Effect of level of feed intake on the development of gastrointestinal parasitism in growing lambs

Veterinary Parasitology 104 (2002) 327–338

Effect of level of feed intake on the development of gastrointestinal parasitism in growing lambs J. Valderrábano∗ , R. Delfa, J. Uriarte Servicio de Investigación Agroalimentaria, Diputación General de Aragón, Apartado 727, 50080 Zaragoza, Spain Received 22 June 2001; received in revised form 18 October 2001; accepted 30 October 2001

Abstract A study was conducted to examine the role of nutrition in the development of gastrointestinal (GI) parasitism, performance and pathophysiology of parasitism in female lambs. Forty-four months old ewe lambs received for 6 weeks 0, 1500 or 7000 L3 larvae of Teladorsagia circumcincta in two doses per week. The animals were given access to a pelleted diet to meet energy requirements for gaining weight close to their potential (H) or approximately 50% of it (L). The level of protein in the diet was 20–30% above requirements for both planes of nutrition. Measurements of body weight, serum pepsinogen concentration and faecal egg counts were taken at weekly intervals. All animals were slaughtered 2 weeks after the last larval dosing for estimation of worm count, carcass composition and reproductive tract development. Voluntary intake of infected animals decreased by approximately 10%. Both nutrition and infection influenced lamb performance and carcass composition. However, no direct effects on reproductive tract parameters due to the presence of infection were observed. The level of L3 infection showed a significant effect not only on faecal egg counts and on worm burden, as it could be expected, but also on the proportion of females to males, which increased significantly with the level of infection. While the plane of nutrition did not show a significant effect on faecal egg counts or on worm burden, surprisingly, both female worm size and their fecundity decreased significantly with the level of nutrition. This response was accompanied by a significant increase in the concentration of circulating eosinophils suggesting that the immune response of lambs consuming high levels of energy was enhanced compared to that of lambs kept on a restricted diet. Results herein show that in young female lambs fed on adequate levels of protein, an improvement in energy supply does not only improve carcass characteristics but clearly enhances the development

∗ Corresponding author. Tel.: +34-976-716441; fax: +34-976-716335. E-mail address: [email protected] (J. Valderr´abano).

0304-4017/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 0 1 7 ( 0 1 ) 0 0 6 3 8 - 0

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of resistance to GI nematodes infection. This may have decisive management implications for the control of parasitic infections in sustainable production systems. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Teladorsagia circumcincta; Sheep–nematoda; Energy restriction

1. Introduction Gastrointestinal (GI) nematode infection is one of the main limiting factors for developing sheep production systems based on pasture (Uriarte and Valderrábano, 1990). GI parasites may decrease voluntary feed intake and the efficiency of nutrients utilisation (Coop et al., 1977; Dakkak, 1990) and hence on lamb performance. Previous results (Llorente, 1999) have shown that parasites may significantly reduce ewe lamb body weight and may have important effects on their future reproductive life. Infection in sheep is predominantly regulated by acquired immunity (Adams, 1989) which controls the impact of GI nematode parasitism on life-time productivity of grazing animals (Van Houtert and Sykes, 1996). The nutritional status of the host has long been considered to be an important factor influencing the host–parasite relationship and the pathogenesis of parasitic infections. However, the mechanisms underlying this fact are far from clear. The interactions between the nutritional status of the host and pathophysiology induced by parasitism have recently been reviewed by Coop and Kyriazakis (1999) and it offers interesting possibilities as a means of reducing anthelmintic usage for the control of nematode infections in sustainable production systems. Most of the experimental work concerning nutrients/expression of immunity has been focused on the effect of protein nutrition on the resilience (Albers et al., 1987) and resistance of the host to parasitic infection. As most of the effector mechanisms of the immune system are proteinaceous in nature, it is possible that infection would increase protein demand for intestinal tissue and plasma synthesis (Bown et al., 1991). In this respect, information available suggests that protein supply may improve resilience, providing the animals are fed an adequate plane of nutrition, but it does not appear to enhance the development of resistance (Jackson, 2000). While much attention has been devoted to the effect of protein supplementation on immune responses, little information is available on the effect of energy supplementation on host resistance to parasites. While a positive effect of energy supplementation on faecal egg excretion has been recorded in some instances (Ferre et al., 1995; Garcia-Perez et al., 1994), no response was observed in others (Donaldson et al., 1998). More recently, Koski et al. (1999) have provided evidence that even mild energy restricted diets, without concurrent protein malnutrition, can modulate protective immunity in mice from activation early during a primary infection to expression of acquired immunity during reinfection in both systemic and gut associated lymphoid tissues. Whether such an effect would occur in sheep may have important management implications for the control of parasitic infections in sustainable production systems. The aim of this study was to examine the development of GI parasitism, performance and pathophysiology of parasitism in ewe lambs fed at two planes of nutrition.

