Chapter VI: Part 6

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Manson’s own epoch-making researches on _Filaria_--another human parasite whose intermediate host is the mosquito--no doubt strengthened his faith and helped to encourage Major Ross, who in 1895 began in Secunderabad a series of investigations, which, after much weary work, were crowned with brilliant success. The difficulties of the work were very great. Hardly anything was known about the great number of gnats and mosquitoes which are found all over India, and it was often impossible to have them accurately determined. Then no one could predict the appearance of the parasite within the body of the mosquito--if it were there--or in what part of the body it should be looked for. The mosquito had to be searched cell by cell. The difficulty of dissecting a mosquito is great even in temperate climes, and when we recollect that hundreds of all the available species were dissected in the most malarious districts in India, we must recognize that it was only a faith akin to that which moves mountains which sustained the courage and stimulated the perseverance of the tireless worker. For nearly two years and a half Major Ross searched in vain. No matter what species of mosquito he worked at, the results were negative. A less determined man would long ago have abandoned the research; Major Ross only tried new methods. At Sigur Ghat, near Ootacamund, a peculiarly malarious district, he noticed for the first time a mosquito with spotted wings which laid boat-shaped eggs. Shortly afterwards he was able to feed eight specimens of this mosquito on a patient whose blood contained the parasites in the gametocyte stage--and it should have been mentioned above that all mosquitoes dissected were first fed upon the blood of malarious patients. Six of these insects were searched through and through, organ by organ, but without result. The seventh showed certain unusual cells in the outer surface of the stomach, which contained a few granules of the characteristic black pigment or melanin of malarial fever. The eighth and last specimen showed the same characteristic cells with the same characteristic pigment; but the peculiar cells, quite unlike anything hitherto met with in the mosquito’s body, were larger and further developed. ‘These fortunate results practically solved the malaria problem.’

Without following in detail the various stages of the further investigations carried on by Major Ross, we must endeavour to give an account of the final results obtained by him and later investigators. Being unable to obtain material for the study of malaria in man owing to the scare caused by the outbreak of plague amongst the natives, Ross worked out the life-history of an allied organism which causes malaria in birds. It is to the brilliant researches of the Italian school--prominent among whom are Grassi, Bastianelli, and Bignami--that we owe the first complete accounts of the life-history of the human parasite. It has already been explained that some of the parasites do not form spores, but persist in a more or less unchanged condition whilst in the blood of man as gametocytes. We have also seen that when removed from the human body some of these gametocytes throw off actively mobile filiform bodies. In 1897 MacCallum of Baltimore showed what these filiform bodies really are. Certain of the gametocytes do not produce them, but lie passively still on the microscope-slide, or in the blood within the mosquito’s stomach. These are destined to form the female cell; the filamentous bodies which break off from the first-named gametocyte were seen by MacCallum to fuse with them, and, in fact, to play the part of the male cell or spermatozoön. This, in fact, happens when a mosquito feeds on a malarious patient. The gametocytes, unchanged in the blood of man, as soon as they reach the stomach of the insect, swell and burst from their red corpuscle. The male gametocyte throws off the filiform bodies, which actively swim about seeking a female gametocyte (Fig. 2, 1). When found they fuse with it, and thus produce a fertilized cell or zygote (Fig. 2, 3). This zygote is produced on the microscope-slide, and in the alimentary canal of certain mosquitoes, but so far as is known at present it undergoes further development only in the stomach of the various species of the mosquito genus _Anopheles_. In all other cases it dies or is digested. In _Anopheles_, however, the zygote travels to the walls of the stomach, pierces the inner coats and comes to rest underneath the muscular tunic which ensheaths that organ (Fig. 2, 4 and 5).

At first the zygote is very small, about the size of a red blood-corpuscle; but it grows, and in the course of about a week it has, roughly speaking, increased to five hundred times its original bulk (Fig. 3, 1 and 3). Its contents have not only increased, but have divided into some eight or twelve cells, called meres; and each of these meres has given off round its periphery a number of filiform cells, called blasts (Fig. 3, 2). The structure of the mere, with its coating of blasts, may be easily understood by a zoologist when it is mentioned that it very closely resembles that stage in the formation of the spermatozoa of the earth-worm just before the spermatozoa separate themselves from the blastophor; the lay mind may gain a better idea of its appearance by recalling the head of a mop. As the zygote, still resting on the outside of the mosquito’s stomach, matures, the cells which are giving rise to the blasts diminish in size and disappear, leaving the capsule packed with thousands of minute filiform slightly spindle-shaped blasts (Fig. 3, 3). Then the capsule bursts and the blasts make their way into the body-cavity, or space between the stomach and the wall of the mosquito’s body. It is not known whether they have any movement of their

FIG. 3.--FORMATION OF THE BLASTS OF HÆMOMENAS PRÆCOX (ROSS) WITHIN
THE BODY OF THE MOSQUITO ANOPHELES. MAGNIFIED 2,000 TIMES. AFTER
ROSS AND FIELDING-OULD.

