Pearls & Parasites

by Sir A. E. Shipley

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.