Breeding for Resistance to Infectious Diseases of Small Ruminants in Europe

Recent studies of the susceptibility to helminth infections of sheep and goats have focussed on artificial infections of lambs, kids and adults with several nematode species. Intra-breed studies have been orientated towards the elaboration of methods to investigate resistance acquired by lambs after natural or artificial monospecific or mixed infections with Teladorsag;a circumcincta and Haemonchus contortus. Repeatability, heritability and correlations between types of infection (natural versus experimental) and between resistance to different species have been estimated. Possible use of blood parameters to complement faecal egg counts has also been investigated. Genetic studies on the resistance of sheep against intracellular bacterial diseases have concerned the identification of genes involved in the control of sheep salmonellosis. One of those genes could be the equivalent of the mouse ItylBcglLsh gene(s) that control susceptibility to murine infections with Salmonella. Mycobacterium and Leishmania respectively. Research in the United Kingdom has increased our knowledge of the role of the PrP protein in the susceptibility of sheep to scrapie and its role in related diseases of other animal species and humans. Research on resistance and immune responses of sheep to infectious and parasitic diseases in Europe is considered important because the sheep is an important livestock species and because it is a model species for ruminants and even for humans. Europe is characterised by high ecological diversity caused by variable climate: from 'Atlantic' climate in the western part to 'Continental' in the East; and from 'Sub Polar' in the North to 'Mediterranean' in the South. Further diversity is caused by the presence of mountain chains and both humid and dry plains. There is a high diversity of small ruminant breeds which are specialised for meat, milk and wool production. Management systems are also highly diverse. Small ruminant systems of production commonly are based on pasture and therefore parasitic diseases caused by helminths are of primary importance. Adults are treated with anthelmintic between one and three times per year and up to four times in young animals, to control gastro-intestinal nematodes. Commonly, young animals are treated each year to control cestodes and Breeding for Resistance to Infectious Diseases in Small Ruminants coccidia. Fascioliosis and dicrocoeliosis are also important; and protostrongylosis occurs in goats and sheep in the Mediterranean areas. Strains of nematodes that are resistant to anthelmintics have developed in France, Germany, Holland, Switzerland and the United Kingdom. Resistance to the benzimidazoles is predominant, but the true prevalence of resistance in farms is poorly documented. Recent evidence confirms that this phenomenon is increasing in Holland (Borgsteede et al. 1991) and the United Kingdom Oackson 1993). The need to reduce the use of drugs has also involved interest in alternative and complementary methods of control such as increasing genetic resistance to nematode parasites (Cabaret and Gruner 1988; Gruner and Cabaret 1988; Gruner 1991; Stear and Murray, 1994). Investigations in Europe on genetic resistance have concerned resistance to two important nematode species, Teladorsagia circumcincta and Haemonchus contortus. Intensive production systems for lambs include indoor management where the main parasitosis is coccidiosis, with associated diarrhoea and loss of production. Control measures include management and prophylactic medication (Gregory 1989), but investigations are orientated towards characterisation of common antigens from different species for vaccination. Very little is known about the genetic resistance of small ruminants against these diseases. In most European countries bacterial diseases caused by intracellular bacteria have probably the highest incidence of all infectious diseases. They induce direct losses through abortion of ewes and does, septicemia and pneumonia in lambs and kids (for example, Chlamydia, BruceI/o, Salmonel/a and Listeria) or through the more specific lesions induced by Listeria monocytogenes (encephalitis), Corynebacterium pseudotuberculosis (caseous lymphadenitis), or Mycobacterium paratuberculosis (chronic enteritis). Their increasing economic importance stems from their effects on humans, which is linked mainly to their transmissibility through contact with livestock and consumption of milk and meat. Control of the most important infectious diseases is orientated towards the development of vaccines and efficient preparations are used against brucellosis (Plommet et al. 1987), salmonellosis (Pardon et al. 1990a, 1990b) and chlamydiosis (Souria u et al. 1988). Investigations on the genetic resistance of sheep to intracellular bacteria such as Salmonel/a have been initiated in France, with the objective of finding a major gene for resistance. An additional aim is to increase our knowledge of the immune mechanisms involved in resistance to intracellular bacteria in ruminants and to demonstrate the feasibility of genetic improvement of innate resistance to such pathogens of worldwide importance.