Fasciola hepatica: role of developmental stages in the rat's resistance to challenge

Rats were sensitized by subcutaneous implantation of either metacercariae, 4 week-old juveniles, adult worms, or eggs of Fasciola hepatica and then challenged with 30 metacercariae 2 weeks later. Worm burdens were determined 8 weeks after challenge. Apart from adult worms, all implanted stages conferred a significant degree of protection on the recipients. The effectiveness of adult worm implants was not improved by using worms from different sources (sheep and cattle rather than rats) nor by extending the period of sensitization prior to challenge.     

Studies on the viability of metacercariae of Fasciola gigantica.

The viability of metacercariae of Fasciola gigantica was tested by in vitro and in vivo methods. In vitro testing was based upon the motility of juvenile flukes within the inner cyst as examined under the light microscope. In vivo testing was undertaken through experimental infections of rabbits (two groups) and natural definitive hosts, lambs (one group). In the first group, out of six rabbits each given 25 metacercariae, worm establishment only took place in one rabbit with a single fluke recovery on 60 days post infection. In the second group of six rabbits each given 200 metacercariae, five were infected, with two or three flukes per host. All the lambs given 250 metacercariae became infected showing prevalences of 7.2-40% in comparison with rabbits in which low prevalences (0-4%) were recorded. The results indicated that even viable metacercariae which were already tested in vitro could not readily establish in rabbits. Such variability in worm establishment suggests that immunological and chemotherapeutic studies in rabbits infected with F. gigantica are likely to be unreliable.     

Immunoprophylaxis against Fasciola hepatica in rabbits using a recombinant Fh15 fatty acid-binding protein.

Previous studies of ours have demonstrated that a recombinant protein (Fh15) related to fatty acid-binding proteins did not induce significant protection in rabbits challenged 2 or 4 wk postimmunization over nonimmunized controls. In the current study, rabbits were immunized with Fh15 and challenged with Fasciola hepatica metacercariae 12 and 20 wk later. In the current study in which longer lag periods for challenge infection after the second immunization were used, worm burden reductions compared to adjuvant controls were a significant 43% and 76%, respectively. Importantly, rabbits immunized with Fh15 had significant numbers of immature flukes, 66% in the 12-wk period and 84% in the 20-wk lag period as compared to controls. In addition, liver lesions were clearly diminished in the vaccinated rabbits. Enzyme-linked immunosorbent assay absorbance values showed that immunized rabbits developed high antibody levels to Fh15 from 8 wk after the first immunization and did not increase after challenge. These results suggest that a recombinant F. hepatica molecule related to fatty acid-binding proteins induces protective (worm burden reductions), anti-fecundity (immature flukes), and anti-pathology (less liver lesions) effects in rabbits and may serve as a model for the immunoprophylaxis of fascioliasis.     

The early migratory behaviour of young Fasciola hepatica in sensitized mice

Earlier work has shown that when mice are sensitized with irradiated metacercariae the numbers of immature flukes that can be recovered from the peritoneal cavity 2 days after reinfection with normal metacercariae is significantly less than the numbers recovered from non-sensitized control mice. Experiments are now described which investigate the reason for this difference. An inflammatory cellular reaction, most marked in sensitized mice occurs in the intestinal wall but this does not delay the migration of challenge flukes into the peritoneal cavity. No effective protective mechanism operates at the intestinal wall because similar numbers of flukes are present in the livers of sensitized and non-sensitized mice at 12 and 14 days after infection. When livers of sensitized and non-sensitized mice were examined 2 days after infection significantly more flukes had already reached the liver in the sensitized group. This indicates that immature flukes migrate more quickly from the peritoneal cavity in mice previously sensitized with irradiated metacercariae and would account for the difference in the number of flukes recovered from the peritoneal cavity of sensitized and non-sensitized mice at 2 days after infection.     

