This thesis describes a series of three experiments designed to estimate the nutritional cost of the immune response to the gastrointestinal nematodes Trichostrongylus colubriformis and Teladorsagia circumcincta in sheep. For each experiment, animals were allocated hierarchically by liveweight into one of four groups that were either infected (group IF), similarly infected and concurrently immuno-suppressed with weekly intramuscular injections of 1.3mg kg liveweight (LW)-1 of methylprednisolone acetate (group ISIF), immuno-suppressed only (group IS) or remained as controls (group C). Body composition of all animals was estimated using x-ray computer tomography prior to infection and at the conclusion of each study with bodyweight and faecal nematode egg counts (FEC; eggs gram-1 of fresh faeces (epg)) measured along with blood samples taken for the determination of levels of serum proteins, phosphate and antibodies. In the first trial (Chapter 3), the nutritional cost of both the acquisition and maintenance of immunity to gastro-intestinal nematodes was investigated using immunologically naive 5-month-old lambs and immunologically competent 17-month-old ewes during infection with 2,000 and 4,000 L3 infective T. colubriformis larvae d-1, respectively (80 L3 T. colubriformis larvae kgLW-1 d-1). Profiles of FEC and comparative worm burdens at slaughter indicated an effective immune response was maintained in IF ewes and developed in IF lambs while successfully suppressed in both ISIF lambs and ISIF ewes and was confirmed by serum antibody titres. The typical reduction in voluntary feed intake as a consequence of infection was observed in IF lambs (0.30, p&lt0.001) but not in IF ewes, ISIF lambs or ISIF ewes, and appeared to be associated with L3 IgA. Gross efficiency of use of metabolizable energy (ME) for net energy (NE) deposition was reduced by 0.20 in lambs during acquisition of immunity and by 0.16 in ewes maintaining an established immunity. Infection in immuno-suppressed animals reduced efficiency by 0.05 and 0.15 for lambs and ewes. These findings allowed the hypothesis that the reduction in feed intake and nutrient utilization in young parasitized sheep is caused by physiological signalling associated with the acquisition phase of the host immune response to infection, rather than simply the damage caused by the parasite per se. The second trial (Chapter 4) investigated the influence of metabolizable protein (MP) supply on the metabolic disturbances associated with the acquisition phase of the immune response during infection with 2,000 L3 T. colubriformis d-1. Groups of lambs were offered either a low protein (L; 62g MP kgDM-1) or high protein diet (H; 95g MP kgDM-1). Patterns of total daily egg excretion indicated that an effective immune response was developed in HIF, but not LIF, HISIF nor LISF and was confirmed by comparative worm burdens. The proportionate reduction in feed intake in immunologically normal animals was reduced through the provision of additional protein, being 0.12 in HIF and 0.23 in LIF. Regardless of diet, infection did not cause a reduction in feed intake in immuno-suppressed animals (p&gt0.05). Infection proportionately reduced the gross efficiency of ME utilization in immunologically normal animals by 0.23 in HIF (p=0.09) and by 0.51 in LIF (p=0.01), but not in immuno-suppressed animals. Immuno-suppression did not suppress serum L3 IgA levels in seven of the eight HISIF and four of the eight LISIF animals. Furthermore, only four out of the eight immunologically normal animals from both the HIF and LIF groups displayed an L3 IgA response. Consequently, regardless of immuno-suppression treatment, animals were termed as IgA responders (HR or LR) or non-responders (HN or LN). Feed intake was proportionately reduced from day 22 by 0.15 in HR (p=0.03) and by 0.32 in LR (p=0.01), but was not significantly reduced in HN or LN. Gross efficiency of ME utilization was significantly reduced for LN animals only, being proportionately 0.59 (p&lt0.01). These findings allowed the conclusion that additional MP reduced the consequence of immunological signalling that was displayed in reduced feed intake and in nutrient utilization, both of which appeared to be associated with an IgA response. It is hypothesized that the lessening of nutritional disturbance observed in high protein and immuno-suppressed animals could be a consequence of altered physiological