Fibrinogen-degrading proteins from Haemonchus contortus used to vaccinate sheep.

Sixteen nonsibling sheep, approximately 12 months old, that were raised in a helminth-free environment, were used for 2 protection studies 6 months apart. Sheep were vaccinated weekly for 5 weeks by IM injection of fibrinogen-degrading proteins derived from the intestinal tract of adult Haemonchus contortus. Ten days after the last vaccination, sheep were given 2,500 infective H contortus larvae by intraruminal injection. Vaccinated sheep produced specific antibodies, and were protected from the worm challenge. Significant differences in mean fecal worm egg counts for 56 days after worm challenge, in mean numbers of H contortus worms, and female fecundity ratios at necropsy were detected in vaccinated sheep, compared with those in control sheep. These data suggest that the fibrinogen-degrading proteins have a protective role in vaccination of sheep against H contortus.     

Comparison of naturally acquired parasite burdens among Florida native, Rambouillet and crossbreed ewes.

The response to natural infection by gastrointestinal nematodes was evaluated in three groups of lambing mature ewes: 16 Florida Native; 21 Rambouillet; and 18 1/2 Florida Native x1/2 Rambouillet (F1 generation). Fecal egg counts (FEC), packed cell volume (PCV) and plasma protein levels were determined periodically through a one-year period. The highest FEC was recorded in Rambouillet ewes, with most of the means higher than 1000 eggs per gram (EPG). Florida Native and F1 ewes showed similar trends in FEC, with most of the means below 500 EPG. The highest mean PCV values were recorded in Florida Native ewes, followed in decreasing order by F1 and Rambouillet ewes. F1 ewes showed the lowest plasma protein levels and Florida Natives the highest. Fecal cultures revealed the highest percentages of Haemonchus larvae in Rambouillet ewes (>64%) and the lowest percentages in Florida Native ewes. Florida Native and F1 ewes were more resistant to gastrointestinal nematode infections than were Rambouillet ewes. High negative correlation coefficients between FEC x PCV and FEC x plasma protein levels coincided with peaks in mean FEC of Rambouillet (May and July), Florida Native and F1 ewes (October). This study showed that crossbreeding Rambouillet and Florida Native sheep resulted in the production of F1 ewes resistant against gastrointestinal nematode infections.     

Inheritance of fecal egg count and packed cell volume and their relationship with production traits in sheep infected with Haemonchus contortus

This study describes responses to artificial infection with Haemonchus contortus in ewes and lambs of 50% Dorset, 25% Rambouillet, and 25% Finn-sheep ancestry and provides estimates of genetic parameters for measures of parasite resistance. One hundred ninety-eight ewes out of 64 sires, and 386 lambs out of 25 sires were evaluated in autumn and spring of 2 yr. Ewes were dewormed shortly after weaning their lambs and lambs were dewormed at about 120 d of age. One week after deworming, ewes and lambs were dosed with approximately 10,000 infective larvae of H. contortus. After infection, BW, fecal egg counts (FEC), and packed cell volume (PCV) were measured weekly for 7 wk in lambs kept in drylot and fortnightly for 11 wk in ewes on pasture. Summary traits were defined as initial PCV, mean BW across all times, and means for PCV (MPCV) and log-transformed FEC (MLFEC) at wk 3 to 7 after infection for lambs and wk 3 to 11 after infection for ewes. Ewes and lambs did not lose weight overall in any year or season, but there was no consistent effect of year or lambing season on mean LFEC or mean PCV during infection in either ewes or lambs. Yearling ewes were less resistant to infection than older ewes, with lower PCV (P < 0.05) and higher LFEC (P < 0.05). During infection, PCV was positively correlated with BW and negatively correlated with LFEC in both ewes and lambs. In lambs, heritabilities were 0.39 (P < 0.01) for PCV, 0.10 (P < 0.05) for LFEC across all measurement times, and 0.19 (P < 0.01) for three measures of LFEC taken at the peak of infection. Heritability estimates for ewes were 0.15 (P < 0.05) for PCV and 0.31 (P < 0.01) for LFEC. Repeatabilities for LFEC and PCV across measurement times were moderate in ewes and lambs. Correlations between dam and lamb records for MLFEC were generally low, suggesting different mechanisms of resistance in lambs and nonlactating ewes. Ewes with higher genetic merit for growth as lambs were less resistant to infection as adults, but genetic merit for fertility and prolificacy were not related to parasite resistance. Lambs with higher genetic merit for body weight were more resistant to infection. Selection for resistance to H. contortus is therefore possible and should not adversely affect growth of lambs and fertility of ewes in this production environment.     

