Chapter VII: On the Prevention of Malaria (1)

“Fever,” _i.e._, malaria, is responsible for so large a share of the sickness peculiar to tropical countries that the subject of its prevention requires especial and separate consideration. Up to a few years ago the causation of malaria was a complete mystery. We had known for some score of years or more that the disease was due to the presence of certain minute animalcules in the blood, but as to how they got there, or the manner in which they passed from man to man, we had not the remotest idea. There was a general opinion that the seeds of the disease were carried by the air in the form of what was pretentiously spoken of as a miasma--a formidable word which served well enough to hide from the profane vulgar the fact that no one could define, or in fact had the vaguest notion as to what a miasma might be. It was also popularly known in many parts of the world that miasmata found great difficulty in getting through a mosquito net; but by the majority of the profession these traditions were looked upon as laic fables, unworthy of scientific attention, though there were not wanting observant practitioners of tropical medicine who were willing to admit the efficacy of the protection afforded by a mosquito net, and who even attempted to account for the fact by all sorts of lame physical explanations, barring only the simple one that a mosquito net serves very fairly the purpose for which it is designed, viz., of keeping out mosquitoes. The condensed moisture of the dew on the fluff of the meshes in some way attracting the germs or dissolving some assumedly gaseous emanation, was a favourite so-called explanation, and was, I believe, that adopted by the late Prof. Maclean, of Netley, who in his lectures was always careful to impress upon us the protection afforded by mosquito nets, as a well-established, though ill-understood, fact.

This was before the date at which the French military surgeon, Laveran, discovered the fact that malaria was due to the presence in the blood of certain animal parasites (_Protozoa_), and although this discovery was made in 1880, it was many years before its truth was accepted by the general body of the medical profession, who have somehow always exhibited a curious reluctance to admit the harmfulness of animal parasites.

Some years later Sir Patrick Manson, F.R.S., then a hard-working doctor in practice in China, made the remarkable discovery that the blood-worm disease, which is very common in those parts, was conveyed from man to man by the agency of mosquitoes; and as the parasitic origin of malarial fever became more and more firmly established, the idea suggested itself to him that this disease, too, might very well be transmitted by the intervention of the same insects.

At this time Sir Patrick had left China, to work harder than ever in London, so that he was unable to personally test the truth of his surmise, which he, however, communicated to Major Ronald Ross, I.M.S., who after prolonged work, was able to establish the truth of Manson’s suggestion, which was a remarkable instance of the value of imagination in science. Almost immediately after Rossi’s work was confirmed and amplified by Prof. Grassi, of Rome, and by several other naturalists, and the fact that malarial diseases are communicated by the agency of mosquitoes, and can be carried from man to man in no other manner, is now absolutely established. Medical science has commonly to accept, as a working theory, whatever hypothesis of the causation of disease may appear most tenable, but in the present case there is no room for doubt, and, like the protective power of vaccination, the carriage of malaria by mosquitoes only, may be taken as one of the few absolutely proven facts of medicine.

In saying as much, it is not implied that the reader may not on his travels meet with medical men sceptical or hostile to this theory, but this is because the training required to appreciate the cogency of the facts adduced in proof is that, not of a medical man, but of a naturalist, and though the profession of medicine numbers in its ranks many distinguished naturalists, it is quite possible to gain the highest qualifications without acquiring any knowledge of zoology sufficient to render the student capable of really forming an opinion on such a point; for, as a matter of fact, the five years of medical training are so overburdened with absolutely necessary medical subjects, that any critical knowledge of the associated subjects of chemistry, physics, and biology must needs be left to the after years of those to whom good fortune affords sufficient leisure to admit of their attacking the fringe of these great subjects when they are no longer _in statu pupilaris_. Hence, especially among the older hands, there are numbers of medical men who would regard the above statements as premature, but the reader will find it difficult to find any naturalist who entertains any doubts on the subject. Many details undoubtedly remain to be worked out, but the broad data may be taken as absolute facts, of a character that future investigations can only amplify.

These facts may be shortly stated as follows:--

(1) Malarial fevers are caused by the presence in the blood of minute animal parasites. There are several species of these, corresponding to the various types of fever; but the life-history of all is broadly the same.

(2) These animalcules multiply in the blood, and when they have become sufficiently numerous, determine an attack of fever, but while in this stage cannot pass from the blood of one human being to another, except by the somewhat difficult vivisectional experiment of injecting the living blood of an infected person into the vessels of a healthy subject, a process which cannot occur in Nature.

(3) Large numbers of the malaria animalcules are destroyed by what may be called the vital powers of the patient’s blood, and the question whether an untreated case of malaria dies or recovers, depends on the outcome of the struggle between the parasites and the vital forces of their host.

(4) The process of multiplication of the parasites within the human body is by simple division, or non-sexual breeding of the single cell of which each parasite consists; but by a well-known law of the life-history of this class of animalcules this method of multiplication cannot continue indefinitely without the intervention of a period of sexual multiplication; and this can not occur within the human subject under any circumstances, but only in the bodies of certain species of mosquitoes; so that the disease always tends to wear itself out, provided the strength of the patient holds out sufficiently long; and the parasites are unable to find their way into the blood of other human beings by natural processes, unless mosquitoes of certain special species be present.

