Small yellowish ulcers were also found in the ileum. The points of injection were occupied by encysted, partly liquefied masses.

No. 223 was found dead July 2. At the points of injection encysted masses were found, the contents of one of which were discharging through an opening in the skin. The mucosa of the entire large intestine deeply congested. Scattered ulcers of varying age and size in the cæcum and colon. Bacterium in spleen.

No. 222, after a period of unthriftiness, was found dead July 1. The autopsy revealed a chronic broncho-pneumonia, with pleuritic adhesions of right lung. The mucous membrane of the cacum and colon, besides being studded with a large number of broad shallow ulcers, was deeply and uniformly congested, the .congestion involving also the lower portion of the ileum. On both thighs an encysted semiliquid mass indicated the seat of the inoculation. This case suggests the probability of a double infection, the first represented by the ulceration, the second by the more recent inflammation of the mucous membrane.

Of the second lot, which had received 4 of the first inoculation, all succumbed to the infection. No. 226 died July 3. The characteristic lesion was extensive ulceration, together with deep congestion of the mucosa of large intestine. Encysted masses at the points of inoculation. A considerable number of bacteria of hogcholera in the spleen.

No. 228 died July 13. In this animal the mucosa of cæcum and colon presented a continuous mass of necrosed blackish tissue, the ileo-cæcal valve being enlarged to twice the nomal size. A few scattered yellowish ulcers in the lower portion of the colon. No. 215 died July 10, probably affected in the same way, though no post mortem examination was made.

No. 229 died June 27. In this case the lymphatic glands were in general deeply congested; ecchymoses beneath the serous membranes. Pigmentation of the mucous membrane of the stomach, duodenum, ileum, and large intestine from former extravasations. Several large ulcers on the valve and some others in colon. Ulcers in the cardiac portion of the stomach. Encysted masses at the point of inoculation. Nos. 224 and 225 were penned with the above eight animals as checks. No. 225, after being sick for a few days, was found dead June 27. The mucosa of the cæcum and upper half of the colon is extensively pigmented and ulcerated, the lower half deeply congested. The ileum is also ulcerated for 5 or 6 feet from the valve. Many of the ulcers are so deep as to have produced inflammation of the serous membrane and thickening of the intestinal walls. The other check (No. 224) lived over two months after exposure, being unthrifty during this period. On post mortem the mucosa of large intestine was considerably pigmented and scars of healed ulcers were present. A large suppurating wound of the lower jaw, involving the bone, may have contributed towards the fatal issue.

These inoculations having failed to produce immunity from natural infection, a second experiment was tried by augmenting the dose of strong virus used for the second inoculation. Thus Nos. 239, 242, 244, and 245 received each c for the first inoculation May 27, No. 243 being retained in the same pen as a check. Of these No. 239 died of hog-cholera as the result of the inoculation. The remaining three, received two weeks later, on June 10, 2° each of strong virus. The cultures were prepared in beef infusion with 1 per cent. peptone. They were usually the third or fourth culture, not more than one day old. A second lot (Nos. 240, 254, 255, and 256) were inoculated at the same time and in the same way, with this exception, that the second dose was increased to 3°. On June 24 all were placed in the large infected pen.

No. 239 died June 2, within six days after receiving fee of the culture and as a result of the inoculation. The lesions were those of a very acute case, engorged spleen and lymphatics, intense congestion of the mucosa of the large intestine and of the intestinal tract in general. The lungs were likewise engorged and dotted with extravasations. This animal was eating and apparently well on the morning of death. The spleen was crowded with the bacterium of hog-cholera, and pure cultures of the microbe were obtained from it.

No. 242 died July 17. The characteristic lesions of hog-cholera were found in it; extensive ulceration of the cæcum and colon; engorgement of spleen and lymphatic glands with blood. Encysted masses at the point of inoculation. No. 224 succumbed July 9 with practically the same lesions, besides the presence of a considerable quantity of serum in the abdominal cavity.

The check to this lot died July 18. The depth of the ulcerations in the cæcum and colon had implicated the serous covering, so that adhesions had formed between the cæcum and abdominal walls. Punctiform ecchymoses on serosa of ileum; the mucosa not affected. The mucosa of cæcum was found completely ulcerated, the necrosis stopping abruptly at the edge of the valve; in the colon the necrosis resolved itself into large isolated ulcers.

Of the second lot No. 240 died July 10. At the place of inoculation a firm pale yellowish mass about one inch long was found. The lower portion of ileum, the cæcum, the upper portion of colon, contained ulcers of different sizes. The duodenum was occluded by a clot of blood. No. 254 died the same day with lesions of a similar character. No. 255 died July 20. The spleen in this case was greatly augmented in size and gorged with blood. The right lung was congested and adherent to wall of thorax; considerable effusion in this pleural sac. The cæcum and upper portion of colon covered with deep blackish ulcers. A few small ulcers in ileum.