The intensity of resistance by mature Merino ewes against Haemonchus contortus and Tichostrongylus colubriformis in single-species and combined-species infection

Three-year-old, non-lactating and non-pregnant Merino ewes, raised on pasture under a program of strategic treatment with anthelmintic and found to be extremely resistant to "trickle" infection with Haemonchus contortus, were given single-dose infections with either H. contortus or Trichostrongylus colubriformis or both species together. The purpose was to ascertain the intensity of protective immunity against the 2 parasites in sheep with immunity acquired from a presumably slight exposure to infection. To provide a criterion, some infected ewes were immunosuppressed with corticosteroid, dexamethasone. Untreated ewes were extremely resistant to challenge infection with either 15,000 or 150,000 H. contortus or 15,000 T. colubriformis. Surprisingly, when mixed infection was given, egg counts for H. contortus were significantly elevated compared with infection by that species alone. Antibody to antigens from infective larval and adult H. contortus was measured in serum by enzyme-linked-immunosorbent assay (ELISA) during the course of infection. Serum titres against larval antigens were significantly depressed when infections with either H. contortus or T. colubriformis were permitted by immunosuppression with dexamethasone, whereas those against adult antigen were depressed when infection with T. colubriformis was permitted.     

Preliminary observations on the potential of gut membrane proteins of Haemonchus contortus as candidate vaccine antigens in sheep on naturally infected pasture.

In a trial lasting 11 months in South Africa, faecal egg counts and haematocrits of sheep vaccinated with gut membrane proteins of adult Haemonchus contortus were compared with unvaccinated controls grazing pasture contaminated with the parasite. Vaccination reduced egg output by >82% on average during one 4 month period of the trial and simultaneously significantly reduced the degree of anaemia and deaths due to haemonchosis. Although vaccine immunity was not sufficiently long lasting to prevent a surge in egg output which occurred after the onset of a period of irrigation, re-vaccinating the sheep at this point cleared their newly acquired infection and rapidly restored protection to approximately the level observed beforehand. It was clear that a vaccine based on parasite gut membrane proteins could offer substantial benefits in the control of natural haemonchosis.     

Genetic analysis of the potential role of IgA and IgE responses against Haemonchus contortus in parasite resistance of Creole goats

Small ruminants are affected by gastrointestinal nematode (GIN) infection. A promising alternative strategy for control of GIN infection is to increase the level of resistance in the population by taking advantage of the host's immune response. Immunoglobulin A (IgA) and E (IgE) are known to be involved in immune response to GIN. The aim of this study was thus to investigate genetic parameters of IgA and IgE responses against Haemonchus contortus in Creole kids naturally challenged at pasture and to determine the relationship with other resistance criteria such as faecal egg counts, packed-cell volume, eosinophil counts and bodyweight. Variance and covariance components for genetic and residuals effects for each trait were estimated on 3862 males at 11 months of age. Heritability estimates for IgA and IgE ranged between 0.15 and 0.57. Strong positive genetic correlations were observed between either IgE or IgA responses against L3 and adult excretory/secretory products (ESP) antigens of H. contortus, suggesting that the humoral immune response is not specific to the life cycle stage of the parasite suggesting that there is substantial cross recognition between the different parasite antigens. Heritability estimates for faecal egg count (FEC), packed-cell volume (PCV) and bodyweight (BW) were in accordance with previous results in Creole kids. Blood eosinophil counts were found moderately heritable and negatively correlated with FEC, suggesting that this cell population plays a role in resistance to nematode parasite infection in Creole goats. IgA response was positively correlated to FEC, in contrast with the negative correlation between IgE against L3 of H. contortus and FEC. In Creole goats, IgA response against L3 or ESP of H. contortus would rather be associated with the worm burden than an immune protective response. The immune response involving activity of IgE against L3 of H. contortus may be one important pathway for development of resistance to gastrointestinal nematode infections in Creole goats.     