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2. Material and methods 2.1. Experimental animals and conditions Forty Rasa Aragonesa female lambs (4 months old and 22.7 ± 0.465 kg BW at the beginning of the experiment) that have been raised indoors were used in this trial. Throughout the trial (10 weeks) sheep were held indoors on individual, slatted floor cages with external feed and water troughs. A pelleted diet (see composition and mean analysis in Table 1) was offered in amounts described in the experimental design. The diets were formulated to meet energy requirements for the female lambs (AFRC, 1993) allowing them to express Growth Rates either close to their potential or approximately 50% of it which compares with those usually found under grazing conditions. The level of protein in the diet was above requirements, not being a limiting factor for any of the experimental treatments. All lambs had free access to mineral blocks throughout the experiment. At the start of the trial all animals were treated with an anthelmintic (Fenbendazole 5 mg/kg BW). 2.2. Experimental design All lambs were allowed ad libitum feed intakes for a preliminary period of 5 days to determine voluntary intake of each lamb. Following this period, lambs were stratified on the basis of body weight (BW) and allocated randomly in six groups, two of six and four of seven lambs each. These were fed either ad libitum (H) (n = 20) or 0.6 × ad libitum (L) (n = 20). Lambs in each plane of nutrition received by mouth 0 (n = 12), 1500 (n = 14) or 7000 (n = 14) L3 of Teladorsagia circumcincta per week. Infective larvae were suspended in 5 ml water and given in two doses per week from week 2 to 8 of the experiment.

Table 1 Composition (g/kg) and mean analysis of experimental diet (g kg DM−1 ) Component Maize Barley Wheat Soya bean Sugar cane molasses Beet pulp Fat Minerals Dry matter Crude protein Ash Mj ME

221 348 100 223 40 30 10 28 900.5 175.5 82.8 13.25

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2.3. Measurements The lambs were weighed once a week before being offered the daily ration, and immediately before being slaughtered. Individual lamb GRs were calculated from regressions of Body Weight upon time. Refused feed was weighed twice a week and from this daily feed intake was calculated. The average intake of the uninfected animals in nutritional group H were used to calculate the amount of pellets to be offered to lambs in group L the following week. Worm egg counts on faecal samples (epg), collected directly from the rectum, were performed weekly using 3 g of faeces according to the McMaster method modified by Raynaud (1970). Serum pepsinogen levels were determined according to the technique of Kerboeuf (1979) and results were expressed as the number of milliunits of tyrosine. Two weeks after the last larval dosing, blood samples were taken and all animals were slaughtered. Differential leucocyte counts were performed using a quick stain technique (Diff-Quick® , American Scientific Products). The carcass conformation and fat cover of the resulting carcass were assessed (European Commission, 1992). The shoulder dissection (Fisher and de Boer, 1994) into fat muscle and bone were used to estimate carcass composition. Reproductive organs were removed from carcasses and their dimensions and weights recorded. Necropsy and worm count procedures were carried out according to the method described by the Ministry of Agriculture, Fisheries and Food (MAFF, 1971) with minor modifications. Briefly, the abomasum was separated, opened and washed in warm water. Subsequent washings were brought up to 2 l and two aliquots of 10% each taken. Worms recovered from one of the aliquots were distinguished as adult male and female nematodes and immature stages. The length of the first 50 adult female recovered from the second aliquot were measured by image analysis (Foster-Findlay PC-Image). Female worms fecundity was calculated as the last epg before slaughter divided by the number of adult female worms counted (Kahn et al., 2000). 2.4. Statistics Statistical analysis followed the general linear models procedure (SAS, 1998) to assess the significance of plane of nutrition (H and L), level of infection (0, 1500 and 7000 L3 ) and their interactions (Ntr × Inf) for variables not subjected to repeated measurements such as mean intake, GR and measurements taken on the carcass. The comparison between means groups was determined by Duncan’s multiple range test. When no significant effects were observed between 1500 and 7000 L3 treatments, they were pooled and the analysis performed for two groups (uninfected control and infected). The epg and plasma constituents were analysed using a repeated measures analysis of variance. For the purpose of normalising the data, a logarithmic transformation was performed. This transformation was carried out with all the variables where the quotient variance/mean was above 1, and therefore they are near a binomial negative distribution (Smith and Guerrero, 1993). The transformation log(n + 1) was used for variables which presented some values of 0. The transformed data were used for statistical analysis, however, results are reported as the actual mean values.

