Part 13

It is very doubtful if the male _Anopheles_, which can easily be distinguished from the female by its bushy feathered tentacles, quite visible to the naked eye, ever sucks blood. The habit in the female is possibly prompted by a desire to obtain material for the growth of the ova. Out of the numerous genus _Culex_ only four species are known in which the male bites; and it is probable that malaria is always conveyed from man to man by the activity of the female. It is difficult to say how long mosquitoes live in the imago state--certainly, if fed, for many weeks. The earlier collectors, not knowing how to feed them, used to cork them up in glass tubes, and then, noticing in a day or two that the poor insect had died, retired to their studies and wrote moral essays on the brevity of life, or learned treatises on the duration of life in relation to the methods of ovipositing. Now we feed the imagos--as a rule, on bananas--and they live well in confinement. The fertilized female survives the winter, hibernating in some dusky corner, and it is probable that some of the eggs also carry the species over the cold months from autumn to the following spring.

It should, perhaps, be mentioned that the infected mosquito does not transmit the parasites to its offspring. This was an important point to ascertain, because it is known that the tick which causes Texas fever does transmit its parasite to the young ticks, and they in turn communicate the disease to the oxen. A somewhat similar case of the transference from parent to offspring of an organism causing disease is that of the Pébrine, caused by a parasite which attacks silkworms, and which is conveyed by the infected ova from one generation to another.

The above short résumé of the life-history and habits of _Anopheles_ has been given as a prelude to the important question: What can be done to diminish malaria? A few years ago, before we understood the cause of the disease, much had been done to lessen it. While aiming at other objects, we drove malaria out of England by draining. Now that we know the secret of the disease we can direct our efforts more intelligently. There are two points exposed to attack. The first is the sporulating organism in the blood of man, the second is the insect. If we could eliminate the organism from man, the mosquito would be saved much suffering, and would be powerless to infect man; or, if we could prevent the mosquito from access to man, either by guarding him against its bites or by killing off the insect, the hæmotozoön would, in the course of time, gradually die out.

Both methods should be tried. Malarious patients should, so far as possible, be treated with quinine, and no effort should be spared to free their system from the parasite. Special precautions, such as hanging up mosquito curtains, etc., should be taken to prevent the access of the mosquito to the patient; otherwise he acts as a centre of infection. It is almost equally important to protect the healthy man living in a malarious place. The mosquito net must be carefully made, and let down over the bed well before sunset; its free edges should be tucked under the mattress, and the greatest care should be taken to prevent the ingress of a mosquito, especially when slipping within the curtains. Punkahs should be employed as much as possible; they certainly tend to keep the _Anopheles_ at a distance. In the summer of 1899 an experiment was initiated by Sir Patrick Manson which must convince even those least open to conviction that malaria is preventable if proper precautions be taken. That the bite of an infected mosquito can convey malaria may be taken as proved by the voluntary submission of Mr. T. P. Manson to the experiment, as recounted in the _Times_.[5] This gentleman allowed himself to be bitten, in this country, by insects previously fed on malarious patients; and in due course the disease--tertian ague--showed itself in him. To prove the other side of the case required even more courage and endurance. During the spring of 1899, Dr. Low and Dr. Sambon, of the London School of Tropical Medicine, with Signor Terzi, an Italian artist, and two servants, have been living in a mosquito-proof hut, near Ostia, in the Roman Campagna, and remained in perfect health. The spot selected for this experiment is so malarious that the Romans regard spending a single night there as equivalent to contracting a virulent type of malaria. Yet, when Professor Grassi and several other experts visited the mosquito-proof hut on September 12, 1900, they found the inhabitants in perfect health--a fact which they telegraphed, with their salutations, to Sir Patrick Manson, ‘who first formulated the mosquito-malarial theory.’ The conditions under which Dr. Low and Dr. Sambon and their Italian companions lived were all directed to the avoidance of being bitten by mosquitoes. During the daytime they were allowed out of their hut, because the chance of being bitten in broad daylight is so small that it may be neglected; but they were ‘gated’ an hour before sunset, and were not allowed out until an hour after sunrise. The mosquitoes were kept out of the hut by the use of wire-gauze doors and windows. By these precautions contact between mosquito and man has been avoided, and man has now lived for months in one of the most malarious spots in Europe without acquiring a trace of malaria. It is most satisfactory to record that a similar success has attended the efforts of the Italian authorities to improve the state of things in the great plain of Salerno. Visitors to Paestum and Battipaglia cannot fail to have noticed how malaria has marked that district as its own. By taking such precautions as are indicated above, the peasants and railway signalmen have, during the last few years, for the first time, escaped the disease; whilst for the first time newcomers to the district have failed to contract it. The intelligent activity of the Italian Government, and the well-known interest taken in the question by the King and Queen of Italy, cannot fail to have a profoundly beneficial effect upon the lives of some of the poorest and most hard-working of European peasantry.

