FIELD TESTS WITH SERUM FOR THE PREVENTION OF

HOG CHOLERA.

By W. B. NILES, D. V. M.,

Inspector in Charge of Field Station, Biochemic Division.

INTRODUCTION.

Experiments which had for their object the production of immunity from hog cholera were carried on by the Bureau of Animal Industry during the years 1903-1906. These experiments, which were described in Bulletin 102 of the Bureau of Animal Industry,a demonstrated that shotes weighing from 25 to 75 pounds could be successfully immunized by the use of hyperimmune serum alone or by the simultaneous use of the serum and virulent hog-cholera blood. This much having been proved, it remained to test the practical value of this method under normal farm conditions, and it seemed very desirable in this connection to learn what could be accomplished by treating animals of different ages, located on farms in different localities, and kept under varying field conditions.

Early in the fall of 1907 the conditions were favorable for an extensive practical test; that is to say, a considerable quantity of hyperimmune serum had been prepared, virulent hog cholera had appeared in several neighborhoods not far distant from the Bureau station (near Ames, Iowa), and the farmers were anxious to cooperate in testing the serum treatment. The work of testing the serum treatment under practical conditions was consequently undertaken.

The field tests were planned to gain information on at least three important points: (1) To determine what could be accomplished toward diminishing the loss in a herd in which hog cholera had already appeared; (2) to ascertain what could be gained by treating a healthy herd after it had been exposed to the disease, but before any hogs had become sick; and (3) to determine whether the spread of

a"Further Experiments Concerning the Production of Immunity from Hog Cholera," by M. Dorset, C. N. McBryde, and W. B. Niles.

The term "hyperimmune," used in this paper, refers to a hog, already immune to hog cholera, whose immunity has been increased by the injection of large doses of blood from hogs affected with hog cholera. Similarly, "hyperimmune serum" means the serum procured from the blood of such hyperimmune hogs.

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hog cholera could be arrested by treating all healthy herds on farms bordering on a center of infection which had not been exposed to the disease at the time of treatment. It was, in addition, considered desirable to test the effect of the treatment on old hogs and on sucking pigs, also to determine the size of dose for hogs of different ages, and whether the possibility of causing disease by the simultaneous use of hyperimmune serum and disease-producing blood would render the simultaneous treatment of uninfected herds impracticable.

Before discussing the various phases of the experiments which were carried out to determine these points we will refer briefly to the material used in the vaccinations.

THE FIELD TESTS.

DESCRIPTION OF SERUM USED.

All of the serum used in the experiments described in this paper was prepared in accordance with the methods described in Bulletin 102. It will be remembered that this serum is prepared by injecting immune hogs with virulent blood obtained from hogs sick of hog cholera. The virulent blood is usually injected subcutaneously into the immune.

Two methods of hyperimmunization are employed, known as the "slow method" and the "quick method." The slow method consists in the injection of three successive doses of disease-producing blood. For the first dose 1 c. c. of disease-producing blood is given for each pound of body weight. After an interval of about a week, 2 c. c. of virulent blood for each pound of body weight is administered, and after a second interval of a week, 5 c. c. of virulent blood for each pound of body weight is given the immune. The hog is then allowed to recover completely from the last injection, which usually requires about ten days; blood is then drawn and defibrinated, and the serum is ready for the treatment of nonimmune animals.

The second method of hyperimmunization-the quick method— consists in administering to an immune hog at one time 10 c. c. of disease-producing blood for each pound of body weight. The animal is allowed to recover from the effects of the injection, and its blood is then drawn and the serum used for the treatment of nonimmunes.

Blood is drawn from the hyperimmune animal by cutting off its tail. Several bleedings are made at intervals in this way. We thus have what are known as the first, second, third, and fourth drawings of blood from the hyperimmune.

In order to avoid the needless labor and expense which would have resulted had the serum from each hyperimmune been tested separately, it was decided to mix the serum obtained from several different hyperimmunes and then test the mixture. Such mixtures were

designated "mixed serum No. 1," "mixed serum No. 2," and so on. The potency of the different mixed serums was determined by injecting small pigs with 15 to 20 c. c. of the serum together with 2 c. c. of disease-producing blood, the virulence of the latter being tested by injecting check pigs with a similar dose at the same time. If the pigs which received serum in conjunction with the disease-producing blood survived and those which received disease-producing blood without serum died, the serum was regarded as efficient, or potent.

In the tests to be described later, five different mixed serums, numbered from 1 to 5, were used. These five mixed serums consisted of serum obtained from different immunes, as follows:

Mixed serum No. 1 consisted of the first and second drawings of blood from two hogs hyperimmunized by the slow method. These hogs were hyperimmunized early in the season, and as some tested serum was needed for special experiments, the first and second drawings of blood were mixed, tested, and used for these experiments. Mixed serum No. 1 represented, therefore, only two drawings, and the amount of this serum consequently was small.