Winter Resistant Oocysts in the Pasture as a Source of Coccidial Infection in Lambs

Lambs born in pens with slatted floor were brought out at 2–5 weeks of age on pastures heavily grazed by sheep the previous years. About 16 days later the oocyst output of the lambs rapidly increased to high levels. Lambs on pastures which never had been grazed by sheep earlier, had moderate oocyst counts. Between 11 and 25 days after the beginning of grazing there were significantly more lambs with diarrhoea on permanent pastures compared with pastures never grazed by sheep earlier. It was found that lambs were heavily infected during the first 2 days on permanent pastures. Thirteen housed lambs were given 10–50 g of soil from a permanent pasture as a water suspension by a stomach tube. Fifteen days later there was a steep rise in the oocyst output in most of them, and 11 of the 13 lambs developed diarrhoea and 2 died. None of 10 lambs given uninfected soil and none of 12 untreated controls showed diarrhoea and the oocyst output remained on a moderate level. It is concluded that oocysts which have survived the winter in the pasture are the main source of infection with Eimeria spp. in lambs with this kind of management. Soil-eating is the most likely source of infection during the first days on pasture.     

Induction of immunity in sheep to Fasciola hepatica with mimotopes of cathepsin L selected from a phage display library

An M13 phage random 12-mers peptide library was used to screen cathepsin L mimotopes of Fasciola hepatica and to evaluate their immunogenicity in sheep. Seven clones showed positive reactivity to a rabbit anti-cathepsin L1/L2 antiserum in ELISA, and their amino acid sequences deduced by DNA sequencing were tentatively mapped on the protein. Twenty sheep were randomly allocated into 4 groups of 5 animals each, for immunization with 1x10(14) phage particles of clones 1, 20, a mixture of 7 clones and PBS, without adjuvant at the beginning, and 4 weeks later. All groups were challenged with 300 metacercariae at week 6 and slaughtered 16 weeks later. The mean worm burdens after challenge were reduced by 47.61% and 33.91% in sheep vaccinated with clones 1 and 20, respectively; no effect was observed in animals inoculated with the clone mixture. Also, a significant reduction in worm size and burden was observed for those sheep immunized with clone 1. Animals receiving clone 20, showed a significant reduction in egg output. Immunization induced a reduction of egg viability ranging from 58.92 to 82.11%. Furthermore, vaccinated animals produced clone-specific antibodies which were boosted after challenge with metacercariae of F. hepatica.     

Oesophagostomum columbianum: gamma-irradiated third-stage larvae for immunization in lambs

40-kR gamma-irradiated third-stage larvae of Oesophagostomum columbianum were used for the immunization of Kashmir Merino lambs. Male lambs (aged from 8 to 12 weeks) were immunized in two separate experiments by two doses of irradiated larvae, given 21 days apart, and subsequently challenged with normal larvae. Judging by the establishment of worms resulting from the challenge infections in the immunized and control groups of lambs in the two experiments, a high degree of immunity was shown to develop in young lambs vaccinated with 500, followed 21 days later with 2000, 40-kR irradiated larvae. Lambs from the immunized groups showed more nodules in the intestine, a high percentage of which were positive for histotrophic stages of O. columbianum, than did controls. The importance of this finding in relation to the possible use of a vaccine for the immuno-prophylaxis of oesophagostomiasis in sheep and other animals is discussed.     

Studies on the abomasal pathology of immunized and non-immunized sheep infected with Haemonchus contortus

Five adult ewes and five lambs were repeatedly immunized with weekly doses of 10,000 irradiated Haemonchus contortus L3 before challenge with 100,000 and 10,000 normal larvae respectively. Two groups of non-immunized ewes and lambs were similarly challenged and one animal from each of the four groups killed on days 3, 5, 7, 10 and 24 post-challenge. The cellular changes in the abomasal mucosa were less marked in the non-immunized groups than in the immunized animals and appeared later in the lambs than in the ewes. Thus, in the immunized ewes increases in the numbers of mast cells and eosinophils were evident within five days of challenge whereas similar changes appeared later in the immunized lambs. Also marked lymphoid aggregates at the base of the mucosa and in the submucosa were detected only in the immunized ewes. However, both immunized and non-immunized ewes showed rises in the numbers of IgA plasma cells after challenge which were not evident in either group of lambs.     

On the viability of Fasciola gigantica metacercariae ingested by Lymnaea auricularia

The effect of ingestion by Lymnaea auricularia on the viability and infectivity of Fasciola gigantica metacercariae was studied. The cyst wall was unaffected by the snail's digestive processes. Two rabbits, each infected with 50 ingested metacercariae, died at 83 and 87 days post-infection. Eight and 10 immature flukes were recovered from the livers, indicating that the metacercariae had remained infective after passing through the intestine of the snail.     