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Mean number of adult male and female worms recovered from the abomasum of lambs infected with 1500 and 7000 L3 were compared by X2 test.

3. Results 3.1. Feed intake and lambs performance No significant effects on lambs performance were found between 1500 and 7000 L3 level of infection. Thus, the analyses were done on non-infected control versus infected lambs. Differences between both ad libitum (1070 g per day) and restricted (616 g per day) planes of nutrition established, had significant consequences (P < 0.001) on lambs GR, carcass internal length (L), cold carcass weight (CCW) and internal fat (Table 2). Carcass conformation and fatness were also significantly affected (P < 0.001) by level of nutrition. The daily requirements for a 30 kg ewe lamb growing at 250 g per day given a complete diet are 13.1 megajoules of metabolisable energy and 104 g metabolisable protein (MP) (AFRC, 1993). A daily allowance of either 1070 or 616 g dry matter per day supplied adequate energy to cover GR requirements. The amount of protein supplied was 135 and 81 g MP for treatments H and L, respectively, which represents between 20 and 30% more than the apparent requirement of MP for 230 and 110 g daily GR. Presence of infection showed a significant decrease (P < 0.05) in lambs voluntary intake. A similar effect was observed for all parameters studied though significant differences (P < 0.05) between control and infected lambs were only found for CCW and L measurements (Table 2). These effects were, however, more evident in animals fed ad libitum than on restricted treatments. No significant interactions were observed between plane of nutrition and level of infection for any of the parameters studied. Analysis of thoracic limb dissection was in agreement with previous results showing a significant effect of plane of nutrition on thoracic limb joint, muscle, bone, subcutaneous, intermuscular and total fat weighs (Table 3). Table 2 Effect of level of nutrition on lambs performance Infection

Intake (g per day) GR (g per day) CCW (kg) Internal fat (g) Carcass length (cm) Conformation Fat cover a

Nutrition level: H

Nutrition level: L

Control

Control

1070 231.9 20.1 1647 62.2 7.3 9.5

Residual standard deviation. NS: non significant. ∗ P < 0.05. ∗∗∗ P < 0.001.

Infected 960 186.4 18.3 1327 60.4 6.3 8.4

616 95.4 15.3 798 58.3 5.3 6.5

RSDa

Infected 614 104.9 14.7 751 57.6 5.6 6.1

68.5 31.81 1.68 308.7 1.70 1.04 2.01

Effects Ntr

Inf

∗∗∗

∗

∗∗∗

NS

∗∗∗ ∗∗∗ ∗∗∗ ∗∗∗ ∗∗∗

∗

NS ∗

NS NS

Ntr × Inf NS NS NS NS NS NS NS

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Table 3 Weight (g) of thoracic limb components Infection

Thoracic limb Muscle Bone Subcutaneous fat Intermuscular fat Total fat

Nutrition level: H

Nutrition level: L

Control

Infected

Control

Infected

1104.0 692.5 251.2 76.2 63.2 139.3

1009.1 636.3 232.6 58.4 65.8 124.1

945.0 590.2 217.7 53.7 53.3 107.0

919.6 585.8 218.0 42.1 47.4 89.6

RSDa

Effects Ntr

76.71 48.15 19.46 20.90 14.67 29.43

∗∗∗ ∗∗∗ ∗∗∗ ∗∗∗ ∗∗ ∗∗

Inf ∗

0.07 NS ∗

NS NS

Ntr × Inf NS NS NS NS NS NS

a

Residual standard deviation. NS: non significant. ∗ P < 0.05. ∗∗ P < 0.01. ∗∗∗ P < 0.001.

