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

biological investigation of disease by the discoveries which led to the formulation of the cell theory in 1840 and the brilliant work of Pasteur on fermentation,7 but it was not until 1878 that it was definitely proved that a disease of cattle called anthrax was due to a species of bacteria. What should be regarded as such proof had been formulated by Henle in 1840. To prove that a certain sort of organism when found associated with a disease is the cause of the disease, three things are necessary:

1. The organism must always be found in the diseased animal and associated with the changes produced by the disease.

2. The organism so found must be grown outside of the body in what is termed pure cultures, that is, not associated with any other organisms, and for so long a time with constant transfers or new seedings that there can be no admixture of other products of the disease in the material in which it is grown.

3. The disease must be produced by inoculating a susceptible animal with a small portion of such a culture, and the organism shown in relation to the lesions so produced.

It is worth while to devote some attention to the disease anthrax. This occupies a unique position, in that it was the first of the infectious diseases to be scientifically investigated. In this investigation one fact after another was discovered and confirmed; some of these facts seemed to give clearer conceptions of the disease, others served to make it more obscure; new questions arose with each extension of knowledge; in the course of the work new methods of investigation were discovered; the sides of the arch were slowly and painfully erected by the work of many men, and finally one man placed the keystone and anthrax was for a long time the best known of diseases. Men whose reputation is now worldwide first became known by their work in this disease. It was a favorable disease for investigation, being a disease primarily of cattle, but occasionally appearing in man, and the susceptibility of laboratory animals made possible experimental study.

Anthrax is a disease of domestic cattle affecting particularly bovine cattle, horses and sheep, swine more rarely. The disease exists in practically all countries and has caused great economic losses. There are no characteristic symptoms of the disease; the affected cattle have high fever, refuse to eat, their pulse and respiration are rapid, they become progressively weaker, unable to walk and finally fall. The disease lasts a variable time; in the most acute cases animals may die in less than twenty-four hours, or the disease may last ten or fourteen days; recovery from the disease is rare and treatment has no effect. It does not appear in the form of epidemics, but single cases appear frequently or rarely, and there is seemingly no extension from case to case, animals in adjoining stalls to the sick are not more prone to infection than others of the herd. On examination after death the blood is dark and fluid, the spleen is greatly enlarged (one of the names of the disease "splenic fever" indicates the relation to the spleen) and there is often bloody fluid in the tissues.

Where the disease is prevalent there are numbers of human cases. Only those become infected who come into close relations with cattle, the infection most commonly taking place from small wounds or scratches made in skinning dead cattle or in handling hides. The wool of sheep who die of the disease finds its way into commerce, and those employed in handling the wool have a form of anthrax known as wool-sorters' disease in which lesions are found in the lungs, the organisms being mingled with the wool dust and inspired. In Boston occasional cases of anthrax appear in teamsters who are employed in handling and carrying hides. The disease in man is not so fatal as in cattle, for it remains local for a time at the site of infection, and this local disease can be successfully treated.

The beginning of our knowledge of the cause dates from 1851, when small rod-shaped bodies (Fig. 17) were found in the blood of the affected cattle, and by the work of a number of observers it was established that these bodies were constantly present. Nothing was known of their nature; some held that they were living organisms, others that they were formed in the body as a result of the disease. Next the causal relation of these bodies with the disease was shown and in several ways. The disease could be caused in other cattle by injecting blood containing the rods beneath the skin, certainly no proof, for the blood might have contained in addition to the rods something which was the real cause of the disease. Next it was shown that the blood of the unborn calf of a cow who died of the disease did not contain the rods, and the disease could not be produced by inoculating with the calf's blood although the blood of the mother was infectious. This was a very strong indication that the rods were the cause; the maternal and foetal blood are separated by a membrane through which fluids and substances in solution pass; but insoluble substances, even when very minutely subdivided, do not pass the membrane. If the cause were a poison in solution, the foetal blood would have been as toxic as the maternal. The blood of infected cattle was filtered through filters made of unbaked porcelain and having very fine pores which allowed only the blood fluid to pass, holding back both the blood corpuscles and the rods, and such filtered blood was found to be innocuous. It was further shown that the rods increased enormously in number in the infected animal, for the blood contained them in great numbers when but a fraction of a drop was used for inoculation. Attempts were also made with a greater or less degree of success to grow the rod shaped organisms or bacilli in various fluids, and the characteristic disease was produced by inoculating animals with these cultures; but it remained for Koch, 1878, who was at that time an obscure young country physician, to show the life history of the organism and to clear up the obscurity of the disease. Up to that time, although it had been shown that the rods or bacilli contained in the blood were living organisms and the cause of the disease, this did not explain the mode of infection; how the organisms contained in the blood passed to another animal, why the disease occurred on certain farms and the adjoining farms, particularly if they lay higher, were free. Koch showed that in the cultures the organisms grew out into long interlacing threads, and that in these threads spores which were very difficult to destroy developed at intervals; that the organisms grew easily in bouillon, in milk, in blood, and even in an infusion of hay made by soaking this in water. This explained, what had been an enigma before, how the fields became sources of infection. The infection did not spread from animal to animal by contact, but infection took place from eating grass or hay which contained either the bacilli or their spores. When a dead animal was skinned on the field, the bacilli contained in the blood escaped and became mingled with the various fluids which flowed from the body and in which they grew and developed spores. It was shown by Pasteur that even when a carcass was buried the earthworms brought spores developed in the body to the surface and deposited them in their casts, and in this way also the fields became infected. From such a spot of infected earth the spores could be washed by the rains over greater areas and would find opportunity to develop further and form new spores in puddles of water left on the fields, which became a culture medium by the soaking of the dead grass. The contamination of the fields was also brought about by spreading over them the accumulations of stable manure which contained the discharges of the sick cattle. The tendency of the disease to extend to lower-lying adjacent fields was due to the spores being washed from the upper fields to the lower by the spring freshets. Meanwhile Pasteur had discovered that by growing the organisms at higher temperatures than the animal body, it was possible to attenuate the virulence of the bacilli so that inoculations with these produced a mild form of the disease which rendered the inoculated animals immune to the fatal disease. The description of Pasteur's work on the disease as given in the account of his life by his son-in-law is fascinating.

Hides and wool taken from dead animals invariably contained the spores which could pass unharmed through some of the curing processes, and were responsible for some of the cases in man. Owing to the introduction of regulations which were based on the knowledge of the cause of the disease and the life history of the organism, together with the prophylactic inoculation devised by Pasteur, the incidence of the disease has been very greatly lessened. Looking at the matter from the lowest point of view, the money which has been saved by the control of the disease, as shown in its decline, has been many times the cost of all the work of the investigations which made the control possible. It is a greater satisfaction to know that many human lives have been saved, and that small farmers and shepherds have been the chief sharers in the economic benefits. The indirect benefits, however, which have resulted from the application of the knowledge of this disease, and the methods of investigation developed here, to the study of the infections more peculiar to man, are very much greater.