signalling during the immunological cascade. The third trial (Chapter 5) utilized lambs infected with the abomasal parasite T. circumcincta to explore the possibility that the reduction in feed intake and nutrient utilization is a universal phenomenon of the acquisition phase of the immune response to nematode parasites inhabiting different organs along the gastrointestinal tract. In addition, immunological changes at the site of parasite infestation in the abomasal mucosa were measured from serial biopsy tissue samples taken from a further twelve animals that were surgically fitted with an abomasal cannula and either infected (CIF) or concurrently infected and immuno-suppressed as described previously (CISIF). The development of immunity in IF animals was accompanied by a 0.17 proportional decrease in feed intake between days 15 to 28 of infection (p&lt0.05) and a 0.20 proportional reduction in nutrient utilization (p=0.07), none of which were observed in ISIF animals. While FEC and worm burdens indicated successful immuno-suppression in ISIF animals, both serum IgA and total antibody production were not reduced. The development of immunity in CIF was reflected in an increase in both mast cells and globule leukocytes in serial abomasal tissue biopsies, both of which were reduced in CISIF (p&lt0.01 for both). In serial biopsy tissue, immuno-suppression did prevent a rise in tissue IgA that was apparent in CIF animals (p&lt0.01) although these changes were not reflected in serum IgA levels. It appears that the alleviation of the reduction in feed intake and nutrient utilization in young lambs through the use of corticosteroid induced immuno-suppression may be a universal phenomenon for both intestinal and abomasal parasites, but the association with and/or role of IgA during infection with T. circumcincta is unclear. In summary, the reduction in feed intake and nutrient utilization in sheep during infection with both the abomasal nematode T. circumcincta and the small intestine nematode T. colubriformis appears to be associated with a component(s) of the acquisition phase of the host immune response, rather than, as conventionally assumed, the direct mechanical damage of the parasite per se. It is hypothesised that the nutritional disturbance as a consequence of infection in young lambs may be the result of pro-inflammatory cytokines involved in immunological signalling that may also be associated with the production of IgA, the effects of which can be reduced through the provision of adequate MP. These studies provide evidence that the immune response to gastrointestinal parasites is nutritionally costly to the animal and have implications for application of manipulations that are intended to promote the development of a strong immune reaction in high producing animals.

Two indoor feeding experiments were conducted at the Animal Physiology Unit (APU) of Massey University, involving young sheep, to investigate the effect of feeding forage willow upon the death of established parasites and upon parasite fecundity, using chaffed lucerne as the control diet. Experiment 1: Twenty-four (24) parasite-free weaned hoggets weighing 29.9 ±1.2 kg (SD) were individually penned and fed chaffed lucerne ad libitum during a preexperimental adaption period of 5 weeks. They were then fed either lucerne chaff or chopped willow for a further 5 weeks (n = 12/group) and intakes were adjusted such that the DMI of the two groups was similar during weeks 9 & 10. All lambs were infected with L3 larvae parasites comprising 20,650 Teladorsagia, 1,320 Trichostrongylus and 330 Cooperia through oral drenching 12 days before willow feeding started. This was done after confirmation that the sheep were free of nematodes through FEC analysis. Total faeces were collected for 3 day periods towards the end of weeks 9 & 10, to measure diet digestibility and total faecal egg excretion. The sheep were slaughtered at the end of week 10. Voluntary feed intake (VFI), FEC and liveweight were measured weekly, whilst burdens of individual parasites and carcass characteristics were measured after slaughter. Duplicate samples of each feed offered and individual animal refusals were taken daily and pooled weekly per animal for chemical analysis. Female worm fecundity was calculated by two methods. Blood samples for immunological analysis were collected on days 20, 34, 51 and 70, and analysed for components of white blood