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The Commercial Possibilities and Limitations of Breeding for Disease Resistance¹

University of Wisconsin

Abstract

There are three principal means of combating infectious diseases: prevention (prophylaxis), curative measures (therapy) and breeding (genetic control).

The attempt to apply genetic control by breeding for increased natural (hereditary) resistance faces many difficulties, especially in the larger farm animals. Some of the facts to be considered are:

1. Natural resistance to many diseases is apparently more prevalent in wild species than in the domesticated races.
   
2. To incorporate general resistance in a breed it must be found in some members of the breed or be brought in by crossing. The rules of most breed associations effectively prevent introduction of resistance by crossbreeding.
   
3. There is commonly difficulty in distinguishing natural from acquired immunity, thus interfering with the selective process.
   
4. While genetic resistance may in some cases be conditioned by a single gene pair it is usually more complex.
   
5. Resistance to infections is commonly specific, requiring selection for each disease. This greatly limits what can be accomplished.
   
6. Selection for resistance adds to the characters already being selected for, complicating the breeding program.
   
7. Variability (mutation) of the pathogen may upset the results of long selection.
   
8. Constant selection will be required to retain resistance after it has been attained.
   

The following general principles are offered:

1. Where methods of prophylaxis or treatment are known and readily applicable they will in general be preferable economically to attempting to breed for resistance. It must be borne in mind, however, that unless the pathogenic agent can be eliminated completely the necessity for control, such, for instance, as by vaccination, will be a continuous one. Under such tratment the stock is likely to come to have less natural resistance rather than more.
   
2. The genetic method can be much more readily applied to animals which produce a large progeny and in which the value of the individual is relatively small. Progress depends on selection of large numbers and elimination on a large scale. Such elimination is scarcely practicable in the larger animals, for the commercial value of the individual is apt to be too high, and reproduction is so slow that the female breeding stock cannot be maintained if selection is too rigid.
   
3. From the nature of the case, the number of diseases which might be selected against in any stock is strictly limited. This is another reason for restricting the genetic method to those diseases which cannot readily be brought under control by other means.
   
4. While in general, as pointed out, the other methods tend away from genetic improvement, it is entirely possible that, at least in certain cases, they may be made to work in harmony and towards the same end. The whole subject is one on which we sadly need more knowledge. The relative ease with which prophylactic and therapeutic measures can be worked out and applied has led to neglect of the natural resistance which would be better if it can be attained. While it may not be practicable to attempt to make a herd of the larger farm animals wholly resistant directly by selection it is possible the end might be approached by other means. If a special breeding herd were established in which the genes for resistance were made practically homozygous, sires from this stock used on the rest of the herd would tend to raise the general resistance level. While at the same time such sanitary or therapeutic measures could be used as seem indicated. As the resistance level rose the need for treatment would correspondingly decline.
   

¹ Paper from the Department of Genetics, Agricultural Experiment Station, University of Wisconsin. No. 146. Published with the approval of the Director of the Station.