The effects of ivermectin and moxidectin on egg viability and larval development of ivermectin-resistant Haemonchus contortus

The in vivo effects of ivermectin and moxidectin on egg viability and larval development of ivermectin-resistant Haemonchus contortus were examined over time after anthelmintic treatment of sheep. Twenty merino sheep, (12 months old) were allocated to five treatment groups and infected with ivermectin-resistant H. contortus. Thirty one days later, the sheep were treated with intraruminal ivermectin capsules, oral ivermectin, oral moxidectin or injectable moxidectin at the manufacturer's recommended dosages, or left untreated. At various times up to 112 days after treatment, faecal egg counts (FEC) were determined and development rates of infective larvae (L3) cultured in faeces or on agar were measured. Eggs in faecal cultures from ivermectin capsule treated sheep showed reduced L3 development percentages in comparison to faecal cultures from untreated sheep. Eggs from ivermectin capsule treated sheep, isolated from faeces, and cultured on agar showed similar L3 development to eggs from control sheep. These results demonstrate an inhibitory effect of excreted ivermectin in faeces on larval development of ivermectin-resistant H. contortus. L3 development in faecal culture from animals receiving oral ivermectin were reduced for only 3 days after treatment. Faecal egg counts and development of L3 larvae in both culture systems from moxidectin treated sheep were low, due to the high efficacy of the drug. Egg counts in moxidectin treated sheep were reduced by approximately 90% 24h after treatment, before decreasing to almost 100% at 48h, suggesting that the current quarantine recommendation of holding sheep off pasture for 24h after treatment may still lead to some subsequent pasture contamination with worm eggs.     

Relative resistance of Dorper crossbred ewes to gastrointestinal nematode infection compared with St. Croix and Katahdin ewes in the southeastern United States

A study was conducted to evaluate relative resistance of Dorper crossbred (DO), Katahdin (KA), St. Croix (SC), and Hampshire (HA) ewes to natural and experimental gastro-intestinal (GI) nematode infection over a 20-month period. The objective of Experiment 1 was to evaluate breeds for resistance to infection acquired naturally from mixed grass pastures. In Year 1 (May-December 2000) de-worming of ewes occurred during wet, hot conditions in July and during late pregnancy in December. In Year 2 (January-December 2001), ewes were de-wormed after fecal egg count (FEC) for a breed group rose above 1000 eggs per gram (epg) or after blood packed cell volume (PCV) of an individual ewe fell below 20. FEC was determined every 28 days and PCV every 14-28 days. In both the years, ewes were pastured together, except during the 28-days breeding periods, on tall fescue, bermudagrass, or ryegrass, and rotated among pastures dependent on forage availability. Ewes were in good or excellent condition (body condition score of 3-4 out of 5) throughout the study. The objective of Experiment 2 was to evaluate the breeds for relative resistance to an experimental infection with Haemonchus contortus infective larvae. Both PCV and FEC were determined every 7 days from 14 to 42 days after inoculation with 30000 infective larvae per ewe. In Experiment 1, Year 1, FEC was slightly greater and PCV was lower from July to September in DO ewes (breed x time, P<0.001). In Year 2, de-worming occurred 14 days later in DO ewes compared with other breed types. Otherwise PCV and FEC were similar among the hair breeds and higher and lower, respectively, compared with HA ewes (breed x time, P<0.001). In Experiment 2, FEC and PCV were similar among hair breeds; FEC was lower and PCV higher in hair breeds compared with that of HA ewes (P<0.01). Relative resistance of mature Dorper crossbred ewes was comparable to that of Katahdin and St. Croix ewes and superior to that of Hampshire ewes.     

