The endemic diseases of the southern states

Front matter

MEDICAL BOOKS

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^Attnmtt SItbrarg

Preface

The inception of this book was due to the fact that there is no work in existence deahng solely with the endemic diseases of the Southern States.

It must not be inferred that the diseases considered are confined to the states of the South. On the other hand most of them are disseminated throughout the United States and some of them are found in most of the states of the Union. It is true, however, that these diseases are more prevalent in the Southern States.

It has not been deemed advisable to treat of all the diseases that are especially prone to visit our confines at longer or at shorter intervals as the list of such diseases is long and such a work would necessarily be one on tropical diseases in general to which this work makes no pretension. We have concluded that it would be more practical to consider intensively only the endemic diseases of our Southland rather than to devote brief space to each of a large number of tropical diseases the most important of which will soon be of historic interest only.

Filariasis which occurs in an extremely limited area has been omitted.

We are grateful to the publishers for courtesies throughout the publication of the book.

The Authors;

Hot Springs, Arkansas, March, igi6.

Index 541

Chapter I Introduction

History. — The history of malaria may be traced to the age of fable. The story of Hercules and the Hydra is a familiar one. This monster dwelt in the morasses in the neighborhood of the Lake of Lerna, where Hercules was dispatched to destroy him. As each of the nine heads was struck off two new ones appeared. With the aid of his faithful servant, lolaus, who burned each wound caused by the removed head, the beast was finally conquered. Even before the birth of Christ this myth was construed to typify the reclamation of swamp lands, uninhabitable on account of the presence, of malaria. Antipater wrote, "Hercules, the greatest subduer of the foggy atmosphere in times past, was placed among the Gods for having destroyed the Hydra; in other words for having reclaimed the marshy desert." The slaying by Apollo of the Python which arose from the fertile ground after the recession of the flood is similarly interpreted.

More than one thousand years before the birth of Christ malarial disease is mentioned in the Orphic poems, and the tertian and quartan types are alluded to. In the Iliad of Homer, and the Wasps of Aristophanes allusions are made to a fever which was probably malarial. Paludism was probably introduced into Greece from Egypt. According to Groff the word AAT, which is found among the inscriptions of the temple of Denderah, referred to a disease, doubtless malaria, which recurred every year at the same season. 2 17

Hippocrates divided malarial fevers into continuous and intermittent, which he subdivided into quotidian, tertian, and quartan. He recognized the etiologic influence of season, rains, and stagnant water, and the dangers of malignancy, dropsy, and affections of the spleen. Plato describes splenic enlargement, and other early Greek writers undoubtedly refer to malaria.

References by Roman writers to malaria are not numerous, the earliest being that of Plautus, who died 184 B.C. Cato speaks of "black bile and swollen spleen," and Cicero, Varro, Celsus, Livy, and others show unmistakable evidence of a knowledge of the disease.

Passing over the development during the middle ages of the knowledge of malaria, the names of Morton, Lancisi, Sydenham, and Torti appear.

Morton, 1697, gave accurate clinic descriptions of the pernicious and intermittent fevers, and attributed them to miasmatic effluvia. He was an ardent advocate of cinchona whose value was at that period being hotly contested.

Sydenham, 1723, accurately described the malarial fevers. The intermittent fevers he divided into spring and autumn fevers. He justly concluded that the intermittent and continuous forms of malaria were due to the same cause. Sydenham ably defended cinchona, and after clinic experiments with its use, formulated useful rules for its administration.

Lancisi, 1717, stated the etiologic relationship between marshy regions and malaria, and was the first to seek for a microscopic organism as the cause of the disease.

Torti, 1753, wrote an exhaustive treatise upon the various forms of malaria. His classification of the pernicious forms has become classical. Numerous quotations from Torti's" treatise are to be found even in recent works upon malaria.

Varro, 118-29 B.C., expressed the opinion that malarial fever was caused by animals so minute that they could not be seen by the naked eye, and which enter the body with the air through the nose and mouth. Similar opinions were held by Columella, Palladius, and Vitruvius. Rasori is quoted as saying, "For many years I have held the opinion that the intermittent fevers are produced by parasites which renew the paroxysm by the act of their reproduction which occurs more or less rapidly according to the variety of their species." Le Diberder, 1869, maintained that the fever was due to the presence in the blood of animals which preyed upon the blood, and that the paroxysms depended upon reproductive acts between which apyrexia occurred.

