Pearls & Parasites

by Sir A. E. Shipley

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.