HOG CHOLERA.
—The term hog cholera has become quite ambiguous, partly on account of new discoveries concerning the cause of the disease and partly on account of what have been supposed to be two different but curiously related diseases being generally included under this general term. Until within a year or two we have supposed that there were two infectious diseases of hogs recognized under the general terms of hog cholera and swine plague. It now seems probable that we will be able to do away with the term swine plague entirely.
The disease considered here answers to the following requirements: (a) Infectious by association or other natural exposure; (b) the animal before death and the carcass after death show certain accepted symptoms which are clearly recognized as pertaining to cholera; (c) the blood is virulent and capable of reproducing the disease on inoculation into susceptible hogs; (d) attack and recovery confer immunity. It is to be understood that we might easily have diseases among swine where characteristic “a” or even “b” might be present and yet the disease be not true hog cholera.
AN ATTACK OF CHOLERA
One of the familiar attitudes assumed when the hog is affected with cholera. When this far along, not many cases of recovery are observed.
Until within recent years American authorities, bacteriologists and veterinarians alike, have very generally accepted a certain germ, the bacillus of Salmon and Smith, as the specific cause of hog cholera and another somewhat similar germ as the cause of what was supposed to be a distinct but curiously related disease—swine plague. But within a few years workers in the Federal bureau of animal industry have apparently demonstrated that hog cholera is caused by a living germ so small that it passes easily through germ filters which remove all known forms of the bacillus of Salmon and Smith.
It may be interesting to note further that this new germ is so small as to be invisible to the highest available powers of the best microscope. That it is a living organism and not a chemical poison may be very easily demonstrated. The curious relations to this disease of the old bacilli of hog cholera and of swine plague are not well understood, but it seems quite possible that they may play some part in the later development of the disease after the disease processes have been started by the invisible germ. While our old theories and supposed information concerning the cause of hog cholera have been very much disturbed by newer work, it is important to remember that hog cholera is now just as much as before to be recognized as a distinctly infectious disease. It is important to remember also that this infection is absolutely necessary, or there can be no cholera no matter how susceptible animals may be. There can be no cholera without this primary and specific cause any more than there can be plants in our wheat fields without the previous presence of mustard seed. Conditions of soil and climate may favor a rank growth of mustard. Conditions of feed and keep may favor the development and spread of hog cholera. They may decrease resistance and increase susceptibility, but cannot originally cause the disease. It is a rather common experience that hogs kept closely housed and fed, especially with such foods as corn, offer less resistance than do other hogs. In our vaccine work we frequently find hogs of this type which die readily under inoculation with blood of low grade virulence. Hogs of hardier type may become slightly sick or not sick at all with inoculation from the same infectious material. Pampered show herds appear especially susceptible to both natural infection and artificial inoculation.
The farmer, and for that matter the public in general, should bear in mind that the cause of hog cholera is a living organism capable of enormously rapid self-multiplication—actual, though very minute particles of matter. This, fully understood, makes it apparent that infection may be carried in any way that other fine particles of matter may be carried. It thus becomes very apparent that the infection may be carried by sick hogs or upon the legs and bodies of hogs not sick; it may be carried in wagon boxes, in hog racks, in stock cars, or upon shoes and clothing of people. It is very evident that the infection may be carried down stream, especially in small creeks, and give rise to other outbreaks.
So far as the sick hog is concerned, we are quite sure that the blood and the manure are thoroughly infectious and there can be no question concerning the infectiousness of fresh carcasses of dead hogs. Perhaps we should say first of all that we rarely get all of the accepted symptoms of hog cholera plainly shown in one case. It is important to bear in mind that cases vary in virulence from those of very chronic type where hogs live for weeks and finally die or recover, to very acute cases where they die overnight.
The hog coming down with cholera is usually sluggish at first, lying around in the shade and refusing feed. The hair may become rough. The eyes early show symptoms of inflammation, with a sticky discharge. There is usually a suppressed cough. The gait may become irregular and uncertain, especially with the hind legs. After these preliminary symptoms have been shown for a time, the skin becomes red, changing to purple, especially noticeable in white-haired hogs. The hog is then usually within a very few days of death.
As already explained, not all cases are typical. Sometimes hogs die in an outbreak of cholera from undoubted hog cholera, and yet the ante mortem or post mortem symptoms show very little upon which to base a diagnosis. But we may easily demonstrate that these were cases of cholera by injecting their blood into susceptible hogs and by thus producing typical cholera.
THE RESULT OF HOG CHOLERA
A post mortem of a hog dying from cholera will show ulcers like those pictured here. Look for them in the large intestine.