Mitchell, 1849, claimed to have found in the sputa of malarial subjects fungus spores in great numbers, which he believed to have been inspired with marsh air and to have caused the disease.

Salisbury, 1866, announced the discovery in the urine and sweat of malarial patients of a species of alga, palmella, common on the marshy regions along the Ohio and Mississippi Rivers, which he alleged to be the causative element.

Until the true parasite of malaria was discovered, the most widely accepted parasitic theory was that proposed in 1879 by Klebs and Tommasi-Crudeli. These investigators found constantly present in the mud of the Roman marshes a short bacillus. They were able to cultivate it upon fish gelatine, and when injected into rabbits produced a fever similar to malaria. They named it the bacillus malaria.

The malaria parasites were undoubtedly seen and described before Laveran discovered them. In 1847, Meckel, who first discovered malarial pigment, described bodies containing pigment which correspond to the malarial parasites. Virchow, in 1849, ill 3- description of the pigment, depicted cells now known to be parasites, as did also Frerichs in 1866. The pigment was observed also by Dlauhy, Heschl, and Planer. None of these investigators, however, recognized the significance of these bodies, and their parasitic nature was not suspected until 1880 by Laveran, to whom all the more honor is due.

Charles Louis Alphonse Laveran was born at Paris, June 18, 1845. He entered the mihtary service and was assigned to Algeria where his brilliant discovery was made Nov. 6, 1880, and announced to the Paris Academy of Medicine, Nov. 23, 1880. He was using a one-sixth inch dry lens when examining the blood.

Laveran's discovery was not accepted by the medical world until several years later; now it has been confirmed the world over. Among Americans who first corroborated Laveran's views may be mentioned Sternberg, Councilman and Abbott, Osier, James, Dock, Thayer and Hewetson, Barker, Woldert and Welch.

The discoveries of Golgi in 1885 were of great importance with reference to the hfe history of the parasite. He was able to follow tertian and quartan parasites throughout the endogenous cycle of development, and showed that a close relationship existed between certain phases of parasitic growth and certain stages of the paroxysm. Other Italian investigators proved the same for the estivo-autumnal parasites.

A vague suspicion that malaria and mosquitoes were in some definite way connected has been entertained in certain countries for a long period. A definite mosquito theory, however, was born in America. While reference is sometimes made to a paper on the "Mosquital Origin of Malarial Disease," supposed to have been published by Dr. John Crawford in the Baltimore Observer, in 1807, no such article has been found, and the reference is probably erroneous.

In 1848, Dr. Josiah Nott,^^'* of Mobile, published a paper upon yellow fever, in which he maintained the dissemination of that disease by insects, and suggested that malaria was spread by the "mosquito of the lowlands."

The most complete theory was proposed by King,*^° in 1883. His views are supported by nineteen arguments, most of which are incontestable at the present day.

That mosquitoes are agents in the spread of malaria was advanced by Koch in 1884, by Laveran in 1884, by Flugge in 1891, by Manson in 1894, and by Bignami in 1896.

Undertaking the work of Manson's suggestion, and after several years (1895-1898) of toil and discouragement, Ross proved conclusively that certain species of mosquitoes were concerned in the dissemination of malaria. The debt owed him by mankind was acknowledged by the gift of a Nobel prize.

MacCallum in 1898 demonstrated that the flagella represent male sexual elements analogous to spermatozoa.

Bass' discovery of the method of cultivating the malarial parasites is the most important landmark in the history of malaria since the discovery of Ross.

Geographic Distribution. — North America. — In the United States it is chiefly the southeastern portion in which malaria is most prevalent. Along the Atlantic Coast, south of New York and especially the lowlands of Maryland and Virginia, and in the Carolinas, Georgia and Florida the disease occurs frequently. Along the Gulf coast and up the Mississippi River and its tributaries, malaria is widely prevalent. The portions of the States lying along the Appalachian Range are almost exempt, but the disease appears as the Mississippi River and the Atlantic Coast upon either side are approached. West of the Mississippi, Arkansas, Louisiana, and Texas present the most numerous foci of malaria. In portions of Pennsylvania and New York autochthonous cases are not infrequently observed. In the more southern New England States malaria is still encountered, and in some places is even increasing in frequency. In the neighborhood of the Great Lakes malaria is very rare, excepting possibly that of Lake Erie and of Lake Michigan. In the Central States malaria has almost or quite disappeared, except in certain low river valleys. Along the Pacific Coast the disease is not so frequent as along the Atlantic. In Washington it occurs in the Puget Sound Basin and the Columbia River, Chehalis, and the Yakima valleys. In Oregon malaria is found in the Columbia, Williamette, Rogue, and the Umatilla valleys, and in California in the Sacramento, San Joaquin, Tulare, Kern, and Santa Clara valleys. In certain parts of New Mexico malaria is occasionally met with.