Presence of infection showed a significant effect (P < 0.05) on thoracic limb weight and subcutaneous fat and to a lesser extent (P < 0.07) on muscle weight. However, no significant effect of infection was observed on bone and intermuscular fat. 3.2. Reproductive tract Reproductive tract parameters were not significantly affected either by level of nutrition or level of infection (Table 4), exception being its total length (P < 0.001) which appears to be associated with carcass internal length. Interactions between plane of nutrition and level of infection were not significantly affected by any of the parameters studied. 3.3. Plasma constituents While the level of pepsinogen in blood samples was not significantly affected by dietary treatment, it showed an abrupt increase 2 weeks post infection in infected animals, reaching a maximum value of 463 and 869 Um Thyr for lambs dosed with 1500 and 7000 Table 4 Reproductive tract parameters

Weight (g) Length (mm) Width at the horn (mm) Width at the neck (mm) Width at the vagina (mm) Ovary weight (g) Ovary length (mm) a

Residual standard deviation. NS: non significant. ∗∗∗ P < 0.001.

Nutrition level: H

Nutrition level: L

RSDa

Significant

28.4 185 26.9 11.0 28.4 0.53 12.0

25.5 163 25.8 10.5 28.1 0.46 11.6

6.52 16.58 4.14 1.37 3.99 0.189 1.49

NS ∗∗∗

NS NS NS NS NS

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Fig. 1. Pepsinogen concentration for non-infected (1) or infected with 1500 (2) and 7000 L3 per week (3).

L3 , respectively, in week 5–6 after infection. The mean pepsinogen concentration in serum collected from lambs in the 7000 L3 treatment group was always higher than that from lambs in 1500 L3 treatment. Mean serum pepsinogen concentration of both groups of animals was declining during the last 2 weeks after cessation of L3 administration. Pepsinogen concentration was significantly affected by both level of infection and time of sampling. The evolution of pepsinogen concentration with time was significantly different between treatments (Fig. 1), as it was evidenced by the interaction (P < 0.001) between both factors. No significant effect of nutrition or infection levels were observed on the number of leucocytes recorded in blood samples that showed a mean value of 9410 ± 473 cells/mm3 . While the concentration of circulating eosinophil (CEO) in blood samples collected immediately before slaughter from infected lambs L was not statistically different from the uninfected ones, it was significantly lower (P < 0.05) than that from lambs H (Table 5). Table 5 Effect of nutrition and infection levels on worm burden, females length and female worms fecundity Infection level

Total burden Females Number of eosinophil Females/males Female length Number of eggs per female a

Nutrition level: H

Nutrition level: L

1500 L3

1500 L3

1611.4 894.3 247 1.24 12.20 0.274

Residual standard deviation. NS: non significant. ∗ P < 0.05. ∗∗∗ P < 0.001.

7000 L3 6682.8 3917.1 81 1.41 10.89 0.276

1985.7 1105.7 40 1.25 12.55 0.593

RSDa

7000 L3 4225.7 2485.7 45 1.42 11.47 0.506

Effects Ntr

– – – – 0.862 0.1844

NS NS ∗

Inf

Ntr × Inf

∗∗∗

∗

∗∗∗

∗

NS

NS

NS

∗∗∗

∗∗∗

∗∗∗

∗∗∗

NS

NS NS

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Fig. 2. Mean faecal egg counts (epg) of ewe lambs fed ad libitum (H) or 0.6 ad lib (L) and parasitised with 1500 or 7000 L3 per week of T. circumcincta.

This was mainly due to the concentration of CEOs in H-1500 lambs (2.3%) that was about six times higher than that found in lambs kept on restricted diets. 3.4. Faecal egg counts The analysis of faecal egg counts (epg) as repeated measurements show a significant effect of level of infection on epg (Fig. 2). Animals fed on a restricted plane of nutrition always showed higher mean faecal egg counts than those fed ad libitum, but the analysis did not show significant differences due to nutritional level. No significant interaction between level of infection and sampling time was observed, showing that the evolution of epg in time was similar for both planes of nutrition considered. 3.5. Worm burden The plane of nutrition did not show any significant effect on worm burden (Table 5) which, however, increased significantly (P < 0.001) with level of L3 infection. A significant interaction (P < 0.05) between level of infection × plane of nutrition on worm burden was detected showing that the effect of level of infection was more pronounced at low than at high plane of nutrition. The number of T. circumcincta female worms found in the abomasum followed a similar trend to that observed for the total worm burden. The number of immature stage worms recovered was negligible. In one animal from L7000 and another from H1500 treatments less than 20 immature worms were found. Analysis of the total number of mature parasites (males and females) revealed that while the number of female worms was not affected by the level of nutrition, its number increased significantly (P < 0.001) with the level of L3 infection. The proportion of females to males ranged from 1.24 to 1.42 for 1500 and 7000 L3 treatments, respectively.

