—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.