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Second attacks of measles are comparatively rare, even more so than second attacks of smallpox or of scarlet fever. The apparent exemption of adults from attacks of measles is largely if not altogether due to the fact that they have usually suffered an attack during childhood. It has been noticed that regiments of soldiers recruited in cities are less subject to measles than regiments raised in rural districts. This is no doubt due to the greater prevalence of the disease among children in cities, where few escape attack during infancy or the age of going to school.

During a severe epidemic in the Faroe Islands in 1846 scarcely any one escaped except those old enough to have passed through the previous epidemic in 1781.

German measles (Rubella) is a distinct disease froin measles, but its specific character was not generally recognised by physicians until the last half of the nineteenth century. An attack of this disease does not protect the child from the far more dangerous disease, measles.

The mortality from German measles is practically nil. Its prevention is therefore of much less importance, but is to be effected by the same measures, viz.: isolation of the sick and disinfection of all clothing and other objects which have been exposed in the sick-room. The period of incubation in this disease is quite variable, but as a rule it is longer than that of measles. CHAPTER XVI

MALARIAL FEVERS

THE discovery of the malarial parasite may justly

be considered one of the greatest achievements of scientific research during the nineteenth century. We owe it to Laveran, a surgeon in the French army, who made the discovery in 1880 while stationed in Algeria. His painstaking microscopical researches convinced him that the blood of patients suffering from malarial fever contains living ameeboid parasites which in one stage of their development invade the red blood corpuscles and lead to their destruction. Subsequent researches in various parts of the world have made it evident that this blood parasite is in fact the malarial germ and the cause of the phenomena which characterise fevers of this class. It has also been demonstrated that the disease is transmitted to man by mosquitoes of the genus Anopheles, in the bodies of which the parasite passes through certain stages of development, resulting in the formation of a multitude of minute spore-like bodies which are found in the salivary glands of the insect.

Twenty-five years ago the best-informed physicians entertained erroneous views with reference to the nature of “malaria” and the cause of the malarial fevers. Observation had taught them that there is something in the air in the vicinity of marshes in tropical regions, and during the summer and autumn in semitropical and temperate regions, which gives rise to periodic fevers in those exposed in such localities; and the usual inference was that this something was of gaseous form — that it was a special kind of bad air (malaria) generated in swampy localities under favourable meteorological conditions. It was recognised at the same time that there are other kinds of bad air, such as the offensive emanations from sewers and the products of respiration of men and animals; but the term malaria was reserved for the kind of bad air which was supposed to give rise to the so-called malarial fevers. In the light of our present knowledge it is evident that the term is a misnomer. There is no good reason for believing that the air of swamps is any more deleterious to those who breathe it than the air of the sea-coast or that in the vicinity of inland lakes and ponds. Moreover, the stagnant ponds, which are

covered with a “green scum” and from which bubbles of gas are given off, have lost all terrors for the well-informed man, except in so far as they serve as breeding-places for mosquitoes of the genus Anopheles. The green scum is made up of harmless algæ and the gas which is given off from the mud at the bottom of such stagnant pools is for the most part a well-known and comparatively harmless compound of hydrogen and carbon — methane or “marsh gas.” In short, we now know that the air in the vicinity of marshes is not deleterious because of the presence of any special kind of bad air in such localities but because it contains mosquitoes infected with the malarial parasite.

The discoveries referred to, as is usual, have had to withstand the criticism of conservative physicians, who, having adopted the prevailing theories with reference to the etiology of periodic fevers, were naturally skeptical as to the reliability of the observations made by Laveran and those who claimed to have confirmed his discovery. The first contention was that the bodies described as present in the blood were not parasites, but deformed blood corpuscles. This objection was soon set at rest by the demonstration, repeatedly made, that the intra-corpuscular forms underwent distinct ameboid movements. No one witnessing these movements could doubt that he

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was observing a living micro-organism. The same was true of the extra-corpuscular flagellate bodies, which may be seen to undergo very active movements, as a result of which the red blood corpuscles are violently displaced and the flagellate body itself dashes about in the field of view.

The first confirmation in this country of Laveran's discovery of ameboid parasites in the blood of malarial-fever patients was made by myself in the pathological laboratory of the Johns Hopkins University in March, 1886. In May, 1885, I had visited Rome as a delegate to the International Sanitary Conference, convened in that city under the auspices of the Italian Government, and while there I visited the Santo Spirito Hospital for the purpose of witnessing a demonstration, by Drs. Marchiafava and Celli, of that city, of the presence of the plasmodium malaria in the blood of persons suffering from intermittent fever. Blood was drawn from the finger during the febrile attack, from individuals to whom quinine had not been administered. The demonstration was entirely satisfactory, and no doubt was left in my mind that I saw living parasitic micro-organisms in the interior of red blood corpuscles obtained from the circulation of malarial-fever patients. The motions were quite slow, and were manifested by a gradual change of outline rather than by visible

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