At the autopsy of an ordinary case of cholera the first and perhaps the most striking thing seen is the purpling of the skin. On opening the carcass small blood spots may be found under the skin and in the fat cut through. The glands along the intestines are intensely inflamed. The mucous membrane of the stomach is frequently thickened and roughened and in chronic cases there may be ulcers. On opening the intestines we see areas here and there of intense inflammation in the acute cases or numerous ulcers in cases of more chronic type. In very acute cases we find areas intensely inflamed, even bloody in places. The slow chronic cases develop characteristic hog cholera ulcers. These may appear at almost any point on the lining membrane, but more particularly in the blind pouch and around the point where the small intestine connects with the large intestine. On stripping off a very thin transparent membrane covering the kidneys, a typical case of hog cholera will usually show minute red spots on the surface somewhat resembling the covering of a turkey egg, which gives the common name of turkey egg kidney of hog cholera.
Preventing the Disease.
—Clearly there are certain things which the owner of healthy hogs in a hog cholera district should do and a good many things which he should not do. The same is equally true for the man who has sick hogs in a neighborhood where there are uninfected herds. The owner of healthy hogs and his family should keep away from public stock yards, from all pens and yards on other farms whether sickness among hogs prevails or not. It may easily occur that a neighbor’s hogs may appear well but have recently received the infection and be already capable of scattering the disease. We do not know at what period in the development of this disease infected hogs become capable of disseminating hog cholera.
During a hog cholera season the owner of healthy hogs should institute something in the way of private quarantine and pleasantly, perhaps, but firmly, ask visitors, especially stock buyers and threshing machine crews, to keep at a reasonable distance from the pens and yards. It is safer for one man to have exclusive care of healthy hogs during the hog cholera season, and this man should be very careful where he goes with reference to possible infection. Special fencing or other provisions should be made wherever practical to keep dogs out of the pens and yards, for, under certain conditions, dogs become very active agents in spreading the disease.
The owner of a healthy herd should be very careful about buying in hogs for feeding or breeding purposes, and, in the Western states especially, all public stock yards and stock cars must be regarded as possible sources of spread. Hogs coming into the herd for breeding purposes, if by rail, should be shipped in other than stock cars, and should not be unloaded so as to go through stock yards. All new hogs coming on to a farm where the disease has not appeared, should be kept carefully apart from the herd for from two to three weeks after arrival. The disease may thus have time to develop, if the animals have been infected before shipment or en route. It is decidedly worth while to be careful about clean feeding, for it seems probable that this is a common method by which infection enters the body. This being the case, troughs and feeding floors should be frequently disinfected with steam, boiling water, or a very dilute corrosive sublimate solution (1:1,000 dissolved in water), with the troughs subsequently rinsed out with plain water. Or the troughs and feeding floors may be disinfected with any of the coal tar disinfectants if they are used in sufficient strength. These are not poisonous in any probable quantity which hogs would get.
A Disastrous Experience.
—The farmer should be especially careful about buying hogs out of stock yards. Some years ago a certain Minnesota farmer purchased a lot of feeders from Sioux City and took them home to his farm. In about two weeks his hogs commenced dying. A little later hogs previously on the farm began dying. In a little while he was losing hogs at the rate of 25 a day, losing a total of about 200. This loss of 200 hogs was scarcely a drop in the bucket—too small for consideration in comparison with the loss which this outbreak cost the state, for, with some others coming into the state from Iowa and Nebraska, this outbreak cost the state, as carefully estimated, about $1,250,000 during that one year. As soon as the Minnesota farmer here referred to realized that he had cholera and was liable to lose a large portion of his herd, he shipped out a lot of fat hogs ready for market. These were yarded for a time in the public stock yards of his town, and one of them died while waiting for shipment. This hog was left for a day or so in the yard. Later a carload of feeding hogs was shipped in from a point in South Dakota, where they had never had hog cholera. These South Dakota hogs were unloaded into the yards where the fat hog had died some time before, and were sold out from there by auction.
It was a very interesting study to follow the resulting outbreaks; but a very serious matter for the owner and for that entire portion of the state. Practically every farmer who bought hogs at this sale, and very many of those who walked around the yards looking at the hogs, but without buying, had hog cholera on their farms in a very uniform period after the sale. Surely the moral of this tale is so self-evident as to need no further suggestion.
Cleaning Up.
