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.