(5) If, however, a human being infected with malaria be bitten by a mosquito of the appropriate sort, the parasites, sucked into the stomach of the mosquito along with its meal of blood, undergo further development into distinct male and female animalcules, whose union gives birth to myriads of germs which, although incapable of further multiplication within the organism of the insect, find their way into its salivary glands, which are the organs in which the irritating poison of the mosquito is elaborated, and so are necessarily inoculated into the tissues of any human being whom the mosquito may bite, with the result that a new victim becomes infected, and the chain of events commences anew.

(6) The malarial parasites can exist in the mosquito only within certain limits of temperature, and hence the disease is not found in countries where the maximum summer temperature is less than 76° F., or in places so hot that the temperature rises above 86° F. for any length of time, and the comparative healthiness of many hot continental climates is due to the fortunate circumstance that a period of excessive dry heat follows immediately on the cold season.

In considering the measures of prevention detailed below, it must be remembered that their importance is enhanced by the fact that not only malaria, but also the blood-worm disease already alluded to, or filariasis, and yellow fever, have also been shown to require the intervention of mosquitoes for their transmission from man to man, and that although not carried by mosquitoes, the germs of sleeping sickness are also probably conveyed by a winged insect, so that many of the measures detailed below will have a certain value also in the prevention of the last-mentioned disease.

Thus for the maintenance of malarial fever the co-existence of three animal organisms are essential, viz., of man, the mosquito, and the malarial protozoon; and it is obvious that even the temporary banishment of either of the three from any given locality will necessarily put an end to all possibility of the occurrence of fever; for man can be infected only by the mosquito, and the mosquito by man, and the presence of both of the others is necessary for the maintenance of the species for the parasite.

For the proper contrivance, then, of measures of prevention it is essential that we should be well acquainted with the life-history of our two partners in this curious cycle of development, and as that of the parasite has already been sufficiently described for our purposes, it remains only to describe the leading parts of the life-history of the mosquito. Mosquitoes, or, as they are called in England, gnats, are small two-winged insects whose appearance is quite familiar to most people, though midges so closely resembles them, in general appearance, that they are commonly confused with them, but mosquitoes may be easily distinguished by their possessing a long, trunk-like proboscis which is wanting in the midge, as well as by the fact that, if examined with a strong magnifier, it will be seen that their wings and the greater part of their surface are covered with minute, downy scales exactly like those of butterflies, while midges are quite devoid of any such covering. The males carry a pair of beautiful plume-like feelers, while those of the females, though quite as long, show only a few inconspicuous hairs.

Over five hundred species of mosquito have been described; and, as might be expected, their habits vary to some extent; but, speaking generally, they are mainly twilight and nocturnal insects which remain hidden in sheltered places during the day, and come forth to feed and disport themselves in the open at dusk. Owing to the peculiar structure of their mouth-parts, which consist of a long, delicate tube, supported in the midst of a group of lancets, they are incapable of taking solid food, and subsist almost entirely on the juices of living plants and animals, which are sucked up by means of the tube which is introduced into a puncture made by the lancets. With one or two doubtful exceptions, the males of all species live entirely upon the juices of plants, but the females of perhaps the majority of species are not content with so simple a diet, and attack animals of all sorts, puncturing the skin and filling themselves with blood till they are scarce able to fly. No animal is safe from their attacks, and improbable though it may appear, it is recorded, on the authority of a skilled naturalist, that young fish are killed in large numbers by the crowds of mosquitoes which pounce on them as they show their heads and backs at the surface of the water.

With the exception of the fleas and ticks, no insects are so admirably adapted for the conveyance of disease from one animal to another by inoculation; and owing to their enormous numbers, and their capability of flight, their powers of mischief must be far greater than those of either of these other pests. When biting an animal, the mosquito injects into its tissues a fluid specially secreted within its body, which being of an irritating character causes a congestion of blood round the puncture, whereby the insect is secured a full and ample feed of blood. The early life of the mosquito is passed in water, on the surface of which the eggs are laid by the female flying insects. These eggs are of various forms, but are all provided with some arrangement to secure their floating on the surface of the water. In many species they are glued together by their sides so as to form rafts consisting of some hundreds of eggs, while in others, and notably in the special sorts that carry malaria, they are laid separately and float on their sides. In a few rare cases they may be laid on dry surfaces, in situations that will be flooded during the rains. This is notably the case occasionally with the form that carries yellow fever; but for practical purposes it may be considered that the eggs can only be laid on water, and in any case, when the young insects are hatched out, it must needs be into water, as at that stage of their existence they are purely aquatic organisms, and live but a short time if removed from the water. In spite of this they are air-breathers, and in the anatomy of their arrangements for obtaining air, present a curious resemblance to the type of submarine boat that gets its air by means of a small tube reaching just above the surface of the water. The young mosquitoes, or larvæ, are small, wormlike animals, which may often be seen wriggling about vigorously in water, and are especially common, in warm weather, in water standing in small stagnant pools, broken crockery, old tins, and so forth.