Canada is free from paludism except along the northern shore of Lake Ontario.

In Mexico severe forms of malaria occur, particularly in the low coast regions.

Malaria abounds in Central America along the Atlantic Coast and to a less extent upon the Pacific side.

South America. — The eastern coast of South America is more intensely infested with malaria than is the western coast. Venezuela (in the valleys), Guiana, and the greater portion of

Brazil are highly malarial. Portions of Paraguay and of Bolivia afford a great many cases, while the disease is much less prevalent in Uruguay and almost absent from the Argentine Republic. On the Pacific border the deep valleys of Peru and of Ecuador are malarial centers.

The entire island of Cuba is malarial to a greater or less extent, as is also Jamaica. Of the Lesser Antilles, St. Vincent and Antigua are highly malarial, and Barbadoes is exempt. Malaria is said to be almost unknown in the Bermudas.

Europe. — Great Britain, once infested, is now free from endemic malaria. In Germany the disease occurs infrequently in the Rhine and Danube valleys and near the mouths of rivers along the coast. Malaria is met in Holland, chiefly upon the island of Zeeland and in North and South Holland. The valley of the Danube, in Austria, affords a considerable number of cases. There are a few regions in Hungary in which the disease does not occur, but it is especially along the western half of the southern border that it is prevalent. The marshes along the west coast and the south of France give rise to a number of cases of malaria. In Spain and Portugal malaria occurs in the coast regions and in the larger river valleys. The disease is practically unknown in Norway, but is occasionally reported from Sweden, as well as from certain of the islands of Denmark. In Russia it is in the southern portion, particularly along the coasts and along the valleys of the rivers flowing southward, that malaria is encountered. Cases are occasionally observed in the southwest of Switzerland. The portions of Bulgaria most highly malarial are the Danube valley, the coast region, and the southern part. Almost the whole of Italy is sorely afflicted with malaria, as are also Sicily and Sardinia. Greece is the most severely scourged country of Europe. It is said that in the plains of Thessaly, Phthiotis, Acarnania, Boeotia, Elis, Messenia, Argos, and Laconia hardly a single inhabitant escapes the disease.

Asia. — Asia Minor, Arabia, and Persia present foci of malaria, both in the coast neighborhoods and in the interior lowlands. In the swampy regions of Afghanistan and Beloochistan malaria is common and severe. The foothills of the Himalayas, the Duars, and Terai are famous malaria seats. Both the coast regions and the interior highlands of Ceylon are endemic territory. Burmah, Siam, the Malay Peninsula, and French Indo-China are malarial in portions of their extent, and parts of China are intensely infested. Malaria is found in Japan, Formosa, and the PhiHppines, and portions of the East Indies are among the most highly malarial regions of the world.

Africa. — On the west coast the territory, between the Senegal and the Congo Rivers, is headquarters for malaria of malignant type. Approaching South Africa the disease diminishes in frequency and in severity. On the east the region from Delagoa Bay to Eritrea is malarial. In the interior of Central Africa, excepting the high elevations, malaria is widespread. Malaria abounds in Madagascar, excepting upon the northeast coast and the mountainous interior. Reunion and Mauritius are also malarial. In Egypt it is chiefly the region overflowed by the Nile in which the disease is most prevalent. Malaria abounds about the coasts and marshes of Algeria.

In Australia malaria occurs from Cape York to Brisbane, on the east coast, diminishing toward the south. New Zealand is apparently free from malaria, and the Sandwich Islands and most of the other Pacific islands are markedly exempt.

The relative frequency of the forms of malarial infection varies greatly. It may be stated as a general proposition that the quartan is the rarest form, the tertian is the form prevailing in temperate regions, and the estivo-autumnal in warm and hot climates. There are regions, however, in which the quartan predominates, as in certain portions of Italy and of India; in other localities it is the only form of malaria present, as upon the island of Merite, of the Bismarck Archipelago.

