Epidemiology predictions
Selection for resistance has been successful in reducing nematode FEC (Bisset et al., 1997a; Laurenson et al., 2012; Miller et al., 1998; Morris et al., 1995a; Morris et al., 2000) which could potentially reduce pasture contamination with infective larvae and make healthier grazing
environments for subsequent grazing. In lines of Romney sheep, selection for resistance is reported to have considerably reduced both FEC and worm burdens (Bisset et al., 2001; Bisset et al., 1996b; Bisset et al., 1991b) resulting in progeny from the most resistant Romney rams harbouring 70% and 85% fewer worm burdens and FEC, respectively, of a majority of economically important sheep nematodes species compared with susceptible counterparts (Bisset et al., 1991b; Morris et al., 1995b). Morris et al (2000) also reported that resistance reduced FEC by 11-fold difference compared with the high-FEC (susceptible) selection line involving the 1993-1994 crop of lambs. In environments where the highly pathogenic nematode species such as H. contortus are predominant, resistant animals have been associated with lower FEC, higher packed cell volume or low FAMACHA scores (Amarante et al., 1999b; Amarante et al., 2004; Gamble & Zajac, 1992; Matika et al., 2003; Mugambi et al., 1997; Van Wyk & Bath, 2002; Vatta et al., 2002). Such large reductions in FEC would lead to slow larvae build-up on pasture. The few initial studies where lines have been run separately appear to be consistent with the greater epidemiological benefits anticipated with selection for nematode resistance. In the US for example, the Gulf Coast Native sheep and their cross-bred F1 generations (Li et al., 2001; Miller et al., 1998) were reported to have consistently maintained lower FEC when grazed separately from the susceptible Suffolk breed in three consecutive years. In addition, the resistant Gulf Native lambs were found to have fewer H. contortus eggs compared with Suffolk lambs suggesting a greater capacity of the native lambs to control infection of the more pathogenic
nematode. In New Zealand, few farmlet based studies have been undertaken which have reported substantial pasture larval contamination in farmlets separately grazed by susceptible animals compared with those grazed by resistant Romney lambs. Bisset et al. (1997a) reported that farmlets grazed by susceptible lambs had approximately ( ̴) 5-6 fold greater nematode larvae infestation on pasture compared with those grazed by resistant counterparts suggesting that the degree of resistance to Trichostrongyle infection in lambs has a substantial impact on the build-up of larval
infestation on pasture. Leathwick et al. (1998) reported that grazing genetically resistant and susceptible Perendale flocks on separate farmlets resulted in more than three-fold differences in levels of pasture larvae and 9 to 13-fold differences in lamb and ewe FECs. Similarly, Morris et al. (1997) reported 6.0 and 6.4 fold differences in pasture contamination between susceptible and resistant female lambs of 1990 and 1991 when grazed separately.
In most commercial breeding and / or farming situations, resistant and resilient or susceptible line animals may not be run separately. So although selection for resistance appears to have the potential to reduce larvae contamination on pasture through greater removal of L3 by ingestion than excretion of eggs in faeces than susceptible animals, the overall pasture contamination may not be expected to change considerably simply by the presence of a small number of resistant animals in a flock (Bisset et al., 2001; Sutherland & Scott, 2010). Conversely, grazing selection lines with differing responses to parasites, will mask the expected impacts of selection for increased resistance. In contrast, solely grazing animals selected for resistance would reduce pasture contamination which may lead to improved productivity.
Production benefits
Predicted benefits of selection for nematode resistance on production have not always been realized. Correlations between FEC and liveweight gain or breech soiling in lambs under nematode challenge has not been consistent. Morris et al. (1997) reported that weaning weights were not different between resistant and susceptible lambs and that resistance had no apparent advantage over susceptibility in either post-weaning liveweight gain or autumn weights while the lines grazed together. A continued selection of Romney sheep for resistance and susceptibility to nematode infection by (Morris et al., 2000), summarised in Table 2.2, reported no difference in weaning weights, post-weaning gain and Autumn weights for lambs born between 1985-1997 with exception of the period between 1993-1997 which suggested a difference in favour of susceptible lambs. Worth mentioning is that in all these years (1985-1997) resistant and susceptible lines animals were grazed together. This also lead Morris et al. (1995b) to conclude that genetically resistant lambs have no apparent growth rate advantage over the high FEC counterparts when grazed together. Similarly, Morris et al. (2005) reported that genetically resistant Perendale line animals were 6-12% and 24- 26% lower in body weight and fleece weight, respectively, compared with their high-FEC line counterparts.
Initial studies of selection for resilience selection reported that the trait is achievable despite a low heritabilities of between 0.21-0.35 (Bisset et al., 1996a; Bisset et al., 1994). The authors suggested that although genetic correlations were low, the approach would lead to lambs with better growth rates under challenge. Other studies have reported that selection for resilience resulted in an
increased post-weaning weight gain of up to 27% while concurrently reducing the number of anthelmintic treatments by 25-50% (Greer et al., 2009b; Morris et al., 2001). Results from an extended (1994-2007) selection study for resilience to GIN parasite challenge in New Zealand Romney sheep have reported increased average ‘age-at-first-drench’ in elite resilient lines by 23.6 days relative to their control counterparts (Morris et al., 2010) which was accompanied by a 4.5 kg increase in 6-month live weight. In the same study however, the authors reported that resilient ewe lambs under low chemical treatment struggled to maintain live weight in autumn (March) when levels of nematode challenge were the highest. They suggested that highly productive resilient animals selected under minimum anthelmintics can get overwhelmed in conditions where infections are high to the point that they may contribute to greater losses associated with parasitism and the need to treat them. Similarly, Greer (2008) also suggested that there is little or no evidence from the past few hundred years that relatively intense selection for increased productivity leads to increased ability to cope with a parasite challenge in sheep particularly in the temperate regions.
Even in results from the few farmlet studies, there are inconsistencies regarding the actual realized production benefits relative to the predictions. In Perendale sheep, Leathwick et al. (1998) reported 1.7 kg increase in lambs’ live weight compared with susceptible counterparts while fleece weight for hogget and ewes were increased by 5% and 8% respectively, being associated with the benefit of reduced pasture contamination. In another study involving Romney sheep, the authors reported no differences in loss of production resulting from parasite infection between resistant and susceptible selection line animals when grazed separately (Bisset et al., 1997a)
Table 2:2 Least-square mean live weight and weight gains (kg) of lambs in each faecal egg count selection line, by year of birth grouping (Compiled by Morrie et al 2000)
No. of High
FEC
Low
FEC H-L
Traits Years lambs (H) Control (L) Mean s.e.
Weaning weight 1985 to 1992 2108 19.1 18.2 19.0 0.1 0.2 1993 to 1997 1725 18.9 19.3 19.1 -0.2 0.2 Post-weaning gain 1985 to 1992 1900 11.0 10.7 10.7 0.3 0.2 1993 to 1997 1358 9.9 8.7 9.0 0.9*** 0.2 Autumn weight 1985 to 1992 1908 30.3 28.9 29.9 0.4 0.2 1993 to 1997 1360 29.0 27.8 28.0 1.0 *** 0.2 Yearling weight 1987, 1994, to 1996 426 42.3 42.6 40.6 1.7 0.6