Effects of active immunization of ram lambs and bull calves against synthetic luteinizing hormone releasing hormone

Ram lambs and bull calves were immunized against LH-RH by injections given in weeks 0, 6, 12 and 28 (ram lambs, week 0 = 16 to 20 weeks of age) and weeks 0, 6, 12 and 18 (bull calves, week 0 = approximately 4 weeks of age). The testis size of LH-RH-immunized animals was significantly less than that of controls from week 13 onwards in ram lambs and from week 15 onwards in bull calves. When ram lambs were sampled in week 17 and bull calves in week 20, mean plasma gonadotrophin and testosterone concentrations were consistently lower in LH-RH-immunized animals than in controls. Single intravenous injection of synthetic LH-RH or an analogue of LH-RH in week 27 failed to induce LH or testosterone responses in LH-RH-immunized ram lambs. Motile semen samples could not be obtained from any of the LH-RH-immunized ram lambs in weeks 24, 25 and 26 or from 7 of 10 in week 72, but samples of moderate motility were obtained in week 72 from three rams in which LH-RH antibody titres had fallen. No attempt was made to obtain semen from bull calves. After castration there was no increase in plasma LH in LH-RH-immunized rams and only a small increase in LH-RH-immunized bull calves. Mean testis weight was significantly lower in LH-RH-immunized animals than in controls of both species. Thus the normal development of the reproductive system in ram lambs and bull calves was blocked by active immunization against LH-RH. Some evidence was obtained for natural reversal of the effects with time and falling antibody titres. These findings demonstrate the potential of LH-RH immunization as an alternative to castration.     

Effect of praziquantel treatment on pulmonary lesions of rats infected with Paragonimus iloktsuenensis

An experimental pathological study was performed to observe the effect of praziquantel treatment on the pulmonary lesions of the rat lung fluke, Paragonimus iloktsuenensis. The metacercariae were obtained from the freshwater crab, Sesarma dehaani, and 40 rats (wistar) were fed each with 10 metacercariae. On 20 rats praziquantel treatment (100mg/kg/day x 5 days) was done at 5 weeks after the infection while remaining 20 rats were kept untreated for use as controls. The drug-treated rats and the untreated ones were sacrificed 3, 7, 14, 21 or 28 days later for the observation of lung pathology. The rats infected with P. iloktsuenensis showed remarkable pulmonary changes; gross features of hemorrhagic and nodular worm capsules protruded on to the surface of the lung, and histologically local atelectasis, inflammatory cell infiltration, and egg granuloma around the worm capsules each containing one or two worms. Praziquantel treatment of the rats was shown to be highly effective in killing the worms and to lead them to degenerate, as early as in 3 days post-treatment. Almost all worms in the lung were dead and absorbed by the host cells in 21 days post-treatment, except a few living ones seen in a rat of 14-day post-treatment group. In most of the rats treated the pulmonary lesions showed the signs of resolution; regression of worm capsules with mummification of worms, decrease of inflammatory cell infiltration, improvement in the degree of atelectasis, and decreases in the frequency and size of the egg granuloma. From the results it is concluded that praziquantel is highly effective for the treatment of rat P. iloktsuenensis infection in the lung, not only by its direct killing effect of the worms but also due to the excellent resolution capacity of the pulmonary tissues.     

Effect of naloxone on gonadotrophin secretion at various stages of development in the ewe lamb

Spring-born crossbred ewe lambs were raised in a natural photoperiod and saline (N = 6) or naloxone (1 mg/kg) in saline (N = 6) was injected (i.m.) every 2 h for 6 h at 5, 10 and 15 weeks of age and for 8 h at 20, 25 and 30 weeks of age. Blood samples were taken every 12 min during treatment periods. Naloxone had no effect on time to first oestrus (controls 235 +/- 6 days, naloxone 242 +/- 7 days). Mean serum LH concentrations and LH pulse frequency were elevated by naloxone in ewe lambs at 20, 25, and 30 weeks of age (P less than 0.05). The only FSH response to naloxone was a depression of mean serum concentrations at 30 weeks of age (P less than 0.05). LH pulse amplitude was elevated at 5 weeks of age in all ewe lambs and declined thereafter to a nadir at 30 weeks of age in control, but not in naloxone-treated animals (P less than 0.05). LH pulse frequency was elevated at 10 weeks of age in control ewe lambs and in all animals at 30 weeks of age (P less than 0.05). FSH pulse frequency declined from 5 weeks of age in control ewe lambs (P less than 0.05), with very few pulses noted in 25- and 30-week-old animals. We conclude that (1) opioidergic suppression of LH, but not FSH, secretion developed at 20 weeks of age in the growing ewe lambs used in the present study, with no obvious change in suppression before the onset of first oestrus: (2) pulsatile FSH secretion occurred in the young ewe lamb but was lost as the lamb matured: (3) attainment of sexual maturity was preceded by an elevation in LH pulse frequency.     