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The length of T. circumcincta female worms recovered from the abomasum declined significantly (P < 0.001) with both nutrition and infection levels. No significant interaction was observed between these two factors. Analysis of female worms fecundity showed that the number of eggs produced/female was significantly lower (P < 0.001) at high than at low plane of nutrition, while the effect due to the level of infection was not significant. No significant interaction was either observed between levels of nutrition and L3 infection.

4. Discussion 4.1. Lambs performance The plane of nutrition showed a significant effect on lamb performance, as expected. Infected lambs showed lower values than non-infected ones for all parameters studied; DM intake, CCW and carcass length were the most affected (P < 0.05) parameters by the presence of infection. These effects were, however, more evident for animals fed ad libitum than for those fed on a restricted diet, suggesting that the effect of GI nematodes on lambs performance was primarily due to its effects on voluntary intake which decreased more than 10%. Voluntary feed intake depression has been generally recognised as a major feature of the pathogenesis of GI infections. Coop et al. (1977) reported that DM intake of young lambs infected with T. circumcincta was lowered by 20% and the BW gain was reduced with 16%. The anorexic response has been considered as a deleterious side effect of infection; however, recent reports (Exton, 1997) suggests that infection induced anorexia is a host behavioural strategy that organisms have evolved and that modifies numerous metabolic and immunological processes contributing to an optimal homeostatic environment for pathogen elimination. Both nutrition and infection influenced the weights of tissue recorded in the limb joint dissection of lambs. Differences between control and infected lambs were more evident on the thoracic limb joint and subcutaneous fat (P < 0.05) and to a lesser extent on muscle (P < 0.07) weights. Abbot et al. (1988) observed similar effects on carcass composition through dissection of the rib joint of lambs infected with Haemonchus contortus that were kept on two different protein diets. Also, Coop et al. (1985) found a loss in protein deposition and minerals in the whole carcass of grazing lambs exposed to moderate intake of T. circumcincta larvae. No significant effects on reproductive tract parameters studied due to presence of infection were observed. However, mean values of all parameters studied in ewe lambs fed ad libitum were higher than those of animals on a restricted diet, although only the reproductive tract length was significantly affected (P < 0.001) by the level of nutrition, which appears to be associated with carcass length. The indirect effect of GI parasites on BW thus, on the development of the reproductive tract may explain the delay on the onset of puberty in parasitised young females observed by Llorente (1999). Likewise Jeffcoate et al. (1988) reported that Ostertagia challenge of ewes tended to depress ovulation rates. However, further studies with heavier infections over longer periods will be necessary to elucidate this point.

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The serum pepsinogen concentration increased with time in both groups of infected animals showing an increase in the abomasal mucosa permeability. While the level of pepsinogen in blood samples collected from lambs dosed with 1500 L3 per week remained within the range of normality (Kerboeuf, 1979), much higher values were found in lambs infected with 7000 L3 per week indicating the damage caused to the abomasal mucosa. The decline in pepsinogen concentration after cessation of infective larvae administration suggests that the abomasal integrity was being restored. 4.2. Effect of nutrition on parasite population dynamics The plane of nutrition did not show any significant effect on worm burden which, however, increased significantly with the level of L3 infection. The interaction observed (P < 0.05) between nutrition and infection factors suggests that high levels of nutrition were more effective in controlling the number of worms established at low than at high level of infection. The concentration of CEOs, considered as an immunological effector mechanism, supports this fact as the number of these cells in H1500 lambs amounted to three times that of H7000 lambs. A number of the conflicting results of the interaction between nutrition and parasitism have been reviewed by Van Houtert and Sykes (1996) who arrived at the conclusion that in young sheep, nematode establishment was not affected appreciably by diet. The size of female worms recovered from the abomasum of lambs decreased significantly (P < 0.001) with both nutrition and infection levels. This effect may arise as a result of intraspecific competition for food or space and the generation of immune responses (Keymer, 1982). This density-dependence response may also explain the significant increase in the number of females found at high levels of infection where males, that are smaller than females (Denham, 1969), could be also less fitted to compete than females. Worms becoming stunted as adults and a reduction in fecundity of female worms have been assumed as an early manifestation of immunity (Dineen and Windon, 1980; Van Houtert and Sykes, 1996). Surprisingly, these responses were enhanced in lambs fed ad libitum compared to that of those kept on a restricted diet. The different immune response observed between both treatments seems to be due to the energy restriction of lambs L compared to that of lambs H, as the amount of MP protein supplied to both treatments was between 20 and 30% in excess their requirements. These results parallel recent work of Koski et al. (1999) who found that energy restricted diets, without concurrent protein malnutrition, depressed both the acquisition and expression of immunity in mice infected with Heligmosomoides polygyrus. The difference in faecal egg count between both planes of nutrition was not statistically significant though lambs fed on a restricted diet always showed higher epg than those fed ad libitum. The reduction in fecundity of female worms combined with the increase in the number of females associated to worm burden may explain the lack of a significant effect of the level of nutrition on faecal egg counts. The nutritional status of the host has long been considered to be an important factor influencing the host–parasite relationship and the impact of GI nematode infections. Thymus derived (T) lymphocytes are believed to play an important role in the regulation of immune response to GI nematodes. Th2 cells, characterised by increased levels of Il4, Il5 and Il10 cytokines and their effector responses, appear to be involved in resistance to GI nematodes

