anopheles, water wood does not seem to be necessary to the existence of the Culex larve, they being often found in pools, &c., from which it is entirely absent. A further distinction between the two species being that the former appears to be much more delicate than the latter. It is not necossary here to describe the Anopheles larva or mosquito, they being so entirely different from the other species as to be easily recognised after having been once seen. Having satisfied myself as to the existence of the Anopheles in the colony, and having given a brief description of its habitat, &c., I will now pass on to the possibilities of its extirpation from the island. With this object in view, I have been esting the effects of various insecti cides. 1 found that kerosene used as described by the African Commission was very effective in still pools, or in those which were temporarily stagnant; gas tar also proved to be very useful, but in my opinion Jeyes' Fluld is the best of those which I have tried up to the present.

So far as my experiments have gone, I find that one part of Jeyes' Fluid to 10,000 of water kills both Anopheles and Cler larvæ in a few hours, very much weaker solutions destroying them after a longer interval.

Permanganate of potash appears to be of very little use. In & solution of 1 to 1,200 of water, equal to one in five of Condy's Fluid, the larvae lived for twenty-four hours, and were as lively as ever at the end of that time, while in a solution of 1 to 500, equal to nearly one in two of Condy's Fluid, they lived for four hours without there being suy appreciable effect upon them, and even after twenty-four hours more than half of them were alive.

Hence by treating all Anopheles resorts frequently with Jeyes' Fluid, kerosene, or gas tar, during the whole of the dry season, when such pools are comparatively scarce, and by paying particular attention to surface drainage in malarial districts, much might be done toward effecting their partial, if not total, extermination.

In conclusion, I think I may safely say that by working upon these lines, and given the services of half-a-dozen intelligent coolies, some one to look after them, and the necessary supplies of the above-mentioned obemicals, in a very short time it would be possible to cause the Anopheles to become so scarce in Hongkong that should they prove to be the only source of infection by malaria, cases of malarial fever would soon be as rare as cholers, and the finding of the Anopheles larvæ in any part of the island as difficult a matter as looking for the proverbial needle in a truss of hay.