152 CHAPTER VIII.
On the Conditions Influencing the Prevalence of Mosquitoes, and on the Prophylaxis of Malaria.
In the previous issue of this handbook, but Httle space was devoted to this subject ; but though no more than a year has elapsed, so much has been observed and M^ritten on the subject, that it is impossible to give any adequate idea of the present state of our knowledge in a few paragraphs, incidental to the life history of the Culicidce, so that it has become necessary to devote a special chapter to the subject. To-day it may be fairly asserted that the question of the connection between malaria and Mosquitoes has passed beyond the tentative stage, and is indeed no longer a " question," but an ascertained fact. Not only has the casual connection between the Mosquito-carried parasite and malaria been proved be3'ond question of reasonable cavil, but experiments on a large scale in the practical application of our present knowledge to the prevention of malarial disease have shown that, given sufficient intelligent co-operation of the affected population, and sufficient earnestness on the part of the governing authorities, it is quite possible to so avail ourselves of this newly gained knowledge as to greatly diminish if not to actually "stamp out " the disease. Without this co-operation, however, we are helpless, for science can only help those who will consent to help themselves. The primary discovery of Laveran has gradually been so followed up by Marchiafava, Golgi, Plehn, Celli, Grassi, and many other distinguished observers in Europe, as to place Laveran's "epoch making" discovery on the soundest basis ; establishing the fact that we have to deal with not one only, but at least three distinct species of blood parasite and
CONDITIONS INFLUENCING PREVALENCE 153 elaborating and systematising our knowledge of their life history within the blood-vessels of the human subject. Following on these we have the undoubtedly pioneering observations of Major Ross, I.M.S., on the life history of the stage of the parasite which is passed within the mosquito, which are all the more remarkable when the conditions under which he worked are understood. Conditions under w^hich those accustomed only to the luxurious ease of a European laboratory with dozens of assistants well-nigh as able as themselves to lighten the work, would probably find so untenable that it may be doubted if they would succeed in observing at all. But even under the most favourable conditions, initial observations of the sort must needs be always more or less incomplete and that some inaccuracies should be discoverable in Ross's work is merely equivalent to stating that the work was of an initial and pioneering character. Ross's observations have been now confirmed and elaborated by the admirable work of Grassi, but there still remain some points to be elucidated before our knowledge of the life history of the parasite within the mosquito can be considered as complete. Finally, it has been clearly demonstrated in Italy that man can be inoculated with malaria by the bite of infected Mosquitoes of the Genus Anopheles and quite recently Major Andrew Buchanan, I.M.S., has recorded in the April issue of the Indian Medical Gazette, a number of carefully conducted experiments which establish the same fact for India. The Italian observers, having the advantage of working in a country, highly malarious, and yet provided with fully equipped laboratories, and all the resources of an advanced civilisation have been able to conduct their experiments with an exactness of precaution quite unattainable in the semi-civilised haunts of tropical malaria, and to the unprejudiced critic, leave no really fair ground for objection ; but they still leave it open for the superficial objector to suggest that, being inhabitants of a notoriously malarious country, the subjects of experiment may have been infected in some other way than through the agency of the experimental Mosquito bite. The standpoint of these last
154 GNATS OE, MOSQUITOES — CHAPTER VIII objectors is, however, entirely cut away by the truly crucial experiments conducted in the London School of Tropical Medicine under the direction of Dr. Manson which conclusively demonstrate that malaria can be transmitted to man through the agency of Mosquitoes. A number of Anojjheles were allowed to bite a patient suffering from tertian ague in Italy. They were then transported to England and made to bite two healthy young English students. Both these gentlemen developed tertian malarial fever, and the characteristic parasites of the disease were found in their blood.
It is difficult to find in this experiment any possible source of fallacy. It is absolutely conclusive of the fact that this is at the very least one of the methods of the transmission and propagation of the disease ; and a very little consideration will show any one conversant with the data of parasitism that it is also necessarily the only one, saving only by the intravenous injection of the blood of a patient suffering from malaria into the vessels of a healthy subject ; a method hardly likely to occur in nature. The reason for our assurance of this is that the malaria) parasite requires two successive hosts — a liuman being and a Mosquito — to attain sexual maturity and propagation. Ill the blood of the fever patient it multipHes non-sexually ; in the tissues of the Mosquito it does so sexually. Now there are a large number of parasites which have an exactly parallel history, the most familiar being that of the tape-worm, which lives and multiplies asexually in herhivora and other eaten animals, and passes its sexually mature life in the carnivora, and other animal-eating animals. Just as it is possible to introduce asexually multiplying malarial protozoa mechanically into the veins of a healthy man, so would it, doubtless, be practicable, in these days of abdominal surgery, to lay open the intestine and introduce into it a living tape-worm, which would, doubtless, continue to thrive in its new host. But in the ordinary plan of nature, the eggs discharged from the bowel of the ea^w^-animal are discharged in situations when they are hkely to be swallowed by the eaten animal, and in the latter produce the asexually
CONDITIONS INFLUENCING PREVALENCE 155 multiplying bladder worm. This, when swallowed with its eaten host, developes, in the flesh -eating animal, once more into the sexually multiplying tape-w^orm.
