G. A. Rohrer and C. W. Beattie

“ Genetic mapping of the swine genome has already provided markers associated with disease resistance for the two strains of E. coli F18+ and F4+. Once producers are able to identify animals containing the disease-resistant allele (s) at these genomic locations, they can initiate marker-assisted selection to increase the frequency of the resistant alleles and reduce the incidence of disease in their herds. Certainly other loci will be detected and used to genetically improve the resistance of pigs in the relatively near future. ”

“ Although natural selection has been genetically improving disease resistance in pigs for millennia, pathological organisms are also evolving to survive in an ever changing environment and host organism. Humans confound the situation by constantly changing the environment and managerial practices used to raise pigs. Alterations in animal density, vaccinations, and antibiotics have significantly modified this ecosystem and have made the relationship between hosts' genetics and susceptibility to acquired diseases extremely complex. ”

“ A trait with low heritability should not be assumed to be unaffected by genetics. In fact, any trait that differs between breeds or responds to heterosis has a significant genetic component. Heterosis is the difference in performance of crossbred animals from the average performance of purebred animals and is attributed to genes with dominant and epistatic action. Traits most affected by heterosis are those pertaining to fitness (longevity, reproduction rate, etc.) ”

“ This knowledge will allow us to identify new marker traits associated with resistance or susceptibility and to produce the genetic markers useful for marker-assisted selection.
OVERVIEW Selection of pigs for disease resistance to certain pathogens can be quite successful. However, it is quite clear that no one gene, or form of a gene, can confer resistance to all pathogens. Selection for general disease resistance is more complex because the different methods of immune protection are governed by different genes in various locations of the genome. ”

“ Response to selection for disease resistance can also be improved. Implementation of a progeny-testing system when a marker trait has a low heritability will improve the accuracy of identifying superior parents and increase the rate of genetic progress. ”

“ If significant genetic-by-environment interactions exist, pigs selected for improved disease resistance in one environment may be more susceptible to disease when raised in another. These interactions will significantly increase the complexity of a structured selection program. ”

“ Acquired immunity was greater in the high-response line for parasitic infections and rabies, while the low line had superior acquired immunity to bacterial infections. In all cases studied, selection for greater antibody production (AMI) resulted in reduced CMI responses, yielding animals more susceptible to particular pathogens. These results clearly indicate that selection did not result in animals resistant to all types of pathogens, that genetic mechanisms which regulate AMI are not identical to those affecting CMI, and the two different genetic mechanisms may be negatively correlated. ”

“ MAPPING IMMUNE RESPONSE GENES The human genome initiative and construction of genetic maps for livestock have greatly expanded our knowledge of the mammalian genome and increased the number of mapped and sequenced porcine genes. The final goal of the human genome initiative is to completely sequence the entire genome. As this goal is approached, more genes will be identified, and as their function is determined, our knowledge of how genes interact will grow immensely. ”

“ In summary, it is likely that pigs which are less susceptible to pathogens can be selected. It is also likely that there will always be diseases that affect pigs. If we endeavor to improve pigs through selection, we must consider that other pathogens or modified pathogens will develop and will need to be addressed. With a multifaceted defense relying on management practices, antibiotics, and swine genetics, we should be able to overcome these new challenges. ”

“ The situation is further complicated by clear evidence that resistance to certain pathogens can be largely controlled by nonimmunological factors. Identification of the genetic mechanisms that affect susceptibility should be easier for diseases with simple etiologies than for diseases with complex etiologies. ”

“ A few general theories from quantitative genetics form the basis of these genetic mechanisms (Falconer 1981) . First, an animal's phenotype is the result of its genotype and the environment in which it developed. ”

“ Therefore, appropriate selection of marker traits to measure and subsequent weighing of each measure ment in a selection index should be an effective method to improve the general disease resistance of pigs. Obviously, additional research is essential to identify marker traits that are easily measured at low cost, have moderate to high heritabilities, and are highly correlated to an animal 's ability to resist infection by various pathogens. ”

“ These results indicate that selection for AMI should be quite successful in pigs, but that selection for animals with general disease resistance to most pathogens will be quite difficult or even impossible.
DISEASE RESISTANCE IN RUMINANTS Mastitis has been the chief focus of studies of genetic mechanisms of disease resistance in cattle. The etiology of mastitis is quite complex since many pathogens can actually cause the disease and numerous environmental factors can influence its progression, resulting in low heritability. ”

“ Of course, any selective process must take into account the antagonisms it may encounter. It is quite likely that during the selection of animals resistant to a pathogen, the pathogen will evolve to survive in the improved host. ”

“ Breed effects detected for immune response to Bordetella bronchiseptica by both Rothschild et al. (1984) and Meeker et al. (1987a) were similar, with Landrace and Yorkshire breeds reportedly superior to Duroc. Contrary to expectation, neither study detected any evidence for heterosis. Hutt (1958) had previously noted that Canadian producers had reduced the incidence of rhinitis when they deliberately selected against "dish-faced" pigs. However, a systematic effort to pursue this observation is lacking.
Meeker et al. (1987a, b) also evaluated the immune response to vaccination for pseudorabies. ”

“ Differences in bovine lymphocyte antigen associations between immune responsiveness and risk of disease following intramammary infection with Staphylococcus aureus. J Dairy Sci 78:1937-1944.
Meeker, D. L.; Rothschild, M. F.; Christian, L. L.; Warner, C. M.; and Hill, H. T. 1987a. Genetic control of immune response to pseudorabies and atrophic rhinitis vaccines. I. Heterosis, general combining ability and relationship to growth and backfat. J Anim Sci 64:407-413.
___. 1987b. Genetic control of immune response to pseudorabies and atrophic rhinitis vaccines. ”