Etiology. Nothing is more certain than that the disease is due to the introduction into the blood or tissues of the bacillus anthracis or its spores. These microbes are always found in the anthrax lesions and in the blood of the victim in the advanced stages. When grown in bouillon cultures to the hundredth generation they retain their virulence unabridged and determine the same lesions in the animals inoculated. When the infecting culture has been passed through a Pasteur filter the virulence is lost with the removal of the bacilli. The fresh anthrax blood containing bacilli but no spores when subjected to compressed oxygen (50 atmospheres) becomes noninfecting. The same liquid when boiled proves non-virulent.
Certain conditions, however, contribute to the propagation and reception of the bacillus and these may be considered as accessory causes.
Animals susceptible. The receptivity of the animal exposed is of first importance. Young animals are the most susceptible. The small rodents, the mouse, Guinea pig and rabbit are susceptible in the order named, followed by the sheep and horse and these again by the camel and ox. Among the wild herbivora the goat, deer and stag have a high susceptibility. Man is less susceptible yet contracts the disease readily by inoculation, inhalation or ingestion. Swine and dogs are comparatively little susceptible, yet they often contract the disease by eating the carcasses or discharges of anthrax animals. White rats and birds are held to be insusceptible, yet the latter contract the disease readily when the vitality of the system has been reduced by immersing the body in water, or giving antipyrine. A similar result is observed in the otherwise immune frog if the body is heated above the normal cold blooded temperature. The receptivity may vary, however, in the same genus and species. The Algerian sheep is virtually immune from anthrax, perhaps because its ancestors have been so constantly exposed that only the insusceptible strains survived. Swine, birds and carnivora may have similarly acquired a fair measure of immunity by the survival of the fittest. Apart from this, however, a flesh diet is to a certain extent protective, thus Feser’s rats if fed vegetable food proved susceptible to inoculated anthrax, while if fed on animal food they were comparatively immune.
The animal that has survived an attack of anthrax is thereafter strongly immune. This serves to partly explain the apparent immunity of animals bred in an anthrax district, the young animal becoming habituated to infinitesimal doses of the toxins, conveyed in the secretions of the uterine glands or mammæ.
Soil is a factor so far as it preserves and propagates the bacillus. As already stated, soils that are naturally wet by reason of their impermeable character, their position on or near the water level, their conformation in basins which dry out in late summer or autumn, are especially favorable to preservation of the bacillus. Again soils that are specially rich by reason of an excess of decomposing vegetable and animal remains, or because of excessive manuring, tend to preserve and multiply the microbe. Rich, flat meadows below tanneries or abattoirs, and irrigated from these or occasionally overflowed are especially dangerous to stock placed upon them. Soils with an alkaline reaction from the lime, potash or ammonia present are very favorable to anthrax. Wells receiving surface drainage are common factors in carrying infection.
Season is a contributing factor in various ways. Damp seasons sometimes bring the germ to the surface of the soil by the gradual elevation of the water level, or by causing inundations and the deposition of the bacillus on areas of pasture or forage that were previously free, or finally by bringing the earth worms to the surface and leading to the deposition in their casts of the bacillus brought from the infected graves or retentive subsoils. Dry seasons are, however, the anthrax seasons par excellence, as they dry up swamps, fens, ponds, lakes, basins, deltas and bottom lands, and render them available for pasture. The germladen mud of these drying lands is also raised in dust and deposited on the vegetation to be taken in by the animals. Again on the drying basins and bottom lands the tempting green vegetation is often pulled up by the roots with adherent, infecting mud. For the above reasons, even in an anthrax region the malady is most prevalent in the late summer and fall, and in certain valleys like those of Corsica the stock is considered safe until the dry autumn weather demands their removal to the mountains.
Another reason for the summer epizoötics is found in the transference of the germ by flies. House flies, horse-flies, blowflies, mosquitoes, etc., carry the bacillus on their feet, mandibles or piercing apparatus, and even in their stomachs (Bollinger) and transfer it from one individual to another. It is worthy of note that the great majority of cases of local anthrax in man occur on the habitually uncovered parts of the body (face, neck, hands and arms) and start from a centre like the bite of an insect.
Stables, stable utensils, harness, shafts, poles, fodder and litter are familiar bearers of the virus. Butchers’ knives and wagons and surgical instruments are further media of contagion.
Certain conditions of the animal system expose it to attack. The fever and constitutional disturbance which are caused by the extreme heats of summer and autumn are strongly predisposing, especially if alternated with frigid nights as seen on the Siberian Steppes. Privation of water raises animal temperature and thus the drying up of the customary drinking places becomes an important factor. Plethora and starvation are alike predisposing, probably by lessening the resisting power of the system. Overwork and exhaustion predispose, as Roger showed by making the immune white rat turn a wheel until worn out and then successfully inoculating it. The addition of lactic acid to the virulent liquid (1:500) greatly encreases its potency (Arloing, Cornevin and Thomas) and the further addition of fermentescible sugar and rest enhances this still more (Kitt). The production of lactic acid by muscular overexertion is thus a potent accessory cause in fatigue.
Bacillus anthracis. This was first demonstrated in anthrax blood and exudates by Pollender and Brauel in 1849 and 1850 but as they failed to find it in all cases they concluded that it was not the essential cause. Davaine who found the bacilli in 1850 suspected that they were pathogenic and by 1863 he had shown that blood which contained no bacilli was non-virulent, while that in which these organisms were present was constantly infecting. Klebs and Tiegel in 1871 filtered anthrax blood through an earthenware vase and found that the clear filtrate (bacillus-free) was noninfecting.