Environmental Effects on Animal Production: The Nutritional Demands of Nematode Parasite Exposure in Sheep

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1 Environmental Effects on Animal Production: The Nutritional Demands of Nematode Parasite Exposure in Sheep (McClymont Lecture) A. R. Sykes Animal and Food Sciences Division, P.O. Box 84, Lincoln University, Canterbury, New Zealand ABSTRACT: The complex interplay between nutrition and response to nematode parasites in sheep is described. The effect of infection on host metabolism and nutrient requirements are considered as well as the evidence that host resilience and immunity can be moderated by improved protein and trace element nutrition. The conclusion is drawn that host immunity may be dependent on a much greater protein supply than current estimates of requirement or can be supplied by pasture. It is speculated that ewe nutrition in the peri-partum period may have important epidemiological consequences. Prof. McClymont is recognised for his contribution to education in animal production and health and applied nutritional problems of ewe nutrition, particularly adapting the animal to environmental demands, in his specific case drought feeding. At the time of Dr McClymont s work government funded research emphasised basic nutritional studies of animals in controlled environments to provide information about the utilisation of energy in growth and how it varies with the rate of growth, type of ration, development pattern and ultimate mature size. It lead to the metabolisable energy (ME) system, a scheme which. avoids the errors of the previous ones by not muddling the efficiency of the animal with the nutritive value ascribed to food, and at every stage in the calculations one is cognisant of the limitations of existing knowledge (Blaxter, 1962). The limitations were considerable when these estimates were applied to animals in pastoral environments. Geenty and Rattray (1987) argued that relative to pen feeding, maintenance requirements of grazing sheep can increase by 30-80% depending on feed supply, climatic conditions and topography. The National Research Councils of several countries used so-called fudge factors which adjusted precise penbased estimates by 20-50% on arbitrary grounds to account for these environmental demands. At the time of these nutritional studies the epidemiology of the major nematode parasites was being worked out (Brunsdon, 1970; Donald and Waller, 1982; Michel, 1963). Pathophysiological studies of the effect of parasitism in sheep were designed to throw light on the extreme clinical disease emaciation and death, so typical of the era before effective anthelmintics became available. Very large larval doses were used to establish infections which resulted in extreme clinical disease at the time of patency of the worm. During the 1970 s and 1980 s, however, nutritionists began to take an interest in quantifying the pathophysiological effects of parasitism. In order to produce relatively stable levels of infection always difficult because of the dynamic interplay between the incoming larvae and the hosts immune system they began to use trickle (daily) infection of small numbers of larvae. What these demonstrated was that even in subclinical infections major reductions up to 50% - in energy utilisation could occur (Reveron et al, 1974; Steel, 1974; Sykes and Coop, 1976; 1977). It is now reasonable to suggest that those socalled fudge factors used to convert estimates of nutrient requirement from calorimeter to paddock also included the additional cost to the host of combating pathogenic components of the natural environment including nematode parasites. This cost has now been identified as having at least two components- the cost of repair of the gastrointestinal tissue damage caused by the parasite and the cost of mounting an immune response and so limiting or preventing further tissue damage. Moreover, studies are increasingly suggesting that nematode infection of the gastrointestinal tract, rather than reducing ability of the host to digest and absorb nutrients as assumed in the past actually increases nutrient requirement. Moreover, there is increasing evidence that in some circumstances it may be possible to meet this requirement and therefore influence the disease outcome. PATHOPHYSIOLOGY OF INFECTION The pathophysiological/nutritional impact of gastro-intestinal nematode parasitism has been the subject of numerous recent comprehensive reviews (Sykes, 1982; MacRae, 1993; Coop and Holmes, 1996; van Houtert and Sykes, 1996; Coop and Kyriazakis, 1999). A major component of the effect of parasitism in reducing overall efficiency of nutrient use is a reduction in feed intake which can reach 50%, even in subclinical cases. The extent of its contribution to reduced energetic efficiency appears to be greater in infections of the abomasum than in infections in the small intestine (Sykes, 1994; Coop and Kyriazakis, 1999). Increased N flow in the digestive tract occurs and has been attributed to increased leakage of plasma proteins, increased mucous secretion and sloughing of epithelial cells. While these secretions are substantially resorbed in distal sections of the gastrointestinal tract (GIT) they do lead to increased protein synthesis and energy demand in the GIT. In effect energy and protein appear to be preferentially sequestered to the

