Part 14

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.

The Report of Colonel Bruce, which was issued three years ago, shows that the sleeping-sickness which devastates Central Africa, from the West Coast to the East, is also conveyed by a species of tsetse fly. Writing over a hundred years ago of Sierra Leone, Winterbottom mentions the disease. ‘The Africans,’ he says, ‘are very subject to a species of lethargy which they are very much afraid of, as it proves fatal in every instance.’ Early last century it was recorded in Brazil and the West Indies; and in all probability the deaths which our slave-owning ancestors used to attribute to a severe form of home-sickness, or even to a broken heart, were in reality caused by sleeping-sickness. The severity of the disease, which always terminates fatally, is shown by the fact that in a single island--Buvuma--the population has recently been reduced by it from 22,000 to 8,000, whilst whole districts have been almost depopulated. In one year the deaths in the region of Busoga reached a total of 20,000; and it is calculated that although the disease was only noticed in Uganda for the first time in 1901, that by the middle of 1904 100,000 people have been killed by it. The disease is caused by the presence of a second species of _Trypanosoma_ in the blood and in the cerebro-spinal fluid. The existence of this parasite has now been proved in all the cases recently investigated. Apparently the _Trypanosoma_ can live in the blood without doing much harm, and only when it reaches the cerebro-spinal canal does it set up the sleeping-sickness. It is also found in great numbers in the lymphatic glands, especially those of the neck, which in patients infected by the parasite are usually swollen and tender. From the similarity of the parasite to that causing the cattle disease of South Africa, the idea at once arose that the _Trypanosoma_ was conveyed from man to man by a biting insect. Along the lake shores a species of tsetse (_Glossina palpalis_) abounds; and it was noticed that if the fly, having fed off a sleeping-sickness patient, bit a monkey, the monkey became infected. Further, flies which were captured in a sleeping-sickness district were also capable of conveying the disease to healthy monkeys. The proof that sleeping-sickness is due to a _Trypanosoma_ known as _T. gambiense_ present in the cerebro-spinal fluid of the patient, and that it is conveyed from man to man by _Glossina palpalis_, seems now complete. Fortunately, like its congener, _G. palpalis_ is confined to certain districts. The knowledge of these, and of the habits of this species of fly, will suggest preventive measures; and the brilliant research of Colonel Bruce and his colleagues, Captain Grieg and Dr. Nabarro, may yet save the much-tried African continent from the most fatal of recent diseases.

Finally, we come to a last class of disease which is of the utmost interest to the agriculturist and settler, and yet at present is but little understood. These diseases are caused by various species of a Protozoon named _Piroplasma_, and the diseases may collectively be spoken of as piroplasmosis. When they are present in cattle they are spoken of in various parts of the world as Texas fever, tick fever, blackwater, redwater, and many other French, German, Italian, and Spanish names. Heartwater in sheep is a form of piroplasmosis. Horses also suffer, and the malignant jaundice or bilious fever, which makes it impossible to keep dogs in certain parts of this country, is also caused by a _Piroplasma_. Finally, under the name of Rocky Mountain fever, spotted or tick fever, the disease attacks man throughout the west half of the United States.

The organisms which cause the disease live for the most part in the red blood-corpuscles, but they are sometimes to be found in the plasma or liquid of the blood. Unfortunately, we know but little about the life-history of the _Piroplasma_, or of the various stages it passes through, but we do know how it is transmitted from animal to animal and from man to man.

We have seen that the carrier or ‘go-between’ in the case of the malaria is the mosquito, and in the case of the sleeping-sickness is the tsetse fly. The _Piroplasma_, however, is not conveyed from host to host by any insect, but by mites or ticks, members of the large group of Acarines, which include beside the mites the spiders, scorpions, harvestmen, and many others.

