lOO GNATS OR MOSQUITOES — CHAPTER VIII where thus more convenient, points of prophylaxis will be dealt with in their own connection.
The necessarily frequent allusions to climatic conditions in India will be more easily understood by occasional reference to the table on pp. 16'2 — 163, the materials of which were kindly supplied me by Mr. J. Eliot, the Meterological Eeporter to the Indian Government. The range of cHmate within the peninsula is very wide, almost every variety of tropical and subtropical condition being represented, from the intense drought of the western Punjab and Rajput desert, with their wide range of annual temperature, to the uniform moist heat of Cochin and the Burman littoral ; and it is obviously impossible to give any complete account within the limits of a page or so of tabulation. The references to the relative salubrity or otherwise of the various places must, moreover, not be taken to apply specially to the town mentioned in the table, but rather to the region of which its climate is representative ; and in speaking of the local malaria as " mild," " virulent," and so forth, I do not refer to the species of parasite (for little else than the sestivo-autumnal fever is to be met with in India), but to the malignancy or otherwise of the type of the disease, which varies greatly in different places, and indeed, from year to year, in the same place. The places included in the list, being selected as fair average examples of the climates or the regions in which they are situate, by no means illustrate the extreme range of variation within the limits of Indian jurisdiction ; and hence do not include such situations as Cherra Punji (said to be the wettest place in the world) where the rain gauge has literally to be graduated to feet ; or Sibi, where, as the tale has it, the smi has such power that the European residents must needs assemble in the club, to sit beneath the billiard table, still wearing their pith hats. InjiiLence of Climate. — There can be no doubt that climate is the most important of all the factors that together contribute to render a given place malarious or otherwise. The genus Anopheles has a world-wide distribution, extending much further north than malaria, which is
CONDITIONS INFLUENCING PREVALENCE 161 unknown in really cold countries, while, given the presence of the malarial blood parasite, no hot climate is free from the disease. If this be absent, there can of course be no malaria, as the disease is no necessary concomitant of a tropical climate, as is shown by the case of the Island of Mauritius, which, previously healthy, suddenly became intensely malarious. In this case there can be little doubt that Mosquitoes of the suitable species were already present, for the endemic developed with a rapidity quite inconsistent with the idea of the establishment of the necessary species from a few chance emigrants. All that was required was the importation of the infected man ; and of this, before the introduction of steam navigation, there was but a precarious chance.
It has usually been suggested that the disease was in this instance imported from India, but the type of the disease seen there is very different. Never in India have I met with cases exhibiting the absolutely classical malarial paroxysm such as I have witnessed in soldiers sent to Natal for change of air from Mauritius.
In other cases the immunity of places where every condition of climate is favourable is due to the absence of Mosquitoes.
In a letter to the Lancet, dated January 18th, 1901, by Mr. H. D. O'Neill, an interesting observation by Robert Louis Stevenson is referred to on the subject of Mosquitoes and their association with filaria and malaria : "In Atuona (Marquesas Islands), a village planted in a shore-side marsh, the houses standing everywhere intermingled with the pools of a taro-garden, we find every condition of tropical danger and discomfort, and yet there are not even Mosquitoes, nor even the hateful day-fly of Wuka-Niva, and fever and its concomitant, the island fe'efe'e, are unknown." It is a long-established fact that the northern limit of malaria corresponds roughly with the summer maximum isotherm of 76° F., or, according to Hirsch, to a mean summer temperature of 15° — 16° C. (60° F.), which is much the same thing. Eecent Italian researches show that the development of the hgemosporidia within the Mosquito 11
Table Showing the Monthly Rainfall and
Mean Temperature of Thirty-One Indian Stations.
164 GNATS OR MOSQUITOES — CHAPTER VIII cannot take place at a lower temperature than 20'' C. (68° F.), or at a higher than 30' C. (86° ¥.), and in the existence of this upper limit we find an explanation of the fact that the hot dry weather in northern India, where for months together the temperature rarely falls as low as this, is, in spite of the unbearable heat, by far the healthiest season of the year, and that during it, primary cases of malaria are practically unknown.
As in these parts of the Peninsula the winter temperature, for considerable periods falls considerably below the lower limit, extreme climates such as we have there, are blessed with two consecutive periods of immunity, malaria being confined to the season of the " rains " and early autumn. Strictly speaking, however. Northern India is sub-tropical. In the Tropics, and especially in the Equatorial region the lower limit is practically never reached and the existence of the higher is of far greater practical importance, as during the drier portions of the year it is, except in certain specially favoured localities, always surpassed for considerable periods. As, however, in the truly Equatorial regions there are two dry and two rainy reasons in each year, the periods of immunity are too short to bring about any very distinct diminution in the amount of sickness due to malaria, as their occurrence is masked by recurrent attacks which always continue to vitiate the statistics of any period of immunity for a longer period than the entire duration of these short intervals.
While then in temperate climates the duration of the winter is the most important climatic factor in securing a prolonged period of immunity, in tropical and sub-tropical regions it is the hot dry weather that exercises a sanitary influence.
