ers have so enlarged and darkened the entire organ as to render them invisible, and because postmortem changes take place rapidly. The enlarged, dark, pulpy spleen which is often encountered in hog cholera outbreaks is of little significance in diagnosis, because it is so frequently the result of other causes. The heart reveals no macroscopic lesions in the vast majority of cases, but the petechia which are sometimes visible on the left auricle, less frequently on the right, and rarely involve the ventricles, are usually caused by hog cholera virus. The lungs are often normal. If the surfaces are dotted with ecchymoses, the fact suggests hog cholera quite strongly, but occasionally these lesions are due to other causes. The laryngeal mucosa is often the seat of petechiæ, which are characteristic of hog cholera. Examination of the intestinal mucosa often aids in making a diagnosis, but the lesions encountered are often difficult to interpret. Certain irritants cause changes which may be confused with those due to hog cholera, and secondary invasion tends rapidly to modify filterable virus lesions so that they are difficult to identify. Ecchymoses and larger hemorrhages, as well as ulcers of recent origin, when distributed near the ileocecal valve and elsewhere in the mucosa of the cæcum and upper colon, may be accepted as supplementary evidence of acute hog cholera. The "button ulcer" is usually associated with the chronic form, but agents other than the filterable virus may be instrumental in producing it. All these hog cholera lesions will not often be found in one animal, but if two or more organs are involved this fact, supported by a history that does not positively deny the presence of the disease, may be accepted as ground for a diagnosis. If more organs are involved the evidence is more conclusive. Animal inoculation. This method is rarely applicable in actual practice, because it is expensive and requires too much time. In very exceptional outbreaks which present atypical features, and in cases involving litigation it may be useful. The essentials of a conclusive experiment may be summarized thus: 1. Blood should be drawn from a hog which has been sick for only a short time, and which carries a temperature near 106° F. 2. The blood should be diluted with sterile water and passed through a filter which retains all microscopic organisms. 3. Enough of the filtrate to represent at least 1 mil of the undiluted blood should be injected into a susceptible pig, preferably one weighing between forty and one hundred pounds. 4. Twelve days previous and subsequent to the date of injection the pig should be protected from extraneous hog cholera infection; all receptacles and instruments used in making the injections should be sterilized. 5. Symptoms of hog cholera should appear in less than eight days following the injection. 6. The pig should die in less than seven days following the appearance of symptoms, or at the end of that time, providing it is still sick, it should be killed. 7. Autopsy should reveal typical hog cholera lesions in at least two organs. 8. In negative experiments the susceptibility of the experimental pig should be proved by a subsequent injection with at least 1 mil of virulent hog cholera blood. Differential Diagnosis Peculiar difficulties are met in the differential diagnosis. Clinical examination of the individual is subject to limitations which are not encountered in dealing with larger animals, and some of the maladies which we seek to differentiate from hog cholera occur so frequently in combination with that disease that we do not always have well defined features upon which to base our conclusions. Thus under certain circumstances we have not only to decide whether a given outbreak is hog cholera or swine plague, but we must also ask ourselves whether the two diseases exist in combination. Laboratory examinations may demonstrate the presence of a bipolar organism, but they cannot in the course of a few hours deny the presence of the hog cholera virus, and thus they are often dangerously misleading. Return mail diagnoses of "swine plague" or "hemorrhagic septicemia" are without value because they ignore consideration of hog cholera virus, which in this country is present in most outbreaks of rapidly transmissible swine disease. Among the diseases from which hog cholera must be differentiated are uncomplicated swine plague, pneumonias due to a variety of causes, septicemias, tuberculosis, anthrax, so-called "flu,' various parasitisms, soap poisoning, brine poisoning, and sudden deaths from such causes as heatstroke and lightning-stroke. Rouget and rinderpest are also to be considered in countries in which they are prevalent. The differential diagnosis cannot be made by rule of thumb, nor is it possible to summarize or tabulate the determining features of these various maladies, so that the inexperienced observer can distinguish among them. Armed with definite knowledge of the diseases with which he is dealing the diagnostician usually reaches his conclusions rapidly and accurately. In the absence of such knowledge, a few rules do not suffice. Having already dealt with the distinguishing features of hog cholera we will confine our remarks chiefly to characteristics which suggest the presence of these other diseases. Swine plague. Swine plague rarely occurs in pure form, it affects only a portion of the animals in a herd, and there are frequent spontaneous recoveries. There is often a history of recent shipping. The incubation period is short (1 to 3 days), acute dyspnea and thumping are prominent symptoms, and the hogs show more distress than is observed in hog cholera attacks. High temperatures are not the rule except during the first few hours of sickness. The characteristic "swine plague pneumonia," if present in several autopsies, speaks for the existence of swine plague, but the absence of the filterable hog cholera virus must be clearly established before a diagnosis of pure swine plague is justified. None of the pneumonias, with the exception of that associated with swine plague, are accompanied by the lesions in other organs which characterize hog cholera. Usually they occur where pigs are kept in very dusty quarters, or where the floors are cold and damp and no dry litter is provided. Recent shipping is a predisposing fac tor, and lung worm and ascarid infestations play an important part in their development. They are sporadic or only slowly transmissible. High |