2 alimentary tract (MacRae,1993), thus increasing maintenance energy requirement. When efficiency of nutrient utilisation is calculated as the ratio of NE deposited to ME available, reduction in feed intake and the diversion of ME to maintenance of the alimentary tract contribute to reductions of efficiency of up to 50% even in subclinical infections (Sykes, 1982). Very recently sophisticated experimental approaches involving catheterisation of the alimentary tract and its vascular bed, coupled with mass isotope tracer kinetics (Yu, 1998; 1999) have shown that sequestration of leucine leaving the alimentary tract before entering the mesenteric and hepatic circulation was increased by 24% and its oxidation by 39%. Although whole body leucine flux was not affected the availability of amino acid for metabolism in peripheral tissues skeleton and muscle was markedly reduced. These findings confirm earlier studies (Symons and Jones, 1975; 1981) in sheep and guinea pigs which showed increased protein synthesis in the gastrointestinal tract. They also explain the change in apparent efficiency of use of energy (Sykes, 1982) especially given that splanchnic tissue normally accounts for 30-45% of whole body protein synthesis despite contributing only 5% of body protein mass (McRae, 1993). Some of the early studies (Sykes and Coop, 1976) were specifically designed to provide an understanding as to whether parasitism could be implicated in the osteoporosis typically seen in growing sheep in parts of Scotland (Nisbet et al., 1962). Infection in the small intestine typically with T. colubriformis (Sykes et al., 1975) or Trichostrongylus vitrinus (Sykes et al., 1977) - did indeed show reduced bone growth and, specifically, reduced bone matrix deposition, undoubtedly a result of the change in priority of protein metabolism described above. However, a very reliable finding of numerous studies of intestinal parasitism has been a hypophosphataemia with resultant impaired calcium and phosphorus deposition in the skeleton. Subsequent studies have shown this to be the result of impaired P absorption (Wilson and Field, 1983). There is no conclusive evidence in these studies for change in Ca absorption, nor for any effect of infection in other sites on Ca or P metabolism. There is some evidence for the disturbance of the metabolism of some other minerals though not necessarily with effect on productivity. Very significant losses of Na into the gastrointestinal tract occur in abomasal and intestinal infections though these can be very effectively reabsorbed (Wilson and Field, 1983). However, though Suttle et al., (1996) observed a direct association between apparent Na status, as judged by Na:K ratio of less than 5 in saliva, and abomasal damage, as judged by plasma pepsinogen, Na supplementation did not improve productivity. Interactions between parasites and trace element nutrition have long been suspected, possibly because of their undoubted independent roles along with feed supply and quality in the clinical disease ill-thrift - July disease in the northern hemisphere (Suttle, 1994). To date there have been few convincing quantitative nutritional studies to show either impairment of absorption or increase in requirement. The ph environment in the abomasum can be changed from the normal ph 2.5 to 6.5 by infection in this organ, and changes in solubility or ability to bind with transporters may be expected. Evidence for impaired absorption is available for Cu, but not, to my knowledge for any other trace element. Bang et al (1990), showed a direct negative relationship between abomasal ph, modified by abomasal infection, and both soluble copper concentration in abomasal liquid and the increase in liver Cu after administration of Cu-oxide wire particles. Reductions in liver Cu concentration correlated with increase in abomasal ph have been induced by infection with Haemonchus contortus (Ortolani et al,1993). WHAT IS THE VALUE OF THIS INFORMATION? The difficulty for nutritionists in incorporating this information into estimates of nutrient requirement is that this host-parasite relationship is an unstable and evolving one, governed on the one hand by variation in incoming larvae and on the other by the state of readiness or competence of the host s immune system though it has been estimated that full immunity itself may cost the equivalent of a 15% loss of productivity (Sykes, 1994). The dependence of the free-living stages of nematodes on climate and the availability of susceptible hosts the immunologically naïve lamb and the immunologically compromised ewe in the peripaturient period -combine to provide typical patterns of larval availability for particular environments. In addition local climate determines the availability of particular nematode genera. For example, the greater ability of Teladorsagia spp. to develop and survive at low temperature means they are more prevalent in cooler temperate regions. They are more likely to generate larger early season larval numbers compared to say Trichostrongylus or Haemonchus spp. which tend to dominate late season larvae and, in the case of the latter, sub-tropical and tropical regions. Given individual nematode species predilection for site in the host the site of parasitic challenge is likely to vary between environments and, from season to season within environments (Suttle, 1994). Assessment of the impact of parasitism on animal performance and the consequences for nutrient requirement or nutritional management may well, therefore, have specific local environmental dimensions, reflecting the particular pattern and genera make up of larval challenge. For example is the rate limiting effect of a particular infection due to red cell loss-induced anaemia, induced protein deficiency or impaired P or Cu absorption? We are, surprisingly, not in a good position to judge this. Does this information help to provide opportunities for non-chemical manipulation of the host-parasite relationship? This raises the concept as to whether the objective should be to improve the ability

