Gastrointestinal Nematode Parasites Research Articles

Control of bovine gastrointestinal nematode parasites using pellets of the nematode-trapping fungus Monacrosporium thaumasium

The viability of a formulation of the fungus Monacrosporium thaumasium associated with ivermectin was evaluated for the biological control of bovine gastrointestinal nematode parasites. Four groups of five calves each were placed in pastures with a stocking rate of 1.6 animal/hectare. In group 1 (control), the calves did not receive any treatment. In group 2, each animal received 20g of pellets of M. thaumasium orally twice a week during a six-month period that began with the onset of the rainy season (October 23, 2000). In group 3, each animal received 20g of pellets of M. thaumasium orally twice a week during the same period as 2, as well as two strategic treatments with ivermectin (200 mcg/kg) on May 10, 2001 and July 5, 2001. In group 4, the animals were treated with ivermectin alone as described for group 3. EPG counts for group 1 were significantly greater (P< 0.01) than those for groups 2 and 3 and the difference at the end of the study period was near 100%. The EPGs of group 4 animals remained high until the first strategic treatment with ivermectin. Values for groups 1 and 4 differed significantly (P< 0.05) from those of groups 2 and 3 from December 2000 onwards. It was concluded that the use of this dose and periodicity of application of M. thaumasium pellets makes the application of anthelminthic treatments unnecessary.

Interleukin-13: A Key Mediator in Resistance to Gastrointestinal-Dwelling Nematode Parasites

Gastrointestinal nematode parasites are one of the most prevalent types of infection worldwide. Evidence from both laboratory and human systems indicates that when resistance is evident immunity is mediated by effector mechanisms controlled by T helper 2 type responses. Moreover, more recent evidence implicates a central role for interleukin 13. We raise the possibility that gut dwelling nematodes may have been an important driving force in the development of Th 2 responses involving IL-13. Moreover, that these parasites have evolved a variety of strategies to avoid destruction and to regulate any potential pathology associated with chronic infection.

Enhanced survival of rats concurrently infected with Trypanosoma brucei and Strongyloides ratti

The interaction between the blood protozoan parasite, Trypanosoma brucei and the gastrointestinal nematode parasite, Strongyloides ratti was studied in outbred white albino rats. Rats were grouped and given either single infection with T. brucei or S. ratti or concurrently infected with both parasites. Blood parasitaemia and packed cell volume, faecal egg/larva output, adult worm burden and survivability were monitored in order to assess the interactive effects of the infections. All trypanosome-infected rats became parasitaemic within 1 week of infection but surprisingly parasitaemia was higher in the single than concurrently infected group of rats. In addition all animals with single T. brucei infection had died by 14 days after the infection, whereas animals with concurrent infection were still alive by day 28 after the infection when the experiment was terminated. Concurrent infection resulted in significant increase in daily S. ratti egg/larval output in faeces ( P<0.01), but lesser number of adult worms were recovered from the intestine of sacrificed rats on day 8 post-infection. Taken together these results suggest that T. brucei and S. ratti interact in a manner that ameliorates their pathogenic effects resulting in a decrease in the level of parasitaemia and intestinal worm burden and in increased life span of the infected rats. These results differ from the classical immunosuppressive attributes of T. brucei and the results are discussed in the context of the possible immune responses that might have contributed to this outcome and the potential significance of the findings in alternative control method of trypanosomosis.

Relative resistance to gastrointestinal nematode parasites in Dorper, Katahdin, and St. Croix lambs under conditions encountered in the southeastern region of the United States

