Ruminant genetic resources and their resistance/tolerance to parasitic diseases

Parasitic diseases like gastrointestinal (GI) nematodes and trypanosomiasis are worldwide an important cause of reduced production efficiency in ruminants, and partly even limiting livestock production in some regions of the world. GI nematodes are among the most important infections faced by livestock, especially affecting poor keepers (Perry et al., 2002). A potential alternative to alleviate the problems is breeding for disease resistance. It has been well established that the ability of animals to acquire immunity and express resistance against diseases varies substantially among and within breeds and is at least partly under genetic control (Miller et al., 1998; Amarante et al., 2004). Beside the scientific evidence, there is much anecdotal evidence pointing to the greater disease resistance of different breeds. For example, when countries enter details of their livestock breeds in FAO’s DAD-IS system, they have the opportunity to indicate whether the breeds have any particularly interesting trait like disease resistance. Currently, four goat and 13 sheep breeds were reported to DAD-IS as having resistance or tolerance to a certain degree against parasitic diseases in general or against specific parasites. Even if these claims are not based on scientific investigations they can be correct. The long evolutionary history for example of nematodes and their ancestors has produced a rich and complex series of coadaptations by host and parasite (Stear et al., 2009). The fundamental theorem of natural selection suggests that evolution will fix genes that improve fitness (Fisher, 1930). Therefore, genes or alleles which are related to parasite resistance should mainly be found in breeds which originate from regions with high parasite burden because of high natural selection pressure. In this context, breeding of such native breeds, which until recently was neglected because these animals were considered to be poorly productive, is receiving increasing attention. However, the genetic component of resistance or tolerance against diseases is still not estimated for most of these local breeds. Therefore, more information is needed which underlines the necessity of conserving these animal genetic resources.