Vaccination against Haemonchus contortus: performance of native parasite gut membrane glycoproteins in Merino lambs grazing contaminated pasture

In a replicated trial, parasitological and antibody responses of grazing weaner Merino sheep were assessed following vaccination with gut membrane proteins prepared from adult worms of the gastrointestinal nematode, Haemonchus contortus. Each vaccinated animal received 100 microg native H11 and 100 microg native H-gal-GP combined together in 5mg Quil A administered intramuscularly on days 0, 34, 80 and 127. Control animals received 5mg Quil A alone on the same days. Vaccinated and unvaccinated control animals grazed pastures contaminated with the parasite from day 34 of the trial, and levels of parasitism were monitored by worm-egg counts (WECs) in faeces and packed cell volumes (PCVs) in blood. The level of larval contamination on pasture was estimated from the worm counts of tracer sheep introduced monthly to the paddocks. WECs and anaemia were significantly reduced in vaccinated animals, and, in contrast to vaccinates, all control sheep required salvage treatment with anthelmintic. By the last 2 months of the trial, pastures grazed by vaccinated animals had significantly lower contamination with H. contortus larvae. Vaccinated animals had high levels of vaccine antigen-specific IgG1 and IgG2 antibodies in plasma, whereas those responses in the control sheep were very low. IgG1 titres in the vaccinated group, but not IgG2 titres, were inversely correlated with worm-egg counts. The levels of systemic IgA and IgE remained low but increased in both groups towards the end of the experiment most probably from exposure to the natural infection from pasture. The results showed that H11 and H-gal-GP behaved like "hidden" antigens producing high levels of protection that were probably mediated through mechanisms involving antibodies, and in particular, IgG1. It was concluded that if similar protective effects could be obtained with recombinant versions of the proteins present in either H11 or H-gal-GP, then the prospects for a commercial Haemonchus vaccine were real.     

Genetic parameters for resistance to Haemonchus contortus in Merino sheep in South Africa

Resistance to natural infections of Haemonchus contortus (wireworm) was studied in Merino sheep. From February 1995 to July 1996 data were collected from the OTK Merino Stud on the farm Wildebeesfontein near Hendrina in Mpumalanga province. From March 1998 to January 2000 another Merino flock at Irene in Gauteng province was included in the study. In both cases, 50 randomly-chosen animals from each group of lambs weaned were sampled weekly for faecal egg count (FEC) until a mean of 2500 eggs per gram faeces (epg) was reached. At this stage, all lambs in the group were sampled for FEC, packed cell volume (PCV) and live mass in the case of the Wildebeesfontein flock. Animals were monitored from weaning to about 1 year of age. Data from 5 different groups of lambs (583 animals and 1722 records in total) with genetic links were accumulated and analysed. Variance components and resulting heritabilities for logFEC and PCV were estimated using a bivariate animal model with repeated records. The estimated heritabilities were 0.24 +/- 0.02 for logFEC and 0.14 +/- 0.02 for PCV. A strong negative correlation (-0.84 +/- 0.06) existed between the 2 traits. The genetic correlation between PCV and live mass for the Wildebeesfontein flock was low (0.28 +/- 0.09), while that between logFEC and live mass was negative, although not significantly different from zero (-0.13 +/- 0.09). These results suggest that FEC can be used as a selection criterion for resistance to H. contortus infections in Merino sheep.     