cells (WBC) and for lymphocyte subsets. Experiment 2: A 2 x 2 changeover experiment was conducted, involving two time periods (Period 1 and Period 2 each of 14 days) with the same diets as used in Experiment 1, fed to 9 individually penned parasite-free young sheep randomly allocated to experimental diets. The parameters investigated were FEC and larvae hatching. Initially, a period of 7 days was allowed for acclimatisation in which both groups were fed on half willow and half lucerne chaff. This was followed by Period 1 with 4 lambs fed lucerne and 5 fed willow, after which the diets were changed over for Period 2. Total faeces produced were collected from all animals on the last day of each period using bagged sheep. A known number of Teladorsagia eggs (500 epg) was then added to faecal samples from these sheep and faeces-egg mixtures were made from which FEC was determined, to see if egg recovery was affected by these diets. Faecal samples for Period 2 with added eggs were also incubated for 10 days to measure hatchability. The recovery of added Teladorsagia eggs in Experiment 2 was 85% in lucerne-fed lambs and 53% willow-fed lambs (P<0.001); these were used as correction factors for Experiment 1 data. Larvae that hatched per gram of wet faeces in Experiment 2 tended to be lower for sheep fed willow than lucerne chaff (71% vs 83% of eggs added; P=0.08). Willow feed offered had lower DM (P<0.001) and CP (P<0.05) content, but had a significantly higher OM content (P<0.01) than lucerne chaff. Condensed tannin content of chopped willow was 27 g/kg DM, with only traces for lucerne. Apparent digestibility for DM (62.4% vs 59.5%; P≤0.05), OM (64.8% vs 59.9%; P≤0.001), DOMD (58.1% vs 55.0%; P≤0.01) and calculated ME (9.48 MJ/kg vs 8.96 MJ/kg; P≤0.01) were higher for the willow diet. VFI was similar for both groups during the adaption period (P>0.05) but declined with the introduction of willow in week 6 (P<0.001) and then progressively increased until it was similar to lucerne-fed sheep in weeks 9 & 10 (P>0.05). Calculated DM intake per head/day during the last two weeks of Experiment 1 was similar for the two groups (P>0.05); while the willow group had higher ME (P<0.01) and CP (P<0.001) intake per animal/day. Liveweight increased for the two groups during the adaption period (P>0.05), then declined for willow-fed lambs in week 6 (P<0.001) but later increased and by week 10 was similar to that of lucerne-fed lambs. The willow-fed lambs had lower carcass GR than the lucerne-fed lambs (P<0.01) when carcass weight was used as a covariate. Adjusted total daily egg production in Experiment 1 was lower in willow-fed sheep than lucerne-fed sheep, due to reductions for Haemonchus spp. (P<0.05) and Teladorsagia spp. (P<0.05). The per capita fecundity for Haemonchus worm spp. (P<0.05) and the in utero fecundity in both abomasal Teladorsagia spp. and small intestinal Trichostrongylus spp. (P<0.001) were lower for willow-fed sheep. There was reduced production of larvae for both Haemonchus spp. and Teladorsagia spp. (P<0.05) in willow-fed sheep. Feeding willow reduced the burden of Haemonchus adult worms in the abomasum (P<0.01) but reduced female worm burden only in Teladorsagia spp. (P<0.05) and reduced Cooperia spp. in the small intestines (P<0.01). Total WBC, total lymphocytes, subsets of lymphocytes and other white-cell groups were not affected by willow feeding (P>0.1). It was concluded that feeding chopped willow to young sheep reduced nematode worm burdens in the abomasum, especially both male and female Haemonchus spp., and reduced female worm burdens of Teladorsagia spp. Female worm fecundity of both species was also reduced by willow feeding. These reductions have been associated with CT content in the willow feed and the reduced worm burdens have been attributed to the death of the established worms by CT, since there was no evidence of immune priming in willow-fed sheep. Compounds present in the faeces of willow-fed sheep have been found to mask some of the nematode eggs, making them invisible by microscopic examination while keeping their viability. It is postulated that this could be due to binding of nematode eggs to insoluble CT associated with indigestible fibre in the faeces of willow-fed sheep. Conventional methods of measuring FEC therefore underestimated nematode eggs present in the faeces of willow-fed sheep and this needs to be checked for other CT-containing forages.