A study was conducted to evaluate the relative resistance to a natural or induced gastrointestinal (GI) nematode infection in hair and wool breeds of lambs. In Experiment 1, natural GI nematode infection was evaluated among fall born Dorper crossbred (DO; n=17) or St. Croix (SC; n=17) lambs. Ewe and wether lambs were weaned at 84±1.1 days of age and dewormed. Fecal egg counts (FECs) and blood packed cell volume (PCV) were determined between weaning and 56 days later. In Experiment 2, natural GI nematode infection was evaluated among spring born DO (7/8 or 3/4 Dorper; n=24), Katahdin (KA; n=26), SC ( n=8), and Suffolk (SU; n=10; wethers only) ewe and wether lambs from weaning (60±1.3 days of age; April) to 115±1.3 days of age (June). In both studies, lambs grazed bermudagrass overseeded with ryegrass, previously grazed with sheep, and were supplemented with 225–500 g of corn/soybean meal supplement. In Experiment 3, the relative resistance was examined among DO ( n=8), KA ( n=8), and SC ( n=9) ewe lambs to induced GI nematode infection (20,000 Haemonchus contortus L3). Lambs were weaned at 62±2.3 days of age, dewormed 22 days later and inoculated 6 days later (Day 0). In Experiment 1, FEC was greater ( P<0.04) at weaning in DO lambs and PCV was similar between DO and SC lambs. In Experiment 2, FEC was greatest and PCV least in DO lambs when compared with other breeds (breed×time, P<0.003). In Experiment 3, FEC was greater in DO and KA lambs compared with SC lambs ( P<0.02). By Day 42, four DO, six KA, and three SC lambs had been dewormed. With a low to moderate challenge, resistance to infection was similar among the three hair breeds examined and greater than the wool breed lambs. When the challenge escalated, St. Croix lambs were most resistant and Dorper crossbred lambs were similar or less resistant than Katahdin lambs.

Efficacy of the nematode-trapping fungus Duddingtonia flagrans against three species of gastro-intestinal nematodes in laboratory faecal cultures from sheep and goats

The ability of the nematode-killing fungus Duddingtonia flagrans to reduce number of infective larvae of three species of gastro-intestinal parasitic nematodes developing in dung was investigated in both goats and sheep. Groups of lambs and kids (12–20 weeks old) were given mono-specific infections of Haemonchus contortus, Ostertagia ( Teladorsagia) circumcincta or Trichostrongylus colubriformis. Following patency of the infections ( t 1) faecal samples were collected for determination of faecal nematode egg count (FEC) and culture of parasite larvae. Groups of animals were then dosed on 2 consecutive days with one of the two dose rates of the fungus (250,000 or 500,000 spores/kg liveweight). One ( t 2) and 5 ( t 3) days after the second dose of fungus samples were again collected for FEC and culture. The number of larvae recovered from the faecal cultures at t 1 and t 3 were used as controls to assess the efficacy of the experimental treatment at t 2. Average efficacy was 78% with group means ranging from 40 to 93%. Dose rate of fungus appeared to influence efficacy against O. circumcincta but not against H. contortus or T. colubriformis. Overall, there were no differences in the efficacy of the fungus against any of the parasite species or in either host animal. The results of this trial indicate the potential use of this fungus as a broad spectrum anti-parasite agent for use in both goats and sheep.

Aplicação de formulação do fungo predador de nematóides Monacrosporium thaumasium (Drechsler, 1937) no controle de nematóides de bovinos

A viabilidade de uma formulação do fungo Monacrosporium thaumasium (Drechsler, 1937) foi avaliada no controle biológico de nematóides parasitos gastrintestinais de bovinos. Dois grupos de sete bezerras cada, mestiças Holandês ´ Zebu, de quatro a seis meses de idade, foram colocados em pastagens de Cynodon dactilon. No grupo A, cada animal recebeu 20g de pellets (formulação granulada) de M. thaumasium via oral, duas vezes por semana, durante quatro meses, com início na estação chuvosa (outubro, 2001). No grupo B (controle), as bezerras não receberam nenhum tratamento. As contagens de ovos por grama de fezes (OPG) e das larvas infectantes encontradas na pastagem do grupo B foram significativamente maiores (P&lt;0,05) do que as do grupo A e a diferença entre o OPG dos animais dos grupos A e B, no final do experimento, foi de 88,8%. O gênero Cooperia foi o mais prevalente em ambas as pastagens. Conclui-se que a aplicação de pellets de M. thaumasium na dosagem e periodicidade de aplicação usadas foi eficiente no controle de nematóides parasitos gastrintestinais de bovinos.

The effect of short-term consumption of a forage containing condensed tannins on gastro-intestinal nematode parasite infections in grazing wether goats

The effect of grazing a forage containing condensed tannins (CT), sericea lespedeza (SL, Lespedeza cuneata, 46g extractable CT/kg DM), on the total fecal egg production of gastrointestinal nematodes (GIN) and egg hatch percentage was compared with a control forage low in CT (rye/crabgrass (RC), 6g extractable CT/kg DM) using Spanish wether goats (47.0kg BW). A crossover experimental design was utilized with groups of five and six animals. Each group grazed one forage species in the first 15 day period and then were switched so they grazed the other forage species during the second 15 day period. The mean fecal egg count (FEC) and total fecal egg output (TFEO) were reduced (2500 versus 710 eggs/g, P<0.0001 and 173 versus 45×104 eggs per day, P<0.01) in goats grazing SL compared with RC. The percentage of eggs developing to L-3 larvae was reduced from 99 to 58.2% (P<0.01) by grazing SL. It is concluded that grazing SL, a CT containing forage, reduced fecal egg production from GIN, reduced hatch and development of larvae and consequently, would reduced pasture contamination by infective larvae, thereby mitigating the need for anthelmintics to control GIN.

