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3. It is not yet absolutely proved that malarial fever (or, as we may perhaps venture to call it, gnal-fever) is not acquired through other channels than the bites of gnats; but there is no good evidence in favour of this supposition, while there are many strong reasons against it. Nor is it yet certain whether some other kinds of gnats, such as species of the genus culez, cannot convey the infection; but as many of these species are often numerous in non-malarious localities, while such of them as have been tried in India and Italy have been shown to be innocent, this supposition also appears to be improbable. On the whole, as regards Freetown, considering that anopheles abound here, that cach insect which has fed on a case of malarial fever may possibly afterwards infect several persons, and that a single infection thus produced may cause relapses of fever for months or years, we think that direct communication of the disease by the bites of anopheles will certainly account for very many of the fresh infections occurring in this place, and may even account for them all.
4. Thus, gnat-fever approximates as regards the mode of infection to the diseases of cattle called Tsetse-fly disease and Texas cattle-fever, which are spread by the bites of the Tsetse fly and the cattle tick respectively; while the fact that anopheles breed in stagnant pools of water at once explains and satisfies many old idens regarding the origin of malaria, its association with rainfall, and its removal by drainage of the soil.
5. It should be understood that only the first infection of gnat-fever is caused by the gnats. Subsequent attacks, which are of the nature of relapses, may occur for years, far away from the original source of the infection, as the result of fatigue, chills, and
so on. •
L
6. The gnat-fever among the troops at Wilberforce has afforded an excellent illustration of the mode in which this disease is spread. Anopheles abound in the barracks. Out of over a hundred of these insects examined by us we have found the characteristic parasites of malaria in twenty-five per cent.; while the same parasites are to be found in about the same percentage of the men.
Modes of Prevention.
7. A.-Drainage of the Breeding Pools of Anopheles.—In view of these facts it is obvious that the most satisfactory preventive of grat-fever would be the obliteration by drainage of the breeding pools of anopheles, if this measure be possible.
8. While the larve of the innocuous gnats of genus culer live mostly in vessels of water, anopheles larva are nearly always found in collections of water in the ground. But not all such collections of water are suitable for them. They in fact require puddles which (a) do not dry up within about 24 hours after the cessation of rain; (b) are not liable to be scoured out by heavy rain; and (c) do not contain small fish. Such pools are comparatively scarce because they can collect only in a few favourable spots; and can be generally detected at sight by the presence of green water-weeds, especially alge, which the anopheles larvæ feed upon,
9. In our experience here, anophiles pools are always found in the neighbourhood Marshes and mangrove of houses, where the adult insects obtain their human food. swamps are frequently supposed to give rise to malaria; but we have never succeeded in finding anopheles larvae in them, and do not think that they provide suitable conditions for the insects.
10. We have found no facts tending to show that the adult anopheles cau fly very far, or can be carried very far by the wind, For instance, we have never but once observed these gnats on Tower Hill, although there are numerous breeding pools to the windward within half a mile of the barracks on the summit of the hill (400 feet).
11. The accompanying map of Freetown shows all the principal anopheles pools which we have been able to discover. Every road shown in the map has been carefully searched, and the puddles which were found to contain anopheles larvae have been entered in red ink. It will be observed that exclusive of some very small puddles close to the larger ones they number only about a hundred for the whole town; but it is right to mention that we may have overlooked some such pools, especially in private premises, while other pools may form at different seasons of the year. At present we observe that the same puddles remain constantly the haunts of the larvae. The flattest parts of the town, especially those where small runnels of water ooze from the ground, as in Grassfields and Sackville Street, show the greatest number of pools. The puddles are usually very small ones..
12. Primâ facie, it seems possible that all these pools can be obliterated by surface drainage at a small cost-certainly at a cost small as compared with that which the
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surface drainage of the whole town would require. Such a measure, it should be observed, would be practically the same as the old measure of surface drainage which has been employed long and often so successfully against malaria. There is this difference however whereas the old method required the drainage of a whole malarious area, the method now proposed demands the drainage only of those small portions of that area which are indicated as being the breeding grounds of the dangerous species of guats. It is therefore possible that where the old method was impracticable on account of the cust, the new method may be found to come within the limits of possible expenditure.
13. Most of the pools in Freetown are to be found in the artificial ditches by the side of the level roads-a kind of construction which it would be advisable to avoid.
14. It may often happen that in some localities anopheles pools are much rarer even than in Freetown, and that drainage would therefore be the cheaper.
15. B.-Destruction of Anopheles Larva with oil. It has been found that a few drops of kerosene oil sprinkled on the surface of a puddle destroy all the anopheles larvæ which it may contain in a few hours.
16. When drainage cannot be employed, or until it can be employed, this method can be used on a large scale at very little cost. It will be necessary to engage especially for this purpose a trustworthy agent who will in the course of time get to know all the haunts of anopheles larva within a given area and will destroy the insects. His work must of course be under proper supervision, and he may be employed also to destroy culez larva for the general comfort of the inhabitants. One person ought to suffice for a considerable area-such, for instance, as that of Freetown-but the efficacy of the measure will evidently depend on the conscientiousness with which he performs his duty.
17. We think it probable that if all or most of the anopheles pools in a locality be systematically treated with oil twice a week for a month or two, such a blow will be struck at the prevalence of the insects there that they will not be able to recover their ascendancy for a long time; but it will evidently be better to continue the measure as long as the larvae are to be found.
18. In very flat localities where drainage will be difficult, oiling the pools may prove to be the only practical measure.
19. C.-Wire gauze Screens to Windows.--It may often happen that the source of anopheles in a locality cannot easily be discovered (and this has actually been the case with respect to the anopheles which are now causing the guat-fever at Wilberforce), or that there is no time and means to discover and deal with the larvæ. In this case another set of measures can be adopted with the object of preventing the insects from biting,
20. Wire-gauze screens fixed to windows would undoubtedly exclude very large number of gnats. After an anopheles bas bitten an infected person, a week must pass before it is capable of infecting other people, and during this week the insect must return at least once to its breeding pools to lay its eggs. It must, therefore, pass several times through the open windows, and wire screens would prevent both ingress and egress. the Roman Campagna experience has taught the peasants to sleep with the windows closed all night to exclude “malaria.' It may be P inted out that the anopheles found here bites during the night only; but it will nevertheless be unsafe, for several reasons, to recommend that removable wire screens should be employed. The mesh of the screens need not be finer than that of an ordinary mosquito-net.
In
21. It would certainly be advisable to use such screens, if possible, in temporary huts provided up country for soldiers or travellers.
22. D.-Destruction of Anopheles by hand.-The adult insects can be easily destroyed in barracks and hospitals as they sleep on the walls by day. Since, as we have experi- enced, one in four of these insects may contain parasites, this measure may often prevent numerous infections or reinfections.
23. E.-Mosquito-nets for Patients. It cannot be too carefully remembered that malarial fever is in reality a catching disease if the proper species of guats are present. Consequently all patients suffering from the disease or having parasites in their blood should sleep in mosquito-nettings in order to avoid communicating the infection to others. This should be especially attended to in hospitals where, if anopheles be present, one case in a general ward may infect many patients; and inasmuch as it is still doubtful whether some other species of gnats besides anopheles cannot carry the infection, we think that patients should always be protected in this manner inmalarious areas. It must be remembered too, that the parasites may remain in the blood for weeks after the fever has left the patient, so that, for instance, a soldier who returns to his barrack after leaving
· hospital may spread the disease among his comrades. This is undoubtedly what is now
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