Now, although we are acquainted with a large number of parasitic life histories of this character, we know of no instance in which a parasite with such a history is capable of maintaining the continuity of the species in any other manner, and it will be indeed astonishing if the malarial parasite should prove an exception to what has been hitherto found to be an unvarying law of parasitism; though of course, however intrinsically improbable, it is within the range of biological possibility, that besides finding its way to its intermediate host along with the venom of the Mosquito, the parasite may also be capable of assuming the form of a resting spore, or some kindred reproductive mechanism, and being in this way conveyed to water, food, &c., through the agency of the living or dead Mosquito. There is not, however, a single recorded fact in the natural history of the disease that suggests the probability of such an occurrence, and practically speaking, everyone possessing any special knowledge of helminthology will be convinced that either the idea that the Mosquito is the alternative host of the malarial parasite is a huge mistake ; or it is, under natural circumstances, the one and only method of infection. There is in reality no tenable middle position. Most of the apparent exceptions depend on the fact that like most other two-host life-history parasites, the host carrying the asexual phase of the malarial parasite may do so for years without any perceptible inconvenience. A bladder worm may have to lie imbedded in the tissues of an ox for years before the animal is turned into beef and devoured by a man.
Then its opportunity has come and it developes into a tape-worm each sexually mature, proglottis of which is a complete, hermaphrodite, sexually mature animal. So with the malarial parasite. An infected person may have no visible symptoms, but lurking in his tissues are the parasites ready to start again on their course of asexual multiplication should any accident bring the resisting power of the host sufficiently low.
156 GNATS OR MOSQUITOES — CHAPTER VIII Hence, persons who have had no recent opportunity of being bitten by Mosquitoes often do develop a typical ague, but the fact remains that they must have been bitten at some time, and as a matter of fact the interval is a concern of but little moment to the parasite. The patient in fact, though apparently' well, has latent malaria ; in other words, he harbours but a harmless number of quiescent parasites, and the exception is only apparent. The fact of the possibility of the transmission of malaria in this way having thus been now conclusively demonstrated, we may take it as certain that every malarial patient has at some time been bitten by an infected Mosquito. Further, it appears probable that only Mosquitoes of the genus Anopheles are capable of acting as the host of the sexual stage of the parasite, but this is not certain.
Now the malarial parasite is responsible for by far the greatest proportion of all sickness and death in the tropics. Cholera and plague are the insignificant enemies that perhaps kill a few thousands a year —in an impressive way it is true ; but the quiet, insidious malaria sweeps off its millions, and so habituated have we, native and European alike, become to the danger, that we have come to look upon the inconvenience of one or more " touches of fever " during the year, as a necessary evil, inseparable from the conditions of tropical residence, and no more to be escaped than the occasional " cold " of more temperate climates. Unfortunately there can, I fear, be no doubt that this fatahstic frame of mind will, for a long time to come, constitute one of the greatest obstacles to sanitary improvement ; for such preconceptions are hard to eradicate, and hence it comes that, while large sums are freely expended in fighting in the dark against the unfamiliar terror of plague, in the case of malaria, where we have already a large basis of solid facts to work upon, our total expenditure on the prevention of malaria appears to be comprised in the vote of the absurd sum of Ks. 30 i^er mensem to provide the salary of a man to destroy Mosquito-larvse with kerosine, by the City fathers of Calcutta. I doubt if India will ever be a pleasant residence for the white man for
CONDITIONS INFLUENCING PREVALENCE 157 the greater part of the year, but nevertheless it would not compare very unfavourably with the temperate zone from a health point of view, could we but do away with malaria. While proposing, in the main, to treat the interdependence of malaria and Mosquitoes as an established fact, it may be well to devote a few words to certain objections that have lately been, in various forms, advanced. The first is that malaria may exist in places or at seasons when Mosquitoes are rare or absent. Now in the first place, putting aside certain little visited islands, gnats or Mosquitoes are to be found practically everywhere, from Greenland to the Equator, and it may be taken as certain that abundance of these insects or their larvae would be found by any one accustomed to the search in every one of the cited cases of their absence. Moreover, it will be generally found that such objections are raised by persons who, however intelligent and highly educated they may be, have had no practice in observations of the kind required for the record of facts bearing on natural history, and they are apt to forget that, in this, as in any other special business, long training, much patience, and a certain aptitude, are