Susceptibility to natural gastro-intestinal nematode infection in different physiological stages in Jamunapari and Barbari goats in the semi-arid tropics

Resistance to naturally acquired gastro-intestinal (GI) nematode parasite infections (predominantly Haemonchus) of 212 Jamunapari and 163 Barbari goats (does) were studied in different physiological stages, namely pregnant, dry and lactating stages in a semi-arid region of India. The faecal egg counts (FECs) for GI nematode infections were examined in different physiological stages in both the breeds in two periods, i.e. an early period (October–November) and a late period (February–March). Analysis was carried out on log e(FEC+100) transformed data, accounting for repeated records on does, during early and late periods. Breed had significant effect on FEC in early and late periods in pregnant, dry and lactating does. Breed by physiological interaction had significant effect on FEC in both the early and late periods. Jamunapari lactating goats had significantly higher FEC than that of dry and pregnant does but there was no peri-parturient rise in FEC in the resistant Barbari goats.

Mapping of chromosomal regions influencing immunological responses to gastrointestinal nematode infections in mice.

This paper reports the results of a genome-wide search for quantitative trait loci (QTL) influencing immunological responses to infection with the gastro-intestinal nematode parasite Heligmosomoides polygyrus in an F2 population created by crossing the resistant SWR and the susceptible CBA inbred mouse strains. Following infections, intestinal granuloma score at post mortem, mucosal mast cell protease 1, and IgE and IgG1 titres were recorded. The susceptible CBA mice had significantly higher IgG1, but significantly lower IgE, mucosal mast cell protease 1 and granuloma scores than SWR mice. Significant QTL were mapped to chromosomes 4, 11, 13 and 17 for granuloma score; chromosomes 12 and 17 for IgE; chromosome 10, 17 and 18 for IgG1 and chromosomes 1, 9, 10, 11, 17 and 18 for mucosal mast cell protease 1. Chromosomes 10, 11, 17 and 18 had QTL affecting more than one trait, and these are most likely to represent single QTL with multiple effects rather than multiple QTL. Some of these QTL map to regions known to harbour genes responsible for the induction of immunological responses to intestinal worms.

Progress and new technologies for developing vaccines against gastrointestinal nematode parasites of sheep.

Despite the identification of highly effective native antigens for vaccination against Haemonchus contortus, particularly 'hidden' antigens derived from the intestine of adult worms, to date similar efficacy has not been shown with recombinant antigens. In addition, progress towards identification of protective antigens from other sheep gastrointestinal (GI) nematode species is limited. Coupled with this is an incomplete understanding of the mechanism of natural immunity to GI nematodes, making selection of appropriate immunization strategies and adjuvants for evaluation of candidate 'natural' antigens problematic. The current explosion in new high-throughput technologies, arising from human studies, for analysis of the genome, transcriptome, proteome and glycome offers the opportunity to gain a better understanding of the molecular pathways underlying pathogen biology, the host immune system and the host-pathogen interaction. An overview is provided on how these technologies can be applied to parasite research and how they may aid in overcoming some of the current problems in development of commercial vaccines against GI nematode parasites.

Is there a common mechanism of gastrointestinal nematode expulsion?

Parasitic gastrointestinal (GI) nematodes are one of the most commonly acquired infections in the world. Although they cause relatively little mortality, infections result in high levels of morbidity that can result in developmental consequences in infected children and cause significant economic loss in infected animals. Over the last 30 years there has been extensive research into the mechanisms controlling the expulsion of gastrointestinal nematodes. Although many of the effector mechanisms that contribute to the loss of the parasite have been defined, we still appear to be some way from understanding the actual cause of parasite loss. Part of this stems from the different responses induced by different gastrointestinal parasites. It is clear that a Th2 response is essential for the expulsion of GI helminths; however, each of the characteristic immunological effector mechanisms induced following infection with these parasites may not be required or may be insufficient in isolation, but together they operate to expel GI helminths. These responses then succeed more efficiently in some cases than in others to induce parasite expulsion. The contribution made by various effector mechanisms to the expulsion of these parasites may therefore be a reflection of both the niche which the parasite inhabits as well as possible evasive/suppressive mechanisms employed by the parasites. In this review the various aspects of parasite expulsion will be described and the controversial issues in the field will be discussed.