When full grown they are about a quarter of an inch in length, and vary in colour from bright green, through brown to black. Two principal forms may easily be distinguished by the difference between the structure of their breathing organs. The first and commonest kind, of which the common English gnat (_Culex pipiens, L._) is a good example, has a long breathing tube projecting from the back close to the tail, so that it looks much as if the hinder part of the body were forked.

As will be seen from the above illustration, only the tip of the breathing tube is kept at the surface of the water, while the body and head hang down into it in a slanting position, so that the head is the most deeply immersed portion of the insect. In the other kind there is practically no breathing tube, the air-vessels opening almost flush with the surface, though in the same part of the body; and hence, in order to keep these openings at the surface, the larvæ must needs lie horizontally at the surface, looking much like a small blackened straw, and on closer inspection are seen to have an outline not unlike the ornamental keyhole plates often seen in old-fashioned furniture. They generally lie with their tails supported against some solid object, such as the side of the saucer in which they have been placed for observation.

Living _Anopheles_ larvæ. Photographed by Mr. T. H. ROYLE, of Rosa.]

Owing to their being confused with the line of optical contact of air and water, and the latter being raised by adhesion into curves over their backs, they do not come out as clearly in the photograph as in a drawing, but they are, nevertheless, sufficiently clearly shown for easy recognition, a matter of some importance, as these are the larvæ of the sub-family _Anophelinæ_, to which belong all the mosquitoes that are concerned in carrying malaria, besides which they are the commonest carriers of blood-worms. The mosquito, however, that conveys yellow fever (_Stegomyia_) belongs to the Culex sub-family.

For any one possessing a microscope these larvæ form most fascinating examples of “pond-life,” as many species, especially when young, are transparent enough to enable one to follow the entire mechanism of their internal organs; but a great deal of interest can be made out with a simple hand-lens.

On the back of each of the rings of the abdomen, or hindmost division of the body, of the Anopheles larvæ may be made out a pair of structures formed like minute palm leaves, the function of which appears to be that of keeping the little creature flush with the surface of the water.

One of these larvæ, magnified about twenty times, with these structures and certain peculiar hairs on the front of the head, which are of value in distinguishing one species from another, more highly magnified, is figured on the preceding page. By the aid of these illustrations the reader should have no difficulty in recognising these organisms when he meets with them. When disturbed, they dart backwards and seek refuge among the _débris_ at the bottom, but cannot remain there long, and soon resume their resting position on the surface.

The larvæ of many species are said to be carnivorous, and even cannibalistic, though I cannot say I have ever observed this personally, and in any case their main provision is found in the minute vegetable organisms which are abundant in the sites where they are common; the Culicine larvæ confining themselves mainly to those that are found completely immersed, while the young Anopheles browses on those floating on the surface, keeping its head screwed round, a full half-turn, so as to bring the mouth uppermost. The abrupt way in which this action is performed and reversed is extremely quaint, so that, as a writer remarks, one is rather surprised at its not being accompanied by an audible click.

The duration of the period of larval existence depends on the temperature of the water and on the abundance or scarcity of food. When first hatched out from the egg, the larvæ are barely visible to the naked eye, whereas when full grown they contain within themselves the entire material of the adult flying insect. Once full-grown, the weather being favourable, they change into “nymphs” or pupæ, not unlike small tadpoles in form, the head and thorax being enclosed in an almost spherical envelope, to which is appended a sort of tail, formed by the abdomen, which is usually kept folded under the body, as shown in the figure below. In this stage the animal breathes through a pair of horns springing from the back of the thorax. The mouth is completely closed, so that the pupa is incapable of feeding, but is in no sense quiescent, as it is capable of lively movements and tries to elude capture almost as briskly as the larva. After a sufficient time has elapsed for the completion of the extraordinary anatomical changes that convert the larva into the adult insect, the pupa-case bursts along the back, and the perfect gnat gradually disentangles itself from its temporary home, and flies away, very shortly to reinitiate the cycle of events by laying a fresh batch of eggs. As has already been remarked, the time required to complete these changes varies greatly in different species and under varying environments, of which climate is the most important factor, but under moderately favourable conditions the time required is a fortnight or three weeks, the greater part of which is passed as a larva. In countries where water freezes during winter for any length of time, all larvæ that have not completed their metamorphosis by the end of autumn, must necessarily perish; and the continuity of the species is maintained entirely by the survival of pregnant females that hide themselves in warm corners and pass the winter in a truly hybernating or dormant condition. Further south, both males and females hybernate, and the dormancy is so much less pronounced that the insects are often tempted to issue from their hiding places on exceptionally warm days. In such climates, which include Italy and most of the sub-tropical zone, the species finds an additional string for its bow in the survival of larvæ, which, though they cannot be said to hybernate in the strict sense of the term, being always lively and alert in their movements, are yet incapable of growing, and appear to remain at whatever size they may have previously attained until the return of warmer weather. Quite recently Dr. Bancroft, of Queensland, has discovered that in some species these wintering larvæ can give birth to small broods of young larvæ, so that in such cases the bow has no less than three strings. Further south again, in truly tropical climates, breeding goes on all the year round.