No. 1, The full-grown zygote dividing up into meres; No. 2, an
isolated mere which has developed its filiform bodies or blasts;
No. 3, the zygote crammed with blasts is bursting; No. 4, the
blasts are making their way into the salivary gland of the mosquito
_a_, through it into the œsophagus _b_, and finally into the
proboscis _c_.

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own, but in some way or another they make their way into the salivary glands of the insect and accumulate in the cells which secrete the saliva. Thence the blasts pass into the salivary duct and down the grooved proboscis of the insect (Fig. 3, No. 4). The next time the mosquito has a meal off a man, some of these blasts will be washed into the man’s blood by the saliva which causes the irritation set up by a mosquito’s bite. It is known that when an infected insect bites a healthy man malaria ensues; and though the blasts have not hitherto been seen to enter the blood-corpuscles, they certainly give rise to the disease, and it can hardly be doubted that they force their way into the red corpuscles and form the young amœbulæ which we described at the beginning of this article.

The appended scheme will perhaps make clear the very diverse phases of the somewhat polymorphic organisms. Those stages which occur in the blood of man are printed in ordinary type, but those which occur in the mosquito are in italics:

The foregoing account of this varied and romantic life-history is no hypothetical one. With the exception that, so far as we know, no one has yet seen the blasts enter the corpuscles and become amœbulæ, every stage in the story has been verified over and over again by competent observers, and their observations are now accepted by all whose opinion in such matters has weight. Further, the facts here recorded are not peculiar to parasites in man. Allied forms of Protozoa attack other vertebrates, and, in fact, the first hæmatozoön whose life-history was thoroughly worked out by Ross was the _Hæmamœba (Proteosoma) relicta_, which causes a malaria-like disease in birds, and is conveyed from one bird to another by means of the common gnat, _Culex pipiens_. Again, the parasite which causes so much loss to stock-owners, the Texas fever organism, _Pyrosoma bigeminum_, is, thanks to the researches of Smith and Kilborne, now known to be conveyed from one ox to another by the cattle-tick, _Boöphilus bovis_. Thus, however strange the life-history of the malarial parasite may seem to the unscientific, it is very much what might have been expected by zoologists who have worked on allied organisms, and it is vouched for in its main features by the most expert workers in England, France, America, Italy, and Germany. The whole literature of the subject of transmission of disease by insects has been ably sifted and brought together by Dr. Nuttall in a monograph whose title is mentioned in the Bibliography.

For two years and a half Major Ross dissected mosquitoes, looking for traces of the malaria organism and finding none, but at last found what he sought in a species of mosquito that had hitherto escaped his attention. This means that, like most other parasites, the Hæmamœbidæ will develop in one kind of animal and in one kind only. If taken up by another kind they are simply digested. The mosquito with the

MALE, IN CHARACTERISTIC ATTITUDE.]

FEMALE.

_To face page 146._]

spotted wings and boat-shaped eggs undoubtedly belonged to the genus appropriately named _Anopheles_; and only the species of this genus, so far as we know, are capable of conveying the infection from man to man. In their bodies only will the gametocytes develop. If swallowed by other biting insects or by leeches, etc., they disintegrate, and are no more.