The foregoing experiments demonstrate the important fact that pigs cannot be made insusceptible to hog-cholera by subcutaneous injections of pure cultures of hog-cholera bacteria. This method, which was originally suggested and applied by Pasteur to anthrax and rouget, therefore fails in this disease. The experiments have been sufficiently varied and extended to leave no doubt as to this point. The subcutaneous injection of large as well as small quantities of culture liquid, either once or twice, left the animal as susceptible to natural infection as before inoculation, and in a few cases produced a virulent type of the disease. We have already dwelt upon the important fact that the disease can be produced by feeding when subcutaneous inoculation fails. This may also explain the failure of protective inoculation. The disease exerts its severest effects locally upon the mucosa of the large intestine, and in only a few cases is it a real septicemia. The bacteria have the power of rapid multiplication within the vascular system only when exceptionally virulent. Subcutaneous inoculations of cultures, in which they may have become attenuated, are of little avail, because they are speedily destroyed in the connective tissue, leaving only a slight local swelling behind. Other lines of investigation must therefore be followed out before any practical results can be obtained.

HOW CAN HOG-CHOLERA BE PREVENTED?

The measures which must be adopted to prevent the introduction and spread of hog-cholera depend upon our knowledge of the disease as it appears in herds, and more especially upon a study of the cause. This we have demonstrated to be a microscopic plant organism belonging to the class of bacteria, and resembling in a general way those organisms which are the cause of infectious diseases among men. Of this disease the only reliable diagnostic lesion is ulceration of the

large intestine, or the presence of the bacterium in the body of the affected animal, and whatever follows can only apply to the disease produced by this specific organism or bacterium.

It has been shown in this and preceding reports that the disease spreads from one animal to another of the same herd until sometimes only a small percentage of unthrifty animals remain. It extends from one herd to another, and may be carried long distances. The presence of the specific bacterium has been demonstrated in such widely different regions as the District of Columbia, Illinois, and Nebraska. The fact that one animal takes the disease from another does not explain how the virus is transferred. Is it carried directly from one to another in the air, or is it deposited in the soil by one animal to be taken up by another? Is it introduced through a wound in the skin? Is it taken up by the lungs from the inspired air, or is it introduced with the food and drink? These questions cannot be solved by simply observing the disease in herds, hence numerous experiments detailed in this and the preceding report have been directed to a solution of these questions, upon which some rational rules for prevention may be based.

The disease is perhaps never communicated by injuries of the skin, by bites and wounds obtained in other ways. Large quantities of liquid containing the virus can be introduced beneath the skin without fatal results. In the great majority of pigs a local swelling is the only effect.

Whether the lungs serve commonly as an entrance to the virus cannot be definitely stated. All experimental evidence points the other way, and in a large percentage of cases the lungs are intact, while the large intestine is severely ulcerated. We have shown that blood and tissue which have been dried for two months may contain bacteria, which readily multiply when placed in proper media. Hence the dust from pens where the disease exists may contain many bacteria, which, on reaching the lungs, multiply and produce the disease. It is our intention to continue experiments which may throw more light upon this mode of infection."

Perhaps the most common source of infection is the food and drink. That is, the virus enters the alimentary canal, and there produces such extensive ulceration that the animal sooner or later succumbs from gradual exhaustion, septic poisoning, or peritonitis. Or the virus immediately enters the blood from the intestines, multiplies in every organ of the body, and causes death in a few days, or even hours. Such sudden deaths usually occur at the beginning of severe epidemics. Pigs which are fed with the internal organs of those that have died of the disease almost invariably take the disease in a very severe form, and die within one or two weeks after infection. To demonstrate that it was the specific bacterium of hog-cholera in these internal organs, and not some other element, pigs were fed with liquids which contained only this organism, and they produced the most severe disease. These demonstrations, together with the commonly observed fact that the disease seems to exist in the large intestine only, prove conclusively that the virus is introduced largely with food and drink.

It has already been stated that the most common seat of the disease is the large intestine. The food after leaving the stomach passes in a liquid condition through the small intestine, so that this never seems filled; in fact, its only contents are a coating of semi-liquid matter over the mucous membrane. It passes through the small intestine

quite rapidly, but on reaching the large intestine the undigested remains become more consistent, because the liquid is reabsorbed, and are kept here for some time. The bacteria, if not destroyed by the gastric juice, pass quickly through the small intestine, but in the large intestine they begin to multiply and attack the mucous membrane, which they destroy. Hence the feces or discharges of diseased pigs, wherever deposited, scatter larger or smaller quantities of the virus, which may induce the disease over and over again. The discharges, then, must be looked upon as the chief vehicle for the virus. when the disease has taken hold of a herd. Pigs endowed with the well-known habits will not hesitate to avail themselves of the opportunity of becoming infected whenever it is offered. But the discharges are not the only means by which the virus is disseminated and kept alive. We have shown that the bacterium constituting the living virus is a very hardy germ, and one endowed with great powers of multiplication. In the laboratory it has grown luxuriantly in milk and on boiled potato. It grows slightly in hay infusion, even in urine not neutralized. The temperature throughout the summer in most of the States is sufficiently elevated to permit the growth of the bacterium in these various substances, since a temperature of 70° F. is amply sufficient, and temperatures above this point simply favor the rate of increase of the quantity of virus. Even in good drinking water the virus will increase for four or five days and remain alive for months.