The ticks differ from the insect bearers of disease inasmuch as the tick that attacks an ox or a dog does not itself convey the disease, but it lays eggs--for I regret to say here, as with the _Anopheles_, it is the female only that bites--and from these eggs arises the generation which is infective, and which is capable of spreading the disease. The tick which conveys the _Piroplasma_ from dog to dog is called _Hæmophysalis leachi_. The brilliant researches of Mr. Lounsbury have shown that even the young are not immediately capable of giving rise to the disease. The female tick gorges herself with blood, drops to the ground, and begins laying eggs. From these eggs small six-legged larvæ emerge. These larvæ, if they get a chance, attach themselves to a dog, gorge themselves, and after a couple of days fall off. If their mother was infected they nevertheless do not convey the parasite. After lying for a time upon the ground the larval tick casts its skin and becomes a nymph, a stage roughly corresponding with the chrysalis of a butterfly. This nymph, if it has luck, again attaches itself to the dog and has a meal, but it also fails to infect the dog. After a varying time it also drops to the ground, undergoes a metamorphosis, and gives rise to the eight-legged adult tick. Here at last we reach the infective stage; the adult tick is alone capable of giving the disease to the animal upon which she feeds, and then only when she is descended from a tick which has bitten an infected host. Think what a life-history this parasite has! Living in the blood-corpuscles of a dog, sucked up by an adult tick, passed through her body until it reaches an egg, laid with that egg, being present while the egg segments and slowly develops into the larva, living quiescent during the larval stage and the nymph stage, surviving the metamorphosis, and only leaping into activity when the adult stage is reached. This most remarkable story probably indicates that the _Piroplasma_ undergoes a series of changes comparable to those of the malaria organism when it is inside the mosquito; what these stages are we do not at present know, but Dr. Nuttall and Mr. Smedley at Cambridge, and many other observers elsewhere, are at work on the problem, and soon we shall have more light.

With regard to bovine piroplasmosis, Koch, and others have distinguished redwater fever, which is conveyed by _Rhipicephalus annulatus_, and in Europe probably by _Ixodes reduvius_ from the Rhodesian fever, which is conveyed by _Rhipicephalus appendiculatus_, and I regret to say by a species dedicated to myself, _Rhipicephalus shipleyi_.

The heartwater disease of sheep and goats is similarly conveyed by _Amblyomma hebræum_, the Bont tick, and many farmers accuse _Ixodes pilosus_ of causing the well-known paralysis from which sheep suffer in the early autumn; and there are many others, diseases such as the chicken disease of Brazil, which is so fatal to poultry yards, and which is conveyed by the _Argas persicus_.

I will not weary you with more diseases. I think I have said enough to show that within the last few years a flood of light has been thrown upon diseases not only of man and his domestic animals, but upon such insignificant creatures as the mosquito and the tick. I have tried to show how these diseases interact, and how both hosts are absolutely essential to the disease. We can now to a great extent control these troubles; the old idea that there is something unhealthy in the climate of the Tropics is giving way to the idea that the unhealthiness is due to definite organisms conveyed into man by definite biting insects. We have at last, I think, an explanation of why Beelzebub was called the Lord of Flies.

THE DANGER OF FLIES

_And Moses said, Behold, I go out from thee, and I will entreat the Lord that the swarms of flies may depart from Pharaoh, from his servants, and from his people, to-morrow._--EXODUS.

It is one of those facts which not unfrequently occur in science that we know less about the life-history and habits of the commonest insects than we know about scarce and remote species. For instance, the life-history of the common house-fly, one of the most widely distributed insects in the world, is as yet very incompletely known.

It was Linnæus who first described this insect and named it _Musca domestica_, and de Geer who, in the middle of the eighteenth century, first described its transformation. In 1834 Bouché described the larva of the insect as living in the dung of horses and fowls. In 1873 the well-known American entomologist, A. S. Packard, reinvestigated the question, and L. O. Howard has recently written on the subject. In our own country C. Gordon Hewitt is publishing a monograph on the house-fly, which will, when completed, fill a long-felt want. Packard noted that in the August of 1873 the house-fly was particularly abundant, especially in the neighbourhood of stables. He was able to observe the insects laying their ova in clumps containing some 120 eggs in the crevices of stable manure, ‘working their way down mostly out of sight.’ The eggs hatched in about twenty-four hours, but he noticed that those hatched in confinement required from five to ten hours longer, and that these larvæ when hatched were smaller than those hatched out in the open. The eggs are oval and cylindrical, one twenty-fifth to one-twentieth of an inch long and about one-hundredth of an inch wide, and of a dull, chalky-white colour.