No adult mosquito can stand the direct rays of the sun, and hence all of them have to seek the shelter of houses, trees, or some such protection, during the heat of the day; moreover, with very few exceptions, they are nocturnal or twilight insects, so that there is comparatively little danger of being bitten during the hours of daylight. For the majority of species too, extreme atmospheric heat has much the same effect as great cold, so that during periods of intensely hot, dry weather the number of species to be met with in a state of activity is very small, and fortunately those that convey malaria are not amongst the number; but on the other hand, the one or two sorts that possess this tolerance are so enormously prolific that their numbers far exceed those of all other species combined.

Another point of importance is that, for practical purposes, mosquitoes cannot fly far, and hence never wander far from the puddle on which they were born. Being tolerably long-lived insects, it is of course possible for them to slowly spread amongst trees or bushes for considerable distances, a few straying further and further away every night from the place of their birth, but the number that can travel in this way for any considerable distance, is so small as to be scarcely worth consideration; and any considerable expanse of bare, open country is practically impassable to them.

Mosquitoes are most active during the periods of twilight, and especially in the evening, at which time they quit the shelter in which they have dozed away the day, and come out into the open to seek their food, remaining outside for the most part throughout the night, though the females of those species that attack man and animals, it is needless to say, will find their way back into houses in search of their favourite food. As soon, however, as the sun has got well above the horizon they may be observed trooping back into the house, and if a window be closed it is very amusing to watch the numbers that will alight on the glass at this time of the day, and their efforts to find a way through the obstruction. The special importance of keeping all means of entry closed at this particular hour can therefore easily be understood, though in ordinary tropical practice, it is the very time at which every door and window is habitually thrown open.

Mosquitoes are found all over the world, it being a great mistake to imagine that they are confined to tropical climates. Some score of species are to be found in England, and though in high latitudes they cease to be dangerous as carriers of disease, there are perhaps no localities where they are so numerous and troublesome as certain parts of the North American Continent, and in Scandinavia, close up to the Frigid Zone.

Almost any collection of water will serve as a nursery for the larvæ, unless indeed, there be a decidedly strong current; but the situations taken by preference during the breeding season are small stagnant pools and domestic collections of water, such as small tanks, broken crockery, empty tins, &c.; while wintering larvæ prefer the larger ponds and marshes which are permanent throughout the cold weather, and especially select those in which there is sufficient vegetation, reaching to the surface of the water, to afford cover and protection from their numerous enemies. Practically speaking, wintering larvæ will never be found in tanks or ponds devoid of fairly robust vegetation, and it may easily be understood from this fact that the clearing away of reeds, grasses, and weeds of all sorts during the cold weather from all such collections of water which may be found near an inhabited site is a most important sanitary measure. Some species prefer the fairly clean water of marshes and ponds, while others luxuriate in the dilute sewage of the domestic waste water, but it would occupy too much space to go into any detail on this subject, and all that the sanitary amateur need remember in this connection, is that any and every collection of water, capable of standing for ten days or a fortnight, should be regarded as dangerous to health in any country where malaria is known to exist.

For our purposes, it will suffice for the reader to understand the general characteristics of three sorts of mosquitoes. First, there are the common _Culex_ mosquitoes, which are, almost everywhere, far more common than the others. They are usually of a dull grey colour, and with very few exceptions, their wings are quite plain and free from spots. As will be seen from the photographs in the accompanying plate, they sit in rather a humped-up position, and the proboscis is obviously much thinner than the body, its appendages, or palps, being held apart from it. Mosquitoes of this sort cannot convey human malaria, though they are instrumental in conveying a similar disease for certain animals. They are to be found, in greater or less numbers, throughout the year.

The second sort is the _Stegomyia_, which is the genus concerned in the conveyance of yellow fever. These mosquitoes are seldom to be seen except during the rains, and rest in much the same position as the _Culices_, which they resemble closely in form. Their wings are never spotted, and almost all are small insects clothed with jetty-black scales, picked out with an ornamentation of dazzling white lines on the body and spots on the abdomen and legs.

Photographs of living mosquitoes. Above, ♂ and ♀ _Culex_ mosquitoes in profile; in the middle, ventral aspects of the same; and beneath, ♀ and ♂ _Anopheles_ mosquitoes. About twice natural size.]