The word mosquito has no scientific import; derived from the Spanish or Portuguese, it simply means ‘little fly’; it is used popularly to denote a gnat which bites, and most gnats bite when they have a chance. The word is sometimes extended to include certain midges. The Dipterous family, Culicidæ, to which the gnat belongs, contains, according to Major Giles, some 242 species, divided amongst 8 genera. The great majority of species, some 160, however, belong to the genus _Culex_; _Anopheles_ includes 30; whilst the remainder are divided amongst the other 6 genera, none of which are large. The collections which have been made at the British Museum, and which were worked out by Mr. Theobald, contain many species of _Anopheles_ new to science; so that we have now some half hundred species of the genus ‘which has been hopelessly convicted of being the medium by which the malaria parasite is transmitted from person to person.’ According to the last-named authority, we have in England 17 species of _Culex_, and 2 of _Anopheles_, _A. bifurcatus_ and _A. maculipennis (claviger)_, though some authorities are inclined to add a third, _A. nigripes_. Five other species, belonging to the smaller genera of Culicidæ, make a total of some 24 species of gnat or mosquito found in England. _Culex pipiens_, probably the commonest gnat the wide world over, conveys the parasite _Proteosoma_, or, as Ross now calls it, _Hæmamœba relicta_, of the avian malaria from bird to bird; but it will not carry the parasite of human malaria. Indeed, 14 different species of _Culex_ have been tried in this respect, and in each case with negative results. The same nice adjustment of parasite to host is found in _Anopheles_. It will not convey the bird malaria, that is to say, the gametocytes are destroyed in its body, but it is readily infected by the human parasite, and at the present date a considerable number of species have been successfully tried, and this not only in Europe, but in Africa, India, and the United States.

_Anopheles_ is obviously worth studying. It has now been found very commonly distributed in England, _A. maculipennis_ abounding in the eastern counties. Its boat-shaped eggs, laid, not as are those of the genus _Culex_, in little rifts, but singly, give rise to a charming little larvæ, whose diet of minute algæ gives a greenish tint to the centre of the body, which elsewhere is of a brownish hue. When at rest, these small larvæ float on the water parallel with the surface, and not hanging down into the water as does the larval _Culex_. They have a most beautiful arrangement of minute hairs, arranged like the ribs of an umbrella turned inside out, along the upper surface of their backs, and by the action of these hairs they hang on to the surface-film. Their breathing organs open near the tail, but are not produced into the long respiratory tube by which the _Culex_ larva can be so easily recognized. They possess the most marvellous arrangements on the head for setting up currents conveying food to the mouth, and, in fact, they afford one of the most charming objects of ‘animated nature’ that one could desire to watch. After some days, varying in number according to the temperature, the larva turns into one of those curious active Dipterous pupæ which are well known in the case of other gnats. Like the larva, the pupa floats at the surface of the water. When mature its integument splits along the back; then the perfect insect steps out, rests a moment to dry its wings, and sails away into the air.

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.

In Ismailia the disease has been reduced to an amazing extent, and quite recently remarkable results have followed the use of these preventive measures at Port Swettenham, in the Federated Malay States. Within two months of the opening of the port in 1902, 41 out of 49 of the Government quarters were infected, and 118 out of 196 Government servants were ill. Now, after filling up all pools and cleaning the jungle, no single officer has suffered from malaria since July, 1904, and the number of cases amongst the children fell from 34·8 to 0·77 per cent. The only melancholy feature about this wonderful alleviation of suffering due to the untiring efforts of the District Surgeon, Dr. Malcolm Watson, is that his fees for attending malarial cases have dropped to zero.

Thus a considerable degree of success has attended the efforts of the sanitary authorities, largely at the instigation of Major Ross, all over the world, to diminish the mosquito plague. It is, of course, equally important to try and destroy the parasite in man by means of quinine. This is, however, a matter of very great difficulty. In Africa and in the East nearly all native children are infected with malaria, though they suffer little, and gradually acquire a high degree of immunity. Still, they are always a source of infection; and Europeans living in malarious districts should always place their dwellings to the windward of the native settlements. Knowing the cause, we can now guard against malaria; mosquito-nets and wire windows and doors are a sufficient check on the access of _Anopheles_ to man. If they could only be kept permanently apart, we might hope for the disappearance of the parasite from our fauna. In relieving man from the pest, all lovers of animals will rejoice that we are also relieving the probably far more acute sufferings of one of the most delicate and beautiful insects that we know.

Another elegant little gnat, _Stegomyia fasciata_, closely allied to _Culex_, with which, until recently, it was placed, is the cause of the spread of that most fatal of epidemic diseases, the yellow fever. Like the _Culex_, but unlike the _Anopheles_, _Stegomyia_ has a humpbacked outline, and its larva has a long respiratory tube at an angle to its body, from which it hangs suspended from the surface-film of its watery home. It is a very widely distributed creature; it girdles the earth between the Tropics, and is said to live well on shipboard. It breeds in almost any standing fresh water, provided it be not brackish. The female is said to be most active during the warmer hours of the day, from noon till three or so, and in some of the West Indies it is known as the ‘day-mosquito.’