Segregation of natural and experimental gastrointestinal nematode infection in F2 progeny of susceptible Suffolk and resistant Gulf Coast Native sheep and its usefulness in assessment of genetic variation

Gastrointestinal nematode parasitism is a concern to small ruminants worldwide. Productivity has been compromised because such nematodes, particularly Haemonchus contortus, have developed resistance to available anthelmintics. Some sheep breeds and lines within breeds are relatively resistant to infection, a trait that may be useful for developing control strategies. Suffolk sheep, which are susceptible to infection, were crossed with Gulf Coast Native sheep, which are more resistant to infection, to produce F1 progeny. F1 rams were bred to F1 ewes which produced 227 F2 offspring. These F2 offspring were evaluated for variability in infection levels, based on fecal egg count (FEC) and blood packed cell volume (PCV), under two natural infection conditions (one at weaning and another after a summer grazing period) and one experimental infection. The range of both FEC and PCV was large for all three infection periods with annual variation. Overall, the range for the three infection periods, respectively, were 167-149,933, 0-31,400 and 17-114,667 eggs per gram (EPG) of feces and 8.7-37.0%, 7.3-33.0% and 8.3-36.0%. This segregation of infection is what would be expected of F(2) progeny from susceptible and resistant parent breeds. Heritabilities of FEC and PCV for the three infection periods, respectively, were 0.15, 0.29 and 0.12, and 0.11, 0.22 and 0.12. Based on segregation of infection, larger heritabilities and maternal environment effects that declined after weaning, the summer natural infection was probably the best model for assessing genetic variation.     

Control of gastrointestinal nematodes with copper oxide wire particles in a flock of lactating Polypay ewes and offspring in Iowa, USA

Copper oxide wire particles (COWP) have been used to reduce infection of Haemonchus contortus in hair breed lambs in southeastern USA without signs of copper toxicity. However, copper sensitivity among breeds and regions varies. The objective was to determine the effectiveness and safety of COWP in lactating Polypay ewes and their offspring grazing alfalfa/bluegrass pasture in a rotational grazing system. Mature Polypay ewes were administered 0, 0.5, 1, or 2 g (n=8 or 9/dose) COWP approximately 60 days after lambing in mid-July 2005. Their offspring were administered 0 (n=6), 0.5 or 0.75 g (n=9), 1 or 2 g (n=6) COWP 2 weeks later in late July. The primary gastrointestinal nematode was H. contortus (70%). Between Days 7 and 35, FEC were greater in 0 and 0.5 g COWP groups compared with ewes administered 2 g COWP (COWP x day, P<0.004). PCV decreased in all groups of ewes between Days 0 and 21 (day, P<0.001). Aspartate aminotransferase (AST) activity, a measure of liver copper levels, and body weight was similar among groups of ewes. FEC decreased within 7 days in COWP-treated compared with untreated lambs and remained low throughout experiment (COWP x day, P<0.05). PCV increased in COWP-treated lambs between Days 7 and 35 and decreased in untreated lambs between Days 0 and 21 (COWP x day, P<0.009). AST activity was similar among groups of lambs. Administration of 2 g COWP to ewes prevented a rise in FEC, but a dose of 0.5 g was ineffective as an anthelmintic. Administration of all doses of COWP to lambs decreased FEC and increased PCV compared to untreated lambs. There were no signs of copper toxicity in ewes or lambs. Alternative suppression of H. contortus infections may be necessary in ewes, but COWP was effective in H. contortus management for lambs.     

Evaluation of the phenotypic performance of a Red Maasai and Dorper double backcross resource population: indoor trickle challenge with Haemonchus contortus

