been a sharp reaction, and there are indications at present that the trend of opinion may even swing too far in the opposite direction. The diseases that complicate hog cholera present very real problems, and experimental work looking toward a deeper understanding of them is one of the immediate needs of the present day.

CHAPTER II

NATURE AND CAUSE OF HOG CHOLERA

HOG cholera is an acute, communicable, febrile disease which attacks swine of all breeds and ages, but does not affect other domesticated animals, or man. It is a septicemia. Occasionally a peracute form of the disease is recognized during the first days of an outbreak and chronic hog cholera · is frequently observed among the stragglers that survive the more severe and rapidly terminating forms. In the individual, the disease is characterized by sudden onset, inappetence, chilling, very high fever, arched back, a disposition to hide in the litter, constipation followed by diarrhea, general weakness in the later stages, accompanied by purplish discolorations of the skin covering the belly, ears and snout. In the herd, the onset is relatively slow, the first death usually preceding subsequent ones several days, but after the first week the outbreak rapidly gains momentum, and in a comparatively short time all hogs become infected. The mortality ranges between 80 and 100 per cent with a strong tendency to approach the latter figure.

Young pigs, especially those farrowed and nursed by immune mothers, are often immune to cholera during the first few weeks of life, and a general impression that all pigs nursing immune sows are likewise immune seems to have gained ground. This impression is not in accord with the facts, for we have seen individual pigs born of immune mothers and suckled by them, dead of hog cholera on the seventh day following birth, and under like conditions of birth and sustenance we have frequently seen entire litters succumb to the disease before attaining an age of four weeks. Among older hogs raised in localities where hog cholera is not prevalent, the "natural immunes” so frequently mentioned are by no means common, and it is probable that in places where they are found in considerable numbers they owe their immunity to the fact that they are exposed to cholera as young pigs, and suffering only a slight reaction, are rendered immune. As a general rule, young shoats, old hogs, and sucking pigs are most susceptible to cholera in the order named, and, as would be expected, recoveries from the disease are less frequent among young shoats, and more frequent among old sows and sucking pigs.

The cause of hog cholera is a filterable virus, probably an organism too small to be visible with the highest magnification now obtainable, and possibly possessed of characteristics which prevent it

from taking stains that render bacteria more plainly visible. The virus readily passes porcelain and infusorial earth filters which retain all visible bacteria, but it is itself retained by the finest porcelain filters. It does not pass through colloid membranes. In the human subject, measles, mumps, scarlet fever and smallpox are among the diseases caused by filterable viruses, while among animals rinderpest, foot-and-mouth disease and rabies are some of the diseases that fall in the same group. The classification is a rather loose one, being based entirely on the fact that these viruses will pass filters that retain visible bacteria, rather than on morphological or cultural characteristics.

There is no conclusive evidence that hog cholera virus has been propagated outside the bodies of infected swine. After a hog has been exposed to the disease and actually infected, the virus appears in the blood stream in about four days, and thus all vascular organs harbor it during the attack. In the later stages of a few chronic cases, we have found the blood free of the virus, but we do not know whether this is the rule, nor is there definite knowledge of the part played by "carriers" in harboring it. It is eliminated through the excretions. The urine is regularly infectious, the feces may or may not contain it, and the discharge from the eyes and skin ulcers is infectious

at least in some instances. Just how any one of the filterable viruses operates to produce disease is quite unknown, but it is certain that hog cholera virus has a selective action for epithelial and endothelial cells.

Virulence. Hog cholera virus produces specific disease only in swine, and very small quantities of infected material are sufficient to cause death in susceptible animals. According to King, subcutaneous injections of 1/86 of a mil of virulent blood produced the disease, while lesser amounts produced only a mild reaction, or none at all. Natural infection usually occurs by way of the digestive system, but the disease is readily produced by subcutaneous, intravenous or intra-abdominal injections of small quantities of virulent material.

Resistance. Most of the natural influences to which hog cholera virus is subjected do not operate to destroy it rapidly. Drying, sunlight, and low temperatures seem to have no immediate attenuating effects, although it is a fact that most infected yards which remain uninhabited from three to six months do not endanger susceptible pigs placed in them. There is, though, a considerable tendency for hog cholera to recur on old infected farms, and this fact indicates that there are exceptional cases in which the span of life of the virus is greatly prolonged.