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while Th1 phenotype are mainly required for resolution of intracellular parasites (Finkelman et al., 1997). In this respect, results of Koski et al. (1999) indicate that energy deficits lowered both Th1 and Th2 cytokines (Il4 and Il5) and their effector responses (IgE, IG1 and eosinophiles) while protein malnourished hosts required more severe dietary restrictions to decrease Th2 (Il4) (Ing et al., 2000). Koski et al. (1999) conclusion on the different roles of energy versus protein deficits in modifying Th1/Th2 profiles during GI infection may explain the sometimes conflicting results reported on the effect of supplemented diets on the immune response to GI nematodes. The present study does not allow a deeper analysis of the mechanisms underlying host–nutrition relationship. However, results herein show that in young female lambs fed on adequate levels of protein, an improvement of energy supply does not only improves carcass characteristics but clearly enhances the development of resistance to GI nematodes infection. These results have special relevance in grazing animals. Under these conditions where protein is not usually a limiting factor, energy supply can be easily added with simple management techniques.

Acknowledgements The authors thank Mr. E. Morago and Mrs. A. Guillén for assistance with the field work. This work has been partly supported by a INIA Project No. SC00-060 and by the European Commission (DGVI) Project No. FAIR3 CT96 1485, as part of a collaborative programme between Scotland, France, Greece and Spain. References Abbot, E.E., Parkins, J.J., Holmes, P.H., 1988. Influence of dietary protein on the pathophysiology of haemonchosis in lambs given continuous infections. Res. Vet. Sci. 45, 41–49. Adams, D.B., 1989. A preliminary evaluation of factors affecting an experimental system for vaccination and challenge with Haemonchus contortus in sheep. Int. J. Parasitol. 19, 169–175. AFRC, 1993. Energy and Protein Requirements of Ruminants. CAB International, Wallingford. Albers, G.A.A., Gray, G.D., Piper, L.R., Barber, J.S.F., Le Jambre, L.F., Barger, I.A., 1987. The genetics of resistance and resilience to Haemonchus contortus infection in young Merino sheep. Int. J. Parasitol. 17, 1355–1367. Bown, M.D., Poppi, D.P., Sykes, A.R., 1991. The effect of post ruminal infusion of a protein or energy on the pathophysiology of Trichostrongylus colubriformis infection and body composition in lambs. Aust. J. Agric. Res. 42, 253–267. Coop, R.L., Kyriazakis, I., 1999. Nutrition–parasite interaction. Vet. Parasitol. 84, 187–204. Coop, R.L., Sykes, A.R., Angus, K.W., 1977. The effect of daily intake of Ostertagia circumcincta larvae on body weight, food intake and concentration of serum constituents in sheep. Res. Vet. Sci. 23, 76–83. Coop, R.L., Graham, R.B., Jackson, F., Wright, S.E., Angus, K.W., 1985. Effect of experimental Ostertagia circumcincta infection on the performance of grazing animals. Res. Vet. Sci. 38, 282–287. Dakkak, A., 1990. Strongyloses gastro-intestinales et malabsorption des nutriments. Ann. Parasitol. Hum. Comp. 65, 73–76. Denham, D.A., 1969. The development of Ostertagia circumcincta in lambs. J. Helmint. 3/4, 299–310. Dineen, J.K., Windon, G.G., 1980. The effect of sire selection on the response of lambs to vaccination with irradiated Trichostrongylus colubriformis larvae. Int. J. Parasitol. 10, 189–196. Donaldson, J., van Houtert, M.F.J., Sykes, A.R., 1998. The effect of nutrition on the periparturient parasite status of mature ewes. Anim. Sci. 67, 523–533.

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