Mixed serum No. 2 was drawn during July and August, 1907, and consisted of the total serum secured from eight hyperimmune hogs, four of which were hyperimmunized by the slow method and four by the quick method. Each of these hogs was bled three times. from the tail at intervals of one week. One week after the last bleeding they were bled to death from the carotid artery. They ranged in weight from 100 to 225 pounds at the time hyperimmunization was begun, and the total amount of serum obtained from them was 30,698 c. c., or an average of about 3,800 c. c. from each animal.

Mixed serum No. 3, drawn during August and September, 1907, was the total product from four hogs hyperimmunized by the slow method. Their respective weights when first injected with the disease-producing blood were 100, 200, 220, and 325 pounds, and the total amount of serum obtained was 17,877 c. c., or an average of 4,469 c. c. from each hog. It is of interest to note that the immune hog weighing 325 pounds required a large amount of disease-producing blood for hyperimmunization and was too large for convenient handling, but yielded a very large amount of serum, a total of 5,641 c. c. being secured; the three tail bleedings yielded about 1,000 c. c. each, and 2,600 c. c. was obtained from the carotid artery when the animal was killed. While the amount of hyperimmune serum secured from these four animals was large, it was shown by tests to be up to the standard in potency.

Mixed serum No. 4 consisted of small lots of serum secured from several hyperimmune hogs. As the serum supply at the station was running low and there were numerous demands for the vaccination of

herds in different parts of the county, it was decided to mix and test the numerous small amounts of serum on hand. The mixture contained (1) a considerable quantity of serum prepared in 1906, consisting of a small amount each of second, third, fourth, and fifth tail drawings from eight immune hogs, four of which had been injected with disease-producing blood by the slow method and the others by the quick method; (2) the carotid, or last, drawing of blood taken from three hyperimmune hogs in July, 1907, together with the fourth tail drawing of blood from two of these; and (3) a few odd lots of serum left over from previous experiments. Approximately one-half of the mixture was prepared in 1906 and the remainder in 1907.

Notwithstanding the fact that No. 4 serum consisted in great part of serum more than one year old and a considerable part of it was carotid blood without the corresponding tail drawings, it proved potent when used on experimental shotes at the station and also for the vaccination of herds in the field, as will be shown later.

Mixed serum No. 5 was drawn in October and November, 1907, and represented the total amount of serum secured from two hogs hyperimmunized by the quick method. The animals were bled three times from the tail and then from the carotid artery, as was done in the case of the hogs from which serum No. 2 was obtained. The weights of the two hogs were 150 and 160 pounds, respectively, and the total amount of serum secured was 8,676 c. c.

DISEASE-PRODUCING BLOOD USED.

a

The virulent blood used in the field tests of the simultaneous method was, in most instances, the same as that used at the Bureau station for the hyperimmunization of the immunes. This blood was used for the field tests soon after it was drawn, or else was preserved in sealed glass bulbs which were held for a few days at ice-box temperature. All of the disease-producing blood used in the field tests proved sufficiently virulent to kill check animals in doses of 2 c. c.. save in the case of one lot, which failed to kill the checks in the dose just stated, but rendered them immune on exposure, as was shown later.

LOCATION OF THE EXPERIMENTS.

It will be seen by referring to the map (fig. 22) and to the records of the herds described hereafter, that we were fortunately able to treat hogs on 47 different farms, located in 9 townships and 12 different localities. Many of the herds were in an infected region, which covered a considerable extent of territory, about 10 miles

a In describing the field tests the use of serum alone will be referred to as the "serum" method, while the use of serum in conjunction with diseaseproducing blood will be referred to as the "simultaneous" method.

southeast of Ames, Story County, Iowa. A few were located to the west of Ames, on the edge of Boone County, 20 miles or more from the locality just mentioned, and the rest were a considerable distance from the Bureau station in other directions. The territory covered by the experiments is sufficiently large to show that the infection responsible for the outbreaks came without doubt from different

sources.

A few of the herds were made up of purebred animals and received extra care and attention from their owners, but the majority were ordinary farm herds, which were cared for in the usual way. All of

FIG. 22.-Map of portions of Boone and Story counties, Iowa, showing townships and locations of herds treated for hog cholera.

the leading breeds were represented in the herds treated, and no difference in results could be noted for the different breeds. In this connection it is of interest to note that the treated herds received the same care after treatment as before. The sick animals were not isolated from the healthy ones, and no attempt was made to disinfect premises. The good results obtained may be attributed, therefore, entirely to the serum treatment.

A study of the disease in the different neighborhoods visited shows a marked similarity of symptoms and lesions, which, considered in connection with the uniformity of results, goes to show that we were dealing with one disease only, and also strengthens the writer in the