This thesis describes a series of three experiments designed to estimate the nutritional cost of the immune response to the gastrointestinal nematodes Trichostrongylus colubriformis and Teladorsagia circumcincta in sheep. For each experiment, animals were allocated hierarchically by liveweight into one of four groups that were either infected (group IF), similarly infected and concurrently immuno-suppressed with weekly intramuscular injections of 1.3mg kg liveweight (LW)⁻¹ of methylprednisolone acetate (group ISIF), immunosuppressed only (group IS) or remained as controls (group C). Body composition of all animals was estimated using x-ray computer tomography prior to infection and at the conclusion of each study with bodyweight and faecal nematode egg counts (FEC; eggs gram⁻¹ of fresh faeces (epg)) measured along with blood samples taken for the determination of levels of serum proteins, phosphate and antibodies. In the first trial (Chapter 3), the nutritional cost of both the acquisition and maintenance of immunity to gastro-intestinal nematodes was investigated using immunologically naive 5-month-old lambs and immunologically competent 17-month-old ewes during infection with 2,000 and 4,000 L3 infective T. colubriformis larvae d⁻¹, respectively (80 L3 T. colubriformis larvae kgLW⁻¹ d⁻¹). Profiles of FEC and comparative worm burdens at slaughter indicated an effective immune response was maintained in IF ewes and developed in IF lambs while successfully suppressed in both ISIF lambs and ISIF ewes and was confirmed by serum antibody titres. The typical reduction in voluntary feed intake as a consequence of infection was observed in IF lambs (0.30, p<0.001) but not in IF ewes, ISIF lambs or ISIF ewes, and appeared to be associated with L3 IgA. Gross efficiency of use of metabolizable energy (ME) for net energy (NE) deposition was reduced by 0.20 in lambs during acquisition of immunity and by 0.16 in ewes maintaining an established immunity. Infection in immuno-suppressed animals reduced efficiency by 0.05 and 0.15 for lambs and ewes. These findings allowed the hypothesis that the reduction in feed intake and nutrient utilization in young parasitized sheep is caused by physiological signalling associated with the acquisition phase of the host immune response to infection, rather than simply the damage caused by the parasite per se. The second trial (Chapter 4) investigated the influence of metabolizable protein (MP) supply on the metabolic disturbances associated with the acquisition phase of the immune response during infection with 2,000 L3 T. colubriformis d⁻¹. Groups of lambs were offered either a low protein (L; 62g MP kgDM⁻¹) or high protein diet (H; 95g MP kgDM⁻¹). Patterns of total daily egg excretion indicated that an effective immune response was developed in HIF, but not LIF, HISIF nor LISF and was confirmed by comparative worm burdens. The proportionate reduction in feed intake in immunologically normal animals was reduced through the provision of additional protein, being 0.12 in HIF and 0.23 in LIF. Regardless of diet, infection did not cause a reduction in feed intake in immuno-suppressed animals (p>0.05). Infection proportionately reduced the gross efficiency of ME utilization in immunologically normal animals by 0.23 in HIF (p=0.09) and by 0.51 in LIF (p=0.01), but not in immuno-suppressed animals. Immuno-suppression did not suppress serum L3 IgA levels in seven of the eight HISIF and four of the eight LISIF animals. Furthermore, only four out of the eight immunologically normal animals from both the HIF and LIF groups displayed an L3 IgA response. Consequently, regardless of immunosuppression treatment, animals were termed as IgA responders (HR or LR) or non-responders (HN or LN). Feed intake was proportionately reduced from day 22 by 0.15 in HR (p=0.03) and by 0.32 in LR (p=0.01), but was not significantly reduced in HN or LN. Gross efficiency of ME utilization was significantly reduced for LN animals only, being proportionately 0.59 (p<0.01). These findings allowed the conclusion that additional MP reduced the consequence of immunological signalling that was displayed in reduced feed intake and in nutrient utilization, both of which appeared to be associated with an IgA response. It is hypothesized that the lessening of nutritional disturbance observed in high protein and immuno-suppressed animals could be a consequence of altered physiological signalling during the immunological cascade. The third trial (Chapter 5) utilized lambs infected with the abomasal parasite T. circumcincta to explore the possibility that the reduction in feed intake and nutrient utilization is a universal phenomenon of the acquisition phase of the immune response to nematode parasites inhabiting different organs along the gastrointestinal tract. In