required for the work. A writer, for example, in a well-known Indian lay journal, a propos this question, gravely propounded the astounding statement that Mosquitoes were extinct in India in the rains, and specially troublesome in the cold weather, whereas, without for a moment questioning the good faith of the writer, it is needless to say that exactly the reverse is the case. In all probability he had never made a single definite note on the subject, or troubled himself as to whether few or many Mosquitoes were about, since the time when they made themselves painfully obvious to the newly landed journalist during the time he was acquiring that indifference to their bites which all of us sooner or later develop ; and under such circumstances memory is naturally treacherous. Another argument that has been used is that, whereas the presence of malaria is dependent on that of man, and should therefore be worst where population is thickest, the reverse is the case, as towns enjoy a practical immunity,
158 GNATS OR MOSQUITOES — CHAPTER VIII while some of the most deadly spots in the world have but a scanty population. The reason why the conditions of urban life are unfavourable to the spread of malaria will be dealt with further on, and it is sufticient to point out that the reason why such tracts of country as the notorious Indian Terai are so deserted is that they are too malarious for human occupation. In other words, the local conditions are so favourable to the multiplication of the species of mosquito concerned in the transmission of the disease that the presence of a very small number of infected persons suffices to infect an enormous number of Mosquitoes, and to render well nigh certain the infection of any visitor who exposes himself to the same conditions. Moreover, every place of the sort that I have heard of has always been a tract of close jungle or swamp, in which it is impossible for the traveller to stray from certain beaten tracks, or to make his camp elsewhere than at certain definite halting places, where, however small may be its number, there is always a permanent population, or at the least, passers through are sufficiently numerous to maintain the infection. That in such deserted tracts these foci of intense malaria are purely local I do not entertain a doubt, and that a healthy man who landed from a balloon a mile or two away from them would, though equally pestered by Mosquitoes, take no other harm ; but in such country man can push his way but slowly, and there is always time for those who attempt to open it up to carry the infection with them. Moreover, in comparing the relative salubrity of neighbouring places, it must be remembered that a certain temperature is essential to the development of the parasite in the body of the Mosquito, and that on this account a difference of a few thousand feet above the level of the sea is quite sufficient to account for a place being quite healthy, though but a few miles of horizontal distance from foci of intense virulence. It is this factor that is overlooked by Mr. Guy Marshall, in his paper on " Mosquitoes and Malaria," in the Entomologist, August, 1900, p. 218, who points out that Salisbury, in Mashonaland, though comparatively thickly populated, is much less malarious than the sparsely
CONDITIONS INFLUENCING PREVALENCE 159 populated Umfuli district. Being a skilled entomologist, Mr. Marshall found several species of Anopheles in both localities, and asks why the more populated district should not be the more malarious. He forgets that Salisbury lies nearly 5,000 feet above the sea, more than 2,000 feet above the malarious regions, and that, though elevation per se is no absolute bar to malaria, the temperature of places so elevated, in that latitude south, is always too low for the development of really serious malaria. Curiously enough, we find that almost simultaneously another writer from Mashonaland, Dr. Ch. Todd, demonstrating (Journ. Trop. Med. 1900, p. 92.) that there, as elsewhere, the curve of malaria prevalence follows that of rainfall and therefore of Mosquitoes ; the most rainy months being January and February, and the most feverish, March and April. Before proceeding to the consideration of the prophylaxis of malaria, it will be necessary to examine in detail how far the prevalence of Mosquitoes and therefore of that disease are influenced by climate, cultivation, and the other incidents of everyday human environment. Working as I have in India, it is natural that most of the examples cited should be based on observations made in that country ; but communications, personal and published, from observers in other tropical regions, most of which cannot, however, be quoted in any moderate space, convince me that our experience in that country may be taken as fairly typical of the conditions present in other hot climates.
It will be seen too that, with modifications arising from the differences of oriental surroundings, I come to practically the same conclusions as Professor Celli, in his admirable work on " Malaria in Italy," and if I quote less than might be expected from that work, it is partly because most of the notes on which the present chapter is based were written before I had an opportunity of reading it, but mainly because I thmk that everyone interested in the subject should make a point of reading his book in extenso. In certain cases, however, it is hardly practicable to separate the consideration of the conditions favouring malaria from the practical outcome of the facts noted, and.