Possibilities to breed for resistance to nematode parasite infections in small ruminants in tropical production systems

Abstract

Gastrointestinal nematode parasitism is the most important disease affecting livestock production systems in developing countries, particularly small ruminant production systems. Of particular importance are infections with the strongyle Haemonchus contortus. Integrated disease control strategies are required, including improved management, nutrition and wise use of anthelmintic chemicals. Increasingly, selection of sheep or goats for improved nematode resistance is viewed as a valuable option to complement other control measures. Breeding for resistance is possible because of the existence of extensive genetic variation in resistance, both within and between breeds of sheep and goats. Such breeding schemes are most likely to be based on choice of appropriate breeds adapted to the local environmental conditions, followed by phenotypic selection for resistance. Goal and selection objective traits are likely to include performance (e.g. growth rate) under conditions of parasite challenge, faecal egg count (FEC) and measures of anaemia. With current technologies, genetic markers are likely to be too expensive and logistically difficult to incorporate into breeding schemes in tropical or developing countries. Genotype by environment interactions may be expected, particularly when comparing animals in environments that differ in the extent of parasite challenge or differ in the quality of available nutrition. However, there is no reason to expect antagonistic genetic relationships between performance and resistance, and selection indices should be readily constructed that improve both performance and resistance. If FEC is decreased, then pasture contamination should also decrease, leading to additional benefits for all sheep grazing the same pasture. Finally, breeding for nematode resistance should lead to lasting and sustained improvements in resistance or tolerance. There is no empirical evidence to suggest that nematodes will evolve rapidly in response to resistant hosts, and mathematical models based on genetic and biological principles also suggest that resistance should be sustainable.

Immune and haematological parameters of Blackbelly ewes infected with gastrointestinal nematodes

“…Number of eggs per gram of faeces (EPG) has been the main phenotypic trait used to determine GIN resistance and is a reference to establish the relationship between nematode burden and immune system response. Protective immunity to GIN dependens on innate and acquired immune factors such as eosinophilia and immunoglobulin (Ig) A and G, which could help confi rm the host resistance against GIN (Alba-Hurtado and Muñoz-Guzmán, 2012;Bishop, 2012). For instance, the high association between IgA response and T. circumcincta infection observed by Strain et al (2002) suggests the role of this antibody (Ab) as a possible biomarker for resistance to GIN infections.…”

Functional study of a genetic marker allele associated with resistance toAscaris suumin pigs

“…Genetic markers can identify animals with desired traits for breeding programmes (Charlier et al 2008;Nielsen et al 2009; reviewed by Bishop, 2012). Care should be taken, though, to weigh the advantage against the loss of other beneficial alleles in linkage disequilibrium with the marker (Wolc et al 2012), and ideally marker information from the whole genome (i.e.…”

Molecular identification of abomasal bacteria associated with genetic resistance and susceptibility to Haemonchus contortus infection in sheep

“…The use of anthelmintic drugs to treat GIN infections causes a strong selective pressure among parasitic nematodes, resulting in the emergence of anthelmintic-resistant strains, even to monepantel, the newest drug available (THOMAZ-SOCCOL et al, 2004;TORRES-ACOSTA et al, 2012;VAN DEN BROM et al, 2015). Therefore, alternative control strategies are being explored to decrease the dependency on anthelmintic drugs, reduce production costs, and promote increases in animal production (BISHOP, 2012).…”

“…According to Bishop and Stear (2003), resistance to infection is regulated mainly by the ability of the host to interact with and control the life cycle of the parasite. Most studies of genetic resistance use fecal egg count (FEC) as the indicator trait for resistance (BISHOP;MORRIS, 2007;JACKSON;SADDIQI et al, 2012).…”