addition, immunological changes at the site of parasite infestation in the abomasal mucosa were measured from serial biopsy tissue samples taken from a further twelve animals that were surgically fitted with an abomasal cannula and either infected (CIF) or concurrently infected and immuno-suppressed as described previously (CISIF). The development of immunity in IF animals was accompanied by a 0.17 proportional decrease in feed intake between days 15 to 28 of infection (p<0.05) and a 0.20 proportional reduction in nutrient utilization (p=0.07), none of which were observed in ISIF animals. While FEC and worm burdens indicated successful immunosuppression in ISIF animals, both serum IgA and total antibody production were not reduced. The development of immunity in CIF was reflected in an increase in both mast cells and globule leukocytes in serial abomasal tissue biopsies, both of which were reduced in CISIF (p<0.01 for both). In serial biopsy tissue, immuno-suppression did prevent a rise in tissue IgA that was apparent in CIF animals (p<0.01) although these changes were not reflected in serum IgA levels. It appears that the alleviation of the reduction in feed intake and nutrient utilization in young lambs through the use of corticosteroid induced immuno-suppression may be a universal phenomenon for both intestinal and abomasal parasites, but the association with and/or role of IgA during infection with T. circumcincta is unclear. In summary, the reduction in feed intake and nutrient utilization in sheep during infection with both the abomasal nematode T. circumcincta and the small intestine nematode T. colubriformis appears to be associated with a component(s) of the acquisition phase of the host immune response, rather than, as conventionally assumed, the direct mechanical damage of the parasite per se. It is hypothesised that the nutritional disturbance as a consequence of infection in young lambs may be the result of pro-inflammatory cytokines involved in immunological signalling that may also be associated with the production of IgA, the effects of which can be reduced through the provision of adequate MP. These studies provide evidence that the immune response to gastrointestinal parasites is nutritionally costly to the animal and have implications for application of manipulations that are intended to promote the development of a strong immune reaction in high producing animals.

This thesis describes a series of three experiments designed to estimate the nutritional cost of the immune response to the gastrointestinal nematodes Trichostrongylus colubriformis and Teladorsagia circumcincta in sheep. For each experiment, animals were allocated hierarchically by liveweight into one of four groups that were either infected (group IF), similarly infected and concurrently immuno-suppressed with weekly intramuscular injections of 1.3mg kg liveweight (LW)⁻¹ of methylprednisolone acetate (group ISIF), immunosuppressed only (group IS) or remained as controls (group C). Body composition of all animals was estimated using x-ray computer tomography prior to infection and at the conclusion of each study with bodyweight and faecal nematode egg counts (FEC; eggs gram⁻¹ of fresh faeces (epg)) measured along with blood samples taken for the determination of levels of serum proteins, phosphate and antibodies. In the first trial (Chapter 3), the nutritional cost of both the acquisition and maintenance of immunity to gastro-intestinal nematodes was investigated using immunologically naive 5-month-old lambs and immunologically competent 17-month-old ewes during infection with 2,000 and 4,000 L3 infective T. colubriformis larvae d⁻¹, respectively (80 L3 T. colubriformis larvae kgLW⁻¹ d⁻¹). Profiles of FEC and comparative worm burdens at slaughter indicated an effective immune response was maintained in IF ewes and developed in IF lambs while successfully suppressed in both ISIF lambs and ISIF ewes and was confirmed by serum antibody titres. The typical reduction in voluntary feed intake as a consequence of infection was observed in IF lambs (0.30, p<0.001) but not in IF ewes, ISIF lambs or ISIF ewes, and appeared to be associated with L3 IgA. Gross efficiency of use of metabolizable energy (ME) for net energy (NE) deposition was reduced by 0.20 in lambs during acquisition of immunity and by 0.16 in ewes maintaining an established immunity. Infection in immuno-suppressed animals reduced efficiency by 0.05 and 0.15 for lambs and ewes. These findings allowed the hypothesis that the reduction in feed intake and nutrient utilization in young parasitized sheep is caused by physiological signalling associated with the acquisition phase of the host immune response to infection, rather than simply the damage caused by the parasite per se. The second trial (Chapter 4) investigated the