GENETICS OF HELMINTH RESISTANCE

Authors: Gasbarre, Louis; MILLER, JAMES - LSU, BATON ROUGE, LA

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 8/6/1999
Publication Date: N/A
Citation: Gasbarre, L.C., Miller, J.E. 1999. Genetics of helminth resistance. In: Axford, R.F.E., Bishop, S.C., Nicholas, F.W., Owen, J.B., editors. Breeding for Disease Resistance in Farm Animals. 2nd edition. UK: CABI Publishing. Part II, Chapter 6.

Interpretive Summary: Roundworms of cattle, sheep, and goats interfere with growth and productivity in pastured animals. Resistance to these parasites is strongly influenced by the genetics of the host. The magnitude of the genetic effect is roughly equal to that seen for milk production. One of the biggest problems in using host genetics to control these infections is the difficulty in measuring the infection level in animals. As a result, a key issue in the use of host genetics to control these worms will be the development of tools to accurately identify animals with enhanced or diminished resistance to the parasites. In any group of animals a small percentage of the animals are responsible for most of the parasites released into the environment. This means that control can be accomplished by targeting this small percentage of susceptible animals for manipulation/treatment/removal. No individual animals or breeds of animals are completely immune to the worms, indicating that resistance is a function of how common are the resistance form of the genes within and across breeds. Ways to use genetics to control the parasites include breeding to incorporate desirable forms of the genes into susceptible animals, the development of genetic markers for susceptibility/resistance and their use in making breeding and/or treatment decisions, and the identification of genetic areas associated with immunity followed by procedures that produce animals of the desired type.

Technical Abstract: Gastrointestinal (GI) nematodes interfere with the efficient raising of ruminants by disrupting nutrient utilization, resulting in reduced growth and productivity. The effect of genetics on resistance to GI nematodes has a heritability of approximately 0.3. Although the magnitude of the genetic effect is similar across host species, unique aspects of the biology of the individual parasites require the use of a variety of measures to assess resistance in infected animals, and infer that resistance involves multiple immune mechanisms. A key to using host genetics to control GI nematode infection is the ability to accurately identify animals with enhanced or diminished resistance. The distribution of parasites in groups of animals is such that a small percentage of animals are responsible for the majority of parasites released into the environment; implying that control can be accomplished by targeting this small percentage of susceptible animals for manipulation/treatment/removal. Resistance observed both between breeds and among individuals within a breed is not complete implying that resistance is a function of the gene frequency for the different alleles within and across breeds. One could use marker assisted selection to introgress desirable alleles into susceptible animals. A second approach would be to develop genetic markers for susceptibility/resistance and use these in breeding and/or treatment decisions. A third approach is to identify the loci associated with immunity through the use of candidate genes and linkage analyses, and use techniques such as introgression and transgenic manipulation to produce desired animals.