The following table shows the relative frequency of the types of malaria in various regions :

Authority

Texas

Georgia

Camp WikofE

New Orleans

Baltimore

Panama

St. Lucia

Panama

Italy

Italy

Greece

Bulgaria

Italy

British Malaya

British Malaya

Philippines

India

Cyprus

East Indies

Philippines

India

India

Assam

Japan

Togo

German East Africa German East Africa

Senegal

German East Africa German East Africa German East Africa German East Africa

Moore""

Curry"'

Ewing'^

Charity Hospital Recs.'"*

Thayer and Hewetson'"'

Kendall""

Gray and Low*"

Gorgas"2

Koch"3

Koch"*

Cardimatis and Diamessis"^

MoUow"6

Italian statistics'''"

Wright"*

Watson"'

Craig"'

Hope""

Williamson"'-

Koch'"2

Chamberlain*'^'

Rogers"

Buchanan''"

Bentley''**

Tsuzuki''"

Ziemann*'

Meixner''

Grothusen"

Thiroux and d'Antreville""

Kudicke"

Exner''

OUwig'i

Schornich'^

23 34

74 373 338

4,012

32 202

32,392

S7

1.372

I

3 71

78 191 145

23,520

1,311

74 107

32 102

68 266 118 328 134 130

CHAPTER n

ETIOLOGY OF MALARIA

Malaria is of very complex etiology, depending as it does for its existence upon the life histories of three species of animals. While within the blood of man the parasite is not subject to great variations of environment, no matter what the season or the latitude, nevertheless exposure to cold, wet or heat, dietary or other excesses, will have the effect of awakening latent malaria.

But it is the influence of external factors upon the life history of the mosquito that determines the greatest variations in the prevalence of malaria according to climate, season, temperature, rainfall, altitude, etc.

Climate. — It may be said as a general rule, that the frequency and virulence of malaria increase as we approach the equator. The conditions of warmth and moisture are more propitious for the development of the parasites within the bodies of mosquitoes in tropic than in colder climates; this is especially true of the estivo-autumnal form of the malarial parasite. Exposure to the heat of the tropical sun predisposes to the cerebral forms of pernicious malaria, and undue exposure to the sun's rays is ofttimes sufficient to stimulate sporulation of the parasites of latent malaria.

With respect to latitude Hirsch^" reached the following conclusions as to the northern boundary of malaria in the northern hemisphere. The line starts from 55°N. on the western side of North America, sinks to 45° on its eastern side, rises to 63° or 64° on the western side of the old world (Sweden and Finland), and runs across Northern Asia in about the latitude of 55°.

Long before the discovery of the role of the mosquito in malaria it was known that the disease was not endemic unless the summer temperature maintained a certain average. Hirsch maintained that the summer isobar of S9°-6o.8°F. marks the limit of the occurrence of malarial fever, and that those regions where the mean summer temperature does not reach that height are exempt from the disease. Curiously enough, it has been recently repeatedly demonstrated that this is the lowest temperature at which the parasite wiU develop in the body of the mosquito.

Season. — While relapses may occur at any season, and in certain tropic regions fresh infections may occur during any period of the year, in all temperate and most tropic regions there are seasons during which the disease is especially prevalent. This is commonly known as the malarial season, and varies according to latitude, temperature, rainfall, soil, etc.

The season of primary attacks depends entirely upon the Ufe-history of the malaria-bearing mosquitoes. This season usually begins a few weeks after the first brood of anophehnes appears, which is at the height of summer, and continues, in temperate chmates, until after the nights become cool. In each individual locality the beginning of the season is rather definite, the disease recurring at a certain period each year with more or less exactitude. In most of the regions of the Southern States the malarial season begins in the earlier half of July. In the latitude of Baltimore the most notable increase in cases begins during August. The malarial season in California is from August to October.

Where both tertian and estivo-autumnal malaria are endemic, the malarial season is usually ushered in by cases of the former, the estivo-autumnal variety appearing at the height of the season. The pernicious forms of malaria occur with greatest frequency at the height of estivo-autumnal prevalence. In Italy quartan malaria begins late in the summer and continues late in the fall. In America this variety is too infrequent to justify any definite conclusions. Mixed and multiple infections occur more frequently late in the season than early.

Rainfall. — The influence of rainfall upon the extent of malaria is very decided. Breeding places for mosquitoes are essential in the etiology of malaria, and limited pools, such as result from a fall of rain, are well suited to the taste of the malarial mosquitoes.