Nematódeos resistentes a anti-helmíntico em rebanhos de ovinos e caprinos do estado do Ceará, Brasil

O controle do parasitismo por nematódeos gastrintestinais é feito basicamente com a utilização de anti-helmínticos. Falhas no controle são o primeiro sinal do aparecimento de resistência anti-helmíntica. A real situação da prevalência da resistência anti-helmíntica, em fazendas comerciais de criação de ovinos e caprinos no Brasil, é desconhecida. Esse experimento teve como objetivo, estimar a ocorrência de resistência ao oxfendazol, levamisol e ivermectina em propriedades comerciais de criação de ovinos e caprinos, na região do médio e baixo Jaguaribe, através do teste de redução na contagem de ovos nas fezes acompanhados de coproculturas. O trabalho foi realizado em 25 criações, sendo 16 de ovinos, 7 de caprinos e uma de ovinos e caprinos. Os dados obtidos foram analisados pelo programa estatístico RESO (1989). A prevalência de nematódeos resistentes ao oxfendazol, levamisol e ivermectina em ovinos foi de 88%, 41% e 59%, e em caprinos de 87,5%, 75% e 37,5%, respectivamente. Observou-se que o gênero Haemonchus foi o mais prevalente na população resistente a todos os anti-helmínticos, tanto em ovinos quanto em caprinos, seguido de Trichostrongylus e Oesophagostomum.

Chromosomal regions controlling resistance to gastro-intestinal nematode infections in mice

Quantitative trait loci (QTL) associated with resistance to an intestinal worm in a well-defined murine model are described. These have been identified in an F(2) population derived from resistant (SWR) and susceptible (CBA) parental mouse strains infected with the gastro-intestinal nematode parasite Heligmosomoides polygyrus. Seven QTL located on six chromosomes are described, associated with components of the complex host response and the differential regulation of parasite survival and reproduction. The combined additive effects of the five significant QTL associated with worm survival (total worm count at necropsy) account for about 60% of the difference in worm count between the parental lines. The dominance effect for these five QTL are all in the direction of resistance, supporting the heterosis for resistance established from the mean worm count for the F(2) line relative to the parental lines. It is now possible to identify the comparative chromosomal regions of these QTL in livestock and humans and to consider the possibility of future improved control strategies. These may include breeding of resistant or tolerant livestock, development of vaccines, or identification of new anthelmintic drugs.

Resistance and resilience to gastro-intestinal nematode parasites and relationships with productivity of Red Maasai, Dorper and Red Maasai ✕ Dorper crossbred lambs in the sub-humid tropics

AbstractResistance and resilience to naturally acquired gastro-intestinal (GI) nematode parasite infections (predominantly Haemonchus contortus) were studied in 1785 lambs born over six lambings (1991 to 1996) consisting of 212 Red Maasai, 311 Dorper and 1262 crossbred (Red Maasai-Dorper) lambs in the sub-humid coastal region of Kenya. These lambs were the progeny of 41 Dorper and 35 Red Maasai rams. Live weights (LWT), blood packed cell volume (PCV) and faecal egg counts (FEC) were recorded at 1- to 2-monthly intervals from birth until the lambs were about one year of age. Red Maasai were more resistant and resilient post weaning to infections with GI nematodes than Dorper lambs as shown by their significantly lower FEC and their significantly higher PCV, respectively. An increasing proportion of Red Maasai genes in the crossbred lambs was associated with decreased FEC and higher PCV, but there was no heterosis for logarithm-transformed FEC (LFEC) or PCV. From one month of age Red Maasai lambs were significantly lighter than Dorper lambs by about 1 kg, but Red Maasai lambs had significantly lower lamb mortality rate from birth to 12 months of age (proportionately 0·30 and 0·66, respectively). Heritability estimates from a repeated measures analysis for records taken at 6 and 8 months of age were 0·14 (s.e. 0·05) for PCV from an animal model and 0·12 (s.e. 0·05) for LFEC from a sire model. The heritability estimate for LFEC from a repeated measures analysis including the four measurements recorded between 6 and 12 months of age was significantly higher (P &lt; 0·05) for Dorper-sired lambs (0·15, s.e. 0·05 for an animal model and 0·19, s.e. 0·07 for a sire model) than for Red Maasai-sired lambs (0·00 and 0·01, s.e. 0·02). The phenotypic and genetic correlations between PCV and LFEC were moderately to highly negative and averaged –0·34 and –0·81, respectively. None of the genetic correlation estimates between LWT and PCV and LWT and LFEC for lambs post weaning were significantly different from zero. The heritability estimates for PCV and LFEC have important implications for within-breed genetic improvement programmes: for the Red Maasai, improvement should concentrate on resilience (e.g. selection for high PCV); for the Dorper, selection should be feasible for both improved resistance (low FEC) and resilience (high PCV).