Second only to temperature is the amount and distribution of rainfall. It is needless, however, to go into any lengthened considerations of its effects, as these naturally follow from the premise that malaria is dependent on the Mosquito for transmission from man to man; beyond pointing out that besides being the main natural puddleproducing agency it has the additional effect of keeping
CONDITIONS INFLUENCING- PREVALENCE 165 down the temperature, and so shortening or abrogating the period of immunity due to heat, and that the more evenly it be distributed the more serious will be its effects on public health.
So greatly is malaria favoured by a copious rainfall that in the majority of places the monthly incidence of new infections of malaria may he roughly said to be directly proportional to the rainfall of the preceding month. The only apparent exceptions to this rule occur in places where a heavy rainfall occurs during months too hot or too cold for the development of the parasite ; but of course in such places the rule must be taken to apply only to those months during w^hich the air temperature permits of the maturation of the parasite within the Mosquito. Without encumbering these pages with needless statistics, the two following examples may be taken as fairly typical, remembering always that the figures given probably include a much larger number of recurrent than of primary attacks.
Table Showing the Relation op Rainfall to Admissions to Hospital FOR Malarial Fever (including Recurrent Cases) in Two Indian Prisons.
The figures referring to the Nagpur Central Jail are derived from a paper by Major A. Buchanan, I. M.S., published in the Indian Medical Gazette, and both series represent the averages of ten years. In the case of the Shahjahanpur Jail the average strength of prisoners during the period was 378"7.
Conditions of atmospheric pressure can, per se, have no influence on the incidence of malaria.
166 GNATS OK MOSQUITOES — CHAPTER VIII Hill stations are, of course, as a rale free from malaria, "but the reason of this is not that because the barometer stands some inches below the instrument in the plains, but because they are cool and well drained. Given other favourable conditions, and malaria will develop at any elevation, and as a matter of fact, the writer has observed and treated undoubted malaria in Wakham in the upper Oxus Valley, at an elevation of 9,000 feet above the sea, among the Tajik tribesmen who rarely or never visit places at a lower level. The Influence of Water. — In a certain sense, the malariousness or otherwise of any locality is intimately connected with its water supply ; but the question is one o£ its relative abundance and distribution, and not of its quahty. As has already been pointed out, it is, for biological reasons, in the last degree improbable that there can exist any alternative route of malarial infection than that through the Mosquito, and it was not proposed to enter here into the question on the possibility of drinking water forming a vehicle of infection ; but old fallacies die hard, and as an attempt has recently been made by Captain Leonard Eogers, I.M.S., to resuscitate this one, it may be well to devote some space to the consideration of the arguments brought forward by him. Captain Rogers attempts to show that those parts of Calcutta and neighbourhood which receive a filtered water supply are less malarious than neighbouring suburbs which draw their supply from the river, from tanks, or from wells. He estimates the relative malariousness of the compared localities by what he terms their spleen ratio, a factor which he determines by counting the number of enlarged spleens in a given number of persons whom he regards as fairly representative of the general population.
Now such a method of estimation is obviously open to a variety of sources of fallacy, and is, at least, misleading. As pointed out by Drs. Christophers and Stephens, and confirmed by Koch, the true index of the malariousness of .a place, is the average length of time required for the infection of new comers ; and the most convenient class of immigrant, because always ready to hand, are the young
CONDITIONS INFLUENCING PREVALENCE 167 children. Further, as will be pointed out further on, the mere fact that Calcutta is a great city, in which municipalisation on European lines has been carried much further than in the "up-country" towns mentioned below, is alone sufficient to account for the more crowded portions of the towns which have been provided with a filtered water supply being less malarious than the suburbs, even assuming that Captain Eogers' researches may be taken as conclusive that such is really the case. x\s a matter of fact, experiments on a very large scale have been of late years conducted in certain Indian prisons, where the entire drinking water supply of the prisoners was carefully and systematically boiled in the hope of diminishing malarial disease, but without producing the least effect in the hoped for direction. But apart from this, our experience in Northern India strongly suggests that, so far from diminishing malaria, the introduction of a piped and filtered water supply, has commonly quite the opposite effect. It is unfortunate that before venturing on generalisation from what appears to be such scanty data. Captain Eogers was not at the pains to avail himself of the tabulated information on this point readily available to him in the annual reports of the Sanitary Commissioners of the Northern Provinces, where the effect of the introduction of filtered water supplies has been a matter of anxious observation for a considerable series of years, for it is impossible to imagine that any diminution in malarial fevers and splenic enlargement could fail to show itself in a diminution of the general death-rate. For the benefit of those to whom the above reports may not be readily accessible, the figures bearing on the question are extracted on next page : — The addition of this table to the Sanitary Reports was probably initiated with the view of illustrating the benefits conferred by modern sanitation ; but if this be the case the compilers must have been most disagreeably surprised, for with hardly an exception a rise, and not a fall, of mortality has followed. There are, of course, absolutely no statistics extant of any value whatever as to the absolute number of deaths referable to malaria in any Indian town,