Rain has a twofold effect upon the prevalence of malaria. First, exposure to wet is not infrequently followed by a recrudescence of a former infection. This effect is usually immediate. Second, rainfall produces breeding pools for the disseminators of malaria. The effect of fresh breeding places is not shown immediately. Allowing twenty days for the aquatic stages of the mosquito, ten days for the mosquito cycle of the parasite, and a Hke period for the incubative stage in man, it would be, obviously, several weeks before an increase in malaria could be expected from such a source. This is well exempUfied in the tropics, where so much depends upon rainfall. Here the height of the malaria curve is attained toward the end of the rainy season, or shortly after.

A heavy rainfall in the spring and early summer has long had the reputation of being favorable to the spread of malaria.

While rainfall is essential to the development of malaria, if excessive it may have the opposite effect by scouring breeding pools and destroying the contained ova and young of the mosquito. Moderate rains at short intervals are more productive of breeding pools than heavy downpours at long intervals. Hence, the number of rainy days, as well as the actual rainfall in inches, is a factor in the etiology of malaria.

In very low countries rainy years may be healthy years. This is said to be the case in the Netherlands.

Dew and a high atmospheric moisture were formerly accredited with being factors in the cause of malaria. This was doubtless on account of the well-recognized danger of contracting malaria between sunset and sunrise. Other than as an index of ground-moisture it is doubtful whether atmospheric moisture bears any relation to primary infections with malaria.

Soil. — The chemical composition of the soil has an effect upon the reign of malaria only so far as the relation of the soil to the retention of water is concerned. More depends upon the physical conformation than upon the geologic characteristics of the soil. As a rule, clay soils retain moisture better than the sandy, though there are exceptions. Rocky regions are less apt to harbor breeding pools because of good drainage, but pools upon a rock bed are very persistent. The soil must be of such a character as to retain surface water sufi&ciently long for the aquatic stages of mosquito life to be completed.

More depends upon the nature of the subsoil than upon that of the surface soil. Even where the surface soil is very porous, an impervious subsoil favors the accumulation of surface water by preventing further percolation. Thus the height of the ground-water during the malarial season bears a close relation to the volume of the malarial endemic. Proximity to collections of water, by raising the height of the ground-water, favors the development of malaria.

Topography. — In countries designated malarial, regions entirely free from the disease are not uncommon. In a region within a short distance of a severely scourged locality malaria may be entirely absent. The difference in the prevalence of malaria within limited areas is dependent, in great measure, upon the physical characteristics of the surface of the land.

It has been known for centuries that malaria is partial to low marshy places, swamps, lakesides, low coast levels, and river valleys, and especially the deltas of large rivers. The cleaner the banks and the swifter the current of the streams at all stages the less apt they are to be malarious. Streams with sluggish or no currents, and with weedy banks which foster eddies, are breeding places for mosquitoes.

It is generally believed that salt marshes are never malarious, and that anopheles larvae cannot develop in sea- water. This is, however, not strictly true. DeVogeP^^ has recently shown that anopheles larvae may develop in sea-water evaporated to half its initial volume, and a number of other observers have found larvae in salt water. But marshes of pure sea-water are not nearly so noxious as those of brackish water, a mixture of salt and fresh water, which are famous anopheles breeders. It appears that in some instances, where salt water is inimical to the development of the aquatic stages of mosquitoes, they may gradually become accustomed to the environment.

Altitude. — Malaria is essentially a disease of the lowlands, high altitudes being relatively exempt. This is partially accounted for by the better drainage of elevated altitudes and fewer pools in which malarial mosquitoes may breed. The lower temperature of high altitudes is also a factor in maintaining a low malarial morbidity in these regions.

It is known that anophehne mosquitoes do not fly to great heights. Hence sleeping in an upper story or in a building situated high above the ground gives a measure of protection from malaria. Laborers employed in highly malarial sections, and who sleep in the surrounding hills, even of moderate altitude, often remain entirely free from infection.

A few hundred feet in altitude may show a more marked difference in the prevalence of malaria than as many miles in latitude.

The general rule that malaria is a disease of low countries has some exceptions. This is especially true in the tropics, where the disease may be encountered at very high altitudes. It may be said that the altitude at which malaria may occur varies in inverse ratio to latitude.