There is consequently very little about a pen in which the virus, when scattered in the discharges of infected animals, will not increase in quantity and form a more potent source of infection. It will multiply in the wet soil, in the drinking water, and in the semiliquid food. A gallon of milk inoculated with a minute portion of infectious material and allowed to stand through a warm night in midsummer might be sufficient to produce the disease in at least a dozen animals if fed to them on an empty stomach.

Such are the external conditions which may favor the extension of hog-cholera. The condition of the animals themselves is of great importance in favoring or preventing infection. When pigs are fed with liquids in which the specific bacterium only is present, those that have been deprived of food for some time previous take the disease, while those whose stomachs contain food that is undergoing digestion do not take it readily. If, besides starving the animal, they are fed with some alkaline solution by which the alkalinity of the stomach is increased, the pathogenic effect is still more pronounced. Any disorder of digestion by which the secretion of gastric juice is diminished or checked and the mucus is increased in quantity will increase the susceptibility of the animal to infection, because the alkalinity of the mucous membrane will favor rather than destroy the virus. Any mode of feeding which produces constipation and overdistension of the large intestine is likely to favor the disease, as the virus is retained for a longer time. It multiplies there and destroys the mucous membrane before it is discharged. Keeping these facts in mind, we may formulate a few rules, which must be carefully observed if the disease is to be kept in check.

In the first, place, there should be no communication between infected herds and such as are still free from the disease. The virus may be carried in various ways, even on the shoes of persons. A small quantity thus introduced may multiply in the soil and water until it becomes a center of infection for many animals. Streams

into which sick animals have dropped discharges or in which dead ones have lain must be considered as vehicles of the disease for all herds below the source of infection. This is especially true in warm weather, when the virus multiplies very rapidly and extensively.

When the disease has appeared in a herd, the ground upon which the animals lived at the time must be considered as infected, and it is much safer to remove all the well ones to uninfected grounds than to simply remove the sick ones. But how are we to know that the disease has gained a foothold in the herd? It is quite common for the disease to announce itself by a few sudden deaths. The stricken animals may seem well a day, perhaps only a few hours, before death. Such animals should always be immediately destroyed by careful deep burial, or by burning, which is much better, for the bodies are as a rule crowded with the specific poison of hog-cholera. In order to remove any doubts as to the precise nature of the disease, it is best to examine such animals before burying or burning them. This should be done in a secluded place which pigs cannot reach, and the ground thoroughly disinfected, as will be described later. The disease in the sudden cases can be easily recognized. The spleen is as a rule very black and enlarged. Spots of blood from the size of a pin's head to a quarter inch or more will be seen in the fat under the skin, on the intestines, lungs, heart, and kidneys. The lymphatic glands are purplish instead of a pale pink. When the large intestines are opened they are found covered with these dark spots of blood more or less uniformly and entirely. Often the contents are covered with clotted blood. Any or all of these may be considered as signs of the disease in its most virulent form. In these animals the virus has penetrated into all of the vital organs, and they should be immediately removed and destroyed. It must be borne in mind that for any animal to consume portions of these carcasses would be certain death; that the blood and fluids from these dead bodies contain the virus, and when scattered over the soil or thrown into streams they simply distribute the virus, allow it to multiply, and all the other animals are thereby put in the way of becoming infected.

In many outbreaks the early cases do not succumb so rapidly. They grow weaker, lie down much of the time, eat but little, and usually have diarrhea. Most of such cases may linger for weeks, meanwhile scattering the poison in the discharges. The disease may be recognized in these cases as soon as they are observed to act suspiciously, and there should be no delay in determining at once the nature of the disease. When the animal has been opened the large intestine should be carefully slit up and examined, beginning with the blind or upper end. There will be seen roundish yellow or blackish spots, having an irregular depressed, sometimes elevated, surface. These are well shown in the Second Annual Report of this Bureau, p. 246, or in the Annual Report of the Department for 1885, p. 522. These spots correspond to dead portions of the mucous membrane, and they are frequently seen from the outside as soon as the animal is opened. Sometimes the membrane has been entirely destroyed. Its appearance is well shown on Plates I and II of this report. These slow, chronic cases are apt to spread the disease in the bowel discharges, for in them the virus is chiefly located.

Having determined the existence of the disease, it may not be possible to remove the healthy animals to uninfected quarters after the sick ones have been taken away. Under such circumstances thorough