Impacts of selecting sheep for resistance and resilience to gastro-intestinal nematode parasites : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

This study investigated the impacts of decades of divergent selection for either resistance or resilience to gastrointestinal nematode (GIN) parasite infection in Romney sheep lines and involved a series of three studies. Study 1 (Chapter 3), assessed the variation in timing of development of immunity and physiological maturity in Romney selection lines; one selected for resistance and the other selected for resilience when exposed to natural mixed-species GIN parasite infection with no anthelmintic treatment. From weaning at mean 92 days-of-age, animals (n=53) were sampled for faecal egg count (FEC) expressed as eggs per gram of faeces (epg), saliva for immunoglobulin (IgG and IgA) determination and fasted live weight (LW) every 10 days until 351 days-of-age. Overall, mean back-transformed FEC were consistently low for resistant animals (<200 epg) whereas resilient counterparts’ FEC increased with time to reach a peak of 1400 epg at day 230 for females and 1800 epg for males at day 280 before declining to less than 500 epg by day 300, respectively (P<0.001). Immunoglobulin for T. colubriformis L3-specific IgG responses showed a selection-line by time interaction (P<0.001) reflecting an earlier rise in IgG profiles in resistant compared with resilient line but both reached similar magnitudes (1.53 ± 0.16 OD) before declining from day 330. Resistant lambs reached the threshold IgG of 1-OD indicative of presence of immunity earlier at 220.6 ± 8.8 days-of-age compared with resilient-line animals which reach this threshold 40 days later at 263.4 ± 6.9 days-of-age, respectively (P<0.001). In addition, resistant females were also associated with an earlier indication of sexual maturity depicted by the presence of mating marks by a vasectomised ram compared with their resilient counterparts viz. 263.5 ± 3.7 c.f. 274.4 ± 3.4 days-of-age, respectively, (P=0.048). Mean fasted live weight showed a selection line by time interaction (P<0.001) which reflected greater LW in the early phase of the study in resilient males but increasing for all groups until day 280 before declining and being similar for all groups from day 330. Correspondingly, greater proportions of resilient male lambs (81.3 %) reached the arbitrary slaughter weight of 35 kg by day 215 whereas only 30% of resistant male lambs reached the mark and only 35.7% and 23.1% of resilient and resistant female lambs reached the slaughter weight, respectively. There was a temporal difference in the timing of immunity; resistant lambs developed immunity earlier, but even resilient-line animals developed immunity, although this was delayed. These findings allow for the suggestion that selection for resistance to parasites may have favoured animals with a lighter body weight that develop immunity at a younger chronological age. However, at the same chronological age, resistant animals appear to be physiologically more mature compared with resilient-line animals. Further there was evidence that resilient Romney lambs still retain the capacity to develop an effective immune response to parasite challenge but this takes longer which may explain the higher indication of infection in these animals. This difference in the timing of immune development has implications for breeding programmes as to when to impose selection pressure for resistance to nematode infections without compromising protection. However, the greater growth potential of resilient lambs earlier in their lifetime inspite of increasing levels of parasite infection, provides a short-term opportunity for commercial sheep producers to exploit and target higher premium prices that are often scheduled early in the season but in the long-term this option may pose greater risks for pasture contamination. It was also apparent that a combined selection for growth and immunity can be achieved and would require use of weaning weights of >23 kg followed by sampling for either immunoglobulin G around 220 days-of age or FEC around 270 days-of-age. The second study (Chapter 4) examined the epidemiological advantage of resistance to parasite infection and whether it outweighs the opportunity costs of greater growth potential in resilient Romney selection-line animals that was evident in study 1. Resistant and resilient line animals were maintained in farmlets from birth through until 7-months-of-age. The study involved 4 farmlet-based treaments viz. resistant alone with no drench (RtA), resilient alone with no drench (RlA), resistant and resilient mixed drenched (Mxd+Dr) and resistant and resilient mixed with no drench (Mxd). Lambs were born and remained set-stocked in their respective birth paddocks within each farmlet throughout the study period and sampled monthly for FEC, saliva, live weight, herbage mass (kgDM/ha) and number of infective larvae per kilogram dry matter of herbage (L3 per kgDM) from weaning until around 30 weeks-of-age. Back-transformed FEC (epg) tended to show an interaction between treatment and time (P=0.058) reflecting greater FEC in RlA and Rl+Mxd groups which also increased with time to reach a peak of 800 and 500 epg, respectively, whereas FEC for RtA, Rt+Mxd and Rt+Mxd+Dr groups were consistently low. For Nematodirus Spp. FEC, there was a tendency for an interaction between treatment and time (P=0.090) reflecting a decline in FEC with time in RlA and Rl+Mxdr+Dr lambs whereas in Rl+Mxd group FEC rose on day 180. Strongyle pasture larvae were greatest in RlA and Rl+Mxd lambs viz. 952.0 and 897.7 L3 per kgDM compared with resistant alone (140.4 L3 per kgDM) and mixed drenched groups (78.2 L3 per kgDM) respectively (p=0.028) which increased with time to reach a peak of 2757.5 and 1801.1 L3 per kgDM in RlA and Rl+Mxd, respectively, whereas in RtA and Mxd+Dr groups remained low (P<0.05). There was no interaction between treatment and time (P=0.477). Pasture larvae for Nematodirus spp. showed an interaction between treatment and time (P<0.007) reflecting a rise in larval numbers in farmlets grazed by RlA and Rl+Mxd lambs with two peaks one on 150 viz. 5700 c.f. 2700 L3 per kgDM and the other on day 210 viz. 8700 c.f. 3300 L3 per kg DM despite a declining FEC whereas RtA and Mxd+Dr farmlets had typically less than 300 L3 per kgDM. Overall, cumulative liveweight gain (LWG) was greater in Rl+Mxd+Dr group (19.48 ± 0.91 kg) compared with Rt+Mxd+Dr treatment group (17.22 ± 0.78 kg) respectively (P<0.05) and was similar between Rt+Mxd and Rl+Mxd groups viz. 17.16 ± 0.84 kg c.f. 16.99 ± 0.84 kg (P>0.05). Further LWG was also similar between RlA and RtA viz. 14.42 ± 0.45 kg c.f. 13.85 ± 0.48 kg, respectively, (P>0.05) although this was temporary; being greater in RlA compared with RtA in first 60 days post-weaning but was considerably reduced in the last 50 days in RlA than in RtA representing losses of productive potential of 28.9% and 16.3%. Overall, these results provide further support that selection for resistance does confer epidemiological benefits in reducing larvae contamination on pasture, a consequence of lower worm egg excretion. However, there was no evidence from the two years study to suggest that the considerable epidemiological benefit provides the added advantage for greater growth in resistant animals when compared with resilient under challenge with no anthelmintic treatment, at least in the short term, which suggested that the epidemiological advantage of resistance over the time frame of this study equalled the costs. Consistent with the findings of Chapter 3 (study 1) and Chapter 4 (study 2), Chapter 5 (study 3) investigated faecal (‘parasite’) avoidance grazing behaviour between Romney selection lines. The study was designed to answer whether the differences in parasite loading between selection lines animals could be attributable solely to immunity or whether they may also reflect differences in grazing behaviour. Ten lambs from each selection line that were maintained in separate farmlets and naturally exposed to mixed-species nematode parasites were assessed on distance grazed from either a mud-clay or faecal ball at 130, 150, 180 and 210 days-of-age. Resistant-line animals maintained lower faecal-egg counts compared with their resilient counterparts viz. 25.6 c.f. 771.0 eggs per gram of faeces (epg), respectively, (P<0.001). Mean Trichostrongylus colubriformis L3-specific immunoglobulin IgG was greater in resistant than in resilient animals (P=0.005) and increased with time (P<0.001) but there was no interaction between selection line and time (P=0.130). Overall, grazing distance from the object was greatest for faecal ball, viz. 11.53 ± 1.14 cm compared with mud-clay ball, viz. 4.19 ± 1.00 cm (P<0.001) and decreased with time (P<0.001). Grazing distance tended to be greater for resilient, viz. 8.63 ± 1.14 cm than resistant, viz. 7.09 ± 0.99 cm (P=0.09) but there was no interaction between selection line and ball type (P=0.709) indicating similar levels of faecal-specific aversion between the selection lines. These results suggest that the lesser infection levels in resistant-line animals when grazing is unlikely to be due to reduced exposure to infective larvae as a consequence of greater faecal avoidance. In summary, selection has resulted in animals with divergence in their response to gastrointestinal parasites. However, these studies suggest that common fundamental characteristics still exist, with a magnitude of the difference reflecting the timing of the response to infection. This timing when applied to a field context does result in considerable epidemiological differences which can be attributed to the earlier, but not necessarily more powerful immune response and cannot be attributed to differences in grazing behaviour/ faecal avoidance. The implication of this is that either approach in its entirety has similar outcomes in animal production, however, in combination with anthelmintic the greater growth potential of resilient animals is beneficial. Further, there is some evidence of sexual dimorphism indicating that selection in females may not create the same production difference a factor which could be exploited in selection programmes.