Six F(1) Dorper (D) x Red Maasai (R) rams were mated to both D and R ewes to produce backcross lambs. These six double backcross resource families are being analysed to identify quantitative trait loci that may be controlling resistance to gastro-intestinal (GI) nematode parasites, mainly Haemonchus contortus. After assessing the phenotypic performance of the lambs following exposure to natural infections, the surviving lambs were drenched and moved indoors for an artificial challenge with H. contortus along with straightbred D and R lambs. A total of 1317 lambs were included in the analyses consisting of 523 3/4D, 580 3/4R, 87 D and 127 R. The D lambs were significantly and consistently heavier than R lambs and 3/4D were significantly heavier than the 3/4R lambs. The difference between the backcrosses was about half of that between the straightbreds. Resistance was assessed in terms of faecal egg counts (FEC) and total worm counts (TWC) at necropsy while packed cell volume (PCV) was used to assess resilience to weekly oral doses of 2500 infective larvae of H. contortus. No significant breed differences were observed for log transformed FEC (LFEC). A significant breed difference in PCV was recorded. The backcrosses had the higher values and while no differences were observed between the straightbreds, 3/4D had significantly higher PCV than the 3/4R. Despite the absence of breed differences in FEC the R and the 3/4R had significantly fewer worms than the D and the 3/4D. The D had significantly longer worms than the R and the 3/4D had significantly longer worms than the 3/4R. Worms recovered from D had more eggs than those recovered from R. Similarly worms from 3/4D contained more eggs than those from 3/4R. Thus, on a breed basis the breed with more worms had longer worms. In contrast, when, in a small part of the experiment two doses of larvae were used to check for any breed by dose interactions, worms from the low dose (and hence fewer worms) animals were longer. We postulate that in fast growing hosts like the D, worms also have a better potential for growth and reproduction than in hosts that have less potential for growth. The overall correlation coefficient between PCV and LFEC was -0.67 while that between LFEC and LTWC was 0.72. From these results it is clear that the R do not respond to the artificial challenge the way they do to natural infection suggesting that phenotyping of R and R cross lambs for purposes of selecting those that are resistant or susceptible is best done under natural challenge.     

Influence of protein supplementation during late pregnancy and lactation on the resistance of Santa Ines and Ile de France ewes to Haemonchus contortus

This experiment aimed to evaluate the effect of periparturient metabolizable protein (MP) nutrition on resistance to Haemonchus contortus in single rearing Ile de France and Santa Ines ewes. The restriction-fed iso-energetic diet was calculated to provide either 0.8 (low MP diet) or 1.3 (high MP diet) times MP, from three weeks before parturition until eight weeks into lactation. The ewes were experimentally infected with 1000 H. contortus infective larvae (L3) three times a week (Mondays, Wednesdays and Fridays), starting five weeks before the predicted date for parturition until a total of 15,000 L3 had been administered. While both breeds showed elevated fecal egg counts (FEC), these values were significantly lower for Santa Ines ewes than Ile de France ewes, but were independent of level of MP feeding. The latter also did not affect lamb weight gain and ewe body weight variation in each breed. Packed cell volume and total plasma protein for Santa Ines in all periods were significantly higher than those for Ile de France ewes (P<0.01) but were not affected by nutrition. In contrast, levels of serum IgG and IgA antibodies against somatic H. contortus infective larvae and adult antigens were similar between breeds but higher in animals that received high MP diets (P<0.05). The reduced body score of ewes at the beginning of the experiment probably influenced their high susceptibility to incoming larvae. Since, unexpectedly, MP scarcity was not achieved in this experiment, our data support the view that Santa Ines ewes are more resistant to H. contortus than Ile de France ewes.     

Structural and functional characterisation of the ovine interferon gamma (IFNG) gene: its role in nematode resistance in Rasa Aragonesa ewes

The ovine interferon gamma (IFNG) gene was structurally and functionally characterised, and the association between its variants and the response to Haemonchus contortus infection was examined in resistant and susceptible Rasa Aragonesa ewes in different physiological conditions. Sequential faecal egg counts (FECs) were performed on a naturally infected flock of 400 Rasa Aragonesa ewes and two groups of ewes selected for high (n=38) or low (n=33) FEC, which were subsequently challenged with H. contortus in the spring and autumn. The second challenge infection was performed to explore the extent to which the previously established resistance or susceptibility to H. contortus based on FEC reflected the worm burdens of ewes in different physiological states. The ovine IFNG gene was partially isolated and characterised, including the promoter and other regulatory regions. In addition, DNA sequencing of the resistant and susceptible Rasa Aragonesa ewes revealed 13 polymorphisms (12 SNPs and 1 microsatellite) in the IFNG gene. Association studies showed that a haplotype composed of C (SNP -641) and (GTTT)? (microsatellite) was linked to the FEC and worm burden recorded in spring and autumn challenges, respectively, and thus to susceptibility to H. contortus in adult Rasa Aragonesa ewes. Our results also suggested that the IFNG gene was not responsible for the phenotypic variation in susceptibility to H. contortus but was linked to a QTL segregating near the IFNG gene. No significant differences in IFNG gene expression were found between either resistant and susceptible or pregnant and non-pregnant ewes. Finally, we studied the effects of the polymorphisms on IFNG gene expression, finding that the C/T SNP at -641 and the microsatellite showed significant effects in spleen. The C/T SNP at -641 was located within a possible trans-acting factor binding site. Further work is needed to elucidate the function of this SNP.     

