Some Methods of Control

Although exact methods of controlling the disease have not been worked out in detail, the following may be considered as objectionable and dangerous procedures that should be avoided :

Re-use of second-hand, unsterilized feed bags; admittance of unnecessary visitors to poultry premises; admittance of chick sexers, blood testers, cullers, or any necessary service personnel without proper precautions; the use of unsterilized bird crates; careless disposal of dead birds; performance of post-mortem examinations of poultry on the farm; shipment of sick birds in public conveyances to the laboratory; conducting egg-laying tests and poultry shows without authority from the livestock sanitary officials; failure to observe quarantine and other sanitary regulations; obtaining eggs, chicks, or any birds from infected or exposed flocks; failure to report sick birds immediately to the proper authorities; failure to sterilize or properly dispose of offal and other waste materials from slaughtered birds.

Additional information on the disease may be found in the Proceedings of the forty-sixth, forty-seventh, and forty-ninth Annual Meetings of the United States Livestock Sanitary Association in 1942, 1943, and 1945, the March 1946 issue of Nulaid News, the June 1946 issue of the Journal of the American Veterinary Association, the July 1946 issue of the American Journal of Veterinary Research, and the April 1946 issue of the Cornell Veterinarian.

Hog Cholera

Hog cholera is still the most prevalent and devastating disease of swine. No herd in this country is absolutely safe from it, nor will be until the disease is wiped out. None of the methods that have been used successfully against some other diseases of livestock offers promise of success against hog cholera. Thus, the method used sometimes to eliminate foot-and-mouth disease—the slaughter of all infected and exposed animals—is not practical in the United States, although it is in Canada. The disease is too widespread in this country and too frequently is not recognized until it has spread throughout a community. Nor is the test- and-slaughter method, which promises ultimate eradication of bovine tuberculosis, applicable because no diagnostic test is known for the detection of hog cholera.

Although a method of eradication is not available, the swine raiser is fortunate in possessing a dependable method of control. The serum-virus treatment, developed by the late Marion Dorset, affords complete and life-long protection against cholera to all except an insignificant few of the treated hogs.

By its use, the great epidemics that once caused annual losses as high as 65 million dollars have been prevented, and any hog raiser willing to pay for the required treatment is relatively secure. But the method, since it involves the use of the live virus that causes the disease, offers no hope of eradicating hog cholera. Partly in recognition of that fact, scientists continued their search for a method of prevention that offered promise of being also a method of eradication.

A result of the search was the development of hog cholera vaccines. One of them, also developed by Dr. Dorset, is known as crystal violet vaccine. It seems incapable of causing the disease, but nevertheless it affords a marked degree of protection. It is the virus of hog cholera so treated that it has lost its ability to produce the disease, but it retains its antigenic properties, or ability to confer protection against the disease.

Field tests were begun soon after the vaccine was discovered. They have been continued on a graduated scale as confidence in its value has grown. These tests, although indicating the general dependability of the product, have also indicated the need of improvements.

One of these involved the purity of the product. The early crystal violet vaccines frequently contained contaminants, micro-organisms that had been derived from the virus donor or that had gained access during preparation of the vaccine. It was possible by animal test to determine whether the contaminants present would be injurious to hogs treated with the product. But it was also possible for the contaminants by their growth to produce spoilage in the vaccine and to cause a loss of potency that could not be detected. As soon as the need for a sterile product was recognized, attempts were begun to supply that need. It was found that the use of a glycerol solution of the dye, crystal violet, used in preparing the vaccine, instead of the formerly used water solutions of dye and phosphate, consistently afforded sterile vaccines. Under ordinary conditions of handling they remain sterile. The new method of preparing the vaccine was patented by Dr. F. W. Tilley, of the Bureau of Animal Industry, in 1945, and the patent was assigned to the Secretary of Agriculture.

The development of a sterile vaccine of satisfactory keeping qualities has been the chief recent advance in the product itself. A considerable advance also has been made in the recognition of the value of vaccines as a means of preventing hog cholera. As recognition grew, demand grew, and to supply the demand commercial establishments began preparing them. The use of commercially prepared vaccines was at first subject to certain restrictions imposed under the authority of the virus serum-toxin act. By 1944, however, their merits had become so well demonstrated that the restrictions were withdrawn.

The production and use of the vaccines is not limited to this country. As a result of an extensive series of studies conducted by the British Ministry of Agriculture, crystal violet vaccine is being made available to farmers in England. Research workers in Venezuela also confirmed the value of the vaccine. Indeed, reports from all countries except Canada have been favorable. The degree of protection afforded by the vaccine in Canada was far smaller than elsewhere. Whether the results of tests by Canadian authorities are due to a peculiarly of Canadian pigs or to some other cause remains to be determined.