ACKNOWLEDGMENTS.

In conclusion, the writer desires to express his obligations to Dr. M. Dorset, chief of the Biochemic Division, under whose direction the field tests were made. The writer also acknowledges the valuable services of Dr. L. E. Day, veterinary inspector, who assisted in the injections and autopsies. Valuable assistance in the preparation of the serum and in the treatment of herds was also rendered by H. J. Shore, assistant bacteriologist. The writer is also under obligations to the owners of the treated herds who so willingly cooperated with us and thereby made possible the extensive field tests which have been described.

THE CONTROL OF HOG CHOLERA BY SERUM IMMU

NIZATION.a

By A. D. MELVIN, D. V. S.,

Chief of the Bureau of Animal Industry.

FACTS ON WHICH TREATMENT IS BASED.

As a result of experimental work conducted by the Biochemic Division of the Bureau of Animal Industry and recorded in Circular 43 and in Bulletin 72 of this Bureau, the conclusion was reached that the so-called hog-cholera bacillus is not the true cause of hog cholera, but that this organism plays the part of a secondary invader, the true cause of the disease being a virus which is present in the blood of hogs affected with hog cholera, and which, under certain conditions of filtration, is capable of passing through the finest porcelain filters. Up to the present time this filterable virus has resisted all attempts at artificial cultivation, and we know of its presence only through the effect upon hogs when fluids from sick animals, free from all known bacteria, are injected into susceptible animals. It is a well-known fact that hogs which have recovered from an attack of hog cholera are completely immune when subsequently exposed to the same disease.

These two facts-the presence of the filterable virus in the blood of hogs sick of hog cholera and the immunity in hogs which have recovered from an attack of that disease-form the basis for the preparation of the serum which we have used successfully in immunizing hogs against cholera.

METHOD OF SECURING IMMUNE SERUM.

Without attempting to go into the method of producing this serum in detail, it will be sufficient to say that the protective serum is produced by a process of hyperimmunization carried out as follows:

An immune hog is injected with large amounts of blood from hogs sick of hog cholera. These injections will not produce more than a a This paper was presented at the annual convention of the American Veterinary Medical Association, Philadelphia, Pa., September 10, 1908.

The methods of immunization described herein have been arrived at by the Biochemic Division, and the investigations for the past four years have been under the direct supervision of Dr. M. Dorset, the chief of that division, through whom the Department of Agriculture has had the process patented in such manner as to insure to all the people in the United States the right to its use without the payment of royalty.

transitory effect upon the health of the immune, although they would prove certainly fatal to a susceptible hog. This treatment of immune hogs with large amounts of virulent blood is known as hyperimmunization, and gives to the blood of the immune the power to protect susceptible hogs from hog cholera. After a week or so, when the immune has recovered from the effects of this treatment, blood is drawn from that animal by cutting off the end of the tail. The blood drawing is repeated three or four times at intervals of a week, after which the immune is usually bled to death from the carotid artery. After each drawing from the immune the blood obtained is defibrinated and mixed with a suitable antiseptic. If preserved in sterile bottles, this defibrinated blood, or serum, as it is called, will retain its potency for years.

The protective serum having been obtained from an immune hog in the manner indicated, the potency of this serum is determined by injecting susceptible pigs with varying amounts, and at the same time exposing them to hog cholera along with untreated control animals. In practice it will of course be found best first to collect large quantities of serum and to mix this before testing. A standard serum will thus be secured at a minimum cost.

METHOD OF PROTECTING SUSCEPTIBLE HOGS.

A standard serum of known potency having been secured, either of two methods may be used for protecting susceptible pigs. These are known as (1) the "simultaneous" method and (2) the "serum-alone' method, or simply the serum method.

The first of these, which is to be recommended for use especially in herds which have not been exposed to hog cholera, consists in injecting subcutaneously on one side of the body of the pig to be vaccinated a suitable quantity of serum, and simultaneously on the other side of the body a small quantity of virulent blood taken from a hog sick of hog cholera. Experiments have shown that by this method pigs are given a firm immunity, lasting at least six months and probably longer.

The serum-alone method, which consists simply in the injection of the protective serum without the simultaneous use of virulent blood, appears to confer only a temporary immunity upon the treated pigs, unless they are exposed to hog cholera a short time after receiving the serum, in which case they also acquire a lasting immunity. For these reasons this method is admirably adapted to the treatment of hogs in a herd where hog cholera has already broken out, but which have not themselves shown visible symptoms of disease.

The experiments which are being carried out to determine the curative properties of the serum are not yet complete, but from the results thus far obtained we know that serum in the doses used for immuniza

tion can not be depended upon to cure hogs which already show visible symptoms of hog cholera. Further work along this line is needed. Neither the simultaneous nor the serum-alone method, when properly applied, appears to injure the hog in any way.

COST OF THE SERUM.

In order to determine the cost of producing serum for practical use every item of cost would of course have to be taken into account and allowance made for all sources of revenue. Owing to the conditions under which the work of the Bureau has been carried on-that is, manufacturing serum for experimental use only and utilizing the same force for the production of the serum and for carrying on varied experiments-it is impossible to determine the exact cost of the serum thus far produced. Sufficient work has been done, however, for an estimate to be made. With the dose of serum at 20 cubic centimeters and with the production carried out with strict economy, it seems likely that the cost per dose can be brought within 25 cents. This estimate is based upon the supposition that each hyperimmunized immune will furnish 150 to 200 doses of serum, and that the carcass of the immune after final bleeding will be utilized for food. There seems to be no objection to the use of such a carcass for food purposes, provided the post-mortem examination discloses no reason for rejecting it.

I have recently been informed by Dr. C. E. Marshall, of the Michigan Agricultural College, who has begun the preparation of this serum for distribution to farmers in that State, that it is the purpose to charge at present 2 cents a cubic centimeter for the serum, though he hopes to be able to reduce the price materially before another season. It will undoubtedly prove to be true that the cost of the serum will vary with the conditions of manufacture, and the proportionate cost should decrease as the amount of serum produced increases. In any case it seems certain that the serum can be produced cheaply enough for practical purposes.

RESULTS OF PRACTICAL TESTS OF THE SERUM.

The statements made above concerning the protective power of serum from hyperimmunized immunes are based upon tests on several thousand hogs. These tests were carried out not only in small experiment pens, but in great part upon farms under practical conditions. During the fall of 1907 approximately 2,000 hogs were treated on 50 different farms, a considerable proportion of untreated hogs being left in all cases as a control on the action of the serum." Both methods of treatment were used, and the herd conditions varied

"A report of these field tests appears elsewhere in this volume (pp. 177-217).