Prospects of breeding small ruminants for resistance to internal parasites

Markers/parameters for the evaluation of natural resistance status of small ruminants against gastrointestinal nematodes

“…Among phenotypic parameters, FEC is the most reliable, practical and frequently used indicator to assess the host resistance/tolerance potential against GINs in natural infection (Gray, 1991;Woolaston, 1992;Kemper et al, 2009). In lambs, it is well documented that FEC is highly correlated with worm counts (McKenna, 1981;Eady, 1995;Stear et al, 1995;Douch et al, 1996;Woolaston and Baker, 1996;Good et al, 2006), particularly when infections are of Haemonchus (H.) contortus and Trichostrongylus (T.) colubriformis. Selection based on low FEC has satisfactory upshots as GINs will not adapt quickly to sheep bred for nematode resistance (Kemper et al, 2009).…”

“…Worm count at necropsy According to Woolaston and Baker (1996), resistance status has also been assessed in terms of worm counts at necropsy. Resistance is the ability to suppress the establishment or the growth of worm infection while resilience is the capability of an animal to maintain acceptable health/performance under worm challenge mainly blood-sucking parasites like H. contortus (Albers et al, 1987).…”

“…circumcincta larval antigens (Stear et al, 1995). Live weight and worm burden are other imperative parameters for evaluating the resilience and resistance status, respectively (Woolaston and Baker, 1996;Burke and Miller, 2004), but live weight may vary depending on the breed. As the reliability of individual parameters and trials is generally questioned, so valuable and acceptable facts can be compiled by pooling data from a large number of trials as well as parameters (Woolaston and Baker, 1996); however, in the absence of infection, quantitative trait loci (QTL) may facilitate the selection process, but the trait selected for would be dependent on the results obtained (Miller et al, 2006).…”

Trends and challenges in the effective and sustainable control ofHaemonchus contortusinfection in sheep. Review

“…Below 5°C, movement and metabolism of L3 is minimal favouring prolonged survival as these larvae are enclosed in a double sheath and thus unable to feed to continuously renew their energy. (Albers & Gray, 1987;Woolaston & Baker, 1996). The development of immunity to gastrointestinal (GI) parasitic infections appears to depend on the host's ability to mobilise both cellular and humoral immune effectors locally at the site of infection and systemically in the blood circulation.…”

“…There is a sizable body of evidence for the existence of genetic variation in resistance to gastrointestinal nematode parasites both between and within breeds (Gray, 1997) and selection for parasite resistance have been successfully demonstrated in Australia and New Zealand (Barger, 1989;Bisset et al, 1996;Woolaston & Baker, 1996). The benefits of such selection arise from the effects of having fewer and less developed worms or greatly reduced faecal egg counts, which in turn leads to a reduced impact on production, a decreased requirement for chemical control and a reduced contamination of pasture by infective larvae.…”