influence of metabolizable protein (MP) supply on the metabolic disturbances associated with the acquisition phase of the immune response during infection with 2,000 L3 T. colubriformis d⁻¹. Groups of lambs were offered either a low protein (L; 62g MP kgDM⁻¹) or high protein diet (H; 95g MP kgDM⁻¹). Patterns of total daily egg excretion indicated that an effective immune response was developed in HIF, but not LIF, HISIF nor LISF and was confirmed by comparative worm burdens. The proportionate reduction in feed intake in immunologically normal animals was reduced through the provision of additional protein, being 0.12 in HIF and 0.23 in LIF. Regardless of diet, infection did not cause a reduction in feed intake in immuno-suppressed animals (p>0.05). Infection proportionately reduced the gross efficiency of ME utilization in immunologically normal animals by 0.23 in HIF (p=0.09) and by 0.51 in LIF (p=0.01), but not in immuno-suppressed animals. Immuno-suppression did not suppress serum L3 IgA levels in seven of the eight HISIF and four of the eight LISIF animals. Furthermore, only four out of the eight immunologically normal animals from both the HIF and LIF groups displayed an L3 IgA response. Consequently, regardless of immunosuppression treatment, animals were termed as IgA responders (HR or LR) or non-responders (HN or LN). Feed intake was proportionately reduced from day 22 by 0.15 in HR (p=0.03) and by 0.32 in LR (p=0.01), but was not significantly reduced in HN or LN. Gross efficiency of ME utilization was significantly reduced for LN animals only, being proportionately 0.59 (p<0.01). These findings allowed the conclusion that additional MP reduced the consequence of immunological signalling that was displayed in reduced feed intake and in nutrient utilization, both of which appeared to be associated with an IgA response. It is hypothesized that the lessening of nutritional disturbance observed in high protein and immuno-suppressed animals could be a consequence of altered physiological signalling during the immunological cascade. The third trial (Chapter 5) utilized lambs infected with the abomasal parasite T. circumcincta to explore the possibility that the reduction in feed intake and nutrient utilization is a universal phenomenon of the acquisition phase of the immune response to nematode parasites inhabiting different organs along the gastrointestinal tract. In addition, immunological changes at the site of parasite infestation in the abomasal mucosa were measured from serial biopsy tissue samples taken from a further twelve animals that were surgically fitted with an abomasal cannula and either infected (CIF) or concurrently infected and immuno-suppressed as described previously (CISIF). The development of immunity in IF animals was accompanied by a 0.17 proportional decrease in feed intake between days 15 to 28 of infection (p<0.05) and a 0.20 proportional reduction in nutrient utilization (p=0.07), none of which were observed in ISIF animals. While FEC and worm burdens indicated successful immunosuppression in ISIF animals, both serum IgA and total antibody production were not reduced. The development of immunity in CIF was reflected in an increase in both mast cells and globule leukocytes in serial abomasal tissue biopsies, both of which were reduced in CISIF (p<0.01 for both). In serial biopsy tissue, immuno-suppression did prevent a rise in tissue IgA that was apparent in CIF animals (p<0.01) although these changes were not reflected in serum IgA levels. It appears that the alleviation of the reduction in feed intake and nutrient utilization in young lambs through the use of corticosteroid induced immuno-suppression may be a universal phenomenon for both intestinal and abomasal parasites, but the association with and/or role of IgA during infection with T. circumcincta is unclear. In summary, the reduction in feed intake and nutrient utilization in sheep during infection with both the abomasal nematode T. circumcincta and the small intestine nematode T. colubriformis appears to be associated with a component(s) of the acquisition phase of the host immune response, rather than, as conventionally assumed, the direct mechanical damage of the parasite per se. It is hypothesised that the nutritional disturbance as a consequence of infection in young lambs may be the result of pro-inflammatory cytokines involved in immunological signalling that may also be associated with the production of IgA, the effects of which can be reduced through the provision of adequate MP. These studies provide evidence that the immune response to gastrointestinal parasites is nutritionally costly to the animal and have implications for application of manipulations that are intended to promote the development of a strong immune reaction in high producing animals.