Malaria has been found on Lake Nyssa at an altitude of 1,560 meters; at Colico, 2,500 meters; in the Himalaya Mountains, at 2,000 meters; in the Andes, at 2,500 meters; at Blantyre, at 3,000 feet; German East Africa, at 1,550 meters; at points in Central Africa, at heights of over 5,000 feet; and in some of the high-lying valleys of Syria, at altitudes of 1,200 meters.

Some of the cases in high altitudes reported as malaria may be mistakes in diagnosis; other cases may be malaria contracted in the lowlands. Thus Tosari, at an elevation of 1,177 meters, has been cited as a place where malaria prevailed without the presence of mosquitoes, and this was used as an argument against the "mosquito theory." Koch,'-^ investigating the place in 1899, examined the blood of eighty- two children; in none was the parasite of malaria detected. The only case of malaria found was in a man who, twelve days before the beginning of his illness, had spent the night in a highly malarial place upon the coast.

However, malaria is endemic in certain places of high altitude. Such are Eritrea, in altitudes of 1,750 meters; Upper Tonkin, at 1,000 meters; parts of Madagascar, at 1,100 meters; parts of Reunion Island, 1,200 meters; in Java, at 1,000 meters; and in the Philippines it is said that, while certain valleys are almost free from malaria, the hills in the vicinity are notoriously infected. Wright,'*''^ in British Malaya, found anopheles larvae in pools at an elevation of 2,300 feet.

Earthquakes and volcanic eruptions have been followed by a great development of malaria. Examples are cited of Rome in 1703, in Reggio in 1783, and Palermo in 1828. Remarkable instances have occurred in Peru also. The most recent illustration is that of Amboina, in the East Indies, which had until 183 s been remarkably free from malaria. In that year a severe earthquake occurred, and since then the malaria has increased both in extent and intensity.

Such results can be explained only by an increase of stagnant water following these violent disturbances, probably through the interruption of the flow of ground-water.

Invmdations. — Since very early times overflows have been recognized as a prolific cause of epidemics of malaria. Tacitus, Suetonious, Livy, Dionysius, Cassio, and Strabo, mention such results from inundations of the Tiber. This stream experienced an overflow in 1695, which was described by Lancisi. The water covered a broad area of country, filling ditches, sewers and canals. The following June, July and August were extremely hot. An epidemic of malignant malarial fever ensued and, spreading far and wide, occasioned a great mortality.

Epidemics of malaria following overflows of the Nile, Ganges, Indus, Euphrates, Niger, Senegal, Volga, Danube, Saone, Rhone, Loire, Mississippi, and other rivers have been described.

The immediate effect of an inundation is to check the development of malaria. This is a result of a destructive effect of the flood upon the breeding pools of mosquitoes. It is only after the waters have subsided and pools and marshes are left that the epidemic develops.

Trees and Vegetation. — It was formerly beheved that, while decaying vegetation was the cause of malaria, living plant life greatly retarded its development. Whole volumes have been devoted to this subject. It was supposed that vegetation filtered the miasm from the air. It was argued that if air vitiated by respiration be confined in a bottle containing a living plant and exposed to the rays of the sun, the carbonic-acid gas will be absorbed and the air restored to its original condition, plant life consuming carbon dioxide and exhaling oxygen. So firm was this belief that in the days of ancient Rome trees were protected by law.

It is needless to say that the protective power of living plants was as much overestimated as the faculty of decaying vegetation to cause malaria. Their power of absorbing moisture from the soil is more than outweighed by the shade they afford the ground.

While the clearing of land of trees and vegetation may be followed by an outbreak of malaria, this may be due to the overturning of the soil, which usually goes hand in hand with opening land, and to the hardships attending such labor. The ultimate effect of clearing trees from the land is to diminish malaria by permitting the sun to dry the soil.

If trees have any protective virtues whatever, it is probably through affording shelter and food for mosquitoes. The culture of eucalyptus trees is now known to have no prophylactic effect upon malaria.

Weeds and other vegetation growing in the water favor the development of mosquito larvffi by protecting the surface of the water from agitation by the wind.

Vegetable decomposition bears no relation to the etiology of malaria other than as an index to heat and moisture.

Wind. — The wind was formerly held responsible for transmitting malaria long distances. It was believed that the malaria of Edinburgh was imported by the winds from Holland, and that Italy became malarious through the agency of the African sirocco. The land breezes, especially if they blew over marshy areas, were regarded as more highly noxious than the sea breezes.