—Troughs and feeding floors, at least, and, if practicable, the hog house also, should be kept clean and frequently disinfected during an outbreak. When the outbreak appears to be over, the owner must decide as to just what he will do in the way of disinfection and cleaning up, or whether he will stay out of the hog business for a year and allow the infection to die out. This is, of course, without regard for the possibility of putting in vaccinated and immune hogs. Feeding troughs and feeding floors and the hog house in general, may be disinfected if of reasonably good construction, by a thorough cleaning and then by one of the methods suggested under prevention. If the sick hogs have been kept in an old straw shed or in an old hog house that is about ready to fall down anyway, by all means the best method of disinfection is by burning. Without disinfection or burning the owner cannot be safe in putting in susceptible hogs within much less than a year after the last hog died or recovered. The slow old chronic cases that go dragging around at the end of an outbreak should usually be killed and safely buried, for it is rarely profitable to put such hogs in shape for market. It might possibly be worth while to hold such a one over and nurse them along, in case of valuable brood sows, for hogs having recovered from cholera are usually immune for life.
Brood sows which have had the disease and recovered usually give something more than natural immunity to their offspring. But the degree of immunity so conferred is so variable in degree and uncertain otherwise that it cannot be depended upon as a routine method of establishing immune herds. Yards may be practically disinfected by plowing or by burning off a good layer of straw.
Hog Cholera Vaccination.
—Generally stated, this vaccine consists of two parts: (a) Blood serum from the body of a specially immunized hog; and (b) virulent blood serum from the body of a hog about to die from cholera. The general theory upon which this double vaccine is used is that of giving the animal an infectious disease and at the same time a treatment which enables the animal to resist the infection. When the hog is through with it he is in exactly the same condition as though he had gone through a natural exposure and recovered.
General Method.
—We start this work with certain hogs that are immune usually because they have passed through an outbreak. It has been shown that when such immune hogs are treated with large injections of virulent blood under the skin or into a vein, that they do not usually become sick, but their own blood develops a peculiar property that gives protection to other hogs that are naturally susceptible.
When the blood or rather blood serum from this specially treated immune hog is injected into the bodies of healthy susceptible hogs, the latter becomes likewise immune, but the immunity so gained lasts only a short time, possibly four to six weeks, and is then gradually lost. If we give a small injection of virulent blood at the same time, or soon after the immunizing serum is given, then the treated hog becomes immune for a long period, perhaps for life.
The Serum Hog.
—The specially immunized hog which produces this immunizing serum is known as a hyperimmune, and to save words will be hereafter mentioned as such. The simply immune hog may be prepared for producing serum in either one of three ways. (1) By three rapidly increasing doses of virulent blood serum injected under the skin at intervals of seven to ten days; (2) by one enormously large injection of virulent serum under the skin; (3) by injecting virulent blood in smaller doses directly into the blood circulation.
In this work an ordinary immune hog weighing 100 pounds is given a quart of very virulent blood, a teaspoon of which similarly injected would kill a hog that was not immune. In other words the immune, and especially the hyperimmune hog, have developed certain properties in their blood antagonistic to hog cholera virus.
Vaccination.
—We have two possible methods of vaccinating or immunizing susceptible hogs (a) Serum only. This is by the injection under the skin of serum from the body of a hyperimmune hog and gives immediate but temporary immunity lasting, as already stated, several weeks. If this animal, during the period of immunity, is exposed to natural infection, he becomes protected for a very long period, perhaps for life. (b) Simultaneous. The second method of vaccination consists of injecting immunizing blood serum into one thigh and a small amount of disease-producing serum at the same time, or soon after, into the other thigh, thus giving the animal the cholera and a cure for it at the same time. If the immunizing serum is potent and the virulent serum is really virulent, then the animal so treated becomes permanently immune.
The serum-only method is usually preferred in actual outbreaks and for hogs not yet sick, because this gives immediate protection, and the hogs, being naturally exposed, usually develop a permanent immunity. The simultaneous method of vaccination is preferred where we are very confident of the serum’s potency against the virulent blood, and for hogs that have not yet been infected. It may yet be found wise to use this method even in outbreaks.
Vaccination Does Not Spread Cholera.
—Every intelligent stockman who reads this will probably ask if there is not danger of scattering cholera by this simultaneous vaccination into districts where it has not yet appeared. A considerable amount of direct evidence on this point is better than any amount of theorizing and personal opinions. This evidence all agrees that unless the vaccinated hogs become distinctly sick as a result of the vaccination (which can occur, and does very often), that there is practically no danger of disseminating the disease. This is especially true since all hogs on the farm are supposed to have been treated and are immune, and, therefore, incapable of developing cholera and so spreading the disease. It does occur, even with good serum, perhaps, that an occasional hog may become a little sick, and very rarely even die, as a result of vaccination. But with good serum given in standard dose and virulent blood also given in proper dose, the risk of this is so small that it may be safely disregarded and especially when all hogs on the farm or that may be exposed with such sick hogs have been treated.