The third, or _Anopheles_, sub-family is that concerned in the transmission of human malaria, and, as may be seen by comparing the two lower photographs of the plate with the upper ones, can easily be recognised by their characteristic form and attitude. In these mosquitoes the feelers are long and thick in both sexes, and as they are held habitually in contact with the proboscis, these together appear to the naked eye as a prolongation of the body as thick, or thicker, than the abdomen. Moreover, except in a few species, their position, when resting, forms a singular contrast to that of the _Culices_, the whole body and proboscis being held in one straight line, with the abdomen raised from, and the proboscis pointed almost vertically at the surface on which they rest, and almost touching it with its point, as if they were preparing to drive the latter into it; so that, viewed with the naked eye, they look much like minute black thorns, stuck into the surface on which they are sitting. On closer examination, it will be seen that the wings are not plain, but spotted (the number of plain-winged species being unimportant).

The appearance of spotting is usually produced by alternate lengths of the veins of the wings being coloured in finely contrasted dark brown or black, and white or yellow, and varies, of course, in detail in different species, but the above figures of the wings of some of the commoner Indian forms will give an idea of the general effect, as seen under a moderate magnification, for though the spots are quite visible to the naked eye, it requires a fairly powerful hand lens to make out the details properly.

Like the _Stegomyiæ_, the _Anophelinæ_ are only common during the rains, but stray specimens will be met with for a month or two afterwards; and though they practically disappear during cold weather, a few may be generally met with during the hot dry months of continental hot climates.

From what has been said, it follows that the prevalence of malaria, like that of mosquitoes, must necessarily be seasonal. The survival of the malarial parasite at those times of the year when mosquitoes are scarce or absent, is maintained by the persistence of considerable numbers in a latent condition, in the persons of human beings who have been imperfectly cured of an attack of malaria.

Cases of this sort, which have either been never treated at all, or in which the administration of quinine has been stopped too soon, are extremely common in all malaria-stricken communities, and such persons are always liable to relapses if they are brought below _par_ by any depressing influence, such as chill, too great exposure to the sun, or the incidence of injuries, or of other diseases.

During such periods the latent parasites, which have been perhaps for months lying hidden in the internal organs, will reappear in the blood in large numbers, and there are always a sufficient number of such cases of relapse persisting through the naturally malaria-free season, to suffice to infect the early broods of mosquitoes that come to maturity with the advent of the rains.

This shows that the curing of all cases of malaria is matter by no means confined to the interests of the patient himself, but is of great importance to the entire community of which he is a member; for, despite the fact that the disease is only indirectly transferable through the agency of the mosquito, a case of malaria is as great a danger to his fellow-men as one of any other infectious disease, and should, as far as possible, be dealt with in the same way; only the problem of isolation is in this case easier than usual, as it suffices to prevent mosquitoes getting access to the sick person; and in a properly fitted mosquito-proof house or hospital, malaria cases may quite safely be allowed to mix freely with the healthy, as well as with persons affected with other diseases.

Relapses in persons chronically infected with malaria may occur at any time of the year, but the fresh infections, which always form the bulk of the cases met with, can only happen as the result of a tolerably recent bite by an infected mosquito; and as about a couple of weeks are required for the maturing of the parasite within the insect, and a further period of incubation is necessary within the human subject before the parasites attain sufficient numbers to produce a definite constitutional reaction, fever does not as a rule commence in earnest until some three weeks or a month after the break of the wet season. Once the process is started, infected mosquitoes and malaria-stricken men increase in numbers rapidly, and as numbers of infected mosquitoes survive for a considerable period after the cessation of the breeding season, the prevalence of malaria continues until the advent of cold weather puts a period to the possibility of the survival of the parasite within the insect organism. Hence it is generally quite possible to give a fair idea of the monthly distribution of rainfall in any warm climate from the returns of sickness and mortality, and _vice versâ_.

The natural history of the living organisms that are concerned in the propagation of malaria has been described with some detail, as suitable measures for the prevention of the disease must needs be based on a fairly competent knowledge of the subject, and is the more necessary as it is hardly possible to give any “rule of thumb” directions; because the circumstances on which the prevalence of malaria depends vary so greatly in different localities that, to ensure success, our operations must necessarily be modified in each case to meet local conditions.

In the following remarks, however, it is not proposed to deal with the question of prevention on a large and public scale, but only with such as can be adopted as measures of individual and personal hygiene, as the larger question of provincial and municipal anti-malarial sanitation cannot be adequately dealt with in a short treatise like the present.

Keeping always in view the just described data of the life history of the malarial parasite and of its temporary host, the mosquito, it is obvious that our measures of protection must be based on one or more of the following plans of action, any one of which, could it be carried to complete success, would suffice to “stamp out” the disease. These measures are:--

(1) To destroy the mosquitoes.

(2) To prevent mosquitoes biting man.

(3) To prevent mosquitoes from becoming infected, by isolating all cases of malaria in man.

In practice, however, it is extremely rare that anything like complete success can be attained on either of the three above indicated lines of action, especially as concerns the second and third methods, and though here and there localities may be met with in which the breeding places for mosquitoes are so circumscribed and easily dealt with as to render the actual extermination of mosquitoes practicable, in by far the majority of cases we must be content with a partial success all along the line, by adopting such measures as may be locally most practicable, based on any or all three of the above principles of action.