The organism which causes yellow fever has yet to be found. It seems that it is not a bacterium, and that it lives in the blood of man. It evidently passes through a definite series of changes in the mosquito, for freshly infected mosquitoes do not at once convey the disease. After biting an infected person, it takes twelve days for the unknown organism to develop in the _Stegomyia_ before it is ready for a change of host. The mosquitoes are then capable of inoculating man with the disease for nearly two months. The period during which a man may infect the mosquito, should it bite him, is far shorter, and extends only over the first three days of the illness.

Very careful search has hitherto failed to reveal the presence of the parasite of yellow fever. By its works alone can it be judged. It seems that, like the germ of rinderpest and of foot-and-mouth disease, it is ultra-microscopic, and our highest lenses fail to resolve it. From the course of the disease and the nature of its host, it will probably prove to be something like the organism which causes malaria. The means of warring against _Anopheles_ and _Culex_ are equally applicable in the case of _Stegomyia_, but, since the last-named flies by day, they are more difficult to carry out, and more irksome to endure. By the intelligent application of these preventive measures the Americans have freed Havana for the first time from yellow fever, and have materially reduced the amount of malaria, and they have been equally successful at Panama.

King Solomon sent to Tarshish for gold and silver, ivory, and apes and peacocks, and at the present day people mostly go to Africa for gold, diamonds, ivory, and game. These are the baits that draw them in. Of the great obstacles, however, which have for generations succeeded in keeping that great continent, except at the fringes, comparatively free from immigrants, three--and these by no means the least important--are insignificant members of the order Diptera. We have considered the case of _Culex_ and _Anopheles_; the third fly we have now to do with is the tsetse fly (_Glossina_), which communicates fatal diseases to man and to cattle and domesticated animals of all kinds.

There are at least seven species of the genus which received its name as long ago as 1830, when Wiedemann first described it. Perhaps the best known species is _Glossina morsitans_, which was named by Westwood.

The members of the genus _Glossina_ are unattractive insects, a little larger than our common house-fly, with a sober brownish or brownish-grey coloration. When at rest the two wings are completely super-imposed, like the blades of a shut pair of scissors; and this feature readily serves to distinguish the genus from that of all other blood-sucking flies, and is of great use in discriminating between the tsetse and the somewhat nearly allied _Stomoxys_ and _Hæmatopota_.

The tsetse flies rapidly and directly to the objects it seeks, and must have a keen sense of smell or sight, or both, making straight for its prey, and being most persistent in its attacks. The buzzing which it produces when flying is peculiar, and easily recognized again when once heard. After feeding, the fly emits a higher note, a fact recalling the observation of Dr. Nuttall and the present writer on the note of _Anopheles_, in which animal they observed that, ‘the larger the meal, the higher the note.’ The tsetse does not settle lightly and imperceptibly on the sufferer as the Culicidæ do, nor does it alight slowly and circumspectly after the manner of the house-flies, but it comes down with a bump, square on its legs. Like the mosquito, the tsetse is greedy, and sucks voraciously. The abdomen becomes almost spherical, and of a crimson red, and in the course of a few seconds the fly has exchanged the meagre proportions of a Don Quixote for the ampler circumference of a Sancho Panza. There is a good deal of discrepancy between the reports of the various sufferers as to the pain of the bite. No doubt different persons are very differently affected, and suffer to very varying degrees. Unlike so many of the blood-sucking Diptera, in which the habit is confined to the females, both sexes of _Glossina_ attack warm-blooded creatures.

The fly always seems to choose a very inaccessible portion of the body to operate on--between the shoulders in man, or on the back and belly in cattle and horses; even inside the nostrils in the latter, or on the forehead in dogs. According to Lieutenant-Colonel D. Bruce, R.A.M.C., to whom we owe so much of our knowledge of this fly and its evil work, the female does not lay eggs, but is viviparous, and produces a large active yellow larva, which immediately crawls away to some secluded crevice, and straightway turns into a hard, black pupa, from which the imago emerges in some six weeks. Thus two stages, the egg and the larva, both peculiarly liable to destruction in the Culicidæ, are practically skipped in the tsetse--at any rate, in some species. On the other hand, this advantage is probably to a great extent counterbalanced by the smallness of the number of the larvæ produced, compared with the number of the eggs laid by the oviparous Diptera.