Maturational changes in opioidergic control of luteinizing hormone and follicle-stimulating hormone in ram lambs

Stimulation by naloxone, an opioid antagonist, of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion was examined in spring-born crossbred ram lambs raised under natural photoperiod. Vehicle (n = 6) or 1 mg naloxone/kg vehicle (n = 6) was injected (i.m.) 3 times at 2-h intervals at 5, 10 and 15 weeks of age and 4 times at 2-h intervals at 20, 25, 30 and 35 weeks of age. Blood samples were taken every 12 min for 6 h at 5, 10 and 15 weeks of age and for 8 h at 20, 25, 30 and 35 weeks of age. Naloxone had no effect on age at sexual maturity (controls 239 +/- 23 days; naloxone 232 +/- 33 days). The only significant (P less than 0.05) effect of naloxone on FSH was a greater pulse amplitude in 10-week-old treated lambs than in control lambs. Naloxone treatment resulted in greater LH pulse amplitude at 5 and 10 weeks of age (P less than 0.05), lower basal serum concentration of LH at 10 weeks of age (P less than 0.05), greater LH pulse frequency at 25 weeks of age (P less than 0.05), and greater mean serum concentrations of LH, basal LH and LH pulse amplitude at 35 weeks of age (P less than 0.01) than in the controls. In both groups of lambs, mean and basal FSH, and LH and FSH pulse amplitude were highest at 5 weeks of age and fell with age. LH pulse amplitude was lowest at 35 weeks of age (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

The behavioural, physiological and immunological responses of lambs from two rearing systems and two genotypes to exposure to humans

The behavioural, physiological and immunological responses of lambs from two rearing systems and two genotypes to exposure to humans was assessed during and immediately after testing in an open-field arena. Ninety-six lambs of two genotypes (Scottish Blackface: BF and Texelx(Blue-faced LeicesterxScottish Blackface): T) were used. From birth to weaning one of two management regimes was applied: extensive (E), whereby animals were handled as little as possible or semi-intensive (I), in which lambs experienced a greater level of human exposure. Eight lambs from each of the four treatment groups received an antigenic challenge (Mycobacterium a. paratuberculosis) at 9 weeks of age to allow subsequent testing of immunological reactivity. At 1 and 3 weeks after weaning and 1 year later, lambs were tested in groups of four in a 4.5x4.5 m indoor arena, marked with gridlines at 0.75 m intervals. There were a number of occasions where testing revealed significant effects of genotype, management or their interaction, but in an approximately equal number of instances no significant effects of either genotype or management were observed. Genotype significantly influenced the number of squares occupied in the test arena over a 10-min period before the human entered (100.4 vs. 110.5; sed 2.70 for BF and T lambs, respectively, p<0.001). In relation to the number of new squares entered, there was a genotypexmanagement interaction: BFE lambs entered fewer squares than TE lambs but following semi-intensive management (I) BF lambs entered more squares than T lambs (p<0.05). When a human entered the arena after this 10-min period, while there was a gradual reduction in the number of animals which had not moved over the next 5 min, 66 animals had not moved within the allocated time. Also during this period, BF lambs stood facing the human for significantly longer than T lambs (p<0.05). At the time of arena testing, 12 lambs from each treatment group were fitted with heart-rate monitoring equipment. There were significant differences in heart rate in relation to period of testing, i.e. before (107.9) or after (112.3) the point at which the human entered the arena or when the lambs were walking in the presence of a moving human (126.3 b.p.m.; sed 2.15, p<0.001). When lambs were alone in the test arena, BF lambs had higher heart rates than T lambs (p<0.05). The heart rate of E lambs increased more than that of I lambs when the human entered the pen (9.4 vs. 0.3 b.p.m.; sed 3.95, respectively; p=0.05). Immediately following completion of the behavioural tests, blood samples were collected from subsets of lambs. Plasma cortisol concentrations of BF lambs were greater than those of T lambs (82.0 vs. 53.5 nmol/l; sed 10.18, p<0.01) but there was no effect of management. Blood samples collected from the lambs challenged with a novel antigen prior to weaning showed a genotype but not a management effect on both antibody and cell mediated immune responses, although there was a genotypexmanagement interaction. However, it should also be noted that there were no significant effects of either genotype or management on a number of the indices recorded: latency of lambs to move from the initial entry position in the absence or subsequent presence of a human; length of time one individual was separated from the other three; distance moved in a raceway before stopping; plasma beta-endorphin concentrations; heart rate in the presence of a human. Overall, these results suggest that although differences in responsiveness associated with specific genotypes of sheep can be detected in a test situation, the early life management regime may also have an effect. The results of this study caution against drawing conclusions between studies where different genotypes are employed.     