3 of the host to withstand the pathological effects of infection (resilience) or its resistance to establishment of infection (immunity). There is evidence that nutrition can influence both aspects of host response to infection and this subject has been reviewed (van Houtert and Sykes, 1996; Coop and Kyriazakis, 1999). IMPROVING RESILIENCE There is little doubt that a well fed host is better able than a poorly fed one to withstand the pathological damage described above and maintain productivity. The hypoalbuminaemia in infection with H. contortus (Abbott et al., 1986a, 1986b; Wallace et al., 1995) was reduced when diets with improved protein concentration were offered. Normal growth rate and body protein deposition was restored in sheep abomasally infused with casein during infection with T. colubriformis (Bown et al., 1991). Losses in LWG in young Merino wethers during infection with T. colubriformis were reduced from 43% to 18% as a result of fishmeal supplementation (van Houtert et al., 1995a). Weight gain and wool growth was improved in lambs given a diet enhanced with protected methionine while infected with T. colubriformis (Coop et al, 1997). While these studies may suggest the importance of high quality protein reaching the abomasum, approximately 50% improvement in LWG under challenge with T. colubriformis and H. contortus has been demonstrated as a result of the inclusion of 3% urea in low quality oaten chaff diets (Knox and Steel, 1996). The importance of protein, as opposed to energy replacement, for resilience was demonstrated by Bown et al, (1991), since abomasal infusion of iso-caloric of amounts of glucose had little effect on resilience compared to the casein infusions. Diet selection studies, which have demonstrated that sheep select a higher protein diet when infected with T. colubriformis (Kyriazakis et al, 1994; 1996), tend to confirm the importance of protein for resilience. Coop and Kyriazakis (1999) argue that improved nutrition will be more dramatic in infections with young naïve hosts which result in substantial pathological changes and damage or loss of host tissue. THE IMMUNE RESPONSE Immunity to nematode parasites is typically slow to develop before puberty (Gibson and Parfitt, 1972) and may not be fully established in 2-year old ewes (Familton, 1991). Once established immunity appears to be precariously dependent on continual antigenic stimulation (Dineen and Wagland, 1966; Leyva et al., 1982 ) and is reduced around parturition. This slow to develop and challenge sensitivity contrasts markedly with the rapid development and retained memory characteristic of immune responses to viral, bacterial and even protozoal infections. The speed of development of immunity does vary with particular gastrointestinal parasite and may be a consequence of the degree of contact of the parasite with the host system (Wakelin, 1996) and the rate of infection (Dobson et al., 1990). The mechanisms of immunity to GI nematodes are far from being fully understood but have been the subject of numerous reviews e.g. Wakelin, (1996). In brief thymus derived lymphocytes of the class 2 system ultimately respond to antigens detected by the host with the production of a cascade of cytokines which then stimulate an eosinophilia, a range of antibody production, including IgA, and an inflammatory process in the alimentary tract involving IgE secretion and mastocytosis. Mast cell degranulation and the secretion of mast cell proteases are characteristic of this inflammatory reaction (Douch et al., 1986; Huntley et al., 1995). NUTRIENT DEMANDS OF THE IMMUNE RESPONSE We have only a superficial understanding of the nutrient requirements for these processes but it is becoming clearer that specific nutrients, particularly cysteine (Takahashi et al., 1997) and glutamine (Calder, 1995) may be important. It is becoming increasingly apparent, however, that immunity per se, as opposed to resilience in the face of challenge can be modified by nutrition. PROTEIN SUPPLY A number of studies have provided evidence that increasing dietary protein supply assists the host in developing immunity. In young animals the speed of acquisition and/or expression of immunity rather than innate immunity seems to be improved. Thus while initial establishment of Oesophagostomum columbianum (Dobson and Bawden, 1974), and of T. colubriformis (Bown et al., 1991; van Houtert et al., 1995a) were not affected by dietary protein supply worm burdens and/or faecal egg counts were subsequently reduced as infection progressed. In a study in which sheep had previously received an anthelmintic-abbreviated vaccinating challenge establishment of a subsequent challenge infection with T. circumcincta was reduced by 50% by protein supplementation (Coop et al., 1995). Very recently (Donaldson et al 1999; 2000) have shown that the periparturient relaxation of immunity in the ewe can be markedly influenced by dietary protein. Ewes were trickle infected with T. circumcincta and T. colubriformis and offered a diet providing either 120 (LP) or 200 (HP) g crude protein/kg DM, achieved by addition of fishmeal. The diets were offered at two energy levels, designed to promote either zero or + 50g/d gain in maternal bodyweight during late pregnancy and 100 or 0 g/d weight loss during the first 3 weeks of lactation, respectively. Mean worm burdens after 3 weeks of lactation were 12,020 and 1540 for LP and HP groups but there was no effect of level of feeding (energy). Although energy allowances were adjusted to take account of whether the ewes carried and suckled twin or single lambs those carrying

