2 Rec. de Med. Vet., Mar. 15, 1879.
3 Ibid., Mar. 15, 1880.
The insusceptibility to anthrax is often characteristic of certain individuals or families or of the animals living in a particular district. Thus, Chauveau found that some French sheep, and nearly all Algerian ones, resisted inoculation with a moderate amount of anthrax virus, while the introduction of a maximum amount proved fatal to these as to others. In the same way, it is often noticed that animals living in an anthrax region escape the evil effects of the poison, while strange animals brought in either fall ready victims or for a time do badly until they have become habituated to the locality. In view of the subsequent protective effect on the system of a first and non-fatal attack of anthrax, it is probable that all these examples of immunity in the Herbivora depend on a previous mild attack of the same disease or on the extinction of the more susceptible races. Even in the case of the animals that do badly on first coming into an anthrax district, and recover better health with immunity later, we may well infer that a mild form of the anthrax infection has been passed through.
ETIOLOGY.—The one essential cause of anthrax is the introduction into the system of a specific bacteridian germ (bacillus anthracis or its spores). This is not, as a rule, carried far on the atmosphere, but demands for its propagation contagion, immediate or mediate. Unless, therefore, it meets in the soil the conditions necessary to the preservation and propagation of the germ, it is transmitted with some uncertainty from animal to animal, and thus the disease does not spread widely and rapidly, like an ordinary plague, but tends to become localized in particular districts as an enzoötic.
But its dangers are none the less real nor its existence less to be dreaded. In predisposed localities, where the disease-germ has gained a footing, the animal mortality may exceed that caused by the great plagues, while the risk to human beings is incomparably greater than from any other acute infectious disease of the lower animals. Thus, in San Domingo, in 1770, 15,000 people perished in six weeks from eating the carcases of anthrax animals, and the mortality was only arrested when the meat was legally interdicted. In the worst anthrax years on some of the Siberian steppes as many as one-fourth of the whole human population suffer from the malady. The prevalence and death-rate, however, vary greatly in different localities and seasons. Sometimes only one or two solitary cases of the affection are observed; at other times the disease becomes moderately prevalent, but a lack of virulence in the poison or a previously acquired insusceptibility of the individual protects the great majority of the animals exposed, while at others, still, the poison attacks nearly all exposed to its contagion.
The animal products that mainly convey the disease are the blood, the liquid exudations, portions of the diseased carcase, and the bowel dejections. The virus is most potent when derived from an animal still living or only recently dead, yet under certain conditions (with spore-formation) it may long retain its virulence under the most extreme changes of climate, temperature, dryness, and humidity. Russian hides tanned in England or America frequently convey anthrax, which is known especially as a tanner's malady, and wool and hair sent from Buenos Ayres have repeatedly produced malignant pustule (woolsorter's disease) in Britain and the United States. The preserved scabs of malignant pustule have been often successfully inoculated on the lower animals, so that, like other forms of poison, this seems to be preserved indefinitely by desiccation.
The simple contact of the virus with the slightest abrasion will suffice to convey the disease. It has often been communicated where no lesion of the epidermis could be found, yet the presumption is that even in such cases the cuticle had been in some way wounded. Eating the flesh of animals killed while suffering from anthrax has often conveyed the disease. In an outbreak in Swineshead, Lincolnshire, England, in 1863, I found a dog and a number of swine suffering from eating the bodies of dead bullocks. In 1864 an East Lothian (Scotland) farmer fed his pigs with the offal of a slaughtered anthrax bullock, and lost nearly the whole herd. The carcase of the bullock had been sent to market. About 1860 cattle, and even horses, died yearly on a swampy meadow at Brighton, Mass. On one occasion the owner, John Zoller, fed the offal of a dead bullock to his pigs, which were speedily attacked with anthrax, and as speedily killed to save their bacon (Dr. Thayer). Even when cooked the flesh is not always safe. Of this we have the undoubted case in San Domingo above noticed, the alleged death of 60,000 people in the vicinity of Naples from the same cause in 1617 (Kircher), and the thousands that die on the Russian steppes every anthrax year from eating the sick horses (Rawitch). But in all these, and in the ever-recurring cases in which families suffer from eating anthrax meat, there is the possibility, if not the probability, of the contamination of the meat subsequently to cooking by the knives, forks, tables, and dishes used. The San Domingo slaves had few appliances for cleanliness, much less disinfection, and the Tartars eat their meat from the same board on which it has been chopped up raw.
In accurate experiments it has been found that the bacilli are destroyed by a temperature of 145° F. maintained for five minutes, but the spores are capable of surviving the boiling temperature for five or even ten minutes. The varying power of resistance may be compared to that of the green stalk of the pea and the dry flinty seed. The first is destroyed by a very moderate heat, while the second will sprout after having had boiling water poured over it. The resisting bacillus-spores are never found in the living animal, but may be developed in the blood and tissues after death, and may account for the occasional extraordinary viability of the poison when exposed to a boiling temperature.
Milk, though often used with impunity, conveyed the disease when inoculated by Bollinger, and the same was true of the vaginal mucus. Innocent in the early stages of the disease while the germs are still localized, they become virulent after the bacilli swarm into the blood.
Healthy men and animals often carry the poison, though themselves insusceptible. The question of its conveyance by insects has been much debated, but the constant occurrence of malignant pustule on the uncovered parts of the body goes far to settle the question. Bourgeois long ago noticed that it was most frequent on the face, hands, neck, and arms, and rare on the trunk. In sixty cases recorded by A. W. Bell of Brooklyn, all occurred on the face except two on the hands, one on the wrist, and one on the forearm. The bite of a fly or mosquito had in many of these cases proved the starting-point of the malady. Bollinger has shown the presence of the bacillus in the stomach of such flies as fed on flesh and blood (horse-flies, bluebottles, etc.), and, together with Raimbert and Davaine, has produced anthrax by inoculations with the stomachs, legs, and proboscides of these insects.