and trying to smother the action of virus by large and repeated doses of serum alone previous to simultaneous treatment. In no case have we obtained evidence to justify even a suspicion that follow-up treatment does not produce permanent immunity, and we know of no experimental work that contradicts these results.

The factor of greater safety cannot well be questioned, although it is seldom that hogs with average resistance require follow-up treatment. Simultaneous immunization produces the same result, and is cheaper. But if exceedingly valuable animals are to be immunized, one cannot go amiss in giving a dose of serum alone and following it in a week or two with simultaneous treatment. If the first dose of serum is in the system, already absorbed, when the second dose is given with virus, there can be no question that the hazards are reduced. Exact comparisons of the safety of simultaneous and follow-up treatment are difficult to make, because under ordinary conditions both are nearly 100 per cent effective. We have seen follow-up treatment used in immunizing cattle against rinderpest with losses running less than 5 per cent when simultaneous treatment with the same serum and virus caused such heavy losses as to forbid its use altogether. We do not care to generalize too far on this point, but the principles employed in preparing and using the two se

rums are the same, and we offer the observation for what it is worth.

Follow-up treatment is indicated:

1. For very valuable hogs where the greatest possible safety is required.

2. When the following conditions coexist in the same animal or herd:

A. Immediate protection is imperative.

B. Ultimate permanent immunity is desired. C. Conditions render immediate simultaneous treatment dangerous.

Follow-up treatment is contra-indicated in all cases in which it appears 'that serum alone or simultaneous treatment will be equally effective.

It is in immunizing hogs with resistance obviously below normal, and in maintaining immune herds under somewhat adverse conditions that follow-up treatment renders greatest service. In practice, especially in the East, we constantly encounter the three conditions we have enumerated above, and follow-up immunization relieves us from the necessity of choosing between serum alone which will not produce a permanent immunity, and simultaneous treatment which is positively dangerous at the time when immediate protection is required. In a succeeding chapter the adaptations of follow-up treatment will receive further attention in connection with specific conditions which we meet in the field.

CHAPTER IX

HANDLING HOG CHOLERA IN THE FIELD

HANDLING hog cholera in the field requires application of the principles that have been outlined in preceding chapters. In this chapter our plan is to assume the existence of certain actual conditions which the practitioner frequently meets in the field, and to suggest methods of handling suited to these conditions. We know that in doing this we may invite criticism, for methods of handling hogs are so widely different in various parts of the country that one cannot supply details that will apply everywhere.

In some parts of the South, for instance, where hogs are allowed almost unlimited range, where predatory animals are common, where hog cholera is prevalent, and where a few breeders use simultaneous treatment regularly, others must protect their hogs in the same manner, or lose them. In certain sections of the corn-belt hog cholera is prevalent to such a degree that it is the part of wisdom for practically all breeders to maintain immune herds. In the East, where hogs are rather closely confined, where they are raised in limited

numbers, where hog cholera is not common and its spread is not rapid, most herds do not require immunization. Moreover, in the corn-belt where hog raising is a business, methods of swine husbandry are relatively much better than they are in sections in which it is a mere adjunct to other farming operations. The average corn-belt breeder has had more or less experience with hog cholera, he knows what it means to have it sweep unchecked through his herd, and he is not inclined to be dissatisfied with measures that will check it, even though these measures may not always be perfect in their operation. On the other hand, the Eastern breeder whose herd we are called on to handle very often is having his first experience with the disease, he is inclined to be skeptical as to the merits of protective serum, and to doubt its value if he loses a few animals after it has been administered. Frequently also, the herd is found in unthrifty condition due to poor methods of swine husbandry and heavy parasitic infestation. Virus cannot be used as freely in such surroundings as it can under circumstances where its effects will be more correctly judged.

Despite these differences, though, and despite the fact that methods of swine husbandry have a direct and important bearing on the handling of disease, the principle holds that hog cholera is hog cholera the country over, and not, as some

would have us believe, different according to the section of the country in which we chance to find it. The differences we observe in various parts of the country are due principally to prevailing secondary invaders, as well as to variation in virulence of the hog cholera virus itself, for they are observed also in comparing individual herds or outbreaks in any one section.

While we must accept all these variations and allow for them, the underlying principles employed in handling hog cholera remain unchanged, and it is desirable and necessary in a treatise of this kind to suggest definite working plans, leaving the reader to alter or supply detail as individual cases warrant. We are moved to do this because we have seen young graduates of veterinary colleges who had had good instruction and whose technique in administering serum left little to be desired-we have seen some of these men practically helpless in the presence of outbreaks of cholera which presented disturbing but not unusual features. Likewise men who are accustomed to the routine of vaccinating thousands of stockyard hogs are sometimes confused when they are called on to accept the conditions they meet on the average farm, to prescribe treatment for a mixed lot of swine, and guide the breeder away from future trouble. Mere knowledge of how to vaccinate hogs does not equip one to handle hog