4 twins harboured almost 4 times as many worms as their single-rearing contemporaries, which was interpreted as indicating the additional protein demands of multiple suckling ewes. There is supporting evidence for the view that susceptibility to parasite establishment in the peri-partum period is proportional to reproductive effort in the effect of number of foetuses carried (O Sullivan and Donald, 1973) and the restoration of immunity on termination of pregnancy or removal of lambs (Salisbury and Arundel, 1970; O Sullivan and Donald, 1973). Ecologists studying great tits (Norris et al, 1994) have found that enlarging brood size increases parasite prevalence in males and negative relationships between brood size and serology in flycatchers (Gustafsson et al, 1994) indicate an effect of reproductive effort on immune function. However, while ecologists have so far related immune competence to parasites with energy intake, the more controlled studies with large animals have suggested that protein supply is more important (Bown et al, 1991; Donaldson et al, 1999; 2000). PHOSPHATE SUPPLY Other components of the diet have been implicated in the host immune response. One of the most intriguing reports was that of Coop and Field, (1982). Sheep trickle infected with T. vitrinus while offered diets with 2.75 or 1.88 gp/kg/dm harboured 1240 and 10,980 worms after 12 weeks of infection. They were unable to explain the results, but did observe that the combined effect of the low P diet and infection resulted in rumen P concentration of 0.087g/L, much lower than the 0.4g/L below which rumen microbial protein synthesis is reduced (Gunn and Ternouth, 1994). If substantiated it has implications for the management of parasites in ruminants consuming low quality forages. TRACE ELEMENTS It would be surprising, given fact that immunity involves intense inflammatory responses, if those trace elements concerned directly or indirectly with the metabolism of anti-inflammatory enzymes do not influence the host immune response, positively or negatively. The copper and selenium containing enzymes, superoxide dismutase and glutathione peroxidase, respectively, are prime examples. Very recently evidence for involvement has begun to emerge. Addition of Mo (0.05 mmol/kg DM) to the diets of lambs reduced the number and length of T. vitrinus worms established and the faecal egg counts and number of H. contortus worms recovered from the abomasum (Suttle et al., 1992a and b) after trickle infections with the two parasites. More recently McLure et al., (1999) suggested a critical dietary Mo concentration of 6-10 mg Mo/kgDM for maximum host resistance to T. colubriformis. Sheep on this Mo intake showed a 90% reduction in faecal egg count and worm burdens compared to lambs offered diets with Mo concentrations below or above this range. Correlated responses in intestinal antibody, jejunal mast cell and blood eosinophil numbers were observed. Suttle et al., (1992 a and b) suggested that Mo could enhance the inflammatory response by increasing superoxide radical concentration in the mucosa either directly or by reducing the effectiveness of local trace elementdependent anti-inflammatory enzymes. There is recent evidence that vitamin B 12 /Co status may predispose to high faecal egg counts in pregnant sheep (Gruner and Sykes, unpublished). The co-enzyme forms of the vitamin adenosyl cobalamin and methyl cobalamin are important in the conversion of propionate to glucose and the methylation of homocysteine to form methionine, respectively. Failure of either pathway could decrease critical amino supply to the immune system and the homocysteine cysteine pathway may be particularly important. LESSONS FROM SELECTION EXPERIMENTS During the past 20 years numerous groups have initiated selection experiments designed to improve the ability of the host to withstand nematode parasites. The objective has been to reduce dependence on anthelmintics. In setting objectives most groups have grappled with the concept as to whether to select for resilience or resistance to infection and, for simplicity, have opted to select for the latter through faecal egg count. These experiments have been very successful in meeting the objective of reduced faecal egg counts heritability estimates in the range have been observed (Morris, 1998; Bishop and Stear, 1997). Selection in young susceptible animals has brought corresponding reductions in the extent of the periparturient breakdown in immunity in adults (Morris et al., 1998). Improvements in growth performance have not, however been observed and correlations between parasite resistance and wool growth have been negative (Morris et al., 1997; McEwan et al., 1994). Moreover, when animals have been selected specifically for wool growth decreased resistance to parasites has been observed (Williamson et al., 1995). These findings imply that the immune system and the wool follicle compete for the same nutrients, prime candidates, given the demands of the immune system outlined earlier, being the S-amino acids and particularly cysteine. Nutrition-immune system interactions are an active area of debate amongst ecologists who consider immunological defences against nematode parasites as inherently nutrient expensive. They consider that the host must trade off nutrients devoted to immunological functions against nutrients devoted to other life history activities such as growth, reproduction and survival (Owens and Wilson, 1999; Sheldon and Verhulst, 1996; Keymer and Read, 1991). Very recently (Coop and Kyriazakis, 1999) have put forward a framework which predicts that the functions of growth, pregnancy and lactation will have priority over immune function. Has selection of sheep for increased wool production