In the case, for example, of habitations placed in the midst of canal irrigation where ample and efficient subsoil drainage is impossible or too expensive, the attempt to destroy mosquitoes can be little better than a waste of time and money; for under such conditions the breeding places are so numerous, and appear so constantly in new and unexpected situations, as to defeat the utmost vigilance; and the thorough protection of all habitations against the invasion of mosquitoes, and the careful treatment of all cases with quinine, is all that can be done. On the other hand, in an exceptional case, such as that of the town of Ismailia in Egypt, where the rainfall is practically _nil_, and the breeding places all of artificial origin, the practical extermination of mosquitoes may be so easy a matter as to be effected at a small expense in a single year, with the immediate result of reducing the cases of malaria to a tithe of their previous numbers.

Measures of the first class, _i.e._, the extermination or diminution of the numbers of mosquitoes, can often be undertaken with considerable success by private individuals, the possibilities of success varying, of course, with the extent of the area directly or indirectly under his control.

The resident of a closely inhabited town can, it is needless to say, do no more than contribute his personal mite to the general welfare in this matter, with little chance of reaping much benefit unless his neighbours follow his example; but with the exception of diplomatic officials and merchants residing in places under oriental rule, where the safety of life and property are too badly secured to admit of their living outside town limits, it is rare for any European to be so situated, as in most of our colonies and dependencies the European quarter of the town consists of widely separated villas each surrounded with a garden of some size, with generally a small hamlet of dwellings for native servants and dependents included within its boundaries. In addition to this, there are often open spaces, of considerable extent, between the various “compounds” which, though not directly under one’s control, are so far “no man’s land” that no one will interfere with any one employing on them any of the measures required for anti-malarial sanitation, while the most litigious municipal council is hardly likely to object to the sprinkling of a little paraffin on the roadside puddles. In this way it will generally be possible to effect a good deal within a radius of three or four hundred yards of one’s dwelling, and actual experience has shown me that if all breeding places within such limits can be rendered harmless the number of stragglers that will stray across from places beyond will be too small to be seriously troublesome.

In devising measures for the destruction of mosquitoes it is obvious that while they may be attacked either in the aquatic or aerial stages of their existence, the easily localised larvæ and pupæ will be far more easily dealt with than the elusive flying insect, and it will be therefore best to devote most attention to the destruction of the former.

There are two principal methods of dealing with the insects during their aquatic stage, _i.e._ (1) by doing away with their breeding places; (2) by poisoning them. Of the two it is clear that the former method is, where practicable, the more valuable, as in nearly all cases its effects are more or less permanent. With either object in view, however, the first step is to seek out the breeding places.

As a preliminary measure, all domestic rubbish capable of holding water, such as disused flower-pots, empty tins, &c., should be carefully sought out and destroyed or disposed of by throwing into some depression of the soil which it is desired to fill in. Next, the entire surface should be carefully inspected after a brisk shower of rain, and all such puddles as are of manageable dimensions carefully filled in and levelled.

The amount of labour that can be profitably afforded in work of this sort will depend a great deal on the probable duration of one’s stay in a place; as temporary residents, such as government officials, will find it much cheaper to employ temporary measures, such as the use of paraffin; while in the case of merchants and other permanent residents, the expenditure of a considerable sum on permanent measures will be more remunerative in the long run. The great difficulty often lies in finding spoil wherewith to fill in the depressions, as unless care be exercised, the only result will be to shift the site of the puddle. Sometimes, however, a bank of earth surrounds the compound by way of a hedge; and as in a level country the existence of such an obstruction to surface drainage is most objectionable such banks should always, if possible, be removed and a hedge of wire or bamboo substituted; when the spoil can be advantageously utilised for filling in hollows. Where no localised elevations are to be found, the material required should be obtained by a general very slight removal of the surface. In other cases it may be possible to drain a depression by cutting a shallow gutter to the nearest surface drain.

Another and very important class of breeding place consists of the comparatively large collections of water formed by the various reservoirs, channels, &c., constructed for obtaining and storing water for various domestic purposes, such as wells, tanks, &c., and especially the appliances for watering gardens. Of these the most harmful of all is canal irrigation, which should undoubtedly be never tolerated near a dwelling by any one valuing his health, malaria or no malaria; for the waterlogging of the soil, that is practically inseparable from the system, is either the exciting or predisposing cause of a variety of diseases, many of which, such as rheumatism, consumption and cancer, are perhaps more seriously dangerous than malaria. Sodden and waterlogged sites are notoriously unhealthy all over the world, and chronic ill-health is a high price to pay for a few flowers and vegetables; so that the settler will be well advised to leave irrigated cultivation to those who are constrained to adopt it for a livelihood, and banish it from his own premises; as much is gained by living on a comparatively dry site of however limited extent.