The influence of supplemental vitamins A and E on ovine humoral immune response

These experiments determined if supplemental vitamins A and/or E would enhance ovine antibody responses. All-rac-alpha-tocopheryl acetate was fed to lambs approximately 6 months old (30 to 40 kg) at levels of 33 (controls), 121, 276, 396, and 476 mg/kg of feed (which are total vitamin E levels). Primary and secondary immunizations with 10 mg keyhole limpet hemocyanin (KLH) were given. A nonlinear dose response of serum antibody titers was observed and the 476 mg vitamin E/kg treatment significantly enhanced (P less than 0.05) the peak primary response over controls. Retinyl acetate fed at five levels ranging from 7000 (the control level) to 97,400 IU/kg feed failed to influence antibody production to 10 mg KLH of lambs about 6 months old (29 to 41 kg). There was no detectable response to an ovalbumin antigen (100 mg). Neonatal lambs were injected with retinyl palmitate or the carrier of the injected vitamin. These lambs failed to raise antibody titers to either of the antigens administered (10 mg KLH, 100 mg ovalbumin). This was apparently due to a neonatal period of immune paralysis to certain antigens. A preliminary study showed that no KLH-specific antibodies are detectable in lambs immunized earlier than 7 weeks of age. Lambs in this age range were utilized in the last trial in which four treatments were applied: 3000 mg oral vitamin E, 400,000 IU injected vitamin A, 4 ml of the injectable vitamin A carrier, or no treatment. Half of the animals in each of these groups were immunized with 15 mg KLH and 1 ml Brucella ovis bacterin and the other half served as nonimmunized controls. No significant differences in titers to KLH were observed. Lambs receiving 3000 mg vitamin E or the carrier produced secondary peak anti-B. ovis titers higher (P less than 0.05) than those of the untreated controls.     

Asexual reproduction in Pygospio elegans claparède (Annelida, Polychaeta) in relation to parasitism by Lepocreadium setiferoides (Miller and Northup) (Platyhelminthes, Trematoda)

Life-history theory predicts that parasitized hosts should alter their investment in reproduction in ways that maximize host reproductive success. I examined the timing of asexual reproduction (fragmentation and regeneration) in the polychaete annelid Pygospio elegans experimentally exposed to cercariae of the trematode Lepocreadium setiferoides. Consistent with adaptive host response, polychaetes that became infected by metacercariae of trematodes fragmented sooner than unexposed controls. Parasites were not directly associated with fission in that exposed polychaetes that did not become infected also fragmented earlier than controls. For specimens of P. elegans that were not exposed to trematodes, new fragments that contained original heads were larger than those that contained original tails, whereas original head and tail fragments did not differ in size for infected polychaetes. In infected specimens, metacercariae were equally represented in original head and tail fragments and were more likely to be found in whichever fragment was larger. Despite early reproduction, parasitism was still costly because populations of P. elegans exposed to parasites were smaller than controls when measured 8 weeks later and because exposure to cercariae reduced survivorship of newly divided polychaetes. Taken together, my results suggest that early fragmentation is a host response to minimize costs associated with parasitism.     

Experimentally induced Fasciola hepatica infections in black-tailed deer.