Survival of Red Maasai, Dorper and crossbred lambs in the sub-humid tropics

AbstractThe survival rates of Dorper, Red Maasai and crossbred lambs born over a period of 6 years at Diani Estate, Coast Province, Kenya were compared using the Cox mixed proportional hazards model with a random (frailty) term for sire. Of the 1785 lambs born, proportionately 0·44 died before they were 1 year old. Almost half of these deaths occurred before weaning; a third were associated with mis-mothering and a fifth with gastro-intestinal nematode parasite (endoparasite) infections. Half of the deaths post weaning were associated with endoparasite infections, predominantly Haemonchus contortus. The Red Maasai lambs had a lower risk of death than the Dorper lambs with a relative hazard of 0·27 pre-weaning and 0·25 post weaning. Other crosses and back crosses had relative hazards in between these values and 1; there was no evidence of heterosis. Survival rates were different among years and appeared to be associated to some degree with variations in rainfall. There were highly significant effects of both birth weight and weaning weight on survival. Body weight, together with packed red cell volume and faecal egg count, were also introduced into the proportional hazard model as time-varying covariates. All three variables had major influences on survival. The risk of death over the following month in animals individually treated with an anthelmintic drug pre weaning was reduced by 0·61 compared with those not treated. The sire frailty variance estimate was similar to its standard error pre-weaning but larger post weaning. When adjusted for lamb body weight the sire variance post weaning increased to three times its standard error.

Regulation of the resistance and resilience of periparturient ewes to infection with gastrointestinal nematode parasites by dietary supplementation

Control of gastrointestinal nematode (GIN) parasites has traditionally been based on the premise of chemotherapeutic control with the resultant consequence being the development of anthelmintic resistance by GIN. An alternative premise to the management of GIN parasites is to improve the resistance and resilience of sheep to GIN infection by manipulating the nutritional environment at key periods of a sheep's life. One key period, is the periparturient phase in the reproductive cycle when ewes experiences a temporary loss, or diminution, of immunity to GIN parasites. This phase is associated with an increased requirement for both metabolisable energy (ME) and metabolisable protein (MP). The increased requirement being greater for MP. The loss of immunity to GIN is associated with a diminished cell-mediated immune response in the gut mucosa, the magnitude of which can be regulated by protein nutrition, or more specifically the supply of MP. Experimental studies that have increased MP supply, in housed and grazing periparturient ewes, have demonstrated significant improvements in resistance to GIN infection. Positive gut immune responses to an increased MP supply are believed to occur because the increased MP supply counters the combined pathological consequences of GIN infection per se and the host's immune response, namely a net loss of amino acids. Susceptibility to GIN infection in the periparturient ewe may also be a function of a low priority for use of MP toward immune function but evidence from young animals suggests a disproportionate partitioning of amino acids to the gastrointestinal tract during GIN infection. It is proposed that the priorities for use of MP that predispose the ewe to GIN infection are altered following infection to favour a gut immune response. Despite being under the regulation of MP supply and genetic selection, the loss of immunity during the periparturient period cannot be fully restored by either approaches suggesting that other unidentified factors are involved in the periparturient loss of immunity. Resilience to GIN infection is responsive to both ME and MP supply. The practice of supplementing periparturient ewes to increase MP and ME supply, in order to enhance resistance and resilience to GIN infection, is gaining favour with graziers within some regions of Australia. Other benefits (e.g. increased reproductive rates) arise from such supplementation strategies that improve the cost effectiveness of this approach.

