MALARIA; CHILLS AND FEVER; AGUE; FEVER AND AGUE; SWAMP OR MARSH FEVER; INTERMITTENT OR REMITTENT FEVER; BILIOUS FEVER.—Malaria is a communicable disease characterized by attacks of fever occurring at certain intervals, and due to a minute animal parasite which inhabits the body of the mosquito, and is communicated to the blood of man by the bites of this insect.
In accordance with this definition malaria is not a contagious disease in the sense that it is acquired by contact with the sick, which is not the case, but it is derived from contact with certain kinds of mosquitoes, and can be contracted in no other way, despite the many popular notions to the contrary. Mosquitoes, in their turn, acquire the malarial parasite by biting human beings suffering from malaria. It thus becomes possible for one malarial patient, coming to a region hitherto free from the disease, to infect the whole district with malaria through the medium of mosquitoes.
Causes.—While the parasite infesting mosquitoes is the only direct cause of malaria, yet certain circumstances are requisite for the life and growth of the mosquitoes. These are moisture and proper temperature, which should average not less than 60° F. Damp soil, marshes, or bodies of water have always been recognized as favoring malaria.
Malaria is common in temperate climates—in the summer and autumn months particularly, less often in spring, and very rarely in winter, while it is prevalent in the tropics and subtropics all the year round, but more commonly in the spring and fall of these regions. The older ideas, that malaria was caused by something arising in vapors from wet grounds or water, or by contamination of the drinking water, or by night air, or was due to sleeping outdoors or on the ground floors of dwellings, are only true in so far as these favor the growth of the peculiar kind of mosquitoes infected by the malarial parasites. Two essentials are requisite for the existence of malaria in a region: the presence of the particular mosquito, and the actual infection of the mosquito with the malarial parasite. The kind of mosquito acting as host to the malarial parasite is the genus Anopheles, of which there are several species. The more common house mosquito of the United States is the Culex. The Anopheles can usually be distinguished from the latter by its mottled wings, and, when on a wall or ceiling, it sits with the body protruding at an angle of 45° from the surface, with its hind legs hanging down or drawn against the wall. In the case of the Culex, the body is held parallel with the wall, the wings are usually not mottled, and the hind legs are carried up over the back.
When a mosquito infected with the malarial parasite bites man, the parasite enters his blood along with the saliva that anoints the lancet of the mosquito. The parasite is one of the simplest forms of animal life, consisting of a microscopical mass of living, motile matter which enters the red-blood cell of man, and there grows, undergoes changes, and, after a variable time, multiplies by dividing into a number of still smaller bodies which represent a new generation of young parasites. This completes the whole period of their existence. It is at that stage in the development of the parasite in the human body when it multiplies by dividing that the chills and fever in malaria appear. What causes the malarial attack at this point is unknown, unless it be that the parasites give rise to a poison at the time of their division. Between the attacks of chills and fever in malaria there is usually an interval of freedom of a few hours, which corresponds to the period intervening in the life of the parasite in the human body, between the birth of the young parasites and their growth and final division, in turn, into new individuals. This interval varies with the kind of parasite. The common form of malaria is caused by a parasite requiring forty-eight hours for its development. The malarial attacks caused by this parasite then occur every other day, when the parasite undergoes reproduction by division. However, an attack may occur every day when there are two separate groups of these parasites in the blood, the time of birth of one set of parasites, with an accompanying malarial attack, happening one day; that of the other group coming on the next, so that between the two there is a daily birth of parasites and a daily attack of malaria. In cases of malaria caused by one group of parasites the attacks appear at about the same time of day, but when the attacks are caused by different groups of parasites the times of attack may vary on different days. In the worst types of malaria the parasites do not all go through the same stages of development at the same time, as is commonly the case in the milder forms prevalent in temperate regions, so that the fever—corresponding to the stage of reproduction of the parasites—occurs at irregular intervals.
In a not uncommon type of malaria the attacks occur every third day, with two days of intermission or freedom from fever. Different groups of parasites causing this form of malaria, and having different times of reproduction, may inhabit the same patient and give rise to variation in the times of attack. Thus, an attack may occur on two successive days with a day of intermission.
The reproduction of the parasite in the human blood is not a sexual reproduction; that takes place in the body of the mosquito.
When a healthy mosquito bites a malarial patient, the parasite enters the body of the mosquito with the blood of the patient bitten. It enters its stomach, where certain differing forms of the parasite, taking the part of male and female individuals, unite and form a new parasite, which, entering the stomach wall of the mosquito, gives birth in the course of a week to innumerable small bodies as their progeny. These find their way into the salivary glands which secrete the poison of the mosquito bite, and escape, when the mosquito bites a human being, into the blood of the latter and give him malaria.
Distribution.—Malaria is very widely distributed, and is much more severe in tropical countries and the warmer parts of temperate regions. In the United States malaria is prevalent in some parts of New England, as in the Connecticut Valley, and in the course of the Charles River, in the country near Boston. It is common in the vicinity of the cities of Philadelphia, New York, and Baltimore, but here is less frequent than formerly, and is of a comparatively mild type. More severe forms prevail along the Gulf of Mexico and the shores of the Mississippi and its branches, especially in Mississippi, Texas, Louisiana, and Arkansas, but even here it is less fatal and widespread than formerly. In Alaska, the Northwest, and on the Pacific Coast of the United States malaria is almost unknown, while it is but slightly prevalent in the region of the Great Lakes, as about Lakes Erie and St. Clair.
Development.—Usually a week or two elapses after the entrance of the malarial parasite into the blood before symptoms occur; rarely this period is as short as twenty-four hours, and occasionally may extend to several months. It often happens that the parasite remains quiescent in the system without being completely exterminated after recovery from an attack, only to grow and occasion a fresh attack, a month or two after the first, unless treatment has been thoroughly prosecuted for a sufficient time.