5 and fertility so increased the demand for resources for these functions that the immune system is compromised? Rauw et al., (1998) reviewed the undesirable side effects of selection for high production efficiency in farm animals and concluded that these include greater risk of behavioural, physiological and immunological problems. The process of domestication of animals has increased the total amount of resources available to an animal since some resource consuming traits are no longer required e.g. searching for food and fighting predators and resource supplies have become much more secure and abundant due to agricultural development. However, as suggested by Beilharz (1998) we must expect (as a consequence of selection for production) domesticated animals have become again limited by the environment in many situations. What evidence is there for such an assumption? Although there are considerable differences between breeds of sheep, cattle and goats in their ability to respond to parasitic challenge (Gray, 1991) clear genetic or phenotypic relationships between resistance to parasites and productivity have not been observed (Bishop and Stear, 1997). Nutritional comparisons between resistant and non-resistant sheep are also difficult to interpret because so-called resistant sheep are often less productive and therefore have a lower nutrient requirement. In one such comparison resistance to worm establishment (as judged by faecal egg count) was affected by increasing dietary protein concentration in both resistant Scottish Blackface - and non resistant Finn-Dorset - sheep (Abbott et al, 1985a and b) but resilience was only improved in the Finn-Dorset sheep. The latter sheep, however, had a 15% greater growth rate than the resistant Scottish Blackface sheep, and presumably therefore greater nutrient demand for growth. Interestingly this difference 15% - is comparable to the average difference in growth rate between entire male and female lambs, and farmer experience suggests the former are more susceptible to parasite infection. IMPLICATIONS FOR NUTRITION OF SHEEP IN PASTORAL SYSTEMS More recent studies (Donaldson et al, 1999) suggest that resistance to establishment of nematode larvae is directly proportional to estimated metabolisable protein (MP) supply in ewes in the peripartum period and that the MP requirement for maintenance of immunity may be as great as 350g MP/day, 20% higher than current estimates of requirement (AFRC, 1993). This is a very high requirement for MP which is unlikely to be achieved by sheep in pastoral situations (Robinson, 1990). The likely supply of MP as a function of metabolisable energy(me) content of pasture has been calculated (Fig. 1) using the assumptions of AFRC (1993) of 11g microbial crude protein synthesised/mj fermentable ME (ME x 0.6) for a fresh herbage with ME and CP contents of 10MJ ME and 230 gcp/kgdm, respectively, and assuming protein degradabilities of 0.70 and On the same graph is plotted the ratio of requirement of MP/ME (g/mj) taken directly from AFRC (1993) for a lamb growing at 200 g/d and for a kg ewe bearing and rearing twin lambs. These estimates of requirement are substantially greater than theoretical estimates of MP/ME supply from pasture, particularly during early growth and late pregnancy and lactation. Superimposed are the ranges of intake of MP/ME provided in the experimental studies of (Bown et al 1991) and (Donaldson et al 1999; 2000) in which evidence of protein-induced enhancement of resistance to parasite establishment was observed. Significantly, these responses have all been obtained at MP/ME ratios much greater than the theoretical estimate of requirement and at very much higher than the ratio of MP/ME provided by pasture. The conclusion seems inescapable that modern pastoral farm animals have protein demands at critical times which cannot be supplied by pasture and that immune function against nematode parasites is likely to be compromised. What can be done? Plant breeding may, in the long term provide some solutions. In parallel to these studies there has been increasing interest in the use of novel plants which may provide some protection against parasites (Neizen et al, 1996). These studies have generally shown lower faecal egg counts and worm burdens and faster growth rates in infected lambs when grazing plants containing condensed tannins (CT) (Neizen et al 1993, 1995). It has generally been assumed that increasing protein supply is responsible, due to the effect of CT in protecting plant protein against breakdown in the rumen. This possibility has not been effectively tested, and should be interpreted with caution because there because there is recent evidence for direct anthelmintic activity of CT (Kyriazakis pers. comm). The former hypothesis is, however, supported by evidence for effect of CT in increasing the flow of dietary cystine and methionine