The susceptibility of black-tailed deer (Odocoileus hemionus columbianus) to the common liver fluke (F. hepatica) was studied. Two deer and one sheep comprised each of three experimental groups. Animals in each group were inoculated individually with 250, 500, or 1000 F. hepatica metacercariae. One deer and one sheep given 1000 metacercariae died with lesions consistent with black disease 7 weeks after inoculation. At necropsy 6 or 15 weeks postinoculation, the mean percentage recovery of the inoculum was 38.9% from the deer and 51.9% from the sheep. Fluke eggs recovered from the deer were viable and metacercariae cultured from the eggs were fully infective for sheep. Pathologic changes associated with F. hepatica infection were more severe in the infected deer; consequently, the deer were less resistant to the lethal effects of the parasite than sheep. Considering the experimental results and the fact that naturally acquired common liver fluke infection has been reported infrequently from black-tailed deer, it was concluded that black-tailed deer do not constitute a significant reservoir for F. hepatica in domestic livestock.     

Delayed puberty caused by hyperthyroidism in ram lambs is not a result of suppression in body growth

Over a period of 8 weeks ram lambs (16 weeks old) were made hyperthyroidal (serum thyroxine approximately equal to 150 ng/ml, compared with control approximately equal to 48 ng/ml) by daily subcutaneous injections of thyroxine or maintained at a constant body weight by restriction of the feed intake. Hyperthyroidal and restricted-intake lambs remained at a constant body weight during the period of treatment whilst control rams gained body weight. Testicular growth was normal in restricted-intake lambs but was suppressed in hyperthyroidal animals. Hyperthyroidism, but not feed restriction, was also associated with decrease in LH pulse frequency (1.3 +/- 0.3/12 h compared with controls 4.8 +/- 0.9/12 h. Hyperthyroidal lambs showed normal LH responses to exogenous LHRH. After cessation of treatment testicular growth continued to be suppressed for up to 16 weeks in previously hyperthyroidic rams; thereafter testes began to increase in size but at 30 weeks after treatment were still smaller than those of control rams. It is concluded that elevated thyroxine concentrations directly influence sexual maturation in ram lambs through actions at hypothalamic and/or higher brain centres which control LH secretion. Transient hyperthyroidism during sexual maturation may cause permanent impairment of sexual development.     

A NEUROCHEMICAL STUDY OF FIELD CASES OF THE DELAYED SPINAL FORM OF SWAYBACK (ENZOOTICATAXIA) IN LAMBS

Abstract— Spinal cords of 5-8 week-old lambs affected with the delayed or spinal form of swayback were deficient in myelin lipid components such as cholesterol, cerebrosides and phospholipids and contained less water than cords obtained from clinically and histologically unaffected lambs. The rate of synthesis in vitro of cerebrosides containing hydroxy fatty acids was reduced but the fatty acid composition of this and of the "non-hydroxy cerebroside' fractions were quite normal. Isolated myelin was deficient in copper compared with healthy controls and there was a deficiency in one of its basic protein constituents. This suggested an inherent instability of myelin in affected lambs while the presence of some 4 per cent of cholesterol in the esterified form (none normally present) was indicative of degenerative changes. Five out of eight clinically unaffected lambs obtained from the same flocks proved to be sub-clinical cases when the spinal cord was examined histologically and showed some of the neurochemical abnormalities of the overt clinical disease. When affected lambs were kept at the laboratory until 16 weeks old and fed a copper sufficient diet some of the spinal lesions showed signs of regression and there were fewer neurochemical abnormalities. In particular, cholesterol esters could not be detected in the spinal cord lipids.     

Variation in resistance to haemonchosis: selection of female sheep resistant to Haemonchus contortus.

Seventy female lambs (6-7 months old) which were exposed to natural infections of Haemonchus contortus were designated as responders or non-responders on the basis of 10 weekly cumulative faecal egg counts. Selected responder and non-responder lambs were treated with ivermectin, housed separately and 6 weeks post-housing, seven lambs from each group were given a trickle infection of Haemonchus contortus at 1000 L3 daily for 5 days per week up to 2 weeks and examined weekly for 10 weeks after first infection. Analysis of data revealed significantly lower mean faecal egg counts and non-significantly less weight loss in responder than non-responder lambs. Mean values of haemoglobin, packed cell volume, total serum protein and peripheral eosinophil counts were significantly higher in responders than non-responders. In contrast, serum pepsinogen concentration was significantly less in responders than in non-responders. At 10 weeks post-infection, there were fewer pathological lesions and significantly lower worm burdens in responders than in non-responders. These results demonstrate a distinct resistance in responders to Haemonchus contortus infection.