Two studies were conducted to investigate anthelmintic resistance in goat parasites in New Zealand. In Study 1 parasites from goats on a farm with a long history of problems with anthelmintic efficacy were used to infect sheep for a controlled slaughter study. Nineteen lambs were acquired, effectively drenched and housed. Each was infected with a mixture of larvae comprising Haemonchus contortus, Teladorsagia circumcincta, Trichostrongylus colubriformis and Oesophagostomum venulosum. After 28 days lambs were restrictively randomised into 3 groups based on faecal egg counts. Group 1 was left untreated (n=6), Group 2 (n=6) was given a single dose of abamectin (0.2mg/kg) + levamisole HCL (8mg/kg) + oxfendazole (4.5mg/kg) (“Matrix Oral Drench for Sheep”®, Ancare, New Zealand) and Group 3 (n=7) was treated at twice the dose rate of Group 2. Fourteen days after treatment all animals were killed for total worm counts. The mean burdens of T. circumcincta in Group 1 was 337, in Group 2 was 68 (efficacy 80%) and in Group 3 was 10 (efficacy 97%). The mean burdens of T. colubriformis in Group 1 was 375, in Group 2 was 220 (efficacy 41%) and in Group 3 was 81 (efficacy 78%). Although the worm burdens in these lambs were low, all animals were infected with each of these two species except for T. circumcincta in Group 3 where only 3 lambs were infected. Efficacy against other species was 100%. These results clearly indicate that a single dose of a combination drench was ineffective against two species and even when a double dose was used the efficacy against T. colubriformis was only 78%. In Study 2 a survey of drench efficacy was conducted on 17 goat farms using the DrenchRite® larval development assay. Evidence of concurrent resistance to benzimidazoles, levamisole and ivermectin was detected in T. colubriformis and T. circumcincta on 11/17 and 3/14 respectively. Only 5 of 14 farms had previously undertaken some form of testing for drench resistance prior to this survey. Evidence from these two studies suggests that severe anthelmintic resistance is common on goat farms in New Zealand

The series of experiments described in this thesis were designed to investigate the role of suckling or late weaning in the response of young lambs to nematode infection. All experiments were conducted outdoors with grazing animals and no supplementation but for suckled groups of lambs whose counterparts were weaned to ryegrass – white clover swards. The parasite of interest was mainly Teladorsagia circumcincta solely but with mixed infection of Trichostrongylus colubriformis in one instance. In Chapter 3 (first experiment), the hypothesis that milk per se may have a direct effect on nematode development, rather than an indirect effect through enhancement of host immunity by superior nutrient supply was tested. Sixty, twinborn lambs were used, allocated to one of eight groups formed by either dosing lambs from 42 days of age or not with the equivalent of 1000 or 250 L₃ T. circumcincta larvae d⁻¹ until five days before necropsy, while a twin was either weaned at 39 days of age, suckled as single or twin until necropsy on day 84. The possibility that weaning one of a twin set onto pasture in close proximity to the ewe would cause abnormal ewe and lamb behaviour was tested by replicating the work in twins maintained as twins but in which one twin received equivalent of 250 and the other 1000 L₃ T. circumcincta larvae d⁻¹. This showed no abnormal ewe nursing or lamb suckling behaviour as a result of weaning a twin in a set. Relatively low faecal egg counts (FEC) and a two to three fold lower worm burdens suggest suckling could reduce larval establishment. Inability to detect peripheral titres of immunoglobulins supports this conclusion. An intra worm-population regulation of T. circumcincta, indicated by a pattern of greater egg-laying by a numerically smaller but physiologically better