As a matter of fact, indeed, the possession of a garden of any sort is a more than doubtful benefit in malarious places, for trees and shrubs necessarily form lurking-places for mosquitoes; and even with the greatest care, it is difficult to carry out the necessary watering of the plants without giving rise to puddles; while the various reservoirs, &c., that are almost indispensable, are a constant source of danger unless constantly and minutely supervised.

In India, for example, gardens are commonly watered from wells by means of water lifts of various forms, which are worked either by means of bullocks or by manual labour. In order to facilitate the distribution of the water, masonry channels are usually constructed which carry the water from the well head to all parts of the garden, and as the lift cannot conveniently be worked continuously, a number of small tanks are arranged along the line of channels wherein water is stored, so that it can be dipped out and distributed by means of an ordinary watering can without the gardener ever having to go far to replenish it. Now these small tanks are _par excellence_ the main source of supply of mosquitoes of all species, and therefore of malaria, to the houses to which they are appended. If, therefore, a garden be considered indispensable, all such tanks should be carefully emptied and all deposit cleaned out at least once a week; under which circumstances, though they will probably swarm with larvæ by the end of the time, it is impossible for any of the latter to complete their metamorphoses into adult insects. By far the least objectionable plan of relieving the dust and glare of a bare situation is the cultivation of a well-kept lawn, a few large trees being left and the area of flower beds strictly limited, for as flooding the grass is sure to spoil a lawn, success can only be ensured by careful and moderate waterings at frequent intervals.

Collections of water that are too large to be done away with by filling in or by draining, should be dealt with by oiling the water with paraffin. There are some other agents which may be employed for the purpose, but none of them are so cheap, efficient, and readily obtainable. As an additional advantage, though fatal to all kinds of insects, it is in the quantities employed, not only absolutely harmless to vegetation, but water so treated is actually a most valuable application, owing to its power of destroying other injurious insects. When sprinkled on water paraffin spreads out into an extremely thin film, so that a very small quantity will cover a considerable area. A three-gallon tin, for example, contains enough to cover an area of 100 yards each way, though it may be a day or two before the oil reaches all parts of such a space. It is important to remember that the cheaper and commoner the oil, the better it is for the purpose, and the addition of a little of common bazar _ghi_, or clarified butter, is said to make the oil spread better and render its action more lasting, though the writer has not personally experimented with the mixture.

In the quantities used there is no possible danger of fire, even though wooden structures, such as the piles of bridges, be immersed in the water, as the film is so thin that it is impossible to ignite it; a matter one would have thought sufficiently obvious, were it not that objections have been gravely raised to the use of paraffin on this score.

The method of application must be varied to suit the size and situation of the piece of water to be dealt with. Small puddles, such as those left in the course of roadside ditches, may most economically be dealt with by, as it were, lightly wiping over the surface with a wisp of rags dipped in the oil; filling in being in such cases out of the question, as to do so would be equivalent to obstructing the drainage; and it is quite impossible to maintain an exact and uniform gradient in an unrevetted channel.

For larger collections of water, by far the best appliance is an ordinary gardener’s watering-pot. The oil should be applied mainly along the windward side of the pool by a coolie, who should, if possible, walk out some distance into the water, and should be trained to sprinkle the oil by rapid single sweeps of the rose of the can, a few steps being taken between each sweep, as there is no need to make the loop-shaped areas of water sprinkled in this way continuous, as the oil will spread laterally and join each of them together even if separated by several yards, and no advantage whatever appears to be gained by applying the oil thickly. In exceptional cases, such as the moats of fortifications, where the water is enclosed within high vertical walls, a garden syringe may be required in place of the watering can.

In situations such as the pools in canal beds, ditches, &c., the oiling must obviously be done after each flow of water through the channel; but in most ordinary situations, the effects of a careful oiling may be trusted to last for at least three weeks, as, though larvæ may be beginning to reappear by the end of that period, none of them will have had time to complete their metamorphoses. For the destruction of wintering larvæ, two oilings, one at the commencement, and one towards the end of the cold season, amply suffice; and in Continental hot climates it is needless to repeat the process during the dry hot season, as at that time only artificial breeding places, such as garden tanks, require attention, and these are better dealt with by periodical emptying.

At that season indeed, the undried-up breeding places are so few and far between that people have only themselves to thank if mosquitoes are in evidence at all; and yet in many places there is no season of the year when they are so numerous and tormenting. There are no remaining natural breeding-places, and the pains of a weekly stroll round one’s premises to ensure the emptying and cleaning out of all garden tanks, water-vessels, &c., are all that is required to secure complete immunity at that season of the year, but it is most difficult to induce people to take even this trifling trouble; and the mosquitoes are likely to flourish undisturbed until the existence of breeding places within the premises of any person is treated by the authorities in the same way as other dangerous nuisances; and yet these very people are the loudest in their condemnation of the inertia of the native in sanitary matters, and while they maintain malaria breweries on a scale in which the modest extent of his premises forbids him to compete, inveigh against him for starting a cholera factory on the most modest lines. The native who refuses to avail himself of the protection against plague of a health camp is, they admit, an impracticable fool for his pains; but the _sahibs_ of the civil lines, whose gardens supply a large share of the harmful mosquitoes to the neighbouring city, are merely “common-sense” people who attach no importance to doctor’s fads and “scientific rot” of all sorts. The native, however, has at least usually the excuse that he is unable to read or write; and for the rest, does not pretend to be a very highly civilised person.

