Disease and Its Causes by William Thomas Councilman (early reader books .TXT) 📕

many parasites. Within the mosquito the parasite undergoes a further development into male and female sexual forms, which may also form in the blood, termed respectively microgametocyte and macrogamete. From the microgametocyte small flagellate bodies, the male sexual elements microgametes or spermatozoa, develop and fertilize the macrogametes; after fertilization this develops into a large body, the oöcyst which is attached to the wall of the stomach of the mosquito. Within the oöcyst, innumerable small bodies, the sporozoites, develop, make their way into the salivary glands and are injected into the individual who becomes the prey of the mosquito, and again the cycle of development begins. The presence of the parasite within the mosquito does not constitute a disease. So far as can be determined, life goes on in the usual way, and its duration in the insect is not shortened.

The nature of the parasite which produces yellow fever is unknown, for it belongs to the filterable viruses; the infectious material, however, has been shown by inoculation to exist in the blood, and the disease is transmitted by a mosquito of another species, the stegomyia. The development cycle within this takes a period of twelve days, which time must elapse after the mosquito has bitten before it can transmit the disease. Here again the mutual interdependence of knowledge is shown. Nothing could have seemed less useful than the study of mosquitoes, the differentiation of the different species, their mode of life, etc., and yet without this knowledge discoveries so beneficial and of such far-reaching importance to the whole human race as that of the cause and mode of transmission of malaria and yellow fever would have been impossible; for it could easily have been shown that the ordinary culex mosquito played no rôle. The rôle which insects may play in the transmission of disease was first shown by Theobald Smith in this country, in the transmission by a tick of the disease of cattle known as Texas fever. The infecting organism pyrosoma bigenimum is a tiny pear-shaped parasite of the red corpuscles. Smith's investigations on the disease, published in 1893, is one of the classics in medicine, and one of the few examples of an investigation which has not been changed or added to by further work.

One of the most interesting methods of extension of infection, showing on what small circumstances infection may depend, is seen in the case of the hookworm disease, which causes such devastation in the Southern States. The organism which produces the disease, the Uncinaria, belongs to the more highly developed parasites, and is a small round worm one-third of an inch long. The worms which inhabit the intestines have a sharp biting mouth by which they fasten themselves to the mucous membrane and devour the blood. The most prominent symptom of the disease is anæmia, or loss of blood, due not only to the direct eating of the parasite, but to bleeding from the small wounds caused by its bite. Large numbers of eggs are produced by the parasite which are passed out with the feces, which becomes the only infectious material. In a city provided with water-closets and a system of sewerage there would be no means of extension of infection. The eggs in the feces in conditions of warmth and moisture develop into small crawling larvæ which can penetrate the skin, producing inflammation of this, known in the region as the ground itch. The larvæ enter the circulation and are carried to the lungs, where they perforate the capillaries and reach the inner surface; from this they pass along the windpipe, and then by way of the gullet and stomach reach their habitat, the small intestine. Unfortunately, the habits and poverty of the people in every way facilitate the extension of the infection. There is no proper disposal of the feces, few of the houses have even a privy attached to them, and the feces are distributed in the vicinity of the houses. This leads to contamination of the soil over wide areas. Most of the inhabitants of the country go barefoot the greater part of the year, and this gives ready means of contact with the larvæ which crawl over the surface of the ground. The disease is necessarily associated with poverty and ignorance, the amount of blood is reduced to a low point, and industry, energy and ambition fall with the blood reduction; the schools are few and inefficient; the children are backward, for no child can learn whose brain cells receive but a small proportion of the necessary oxygen; and a general condition of apathy and hopelessness prevails in the effected communities. The control of the disease depends upon the disinfection of the feces, or at least their disposal in some hygienic method, the wearing of shoes, and the better education of the people, all of which conditions seem almost hopeless of attainment. The infection is also extended by means of the negroes who harbor the parasite, but who have acquired a high degree of immunity to its effects and whose hygienic habits are even worse than those of the whites. The organism was probably imported with the negroes from Africa and is one of the legacies of slavery.

The diseases of animals are in many ways closely linked with those of man. In the case of the larger parasites, such as the tapeworms and the trichina, there is a direct interchange of disease with animals, certain phases of the life cycle of the organisms are passed in man and others in various of the domestic animals. A small inconspicuous tapeworm inhabits the intestine of dogs and seems to produce no ill effects. The eggs are passed from the dog, taken into man, and result in the formation of large cystic tumors which not infrequently cause death. Where the companionship between dog and man is very close, as in Iceland, the cases are numerous.