The problem in Africa is more complex, owing to the fact that the native population is thoroughly permeated with the parasite. Mr. Christophers and Dr. Stephens, in their ‘Further Reports to the Malaria Committee,’ have shown that the children of natives are in the great majority of cases infected with malaria. In one village where the _Anopheles_ was found in ‘considerable numbers,’ 90 per cent. of the babies suffered, 57 per cent. of the children up to eight years, 28 per cent. of the children up to twelve years, after which age the children were ‘very rarely infected.’ This is but one example out of many, all tending to show that after a time a certain immunity to the disease is acquired, and, further, that travellers should as far as possible avoid the neighbourhood of native villages, and, above all, decline to sleep in native huts.

The destruction of the mosquito, at any rate in neighbourhoods inhabited by man, is a matter of difficulty, but is worth attempting. To expect to destroy the mature insect seems a vain thing, but the larva can be more easily dealt with. _Anopheles_--unlike the common gnat, which breeds close to houses, in cisterns, garden fountains, old tubs, drains, etc.--prefers rain-water puddles, natural hollows by the roadside, small ponds, and rice-fields. We have occasionally found the larvæ of _Anopheles_ and _Culex_ in the same water in England, but this is probably exceptional. In England, so far as our experience goes, the _Anopheles_ larvæ are usually met with in shallow water easily heated by the sun’s rays; and we have always found them in association with the common green water-weed _Spirogyra_, though they are not known to eat this.

Attention to the standing water round houses or near towns will do much to diminish the scourge of mosquitoes. All pots and pans containing water should be regularly turned out once a week, and puddles should be brushed out. The larva takes some seven days to develop, so that once a week suffices to destroy each brood. All useless water should be drained away and stagnant ponds filled up. The introduction of fish has markedly diminished the number of mosquitoes around the late Mr. Hanbury’s celebrated garden at La Mortela on the Riviera. They eagerly devour the larvæ, and should be made use of in all large areas of water. For smaller areas some ‘culicide’ should be tried, and more experiments in this direction are urgently needed. One of the simplest remedies known is kerosene oil. A piece of rag tied to a stick should be dipped into the oil, and then applied to the surface of the water. The oil diffuses in a fine film over the surface and clogs the breathing tubes of the larval insect; it possibly interferes with the action of the surface tension--at any rate, the larvæ die. Fresh tar has the same effect. This ‘painting’ of the water must be renewed once a week. Wells and cisterns should be kept closed. A more careful selection of the site for houses, and a more liberal use of wire-netting mosquito shutters, will do much to minimize the risk to Europeans in malarious districts.

The various remedies suggested above have been tried with success in different parts of the world. The writer has been assured by an old inhabitant of Colombo that the mosquitoes have distinctly diminished in number in parts of that town since the custom of storing water near the houses was abandoned. During the summer of 1900 the authorities at Sassari in Sardinia claim to have ‘practically exterminated the mosquitoes ... by killing the larvæ in the swamps with petroleum, and the flies with chlorine and other destructive chemicals.’[6]

The extinction of malaria in England is a kind of by-product of the draining operations which restored to the agriculturist large tracts of land in the fen districts and elsewhere. The breeding-places of the mosquitoes were dried up and their numbers materially lessened; at the same time the parasite was killed in an increasing number of patients. Thus the mosquitoes which survived had fewer opportunities of infecting themselves, and as time went on the parasite was ultimately eliminated. _Anopheles_, though in diminished numbers, is still with us, and is especially to be found in those parts of England once infested with the malaria; but the parasite has disappeared.

‘INFINITE TORMENT OF FLIES’

_Where the water is stopped in a stagnant pond, Danced over by the midge._ R. BROWNING: ‘By the Fireside.’

The last few years of the nineteenth and the first few years of the present century are marked in the annals of medicine by a great increase in our knowledge of certain parasitic diseases, and, above all, in our knowledge of the agency by which the parasites causing the diseases are conveyed from host to host.

Chief among these agencies in carrying the disease-causing organisms from infected to uninfected animals are the insects, and, amongst the insects, above all the flies. Flies--_e.g._, the common house-fly (_Musca domestica_)--can carry about with them the bacillus of anthrax, and, if brought into contact with a wounded surface, may thus set up an outbreak of woolsorter’s disease. Flies, ants, and other even more objectionable insects, are not only capable of disseminating the plague bacillus from man to man, and from rat to man, but they themselves fall victims to the disease, and perish in great numbers. They are active agents in the spread of cholera, and the histories of the South African and Cuban wars definitely show that flies play a large part in carrying the bacilli of enteric fever from sources of infection to the food of man, thus spreading the disease. They are also accused of conveying the inflammatory matter of Egyptian ophthalmia, and of the ‘sore-eye,’ so common in Florida, from one human being to another.

