This is one of the best known and most widely diffused pathogenic forms, occurring, as it does, both as a saprophyte and a parasite. As a parasite it gives rise to the conditions variously known as Splenic fever, Anthrax, Charbon, Malignant pustule, and Wool-sorter's disease, and it is sometimes described under the name Bacteride du charbon. It mostly occurs in animals, but is occasionally communicated to man.

The bacteria are rods of considerable size, generally united end to end so as to form longer threads. Each bacillus is about as long as the diameter of a red blood-corpuscle. As seen in the blood the extremities of the bacilli are abrupt, and even slightly concave, so that when two are united there is an oval space which is very characteristic. (See Fig. 143).

The bacillus is capable of living on so many different media and at such different temperatures that its saphrophytic forms are of great importance. It' may be cultivated in many different nutrient media, nutrient gelatine, agar-agar, potato, etc. It grows readily in neutralized or weakly alkaline urine, and in infusions of animal substances in general. On potato it grows very vigorously, forming a dry yellowish creamy layer.

In the animal body it only occurs in the form of the bacilli, which are either single or form short threads of two or three. When grown on the surface of nutrient media so as to be exposed to the oxygen of the air, the bacilli grow out into long threads, which at first sight appear to be continuous, but when properly displayed are seen to be composed of individual bacilli. The threads form parallel groups or more tangled and matted masses. In these threads the formation of spores occurs when the circumstances are favourable. This requires the free access of oxygen, and a temperature between 18° and 40° C, while that between 20° and 25° gives the best results. The optimum temperature is between 28° and 37° C. The spore lies in the middle of the bacillus as a glancing refractive body. The bacillus by and by disappears, leaving the spore free. The spores contrast with the bacilli in being very permanent, as they resist drying and decomposition. Pieces of silk thread impregnated with spores from a culture on potato may be-kept for years perfectly capable of germination.

Fig. 143. - Bacillus of anthrax (6), and of the septicemia of mice (a), x 700. (Koch).

The anthrax bacillus, like some others, requires careful staining, but if due care be taken a watery solution of almost any aniline dye may be made to serve, or Gram's method may be used for double staining. The spores are more difficult to stain. If a cover-glass preparation be first made then it may be floated for about twenty minutes on the surface of a hot alcoholic solution of fuchsine, decolorized in weak hydrochloric acid and again stained with methyl-blue. The bacilli are stained blue and the spores red.

The anthrax bacillus is capable of transmission to a large number of animals, although considerable differences of susceptibility exist. Mice are the most readily infected, and next to them stand guinea-pigs, rabbits, sheep, and cattle. Rats are with difficulty affected, and dogs, birds, and frogs are scarcely susceptible. Frogs may be affected if after inoculation they are kept for a few days at the temperature of the incubator. Man, although susceptible, exhibits a certain degree of resistance to anthrax infection, evidenced by the fact that there is not infrequently after inoculation a local disease (malignant pustule), the bacilli being present in the exudation of the pustule.

The disease may be induced experimentally either by inoculation into the skin or mucous membranes or by introduction into the alimentary canal. For inoculation, the blood of an animal which has died of anthrax or an artificial culture may be used. In the case of the alimentary canal, cultures containing spores must be used, as the bacilli are destroyed by the gastric juice. When an animal is infected it shows the symptoms of an acute specific fever, which usually proves fatal in a few days. The bacilli are found in enormous numbers in the blood, and as a rule they are found only inside the blood-vessels. So abundant are they in some cases that a preparation stained in the proper way so as to render the bacilli prominent appears as if the vessels were injected with a coloured material.' (See Figs. 144 and 145.) It is chiefly in the capillaries that they are to be found, and they are most readily observed in the spleen, the villi of the intestine, and the glomeruli of the kidney. In the latter situation there is not infrequently rupture of the glomeruli and the passage of blood and bacilli into the uriniferous tubules.

Spontaneous Infection with anthrax occurs in animals mostly with their food. It originates chiefly in cattle and sheep, and they acquire it in grazing. There are certain parts of the country where the disease is endemic, the grass apparently containing the microbe. As this infection is by the alimentary canal it is necessary that spores should be present. The bacillus grows on many sorts of vegetable and animal matter on which it may be planted by the blood or discharges of infected animals. Hence the bodies of such animals should, where possible, be buried without opening, and at any rate, precautions taken against contaminating the ground with blood or tissues.

In man the disease has mostly been communicated by handling the tissues of animals which have been affected. Sometimes spore-formation will occur in such tissues, the bacilli growing on the tissue when exposed to the air, and producing spores. Hence spores may adhere to hair, wool, or other material, and be conveyed long distances. Most cases in man have occurred amongst workers in hair or wool, hence the name of Wool-sorter's disease. In several cases occurring in Glasgow the hair had come from Russia.

Fig. 144. - Villus of intestine in anthrax, the bacilli visible as minute threads. x 250. (Koch).

When inoculation occurs by a scratch or wound of the skin in man, or by the pricking of insects which have been in contact with the blood of an affected animal, a local disease results, the Malignant Pustule. A small elevated boil forms of a red or yellow colour, which soon acquires a crust. The pustule has the usual characters of an acute inflammation, the cutis being infiltrated with leucocytes. There are numerous bacilli in the superficial layers of the cutis. There may be around the pustule much oedematous swelling. The infection sometimes passes on to the blood and produces the regular splenic fever.

In the Intestinal canal local lesions are produced similar to those of the skin. They are rare in the human subject, though common in animals. They are seated mostly in the small intestine, and rarely in the stomach or colon. It is also stated (by Eppinger and Paltauf) that occurs by the lungs from inhalation of spores. This has been observed amongst rag-pickers in paper works, the disease manifesting itself as an acute inflammation, affecting bronchi, lungs, and pleura. There are sometimes necrotic areas in the bronchi.

Attenuation of the virus was first attempted by Pasteur, and was attended by very striking results. By cultivating the bacillus at a temperature of 42°-43° C. for about twenty-four days, the infective power is destroyed, while a shorter period suffices to weaken it. By this means a permanent Vaccine is procurable. Immunity has also been procured by using a toxalbumin separated from anthrax cultures, and also, according to Wooldridge, by using a form of fibrinogen obtained from the normal blood.

When animals are vaccinated with the attenuated virus they have a mild form of the disease and acquire a certain degree of immunity to the more severe forms. The practical importance of this fact is seriously diminished because the immunity appears to be only from infection by inoculation, and does not extend to infection by taking the spores into the alimentary canal The latter is, moreover, the "most frequent mode of infection in spontaneous anthrax.