Part 14

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:

AMŒBULÆ. | +----------------------+----------------------+ | | | Sporocyte. Female Gametocyte. Male Gametocyte. | | | Spores | | (in liquid of blood). | | | Female Gametocyte. Filamentous bodies or Amœbulæ. | Spermatozoa. | | | Sporocyte. +----------------------+ | | | Zygote. Spores | and da capo. Meres. | Blasts. | Amœbulæ. | +-----------------------+----------+-----------+ | | | Sporocyte. Female Gametocyte. Male Gametocyte.

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. 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.’

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.