Genetic resistance to endoparasites in sheep and goats. A review of genetic resistance to gastrointestinal nematode parasites in sheep and goats in the tropics and evidence for resistance in some sheep and goat breeds in sub-humid coastal Kenya

Abstract

SummaryThe evidence for both between- and within- breed genetic variation for resistance to gastrointestinal (GI) nematode parasites is reviewed. It is concluded that much of the published research on breed characterisation for resistance suffers from poor experimental design. Prior to the initiation of a 6-year study that has just been completed in coastal Kenya there were no estimates of within-breed genetic variation (i.e. heritabilities) of resistance in sheep or goats in the tropics. This study has confirmed that Red Maasai sheep and Small East African (SEA) goats are more resistant to GI parasites (predominantly Haemonchus contortus) than Dorper sheep and Galla goats. Heritability estimates for logarithm transformed faecal egg counts (an indicator of resistance) in 8-month-old lambs was 0.18±0.08 for all lambs, but higher in the susceptible Dorper-sired lambs (0.35±0.16) than in the resistant Red Maasai-sired lambs (0.06±0.07). This difference in heritability suggests that many centuries of natural selection have fixed most of the genes for resistance in the Red Maasai sheep. The resistant Red Maasai sheep and SEA goats were two to three times more productive than the susceptible Dorper sheep and Galla goats in the sub-humid coastal Kenya environment.

Genetic parameters for faecal egg count following natural nematode infections and correlation with productive traits in Polish Heath sheep

“…This value was similar to the 0.06 reported by Baker (1998) but lower than most other studies (Bishop et al, 1996;Woolaston and Piper, 1996;Gauly et al, 1999). In lambs, FEC heritability ranged between 0.06 for one-month-old lambs and 0.33 for six-month-old animals (Bishop et al, 1996).…”

“…One relatively cheap and simple method of reducing the effects of nematode infection is the selective breeding of nematode-resistant sheep (Baker, 1998;Bishop and Stear, 1999). This requires knowledge of the repeatability and heritability of faecal egg counts as well as genetic correlations with production traits.…”

Nematode control in ‘green’ ruminant production systems

“…However, for most species of ruminant livestock, animals that have evolved to be highly resistant to parasite infection are not generously endowed with desirable productivity traits for wool or meat production. These innately resistant breeds are found in the tropics [11], where the formidable combination of malnutrition, environmental stress, long-term and often massive larval challenge and limited relief by way of effective anthelmintic treatment have imposed the harshest conditions for selection, resulting in survival of the fittest [12]. However, attempts are being made to identify those genes that encode parasite resistance in laboratory animal models [13,14].…”

Genetic Adaptability of Livestock to Environmental Stresses

“…Studies have shown that Trypano-tolerant breeds are more productive than susceptible animals under moderate to high tsetse challenge (Agyemang et al 1997). Similarly many breeds which are resistant to diseases are also better in production profile as compared to other breeds in the similar environment (Baker 1998). This property of disease-resistant breeds make them superior to their counterparts and help them become the breed of choice for introgression of the favorable genes in the other populations through genetic intervention as a measure of protection against the future environmental threat.…”