Impact of non-protein nitrogen supplements on nematode infected sheep

Inadequate nutrition and gastrointestinal nematode parasitism are major constraints to livestock production. In livestock rearing enterprises that rely on low quality roughage for the provision of digestible carbohydrate, the most critical nutritional deficiency is often nitrogen. The provision of non-protein nitrogen in the diet can compensate for this deficiency and enable increased productivity from the available feed resource. Numerous studies have shown that increasing the supply of protein for intestinal absorption can alleviate the detrimental effects of nematode parasites on production and can also result in improved protective immunological responses to infection. In many situations, the practical implementation of such a strategy for improved nematode parasite control is hampered by the high cost and/or unavailability of high quality protein sources for use as ruminant livestock feed. Evidence is presented from studies with young sheep to demonstrate that supplementation with urea can achieve similar qualitative benefits in reducing the effects and level of infection with parasitic nematodes to that achieved with protein supplements. Urea–molasses blocks are a popular low-cost means of delivering non-protein nitrogen and experimental evidence shows that the use of urea–molasses blocks can have a beneficial impact on enhancing the resilience and resistance of sheep to infection with nematode parasites. Consistent with other nutritional studies, urea appeared to confer increased benefits proportional to increasing supplement intake as shown by reduced pathological effects, parasite numbers and faecal egg output. It is suggested that application of these findings in Australian pasture systems may require revision of current systems for the delivery of non-protein nitrogen supplements to increase the level of urea consumed.

Nutritional programming of young sheep to improve later-life production and resistance to nematode parasites: a brief review

Nutritional supplementation, particularly with high quality protein, can increase the resistance and resilience of sheep to infection with gastrointestinal nematode parasites. Nonetheless, this approach to improved nematode parasite control has not been widely adopted, probably because the cost of supplements is usually greater than the immediate production benefits that are conferred. Evidence is presented to support the proposition that long-term effects on liveweight, wool production, reproductive performance and immunity to nematode infection can be programmed through strategic supplementation of sheep during periods of nutritional scarcity. Where optimal efficiency of lifetime production is desired, particular attention should be paid to ensuring that nutritional needs are met and negative impacts of nematode parasitism are minimised during lactation and around the time of weaning.

Metabolic effects of host defence responses during gastrointestinal parasitism in sheep

Following stimulation of the immune system, tissue trauma, or exposure to some stressors, there can be activation of the acute phase response (APR). This is a primitive defence reaction that helps protect the host against noxious insults. Part of the response involvess a change in priorities for nutrient utilisation, when the pro-inflammatory cytokines IL-1α/β, TNFα, IL-6 and IFNα/β override the normal hypothalamic-somatotropic control of nutrient utilisation. The pro-inflammatory cytokines reduce protein synthesis and increase protein catabolism in muscle, induce synthesis of acute phase proteins in liver, and decrease de novo fatty acid synthesis and increase lipolysis in adipose tissue. In the central nervous system there is induction of fever, which increases metabolic rate, and induction of sickness behaviour. There are few reports in the literature of studies on the APR during gastrointestinal nematode (GIN) infection in sheep. However, IL-6 message is present in mucosa during Trichostrongylus colubriformis infection and mast cells in many species contain preformed TNFα. The changes in appetite, growth and nitrogen metabolism seen during GIN parasitism in sheep are in accord with the systemic effects of the APR. This review reports studies on activation of the APR in mastitis and fly strike, and the effects of APR on wool growth in high and low tensile strength genotypes and during Haemonchus contortus infection. The modified capacity of tissues to take up amino acids during pro-inflammatory cytokine perturbation of nutrient utilisation may limit the capacity for dietary supplementation to remedy the cost of host defence. Genetic variation may occur in production of pro-inflammatory cytokines or in tissue sensitivity to pro-inflammatory cytokines during GIN infection and may provide a basis for selection for resilience. Some questions for future research are identified.

Effects of parasitism on the nutrient economy of sheep: an overview

The experimental difficulties of qualifying the effects of gastrointestinal nematode parasitism on the nutrient economy of sheep from field and laboratory studies are discussed. The evidence available is reviewed and the conclusion drawn is that infection in the abomasum or small intestine induces protein deficiency while reducing supply through depression of appetite. It appears that the effects in susceptible animals — in lambs during the phase of initial acquisition of immunity and in ewes during the peripartum period — are largely reversible by protein supplementation. Quantified effects on mineral nutrition are few and are probably site-specific.The possibility that the host immune response per se may be implicated in the characteristic appetite depression is discussed. The need to understand the immune response's demand for specific amino acids is emphasised, particularly if nutritional supplements are to be used effectively in parasite control. A better understanding of the post-ruminal supply of amino acid in grazing animals will also be needed.