430 DISEASES OF SWINE 



this ease the animals were to be placed in direct contact with in- 

 fected animals and on infected premises. Accordingly, it was con- 

 sidered unnecessary to inject any of the hog-cholera virus. This 

 was quickly obtained by the animal through natural channels when 

 placed in an infected feed lot and in contact with infected animals. 

 Yet the immunity produced in this first herd was evidently of a 

 permanent nature, as we find that later observations of this herd 

 showed that the animals remained thrifty and well. Under 

 ordinary circumstances the immunity produced by the serum- 

 alone treatment lasts but about six weeks, or sixty days at the 

 outside. When, however, as in this case, serum is given to animals 

 which are in infected pens or in contact with infected animals, the 

 results are often practically the same as when the virus and serum 

 are both given by injection. That is, we have the development of 

 a permanent immunity. More recent tests have proved, however, 

 that this cannot always be relied on, and it is better policy to give 

 the double treatment to healthy animals intended to be placed in 

 infected lots. 



In treating herds similar to Herd Number Two, above men- 

 tioned, where there has been no appearance of the disease, but 

 where the animals are in close association with an infected herd, 

 the proper procedure is to administer the serum and virus to- 

 gether, thus producing a permanent immunity. Otherwise, if only 

 serum alone is used, the protection afforded is only a temporary 

 one, and after the lapse of sixty days the animals are again sus- 

 ceptible to the disease. 



Herd Number Three. — This herd was located on a farm one 

 mile east of Herd Number One, and the infection in this case 

 probably had its origin on the first-mentioned farm. When the 

 first visit was made on September 14, 1907, 2 old sows had 

 already died from the disease. One of these was cut open and a 

 postmortem examination made, which showed typical lesions of 

 hog-cholera. Several of the late summer pigs were also showing 

 symptoms of the same disease. There were also a number of 

 spring pigs in the herd, and some of these were beginning to show 

 slight symptoms of being unwell. In some of the sick animals 

 the symptoms of hog-cholera were well developed, including a 

 marked diarrhea. Later in the course of the experiment the un- 



