It is difficult to distinguish between the swellings of blackleg and malignant edema, as they resemble each other very closely and both are distended with gas. Malignant edema, however, generally starts from a wound of considerable size; it usually follows surgical operations, and seldom results from the small abrasions and pricks to which animals are subjected in pastures. Inoculation experiments on guinea pigs, rabbits, and chickens will generally disclose the differences between the three diseases above, as all these species are killed by the germ of malignant edema, only the first two species by the anthrax bacillus, while the guinea pigs alone will succumb to the blackleg infection. Hemorrhagic septicemia may be differentiated from blackleg by its affecting cattle of all ages, by the location of the swelling usually about the region of the throat, neck, and dewlap, by the soft, doughy character of the swellings without the presence of gas bubbles, and finally by the characteristic hemorrhages widely distributed throughout the body. Other means of diagnosis, which have reference to the specific bacilli, to the inoculable character of the virus upon small animals, and which are of decisive and final importance, can be utilized only by the trained bacteriologist and veterinarian.

Treatment.—In this disease remedies have thus far proved unavailing. Some writers recommend the use of certain drugs, which seem to have been beneficial in a few cases, but a thorough trial has shown them to be valueless. Others advise that the swelling be opened by deep and long incisions and a strong disinfectant, such as a 5 per cent solution of carbolic acid, applied to the exposed parts; but this procedure can not be too strongly condemned. As nearly all those attacked die, in spite of every kind of treatment, and in view of the fact that when these tumors are opened the germs of the disease are scattered over the stables or pastures, thus becoming a source of danger to other cattle, it is obvious that such measures do more harm than good and should be put aside as dangerous. Bleeding, nerving, roweling, or setoning have likewise some adherents, but the evidence indicates that they have neither curative nor preventive value and therefore should be discarded for the method of vaccination which has been thoroughly tried and proved to be efficacious.

Prevention.—The various means suggested under "Anthrax" to prevent the spread or recurrence of this disease are equally applicable to blackleg, and hence do not need to be repeated here in full. They consist in the removal of well animals from the infected pasture to a noninfected field, the draining of the swampy ground, the burial or burning of the carcasses to prevent the dissemination of the germs over vast areas through the agency of dogs, wolves, buzzards, or crows, the disinfection of the stables and the ground where the animals lay at the time of death, and, if possible, the destruction of the germs on the infected pastures. One of the most effective methods for freeing an infected pasture from blackleg is to allow the grass to grow high, and when sufficiently dry to burn it off. One burning off is not sufficient to redeem an infected pasture, but the process should be repeated several years in succession. This method, however, is in many instances impracticable, as few cattle owners can afford to do it, and the only means left for the protection of the animals is vaccination.

Immunization by vaccination.—Three French veterinarians, Arloing, Cornevin, and Thomas, were the first to discover that cattle may be protected against blackleg by inoculation with virulent material obtained from animals which have died of this disease. Later they devised a method of inoculation with the attenuated or weakened blackleg spores which produced immunity from natural or artificial inoculation of virulent blackleg germs. Their method has undergone various modifications both in regard to the manufacture of the vaccine and in the mode of its application. Kitt, a German scientist, modified the method so that but one inoculation of the vaccine was required instead of two, as was the case with that made by the French investigators. The vaccine formerly prepared and distributed by the Bureau of Animal Industry combined the principle of Arloing, Cornevin, and Thomas, and the modification of Kitt.

By vaccination we understand the injection of a minute amount of attenuated—that is, artificially weakened—blackleg virus into the system. This virus is obtained from animals which have died from blackleg, by securing the affected muscles, cutting them into strips, and drying them in the air. When they are perfectly dry they are pulverized and mixed with water to form a paste, smeared in a thin layer on flat dishes, placed in an oven, and heated for six hours at a temperature close to that of boiling water. The paste is then transformed into a hard crust, which is pulverized and sifted and distributed in packages containing either 10 or 25 doses. This constitutes the vaccine, the strength of which is thoroughly tested on experiment animals before it is distributed among the cattle owners. This vaccine, which is in the form of a brownish, dry powder, is mixed with definite quantities of sterile water, filtered, and by means of a hypodermic syringe the filtrate injected under the skin in front of the shoulder of the animal. The inoculation is usually followed by insignificant symptoms. In a few cases there is a slight rise of temperature, and by close observation a minute swelling may be noted at the point of inoculation. The immunity conferred in this way may last for 18 months, but animals vaccinated before they are 6 months old and those in badly infected districts should be revaccinated before the following blackleg season.

The effect of the vaccine prepared by this bureau in preventing outbreaks of the disease and in immediately abating outbreaks already in progress was highly satisfactory, and it is not to be doubted that thousands of young cattle were saved to the stock owners during the 25 years in which the vaccine was distributed.[6] More than 47,000,000 doses were sent out during this period, and from reports received it is safe to conclude that more than 40,000,000 were actually injected, whereby the percentage of loss from blackleg has been reduced from 10 per cent, which annually occurred before using, to less than one-half of 1 per cent per annum. With these figures before us it is plain that the general introduction of preventive vaccination must be of material benefit to the cattle raisers in the infected districts. Moreover, there is every reason to believe that with the continued use of blackleg vaccine in all districts where the disease is known to occur, and an earnest effort on the part of the stock owners to prevent the reinfection of their pastures by following the directions given, blackleg may be kept in check and gradually eradicated.

Immunization against blackleg is now frequently accomplished by the use of the so-called blackleg aggressin and blackleg filtrates.

NECROTIC STOMATITIS (CALF DIPHTHERIA).

[[Pl. XLIII].]

Necrotic stomatitis is an acute, specific, highly contagious inflammation of the mouth occurring in young cattle, and characterized locally by the formation of ulcers and caseo-necrotic patches and by constitutional symptoms, chiefly toxic.