Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/339

MALARIA 319 mittent and remittent fever, from that of 1557-53 (which spread over all Europe) down to that of 1872, which prevailed simultaneously in Europe, North America, and southern India. The epidemic or pandemic prevalence of intermittent and remittent fever in certain years probably finds its explanation in the meteorology of those years, but no uniform law has been discovered. Whenever malaria has settled endemically in a new locality, there had been epidemics coming and going for some time previously. Malarious Localities. The most malarious localities are the deltas and estuaries of rivers (Ganges, Euphrates, Po, Mississippi, Orinoco), low-lying country that is apt to be inundated (Danubian states), tropical or subtropical forests in which there is a moist atmosphere, with stagna tion of the air and rank vegetation (jungles), tracts of land that have been cleared of trees and have gone out of cultivation, being in more cases dry than wet (Koman Campagna, Tuscan Maremma, many parts of Persia, Asia Minor, and North Africa, including the sites of ruined cities), inland swamps and marshes (Pontine Marshes), and situations on the coast where the tidal and fresh water join to form brackish marshes (mangrove swamps of the West Indian, Central American, Brazilian, and West African coasts). The mangrove is associated with the most pestilential localities; it springs "like a miniature forest out of the greasy mud-banks, the bright green colour of the bushes reminding one of the rank grass in a churchyard" (C. Darwin). In all those localities there is a soil, usually wet but sometimes dry, rich in the products of vegetable decay ; the soil has been either deposited by rivers and tides, or it has formed on the spot out of the undisturbed accumulation of decaying vegetation season after season over a long period. There is, however, a second great class of malarious localities, distinguished by characters that are to some extent the opposite of the foregoing. These are barren rocks (Ionian Islands, Hong Kong, parts of Baluchistan, De Los Islands near Sierra Leone) ; high table-lands more or less barren (Deccan, Mysore, Persia, New Castile); mountainous regions (Andes, Rocky Mountains) ; prairies of North America and savannas of Venezuela and Brazil; sandy plains (North Africa, Rajputana, Sindh). A somewhat excep tional locality for malaria is on board ship at sea ; there are several well-authenticated instances of epidemic outbreaks at sea, in most cases referred to the putrid bilge- water, and in one case to a cargo of wet deals from the Baltic. There are several localities whose exemption from malaria has been thought remarkable. Among these, Singapore has long been noted ; other instances are the Amazon (as compared with its tributaries and with the Orinoco), the pampas of the La Plata and the Parana, marshy parts of Australia, New Zealand, and New Caledonia, and the marshy Bermudas. The explanation given of the exemption of Singapore, where many of the supposed malarial conditions are present, is that the range of temperature (diurnal and annual) is small ; the explana tion for the Amazon is that a wind constantly blows up the river from the sea (not reaching the side streams), which serves to equalizs the day and night temperature and to obviate the nocturnal radiation of heat. Malarious Seasons. In temperate climates autumn is the season when malaria prevails most. " In the autumn, and after the harvest has been gathered, when the ground is covered with its debris, when the rain falls in torrents and when the solar heat has acquired its greatest intensity, all the conditions of greatest quantity of vegetable matter, of moisture, and of highest temperature are united, so that the season which realizes the hopes of the husbandman is the period of pestilence and of his greatest danger " (R. Williams). In the equatorial regions of the East Indies, Africa, and America, the rainy season (May to J uly or August) is mos.t unhealthy, and especially the time of commencement of the rains and the time of cessation ; on the west coast of Africa the months of February, March, and April, which are the hottest months of the year, are at the same time the most healthy. But while autumn and the time of the rains are the malarious season for those localities that are distinguished by wet soil, rank vegetation, &c., it is summer, or the time of extreme heat and drought, that is the unhealthy season for the localities distinguished by dryness of the soil and often by barren ness. The hill fever of the Deccan and Mysore is often most prevalent and most severe in the hottest and driest seasons ; in Algeria there is most fever when the country is parched to a desert. The malarial season in the Tuscan Maremma is from June to the middle of September. In military experience it has frequently happened that malaria has attacked the troops in the hottest weather after camp ing in the dried-up water-courses of uplands, or in parched meadows and sandy levels that are apt to be flooded only in winter. Conditions of Origin. In all localities and at all seasons, it is at or after sunset that the malarial influence prevails, and it tells most when a cold night follows a hot day. Perhaps the most constant fact relating to malaria is that it goes with watery exhalations and with the fall of clew, On wet soils, and over marshes, swamps, and jungles, the aqueous vapour condenses as the air cools ; while on dry surfaces the rapid radiation of heat causes a heavy dew-fall. The occurrence of malaria on bare rocks, parched uplands, and treeless tracts of dry fallow land may have several associated circumstances ; but that which has been most uniformly observed in such localities is great diurnal range of temperature, with rapid radiation of heat after sunset, and copious fall of dew. The " hill fever " of Mysore occurs among bare rocks and stones and brown earth ; at the hottest season (March to June) the diurnal range of the- shade temperature may be 20 to 30, while the rocks in the sun may show a surface temperature up to 220, and undergo a rapid cooling after sunset. The most malarious locality at all times of the year on the Orinoco is around the great cataract, where the banks of the river for some distance are covered with bare black rocks piled to a con siderable height ; the rocky substance and the black surface combine to produce the greatest absorption of heat and the most rapid radiation, and the rocks there, as well as in other parts of South America and in India, are credited by the natives with giving off poisonous exhalations which cause the fever. Among the conditions of origin the pre disposition of the human subject takes a prominent place. Those who have been habituated to extreme heat, and are on occasion exposed to cold and damp, are likely to acquire intermittent or remittent fever ; and those who are poorly clad, housed, and fed are most likely. Fires at night in a malarious locality are a well-known protection from fever; the cover of trees (preventing the radiation of heat) is also a protection. Those who have had ague before are liable to have it again on exposure in a malarious locality, or to chill anywhere. Diffusion of Malaria. On the hypothesis that malaria is a poisonous substance, it is permissible to speak of its diffusion. It acts for the most part only within a few feet of the ground ; in the East Indies the raising of dwellings on piles serves to keep off, or at least lessen, the liability to fever, and the Indians in South America escape it by sleeping in the branches of trees. Although it is not known to act beyond a few feet from the earth s surface, it may produce fever in localities situated at a height of

7000 to 9000 feet above the sea-level. It sometimes acts