Climate and Health in Hot Countries and the Outlines of Tropical Climatology / A Popular Treatise on Personal Hygiene in the Hotter Parts of the World, and on the Climates That Will Be Met Within Them.

by George Michael James Giles

The third, or Anopheles, sub-family is that concerned in the transmission of human malaria, and, as may be seen by comparing the two lower photographs of the [plate] with the upper ones, can easily be recognised by their characteristic form and attitude. In these mosquitoes the feelers are long and thick in both sexes, and as they are held habitually in contact with the proboscis, these together appear to the naked eye as a prolongation of the body as thick, or thicker, than the abdomen. Moreover, except in a few species, their position, when resting, forms a singular contrast to that of the Culices, the whole body and proboscis being held in one straight line, with the abdomen raised from, and the proboscis pointed almost vertically at the surface on which they rest, and almost touching it with its point, as if they were preparing to drive the latter into it; so that, viewed with the naked eye, they look much like minute black thorns, stuck into the surface on which they are sitting. On closer examination, it will be seen that the wings are not plain, but spotted (the number of plain-winged species being unimportant).

Fig. 10.—Wings of various sorts of Anopheles mosquitoes, much magnified, to show the arrangement of the dark and light scales which form the characteristic spots.

The appearance of spotting is usually produced by alternate lengths of the veins of the wings being coloured in finely contrasted dark brown or black, and white or yellow, and varies, of course, in detail in different species, but the above figures of the wings of some of the commoner Indian forms will give an idea of the general effect, as seen under a moderate magnification, for though the spots are quite visible to the naked eye, it requires a fairly powerful hand lens to make out the details properly.

Like the Stegomyiæ, the Anophelinæ are only common during the rains, but stray specimens will be met with for a month or two afterwards; and though they practically disappear during cold weather, a few may be generally met with during the hot dry months of continental hot climates.

From what has been said, it follows that the prevalence of malaria, like that of mosquitoes, must necessarily be seasonal. The survival of the malarial parasite at those times of the year when mosquitoes are scarce or absent, is maintained by the persistence of considerable numbers in a latent condition, in the persons of human beings who have been imperfectly cured of an attack of malaria.

Cases of this sort, which have either been never treated at all, or in which the administration of quinine has been stopped too soon, are extremely common in all malaria-stricken communities, and such persons are always liable to relapses if they are brought below par by any depressing influence, such as chill, too great exposure to the sun, or the incidence of injuries, or of other diseases.

During such periods the latent parasites, which have been perhaps for months lying hidden in the internal organs, will reappear in the blood in large numbers, and there are always a sufficient number of such cases of relapse persisting through the naturally malaria-free season, to suffice to infect the early broods of mosquitoes that come to maturity with the advent of the rains.

This shows that the curing of all cases of malaria is matter by no means confined to the interests of the patient himself, but is of great importance to the entire community of which he is a member; for, despite the fact that the disease is only indirectly transferable through the agency of the mosquito, a case of malaria is as great a danger to his fellow-men as one of any other infectious disease, and should, as far as possible, be dealt with in the same way; only the problem of isolation is in this case easier than usual, as it suffices to prevent mosquitoes getting access to the sick person; and in a properly fitted mosquito-proof house or hospital, malaria cases may quite safely be allowed to mix freely with the healthy, as well as with persons affected with other diseases.

Relapses in persons chronically infected with malaria may occur at any time of the year, but the fresh infections, which always form the bulk of the cases met with, can only happen as the result of a tolerably recent bite by an infected mosquito; and as about a couple of weeks are required for the maturing of the parasite within the insect, and a further period of incubation is necessary within the human subject before the parasites attain sufficient numbers to produce a definite constitutional reaction, fever does not as a rule commence in earnest until some three weeks or a month after the break of the wet season. Once the process is started, infected mosquitoes and malaria-stricken men increase in numbers rapidly, and as numbers of infected mosquitoes survive for a considerable period after the cessation of the breeding season, the prevalence of malaria continues until the advent of cold weather puts a period to the possibility of the survival of the parasite within the insect organism. Hence it is generally quite possible to give a fair idea of the monthly distribution of rainfall in any warm climate from the returns of sickness and mortality, and vice versâ.