developed nematode population in suckled lambs measured in eggs 'in utero' and worm length made interpretation of FEC difficult. Suckling significantly improved weight gain and carcass weights, but early weaning did not reduce resilience to infection. In Chapter 4 (second experiment), 40 pairs of twin lambs, average age of 39 days, were either infected with the equivalent of 1000 L₃ T. circumcincta larvae d⁻¹ or not, while one twin was weaned and the other allowed to continue suckling. Necropsy was carried out on groups of five and six lambs from each of the uninfected and infected treatments, respectively, at mean age of 84, 112, and on six lambs from each group at 140 days of age. This serial slaughter allowed further confirmation of the hypothesis in Chapter 3 but also investigated the long-term effect of suckling on resistance or resilience of lambs at the trial when immune responses were anticipated to be developing. An in vitro direct larval challenge (IVDC) study, to monitor larval establishment, was carried out on tissue explants from necropsied lambs. Suckled lambs consistently showed lower FEC (P < 0.05) and worm burdens (P < 0.05) at every phase of the trial. Within the infected groups, % in vitro larval rejection suggested earlier immune responses in the weaned lambs by day 84, which was not consistent with lower worm burdens in suckled lambs but appeared similar in the subsequent necropsies. Lambs continued to show better growth due to suckling while weaning did not reduce the resilience of lambs confirming observations in Chapter 3. The immunoglobulin profile suggested the commencement of immune responses in lambs from the period after the 84th day necropsy, with significantly greater (P < 0.01) IgA titre in the infected groups, and the suckled lambs towards the end of the trial on day 140. A vaccinating effect of early exposure to parasites was coincidentally revealed as a result of unintentional pasture larval contamination, seen in suckled non-infected lambs shedding fewer eggs and harbouring fewer worms during the later necropsies compared with their weaned non-infected counterparts. In Chapter 5 (third trial), 93 pairs of twin lambs, 47 pairs of which received a vaccinating mixed infection of T. circumcincta and T. colubriformis larvae (60 L₃ / kg W / d) at ratio 40:60, respectively during the period 36 – 103 days of age, were either weaned early on day 51 or later on day 108. All lambs were drenched on day 108 and groups received challenge infections from day 116, at same rate with the vaccinating infection, or not, which ceased five days before respective necropsies. Necropsies were carried out on selected lambs on days 108, 184 and 218. The direct effect of milk on larval establishment appeared to feature only in the T. circumcincta populations on slaughter day 108. The long-term benefit of late weaning for development of resistance was conditional on lambs receiving the vaccinating infection, and appeared to be more pronounced in the small intestine, reflected by a greater reduction of T. colubriformis populations in that organ than of T. circumcincta populations in the abomasum. A negative consequence of enhanced immune response was the suggestion of an increased metabolic cost in reduced performance of lambs. In conclusion, the work provides support to the hypotheses that: (a.) suckling may reduce the establishment of nematode larvae through the direct effect of milk, (b.) may enhance rapid development of host immunity to infection, and (c.) it further suggests that lack of larval experience during suckling may have long term negative implications for host resistance. Finally, it suggests that milk may play little role in the enhancement of host resilience to infection and, on the contrary, that additional metabolic cost may be associated with a more rapid development of immunity resulting from larval challenge while suckling.