The malarial parasite belongs to the Protozoa, of which it is one of the smallest members. Man is its intermediate host, and the anopheles its definitive or final host. In the red blood corpuscle of man it is a unicellar organism, from 1 µ to 8 µ in diameter. It has two methods of reproduction, endogenous by spore formation and exogenous or sexual. The former occurs in man; the latter in the mosquito. Without the latter, the parasite being unable to pass from man to man, would die with its host. In endogenous multiplication spores are formed which separate from the original parasite and gain access to other red blood corpuscles. The large pigmented spheres and the crescent bodies require to enter the stomach of the anopheles to attain full development. In the anopheles the crescents become spherical, flagella are shot out, having a length of 4 to 5 times the diameter of a red blood corpuscle. These represent the male element, while other spheres without flagella are the females. By the fusion of these two a fertilised cell is produced (the travelling vermicule), which now assumes the shape of a spear-head and is actively mobile. The travelling vermicule pierces the stomach wall of the mosquito and develops into a zygote. If an infected mosquito is examined on a succession of days under the microscope, the following stages can be traced. The zygote consists of pigmented spheres 7 to 8 µ in diameter, lying in the muscular fibres of the mosquito. These grow, and become surrounded by a capsule. Smaller spheres form and subdivide, bud-like processes develop on their surfaces; these gradually become sickle-shaped and protrude into the body cavity. They increase in size. until they attain dimensions of from 40, to 60 µ. Eventually they rupture, and the sickle-shaped bodies (sporozooites) escape and are carried in the body fluid of the mosquito to its salivary glands. These sporozooites are about 14 µ long, and human infection is caused by them. They have been traced as far as the end of the proboscis of the mosquito. (See also page [307]).
[CHAPTER XLIII.]
INFECTIVE DISEASES.
The prevention of disease depends largely on a knowledge of its causes. Disease may be due to a personal life not in accordance with physiological laws; or to some cause or causes acting ab extra. With advance of knowledge the number of diseases which can be proved to be caused by a contagium vivum introduced from without is steadily increasing. We have already discussed the influence of habits, of clothing, exercise, sleep, and food on health, and have shown how errors in these respects may lead to disease. It now remains to consider more particularly the prevention of diseases, due to the introduction into the system of contagia.
In the study of such diseases three chief factors require consideration: (1) the contagium itself; (2) conditions of environment, as climate, soil, season, weather, etc., which may favour or impede its spread; and (3) personal conditions which similarly influence it. Of these age, heredity, fatigue, injury, diet, and race are specially important.
The first two groups of diseases given in the Registrar-General’s classification of causes of death are (1) Specific Febrile or Zymotic Diseases, and (2) Parasitic Diseases. The objection to the word “specific” is that, although in most instances diseases in this group are “specific” in the sense that they are caused by a particular microbe, e.g. tetanus, anthrax, tuberculosis, in a few instances the same lesions may be caused by several microbes, e.g. septicaemia (blood-poisoning), pneumonia. “Zymotic” was the name given by Farr, in view of the analogy of the febrile process to that of alcoholic fermentation. In both there is the introduction of a living germ or germs; in both a period of “incubation” in which nothing can be observed; then follows the active disturbance; and in the disease, as well as in the fermenting liquid, the process is stopped, when the microbes have multiplied to a certain extent, a temporary or permanent protection being the result. The best name for the diseases in this group is “Infective.” Parasitic diseases, like ringworm, scabies, or trichinosis, are also infective; but for convenience may be described separately as “parasitic.”
The relation between the words “infectious” and “contagious” requires explanation. A disease like measles or small-pox, which can be transmitted from person to person, without immediate contact between the two, is termed infectious. In these cases the infection is conveyed by mucus expectorated or by dust blown about, or carried in apparel, etc., from the first patient. Such diseases may also, of course, be communicated by direct contact. If direct contact between the sick and well is indispensable for the transmission of a disease it is called contagious. There is no such hard line in nature, although some diseases can be more easily communicated than others. The term contagious is usually applied to parasitic diseases like ringworm and scabies, but even these can be communicated by means of infected articles as well as persons. The word contagious should be abandoned for all the acute febrile diseases. The word infective is used to include all specific febrile diseases, however spread. This word, therefore, includes not only infectious and contagious diseases, but also diseases spread by inoculation, i.e. injection of the infection under the skin. Thus malaria is not infectious from patient to patient; but can be inoculated by the mosquito.
Infective Diseases are either acute or chronic. Of acute infective diseases small-pox and enteric fever are typical examples; of chronic, tuberculosis and syphilis.
It was formerly supposed that in certain diseases the contagium or infective agent grew in external noxious matter, a miasm being produced; while in other diseases contagion was only produced direct from patient to patient; and others originated in either way. Hence the classification of infective diseases into (a) miasmatic, (b) contagious, and (c) miasmatico-contagious diseases. This classification has now been abandoned. Thus influenza and ague were formerly thought to be miasmatic; but the former is spread by personal infection; the latter by inoculation of the contagium by an infected mosquito.
Bacteriology has thrown an immense light on the causation of infective diseases. A large number of these have been proved to be caused by bacteria, and by analogy we infer the same thing for many others. Koch has laid down the following postulates as necessary before it can be stated that a particular disease is directly caused by a given microbe:—