to the small intestine and the irreversible loss of cystine from the body, (McNabb et al, 1993; Waghorn et al, 1994). Incorporation of CT plants into production systems will, however, probably proceed with some caution given the evidence for low feed intake (Lowther and Barry, 1985), for reduced absorption of cystine with some CT containing plants (McNabb et al, 1993; Waghorn et al, 1994) and uncertainty about the effect of variation in ph conditions in the small intestine on the dissociation of different CT from plant protein. There is no doubt, however, that the future will see plants genetically engineered to promote host resistance to parasites. We still have some way to go to provide the precise requirements of the immune system and how best to supply them. THE IMPORTANCE OF EWE NUTRITION The major aim of this lecture has been to reemphasise the need, as in Dr McLymont s day, to carry out nutritional studies in the environment of the production animal because of the potential

6 environmental impact of environment on nutrient demand. However, in summarising I would argue, in particular, the need for a still-better understanding of the nutritional requirement of the breeding ewe since, in an epidemiological sense, it is her failure to maintain immunity in the peripartum period which provides the all-important new seasonal wave of larval contamination. Its timing ensures exposure of the newly weaned and susceptible lamb, and perpetuates the need for anthelmintics. This may well be critical in systems in which protein supply is reduced when lambs are weaned early to reduce competition for pasture with the ewe. To quote one of Prof. McClymont s parasitologist contemporaries H.McL.Gordon parents perpetuate parasites. LITERATURE CITED Abbott, E. M., J. J. Parkins and P. H. Holmes. 1985a. Influence of dietary protein on parasite establishment and pathogenesis in Finn Dorset and Scottish Blackface lambs given a single moderate infection of Haemonchus contortus. Res. Vet. Sci. 38:6-13. Abbott, E. M., J. J. Parkins and P. H. Holmes. 1985b. Influence of dietary protein on the pathophysiology of ovine haemonchosis in Finn Dorset and Scottish Blackface lambs given a single moderate infection. Res. Vet. Sci. 38: Abbott, E. M., J. J. Parkins and P. H. Holmes. 1986a. The effect of dietary protein on the pathogenesis of acute ovine haemonchosis. Vet. Parasitol. 20: Abbott, E. M., J. J. Parkins and P. H. Holmes. 1986b. The effect of dietary protein on the pathophysiology of acute ovine haemonchosis. Vet. Parasitol. 20: AFRC, Energy and protein requirements of ruminants. CAB International, Oxford. Bang, K. S., A. S. Familton and A. R. Sykes Effect of ostertagiasis on copper status; a study involving the use of copper oxide wire particles. Res. Vet. Sci. 49: Barry, T. N. and T. R. Manley The condensed tannin in the nutritional value of Lotus pedunculatus 2. Quantitative digestion of carbohydrates and protein. Brit. J. Nutr. 51: Beilharz, R. G The problem of genetic improvement when environments are limiting. Proc. 6 th World Congress on Genetics Applied to Livest. Prod. Armidale, Australia. Bishop, S. C. and M. J. Stear Modelling responses to selection for resistance to gastrointestinal parasites in sheep. Anim. Sci. 64: Blaxter, K. L Energy metabolism of ruminants. Hutchinson, London. Bown, M. D., D. P. Poppi and A. R. Sykes The effect of post-ruminal infusion of protein or energy in the pathophysiology of Trichostrongylus colubriformis infection and body composition in lambs. Aust. J. Agric.Res. 2: Brunsdon, R. V Seasonal changes in the level and composition of nematode burdens in young sheep. N.Z. J. Agric. Res. 2: Calder, P. C Fuel utilisation by cells of the immune system. Proc. Nut. Soc. 54: Coop, R. L. and A. C. Field Effect of phosphorus intake on growth rate, food intake, and quality of the skeleton of growing lambs infected with the intestinal nematode Trichostrongylus vitrinus. Res. Vet. Sci. 35: Coop, R. L. and I. Kyriazakis Nutrition-parasite interaction. Vet. Parasitol. 84: Coop, R. L. and P. H. Holmes Nutrition and parasite interaction. Int. J. for Parasitol. 26: Coop, R. L., I. Kyriazakis, J. F. Huntley, E. Jackson and F. Jackson The influence of protein and amino acid on the resilience of sheep to intestinal parasitism. Proc. 4 th Int. Cong. For Sheep Vet. Armidale, NSW, Australia. pp Coop, R. L., J. F. Huntley and W. D. Smith Effect of dietary protein supplementation on the development of immunity to Ostertagia circumcincta in growing lambs. Res. Vet. Sci. 59: Dineen, J. K. and B. M. Wagland The dynamics of the host-parasite relationship IV. The response of sheep to graded and to repeated infection with Haemonchus contortus. Parasitol. 56: Dobson, C. and R. J. Bawden Studies of the immunity of sheep to Oesophagostomum columbianum: effects of low-protein diet on resistance to infection and cellular reactions in the gut. Parasitol. 69: Dobson, R. J., P. J. Waller and A. D. Donald Population dynamics of Trichostrongylus colubriformis in sheep: the effect of infection rate on establishment of infective larvae and parasite fecundity. Int. J. for Parasitol. 20: Donald, A. D. and P. J. Waller Problems and prospects in the control of helminthians in sheep in Biology and Control of Endoparasites (Eds. L.E.A. Symons, A. D. Donald, J. K.), Dineen Academic Press, Sydney. Donaldson, J. D., M. F. J. van Houtert and A. R. Sykes The effect of nutrition in the peri-parturient parasite status of mature ewes. Anim. Sci. 67: Donaldson, J. D., M. F. J. van Houtert and A. R. Sykes The effect of dietary fishmeal supplementation on parasite burdens of peri-parturient sheep. Anim. Sci. (in press). Douch, P. G. C., G. B. L. Harrison, D. C. Elliot., L. L. Buchanan and K. S. Greer Relationship of gastrointestinal histology and mucus anti-parasite activity with the development of resistance to trichostrongylus infections in sheep. Vet. Parasitol. 