Once they have emerged from the pupa case, our means of attacking mosquitoes are comparatively feeble, and for those that habitually pass most of their time in the open air practically _nil_.

In the case however of domestic species, which habitually shelter in houses, a good deal can be done in this matter; and, just as in the repression of other troublesome insects, such as fleas and bugs, scrupulous tidiness and cleanliness is by far the most important of all agencies. Useless, rarely dusted draperies and curtains, and untidy collections of clothing hung about on nails and pegs, instead of being kept in properly closed wardrobes, are the things that are mainly responsible for attracting and sheltering mosquitoes within houses, for they will not, if they can avoid it, remain in a well-lighted room, with freshly colour-washed bare walls.

In the tropics, the Italian villa with its frescoed walls and minimum of useless furniture, is the ideal that should be aimed at, and not the elaborate lumber warehouse of an English drawing-room; which, though comparatively harmless in our own climate, is about as well adapted for imitation in hot countries as the coat of the Polar bear in the Zoo is suited to our summer.

Perhaps our most effective agency in dealing with adult mosquitoes depends on the intense objection all species of these insects entertain to smoke.

That it is quite possible to effectually protect animals from mosquito bites by the agency of smoke alone, has recently been conclusively shown in the course of Mr. Power’s experiments on the prevention of horse sickness in South Africa; but, though most semi-civilised people seem to live comfortably enough in a smoky atmosphere, Europeans would find such a state of things intolerable; and the plan can only be utilised to drive mosquitoes out of houses, other methods being relied on to prevent their re-entering. Smoke from almost any source will put mosquitoes to flight, and if sufficiently intense will stupefy them; but certain special materials must be burnt if it is desired to kill them outright, and our _modus operandi_ must be varied according to the means at our disposal, for if we can only expect to annoy the insects sufficiently to drive them out of doors, the latter must be left open during the fumigation; whereas, if it is proposed to kill them, all openings must be closed as completely as possible, so that the fumes may reach the insects in as concentrated a form as possible.

For simply driving out mosquitoes, any fuel that produces a dense smoke, such as damp straw, will serve. Maize cobs are excellent for the purpose, but probably the least objectionable is the burning of a little incense (Hindustani, _Lobán_). To actually kill them, on the other hand, it is necessary to shut up all openings as closely as possible, and to burn certain special substances, such as sulphur, unopened chrysanthemum flowers, the leaves of the neem tree, tobacco, &c., and the degree of success attained will vary a good deal according to the character of the building in which it is attempted; it being obviously extremely difficult to secure an adequate concentration of the fumes in a very pervious structure, like a house with a thatched roof and walls of bamboo matting, such as is often met with in Burmah and the Malay Archipelago. Under such circumstances, the rapid burning of a large amount of the fumigating agent is the only way of meeting the difficulty; and under any conditions, plain sulphur, simply ignited as a powdered mass, or placed on the hot coals of a charcoal brazier, burns so slowly as to be quite useless.

A few instants’ exposure to an atmosphere containing a fairly high percentage of sulphurous acid will kill any insect, whereas they will be merely stupefied or unaffected by prolonged exposure to a weaker mixture of the poison. I am quite aware that sulphur has been decried as almost useless by several observers, but this is simply because they have not used it properly; for to secure success, something must be added to the sulphur which will make it burn quickly, and what is wanted is, in fact, a weak firework. In well-constructed buildings the sulphur pastiles prescribed in the second edition of my work on the “Gnats or Mosquitoes” answer sufficiently well, but subsequent experience has shown that the proportion of nitre is not enough to secure a sufficiently rapid combustion to instantly flood the air with sulphurous acid, and that nothing short of a slow burning firework, such as a Roman candle or Bengal light, will do this in a building roofed with tiles or thatch. Possibly it might be advantageous to raise the proportion of sulphur; and it is undoubtedly better to pack the material as a powder in paper cases, than to mould it into pastiles. Proper precautions against fire must of course be taken, but in fireworks of this sort the sparks do not fly far, and it is quite safe to use them in any ordinary room if the precaution be taken of placing the cases under a low shield, formed of a sheet of corrugated iron supported on four piles of bricks.

The occasional fumigation of a dwelling in this way is a useful adjunct to the destruction of the larvæ; and where combined with proper wire-gauze protection, after the plan described below, may be relied upon to secure immunity from the attacks of insects, even where measures for the destruction of larvæ are impracticable. It has the further advantage that it is fatal not only to mosquitoes, but also to insect pests of all sorts, such as flies, hornets, bugs, or fleas, and even to such more awesome foes as scorpions and centipedes, all of which delight to share the home of the dweller in tropical climes.