The diseases already mentioned are caused by bacteria. But flies also play a part in the conveyance of a large number of organisms which are not bacteria, but which, nevertheless, cause disease, and cause it on the largest scale.

Of all the twenty-two orders into which the modern entomologist divides the class Insecta, that of the Diptera, or true flies, is, perhaps, the easiest to recognize, for it is characterized by one very obvious feature, the presence of the fore-wings only. The hind-wings are replaced by a pair of small-stalked, club-shaped ‘balancers,’ which are readily visible in some kinds of fly--_e.g._, the daddy-long-legs--but in others are by no means conspicuous. Thus it is an easy matter to determine whether an insect be a fly or not. To determine what particular kind of fly it be is, however, a very different affair. At present some forty thousand species of Diptera are known, and have been more or less completely described or figured; and Mr. D. Sharp estimates that this number is ‘only a tithe of what are still unknown to science.’ Further, the group has been rather neglected. Flies, speaking generally, are neither attractive in their appearance nor engaging in their habits, and it is a cause for no astonishment that entomologists have preferred to work at other groups.

In considering the part played by flies in disseminating diseases not caused by bacteria, we can neglect all but a very few families, those flies which suck blood having alone any interest in this connexion.

From the point of view of the physician, by far the most important of these families is the Culicidæ, with over three hundred described species and five sub-families, of which two, the Culicina and the Anophelina, interest us in relation to disease. The gnats or mosquitoes--the name is indifferently used, and has no scientific application--are amongst the most graceful and most beautiful insects that we know, but they have been judged by their works, and undoubtedly are unpopular, and we shall see that this unpopularity is well deserved. Gnats belong both to the genus _Culex_ and to the genus _Anopheles_. The genus _Culex_, from which the order takes its name, includes not only our commonest gnat, often seen in swarms on summer evenings, but some hundred and thirty other species. Members of this genus convey from man to man the _Filaria nocturna_, one of the causes of the widely-spread disease filariasis, one variety of which is the elephantiasis, so common in parts of the tropics. In patients suffering from this disease minute embryonic round-worms swarm in the bloodvessels of the skin during the hours of darkness. Between six and seven in the evening they begin to appear in the superficial bloodvessels, and they increase in number till midnight, when they may occur in such numbers that five or six hundred may be counted in a single drop of blood. After midnight the swarms begin to lessen, and by breakfast-time, about eight or nine in the morning, except for a few strayed revellers, they have disappeared from the superficial circulation, and are hidden away in the larger bloodvessels and in the lungs.

In spite of their incredible number--some authorities place it at thirty to forty millions in one man--these minute larval organisms, shaped something like a needle pointed at each end, seem to cause little harm. It might be thought that they would traverse the walls of the bloodvessels and cause trouble in the surrounding tissues; but this is prevented by a curious device. It is well known that, like insects, round-worms from time to time cast their skins, and the young larvæ in the blood cast theirs, but do not escape from the inside of this winding-sheet; and thus, though they actively wriggle and coil and uncoil their bodies, their progress is as small and their struggles as little effective as are those of a man in a strait-waistcoat.

The causes of the periodicity of the appearance of these round-worms in the superficial bloodvessels are not completely understood, but they appear to have more relation with the usual sleeping hours of humanity than with day and night. In individuals who sleep by day and work by night the _Filaria nocturna_ is found in the bloodvessels of the skin during the day. Thus, whilst between 5 p.m. and 7 or 8 a.m. the vessels of the skin of Cox the Hatter would be well peopled by the round-worms, they would only come to the surface in Box the Printer during the daytime, whilst he was sleeping in the lodgings of Mrs. Bouncer.

One reason of the normal appearance of the creatures in the blood at night is undoubtedly connected with the habits of its second host, the gnat or mosquito. Two species are accused of carrying the _Filaria_ from man to man--_Culex fatigans_ and _Anopheles nigerrimus_. Sucked up with the blood, the round-worms pass into the stomach of the insect. Here they appear to become violently excited, and rush from one end to the other of their enveloping sheath, until they succeed in breaking through it. When free, they pierce the walls of the stomach of the mosquito, and come to rest in the great thoracic muscles. Here the _Filarias_ rest for some two or three weeks, growing considerably, and developing a mouth and alimentary canal; thence, when they are sufficiently developed, they make their way to the proboscis of the mosquito. Here they lie in couples, and it would be interesting to determine whether these couples are male and female. Exactly how they effect their exit from the mosquito and their entrance into man has not yet been accurately observed, but presumably it is during the process of biting. Only inside man they work their way to the lymphatics, and very soon the female begins to pour into the lymph a stream of young embryos, which reach the bloodvessels through the thoracic duct. It is, however, the adults which are the source of all the trouble. They are of considerable size, three or four inches in length, and their presence, by blocking the channels of the lymphatics, gives rise to a wide range of disease, of which elephantiasis is the most pronounced form. We can consider later how the disease can be averted by keeping down the number of gnats and by preventing their access to infected patients.