20: Geenty, K. G. and P. V. Rattray The energy requirements of grazing sheep. In: Livest. Feeding on Pasture (Ed. A.M. Nicol), Publication 10, N.Z. Soc. Anim. Prod. Gibson, T. E. and J. W. Parfitt The effect of age on the development of resistance to Trichostrongylus colubriformis. Res. Vet. Sci. 18: Gray, G..D Breeding for resistance to trichostrongyle nematodes in sheep. In: Breeding for disease resistance in farm animals (Eds. J.B. Owen and R.F.E. Axford), CAB International, Oxford. Gunn, K. J. and J. H. Ternouth The effect of phosphorus deficiency upon ruminal microbial activity and nitrogen balance in lambs. Proc. Aust. Soc. Anim. Prod. 20:445. Gustafsson, L. et al Infections disease, reproductive effort and the cost of reproduction in birds. Philosophical Transactions of the Royal Society, London. Series B. 346: Huntley, J. F., M. Patterson, A. MacKellar, F. Jackson, L. M. Stevenson and R. L. Coop A companion of the mast cell and eosinophil responses of sheep and goats to gastrointestinal nematode infections. Res. Vet. Sci. 58:5-10. Keymer, A. E. and A.. Read Behavioural ecology: the impact of parasitism. In: Host-Parasite Associations:

7 Coexistence or Conflict? (Eds. C.A. Toft, A. Eeschlimann and A. Bolis), Oxford University Press. Knox, M. and J. W. Steel Nutritional enhancement of parasite control in small ruminant production systems in developing countries of South East Asia and the Pacific. Int. J. Parasitol. 26: Kyriazakis, I., D. H. Anderson, J. D. Oldham, R. L. Coop and F. Jackson Long-term subclinical infection with Trichostrongylus colubriformis: effects on food intake, diet selection and performance of growing lambs. Vet. Parasitol. 61: Kyriazakis, I., J. D. Oldham, R. L. Coop and F. Jackson The effect of subclinical intestinal nematode infection on the diet selection of growing sheep. Br. J. Nut. 72: Leyva, V., A. E. Henderson and A. R. Sykes Effect of daily infection with Ostertagia circumcincta larvae on food intake, milk production and wool growth in sheep. J. Agric. Sci. Cambridge. 99: Lowther, W. L. and T. N. Barry Nutritional value of Grasslands Maku lotus grown on low fertility soils. Proc. N.Z. Soc. Anim. Prod. 45: MacRae, J. C Metabolic consequences of intestinal parasitism. Proc. Nut. Soc. 52: McEwan, J. C., K. G. Dodds, G. J. Greer, W.E. Bain, S.J. Duncan, R. Wheeler, K. J. Knowler, P. J. Reid, R.S. Green and P.G.C. Douch Genetic estimates for parasite resistance traits in sheep and their correlations with production traits. N.Z. J. Zool. 22:177. McLure, S. J., T. J. McLure and D. L. Emergy Effects of molybdenum on primary infection and subsequent challenge by the nematode parasite Trichostrongylus colubriformis in weaned Merino lambs. Res. Vet. Sci. 67: McNabb, W. C., G. C. Waghorn, T. N. Barry and I. D. Shelton The effect of condensed tannins in Lotus pedunculatus on the digestion and metabolism of methionine cysteine and inorganic sulphur in sheep. Brit. J. Nutr. 70: Michel, J. F The phenomenon of host resistance and the course of infection of Ostertagia ostertagi in calves. Parasitol. 53: Morris, C. A Responses to selection for disease resistance in sheep and cattle in New Zealand and Australia. Proc. 6 th World Congr. on Gen. Appl. to Livest. Prod. 27: Morris, C. A., S. A. Bissett, A. Vlassoff, R. L. Baker, T. G. Watson, D. M. Leathwick and M. Wheeler Correlated responses in fleece weight to selection for divergence in faecal nematode egg count in New Zealand Romneys and Perendales. Proc. N.Z. Soc. Anim. Prod. 57: Morris, C. A., S. A. Bissett, A. Vlassoff, C. J. West and M. Wheeler Faecal nematode egg counts in lactating ewes from Romney flocks selectively bred for divergence in lamb faecal egg count. Anim. Sci. 67: Neizen, J. H., T. S. Waghorn, W. A. G. Charleston and G. C. Waghorn Growth and gastro intestinal nematode parasitism in lambs grazing lucerne (Medicago sativa) or Sulla (Hedysarum coronarium) which contains condensed tannins. J. Agric. Sci., Camb. 125: Neizen, J. H., W. A. G. Charleston, J. Hodgson and T. S. Waghorn Effect of four grass species on lamb parasitism and growth. Proc. N.Z. Soc. Anim. Prod. 55: Neizen, J. H., W. A. G. Charleston, J. Hodgson, A. D. Mackay and D. M. Leathwick Controlling internal parasites in grazing ruminants without resource to anthelmintics: approaches, experiences and prospects. Int. J. Parasitol. 26: Norris, K., M. Anwar and A. F. Read Reproductive effort influences the prevalence of haematazoan parasites in great tits. J. Anim. Ecol. 63: O Sullivan, B. M. and A. D. Donald A field study of nematode parasite populations in the lactating ewe. Parasitol. 61: O Sullivan, B. M. and A. D. Donald Responses to infection with Haemonchus contortus and Trichostrongylus colubriformis in ewes of different reproductive status. Int. J. Parasitol. 3: Ortolani, E., D. Knox, F. Jackson, R. L. Coop and N. F. Suttle Abomasal parasitism lowers liver copper status and influences the Cu x Mo x S antagonism in lambs. In Trace Elements in Man and Animals eds. M. Anke, D. Meissner and C.F. Mills. Veilag Media Touristik. Owens, I. P. F. and K. Wilson Immunocompetence: a neglected life history trait or conspicuous red herring? Trends in Ecology and Evolution. 