Most of the diseases in animals caused by bacteria and protozoa are not transmitted to man, but there is a conspicuous exception. Plague is now recognized as essentially an animal disease affecting rats and other small rodents, and from these the disease from time to time makes excursions to the human family with dire results. The greatest epidemics of which we have any knowledge are of plague. In the time of Justinian, 542 B.C., a great epidemic of plague extended over what was then regarded as the inhabited earth. This pandemic lasted for fifty years, the disease disappeared and appeared again in many places and caused frightful destruction of life. Cities were depopulated, the land in many places reverted to a wilderness, and the works of man disappeared. The actual mortality cannot be known, but has been estimated at fifty millions. Plague played a large part in the epidemics of the Middle Ages. An epidemic started in 1346 and had as great an extension as the Justinian plague, destroying a fourth of the inhabitants of the places attacked; and during the fifteenth and sixteenth and seventeenth centuries the disease repeatedly raised its head, producing smaller and greater epidemics, the best known of which, from the wonderful description of De Foe, is that of London in 1665, and called the Black Death. Little was heard of the disease in the nineteenth century, although its existence in Asia was known. In 1894 it appeared in Hong Kong, extended to Canton, thence to India, Japan, San Francisco, Mexico, and, in fact, few parts of the tropics or temperate regions of the earth have been free from it. Mortality has varied greatly, being greatest in China and in India; in the last the estimate since 1900 is seven million five hundred thousand deaths. The disease is caused by a small bacillus discovered in 1894 which forms no spores and is easily destroyed by sunlight, but in the dark is capable of living with undiminished virulence for an indefinite time. The disease in man appears in two forms, the most common known as bubonic plague, from the great enlargement of the lymph nodes, those of the groin being most frequently affected. The more fatal form is known as pneumonic plague, and in this the lungs are the seat of the disease.

In the old descriptions of the disease it was frequently mentioned that large numbers of dead rats were found when it was prevalent, and the most striking fact of the recent investigations is the demonstration that the infection in man is due to transference of the bacillus from infected rats. There are endemic foci of the disease where it exists in animals, the present epidemic having started from such a focus in Northern China, in which region the Tarabagan, a small fur-bearing animal of the squirrel species, was infected. Rats are easily infected, the close social habits of the animal, the vermin which they harbor, and the habits of devouring their dead fellows favor the extension of infection. The disease extends from the rat to man chiefly by means of the fleas which contain the bacilli, and in cases of pneumonic plague from man to man by means of sputum infection. The disease once established in animals tends to remain, the virus being kept alive by transmission from animal to animal, and the persistence of the infection is favored by mild and chronic cases.

Chapter IX

Disease Carriers.—The Relation Between Sporadic Cases Of Infectious Disease And Epidemics.—Smallpox.—Cerebro-Spinal Meningitis.—Poliomyelitis.—Variation In The Susceptibility Of Individuals.—Conditions Which May Influence Susceptibility.—Racial Susceptibility.—Influence Of Age And Sex.—Occupation And Environment.—The Age Period Of Infectious Diseases.

We have seen that insects serve as carriers of disease in two ways: in one, by becoming contaminated with organisms they serve as passive carriers, and in the other they undergo infection and form a link in the disease. The more recent investigations of modes of transmission of infectious diseases have shown that man, in addition to serving while sick as a source of infection, may serve as a passive carrier in two ways. For infection to take place not only must the pathogenic organism be present, but it must be able to overcome the passive and active defences of the body and produce injury. Pathogenic organisms may find conditions favorable for growth on the surfaces of the body, and may live there, but be unable to produce infection, and the individual who simply harbors the organisms can transmit them to others. Such an individual may be a greater source of infection than one with the disease, because there is no suspicion of danger. The organisms which thus grow on the surfaces have in some cases been shown to be of diminished virulence, but in others have full pathogenic power. Such passive carriers of infection have been found for a number of diseases, as cerebro-spinal meningitis, diphtheria, poliomyelitis and cholera. In all these cases the organisms are most frequently found in those individuals who have been exposed to infection as members of a family in which there have been cases of disease. The other sort of carrier has had and overcome the disease, but mutual relations have been established with the organism which continues to live in the body cavity. Diphtheria bacilli usually linger in the throat after convalescence is established, and until they have disappeared the individual is more dangerous than one actually sick with the disease. Health officers have recognized this in continuing the quarantine against the disease until the organism disappears. In typhoid fever bacilli may remain in the body for a long time and be continually discharged, as in the well-known case of "typhoid Mary."10

Single cases of certain infectious diseases may appear in a community year after year, and at intervals the cases become so numerous that the disease is said to be epidemic. Such a disease is smallpox. This is a highly infectious disease, towards which all mankind is susceptible. Complete protection against the disease can be conferred by Jenner's discovery of vaccination. The disease becomes modified when transferred to cattle, producing what