We now pass to the second of the diseases carried by gnats, that of malaria.

The parasite which causes malaria is a much more lowly organized animal than the _Filaria_. It is named _Hæmamœba_, and it, too, is conveyed by an insect, and, so far as we know, by one genus of mosquito only, the _Anopheles_. Hence, from the point of view of malaria, it is important to know whether a district is infected with _Culex_ or _Anopheles_. The former is rather humpbacked, and keeps its body parallel with the surface it is biting, and its larva hangs at an angle below the surface of the water, by means of a respiratory tube. _Anopheles_, on the other hand, carries its body at a sharp angle with the surface upon which it rests, and its larva lies flat below the surface-film and parallel with it. The malarial parasite lives in the blood-cells of man, but at a certain period it breaks up into spores, which escape into the fluid of the blood, and it is at this moment that the sufferer feels the access of fever. The presence and growth within the blood-cells result in the destruction of the latter, a very serious thing to the patient if the organisms be at all numerous. If the spores be sucked up by an _Anopheles_, they undergo a complex change, and ultimately reproduce an incredible number of minute spores or ‘blasts,’ each capable of infecting man again if it can but win entrance into his body.

Under normal circumstances, for each _Filaria_ larva which enters a mosquito, one _Filaria_ issues forth, longer, it is true, and more highly developed, but not much changed. The malaria-parasite undergoes, in its passage through the body of the _Anopheles_, many and varied phases of its life-history. As the Frenchman said of the pork, which goes into one end of the machine in the Chicago meat factories as live pig, and comes out at the other in the form of sausages, ‘Il est diablement changé en route.’ The mosquito is as truly a host of the malarial parasite as man, and is as necessary for its full development as is man. Judging by the number and extent of the lesions in the insect’s body, it must suffer far more than man, and it is undoubtedly killed at times, and perhaps fairly frequently, by the parasite.

Whoever has watched under a lens the process of ‘biting’ as carried on by a mosquito, must have observed the fleshy proboscis (_labium_) terminating in a couple of lobes. The labium is grooved like a gutter, and in the groove lie five piercing stylets, and a second groove, or _labrum_. It is along this labrum that the blood is sucked. Between the paired lobes of the labium, and guided by them (as a billiard cue may be guided by two fingers), a bundle of five extremely fine stylets sinks slowly through the epidermis, cutting into the skin as easily as a paper-knife into a soft cheese. Four of these stylets are toothed, but the single median one is shaped like a two-edged sword. Along its centre, where it is thickest, runs an extremely minute groove, only visible under a high power of the microscope. Down this groove flows the saliva, charged with the spores or blasts of the malaria-causing parasite. Through this minute groove has flowed the fluid which, it is no exaggeration to say, has changed the face of continents, and profoundly affected the fate of nations.

It is an interesting fact that, amongst the Culicidæ, it is the female alone that bites. The mouth-parts of the male are weaker, and seem unable to pierce the skin. It has been suggested that a meal of blood is necessary for the development of the eggs; but the evidence for this is not conclusive. There must be millions and millions of mosquitoes in sparsely inhabited or uninhabited districts, in Africa, in Finland, in Northern Asia, and America, which never have a chance of sucking blood; and it is impossible to believe that these millions do not lay eggs.

The female is undoubtedly greedy. If undisturbed, she simply gorges herself until every joint of her chitinous armour is stretched to the cracking-point. At times even, like Baron Munchausen’s horse after his adventure with the portcullis, what she takes in at one end runs out at the other. But she never ceases sucking. The great majority of individuals, however, can never taste blood, and subsist mainly on vegetable juices. In captivity they cannot last longer than five days without food and drink; but they can be kept alive for weeks on a diet of bananas, pineapples, and other juicy fruits.

_Anopheles_ is often conveyed great distances by the wind, or in railway trains or ships; but of itself it does not fly far; about five or six hundred yards--some authorities place it much lower--is its limit. Beyond this distance they do not voluntarily stray from their breeding-places. Both _Anopheles_ and _Culex_ lay their eggs, as is well known, in standing water, and here three out of the four stages in their life-history--the egg, the larva, and the pupa--are passed through. The larva and the pupa hang on to the surface-film of the water by means of certain suspensory hairs, and by their breathing apparatus. Anything which prevents the breathing tubes reaching the air ensures the death of the larva and pupa. Hence the use of paraffin on the pools or breeding-places. It, or any other oily fluid, spreads as a thin layer over the surface of the pools and puddles, and clogs the respiratory pores, and the larvæ or pupæ soon die of suffocation.