14: Rauw, W. M., E. Kanis, E. N. Noordhuizen-Stassen and F. J. Grommers Undesirable side effects of selection for high production efficiency in farm animals: a review Livestock Prod. Sci. 56: Reveron, A. E., J. H. Topps, D. C. McDonald and G. Pratt The intake digestion and utilisation of food and growth rate of lambs affected by Trichostrongylus colubriformis. Res. Vet. Sci. 16: Robinson, J. J The pastoral industries in the 21 st Century. Proc. N.Z. Soc. Anim. Prod. 50: Salisbury, J. R. and J. H. Arundel The relationship between lactation and post-parturient rise in faecal nematode egg counts in ewes. Aust. Vet. J. 46: Sheldon, B. C. and S. Verhulst Ecological immunity: costly parasite defences and trade-offs in evolutionary ecology. Trends in Evolution and Ecology. 11: Steel, J. W Pathophysiology of gastrointestinal nematode infections in the ruminant. Proc. Aust. Soc. Anim. Prod. 10: Suttle, N. F Seasonal infections and nutritional status. Proc. Nut. Soc. 53: Suttle, N. F., D. Knox, F. Jackson, R. L. Coop and K. W. Angus. 1992a. Effects of dietary molybdenum on nematode and host during Trichostrongylus vitrinus infection in lambs. Res. in Vet. Sci. 52: Suttle, N. F., D. Knox, K. W. Angus, F. Jackson and R. L. Coop. 1992b. Effects of dietary molybdenum on nematode and host during Haemonchus contortus infection in lambs. Res. in Vet. Sci. 52: Suttle, N. F., J. Brebner, K. McLean and F. U. Hoeggel Failure of mineral supplementation to avert apparent sodium deficiency in lambs. Anim. Sci. 63: Sykes, A. R Nutritional and physiological effects of helminthiasis in sheep. In: LEA (Eds. Symons, A.D. Donald and J.K. Dineen) Biology and Control of Parasites. Academic Press. Sydney pp Sykes, A. R Parasitism and production in farm animals. Anim. Prod. 59: Sykes, A. R. and R. L. Coop Intake and utilisation of food by growing lambs with parasitic damage to the small intestine caused by daily dosing with Trichostrongylus colubriformis larvae. J. Agric. Sci., Camb. 86: Sykes, A. R. and R. L. Coop Intake and utilisation of food by growing sheep with abomasal damage caused by daily dosing with Ostertagia circumcincta larvae. J. Agric. Sci., Camb. 88: Sykes, A. R., R. L. Coop and K. W. Angus Experimental production of osteoporosis in growing

8 lambs by continuous dosing with Trichostrongylus colubriformis larvae. J. Comp. Path. 85: Sykes, A. R., R. L. Coop, and K. W. Angus The influence of chronic Ostertagia circumcincta infection on the skeleton of growing sheep. J. Comp. Path. 87: Symons, L. E. A. and W. O. Jones Skeletal muscle, liver and wool protein synthesis by sheep infected by the nematode Trichostrongylus colubriformis. Aust. J. Agric. Res. 26: Symons, L. E. A. and W. O. Jones Intestinal protein synthesis in guinea pigs infected with Trichostrongylus colubriformis. Int. J. Parasitol. 13: Takahashi, K., N. Ohta and Y. Akiba Influences of dietary methionine and cysteine on metabolic responses to immunological stress by Escherichia coli lipopolyssacharide injection and mitogenic response in broiler chickens. Br. J. Nut. 78: Van Houtert, M. F. J. and A. R. Sykes Implications of nutrition for the ability of ruminants to withstand nematode infections. Int. J. Parasitol. 26: Van Houtert, M. F. J., I. A. Barger, J. W. Steel, R. G. Windon and D. L. Emery. 1995a. Effects of dietary protein intake on responses of young sheep to infection with Trichostrongylus colubriformis. Vet. Parasitol. 56: Van Houtert, M. F. J., I. A. Barger and J. W. Steel. 1995b. Dietary protein for young grazing sheep: interactions with gastrointestinal parasitism. Vet. Parasitol. 60: Waghorn, G. C., I. D. Shelton, W. C. McNabb and S. N. McCutcheon Effects of condensed tannins in Lotus pedunculatus on its nutritive value for sheep. 2 Nitrogenous aspects. J. Agric. Sci. Camb. 123: Wakelin, D Immunity to Parasites: How parasitic infections are controlled, 2 nd ed. Cambridge University Press, Cambridge. Wallace, D. S., K. Bairden, J. L. Duncan, G. Fishwick, G. Gill, P. H. Holmes, Q. A. McKellar, M. Murray, J. J. Parkins and M. J. Stear The influence of dietary soya bean meal supplementation on resistance to Haemonchosis in Hampshire Down lambs. Res. Vet. Sci. 58: Williamson, J. F., H. T. Blair, D. J. Garrick, W. E. Pomroy, P. G. C. Douch and R. S. Green Parasitological characteristics of fleece-weight selected and control sheep. N.Z. J. Agric. Res. 38: Wilson, W. D. and A. C. Field Absorption and secretion of calcium and phosphorus in the alimentary tract of lambs infected with daily doses of Trichostrongylus colubriformis or Ostertagia circumcincta larvae. J. Comp. Path. 93: Yu, F., L. A. Bruce, A. G., Calder, E. Milne, R. L. Coop, F. Jackson and J. C. MacRae Leucine and protein metabolism across the gastrointestinal tract of sheep infected with Trichostrongylus colubriformis. Amer. J. Anim. Phys. Yu, F., L. A. Bruce, R. L. Coop and J. C. MacRae Leucine metabolism across the gastrointestinal tract of sheep infected with Trichostrongylus colubriformis. Proc. Brit. Soc. Anim. Sci Ratio of ME:MP (g/mj) Livew eight (kg) Pregnancy (w k) Lactation (w k) Fig. 1. The ratio of requirement of metabolisable protein (MP): metabolisable energy (ME) in growing pregnant and lactating sheep from AFRC (1993). Superimposed is the calculated ratio of supply of MP and ME from fresh pasture and, in vertical bars the ratio of supply in experiments in which protein supplements have significantly improved the immune response of lambs (Bown et al, 1991) and sheep in the peri-parturient period (Donaldson et al, 1992, 2000) to nematode larval intake.