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Efficiency of animal production could be increased by reducing losses due to diseases. Therefore, disease resistance is an obvious trait to include in a selection program. However, how to incorporate this trait into the program is a difficult question. While it has been experimentally shown that selection for resistance against specific diseases is effective, it would be impossible to select for resistance to all potential diseases. Also, selection studies in mice show that increasing resistance to one disease can result in increased susceptibility to other diseases. This may be because antagonistic relationships exist among the mechanisms of the immune system. Thus it would be preferable to use general resistance to disease as the selected trait in cattle.
As for any selected trait, disease resistance must meet three criteria. First, there must be a reasonable economic weight placed on disease resistance. There is little doubt that disease costs can be extremely high because of reduced production due to mortality, morbidity, and subclinical infections. Second, genetic variation must exist. Several experiments have established that there is significant genetic influence on disease resistance. Finally, the selection procedure must be accurate in estimating the breeding potential of selected animals. While an accurate method of assessment would be to infect all animals and select those that survive, it would be very costly. A preferred, indirect method of selection would include the use of genetic markers that are associated with, or closely linked to, the genes influencing disease resistance. Potentially, a newborn animal could be tested for these markers and evaluated for lifetime resistance, since an animal's genetic potential is not altered throughout life.
A set of genetic markers that is associated with disease resistance or susceptibility has been identified in humans and laboratory species. These markers are genes that belong to the major histocompatibility complex (MHC). The MHC is a cluster of tightly linked genes discovered in the late 1930's in mice. It was first implicated as the genetic basis for rejection or acceptance of tissue and organ transplants. In 1963, it was also demonstrated that the MHC determines the degree of response made by the immune system against foreign molecules or pathogens. The MHC spans a short segment of chromosome and contains genes that code for variable or polymorphic class I and class II proteins. Class I proteins, found on almost all nucleated cells, are involved in rejection or acceptance of grafts as well as tumor rejection and elimination of virus-infected cells. Class II proteins, found predominantly on cells of the immune system, are involved in regulation of antibody production by the immune system. The MHC of cattle is called BoLA.
Because of the previously defined association of the MHC with disease resistance, a project at MARC is investigating the possibility of using BoLA polymorph isms as genetic markers for assessing disease competence in cattle. To accomplish this objective, several studies of this genetic region are necessary. First, definition of class I and class II proteins or genes coding for these proteins is required. Secondly, the associations with diseases must be defined. Finally, before decisions can be made on which markers to include in a selection program, the associations with economically important traits must be estimated. If an antagonistic relationship exists between a desired MHC marker and a production trait, severity of the associated disease must be assessed before the marker would be included in the selection program.