The genera of the Culicidæ which we have considered are found practically all over the world, but the genus _Glossina_, except that it just reaches Arabia, is fortunately confined to Africa. From the admirable map of the geographical distribution of the fly compiled by Mr. Austen we gather that its northern limit corresponds with a line drawn from the Gambia, through Lake Chad to Somaliland, somewhere about the 13th parallel of north latitude. Its southern limit is about on a level with the northern limit of Zululand. The tsetse, of course, is not found everywhere within this area, and, though it has probably escaped observation in many districts, it seems clear that it is very sporadically distributed. Mr. Austen further thinks that it may occur outside the boundary above laid down, and suggests that the great mortality amongst the horses in the Abyssinian campaign against King Theodore may have been caused by it.

Even where the tsetse is found it is not uniformly distributed, but occurs in certain localities only. These form the much dreaded ‘fly-belts.’ The normal prey of the fly is undoubtedly the big game of Africa, including crocodiles, but they are not the only factor in its distribution; the nature of the land also plays a part. There are the usual discrepancies in the accounts of travellers, especially of African travellers, as to the exact localities the _Glossina_ affects; but most writers agree that the tsetse is not found in the open veld. It must have cover. Warm, moist, steamy hollows, containing water and clothed with forest growth, are the haunts chosen. Even within the fly-belt there are oases, due, perhaps, to an absence of shrubs or trees, where no flies are.

The tsetse fly belongs to the family Muscidæ, the true flies, a very large family, which also includes our house-fly, blue-bottle fly, etc. These flies, unlike _Anopheles_ and _Culex_, are day-flies, and begin to disappear at or about sunset, a fact noted centuries ago by Dante:

‘Nel tempo che colui, che il mondo schiara,
La faccia sua a noi tien meno ascosa,
Come la mosca cede alla zanzara.’[7]

The practical disappearance as the temperature drops has enabled the South African traveller to traverse the fly-belts with impunity during the cooler hours of the night. At nightfall the tsetse seems to retire to rest amongst the shrubs and undergrowth, but, if the weather be warm, it may sit up late; and some experienced travellers refrain from entering a fly-belt, especially on a summer’s night, until the temperature has considerably fallen.

The sickness and death of the cattle bitten by the tsetse were formerly attributed to some specific poison secreted by the fly, and injected during the process of biting. It is now, largely owing to the researches of Colonel Bruce, known to be due to the inoculation of the beasts with a minute parasitic organism conveyed from host to host by the fly. The disease is known as ‘nagana,’ and the organism that causes it is a species of _Trypanosoma_, a flagellate Protozoon or unicellular organism, which moves by means of the lashing of a minute, whip-like process. Since Bruce’s researches a number of _Trypanosomas_ have been found causing diseases in various parts of the world. Thus _T. evansii_ causes the ‘surra’ disease of cattle, horses, and camels in India. _T. equinum_ produces the ‘mal de caderas’ of the horse-ranches of South America, and _T. equiperdum_ is responsible for the North African disease called by the French the ‘dourine.’ _T. theileri_ causes the gall-sickness, and there are others. These parasites were first seen by Gruby, who named them in 1843, in the blood of a frog; they live, not as does the malaria parasite, in the blood-cells, but in the fluid of the blood. The particular species of _Trypanosoma_ which causes nagana is _Trypanosoma brucei_, and it does not attack man, and some goats and donkeys seem also immune; but, with these exceptions, all domesticated animals suffer, and in a great percentage of cases the disease terminates in death. Just as the native children in Africa form the source of the supply of the malarial parasite without appearing to suffer much, so the big game of the country abound in _Trypanosoma_ without appearing to be any the worse. They are, in Lankester’s phrase, ‘tolerant’ of the parasite, and a harmony between them and the parasite has been established, so that both live together without hurting one another. Under a more natural condition of things than at present obtains in South Africa, the big game formed the natural prey of the tsetse; and, indeed, so dependent is the fly on the antelopes, etc., that, in places where the game has been exterminated, the fly has also disappeared. It is from the big game that the disease has spread. In their bodies the harmful effect of the parasite has through countless generations become attenuated, but it leaps into full activity again as soon as the _Trypanosoma_ wins its way into the body of any introduced cattle, horse, or domesticated animal. Whether the _Trypanosoma_ does any harm to the fly, or whether it passes through any stages of its life-history in the body of the fly, is still a debatable point. Possibly it does not, and the proboscis of the fly acts then simply as an inoculating needle.

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Pearls & ParasitesChapter VI: Part 6

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