The epidemiology of abomasal nematodes of sheep in Sweden, with particular reference to over-winter survival strategies

In May 2002, studies on the seasonal patterns of nematode infection of sheep were undertaken on four commercial sheep farms in southern Sweden, which had previously reported problems with nematode parasitism, especially due to Haemonchus contortus. One farm was used for intensive investigation. This entailed the establishment of two replicate groups of sheep, each consisting of 20 ewes and their lambs, on adjacent pasture paddocks. The seasonal patterns of nematode infection were followed by regular (approximately monthly) sampling of both ewes and lambs for nematode faecal egg counts and larval differentiation, and the sequential use of replicate groups of tracer lambs. H. contortus and Teladorsagia circumcincta were the most abundant nematode species, with the former most prevalent in the post-parturient faecal egg counts of ewes. Tracer worm counts showed almost 100% arrested development in the early fourth larval stage for H. contortus as early as mid-summer and the numbers of parasites progressively increased during the season. T. circumcincta also showed high levels of arrested development, but not as early, or as absolute, as for H. contortus. Tracers allocated to the paddocks at the time of turn-out following winter in May 2003, showed virtually a total absence of H. contortus in contrast to exceedingly high infections with T. circumcincta. Results of the three additional monitoring flocks supported these findings. It can be concluded that under Swedish sheep farming conditions, H. contortus has evolved to survive the long, cold winters entirely within the host as the arrested larval stage, relying on the lambing ewe to complete its life cycle. The peri-parturient relaxation of resistance in the ewes triggers the resumption of development to the adult egg-laying parasites to result in pasture contamination and the completion of just one parasite generation/year. In contrast, T. circumcincta can survive well over-winter, both on pasture and within the host.     

Effect of dexamethasone treatment on the immune response of Gulf Coast Native lambs to Haemonchus contortus infection

Neonatal and weaner Gulf Coast Native (Native) lambs were studied to determine whether an immunological basis underlies their natural resistance to Haemonchus contortus infection. Neonatal Native lambs (n = 8) and weaner Native lambs (n = 15) were randomly assigned to a treatment or a control group. Lambs in the treatment group received dexamethasone by intramuscular injection three times a week for 10 weeks (neonatal) and 15 weeks (weaners). All lambs were monitored for fecal egg count (FEC), blood packed cell volume (PCV), and white blood cell differential counts on a weekly basis for the duration of the studies. Neonatal lambs were kept on pasture with their dams and weaner lambs were dewormed at weaning and kept in pens where they received trickle infections. Serum antibody titers to H. contortus whole worm antigen (WWA) were determined using ELISA. Lymphocyte proliferation assays on peripheral blood mononuclear cells (PBMC) were done to assess lymphocyte function. All lambs were vaccinated with killed Brucella abortus strain 19 to assess the effect of dexamethasone treatment on antibody response. All lambs were necropsied at the end of each study to recover the contents of the gastrointestinal tract for nematode enumeration and identification. The results showed that mean FEC and mean PCV of the treatment group was significantly higher and lower, respectively, than in the control group in both neonatal and weaner lambs from weeks 6 and 5, respectively. At necropsy, total nematode count was significantly higher in treatment groups than in the control groups. Serum antibody titers to H. contortus WWA were significantly lower in treated groups than in control groups. Treatment groups showed a consistent depression in lymphocyte percentage being significantly lower from week 6 in both neonatal and weaner lambs. No differences were found in the response of PBMC to mitogen stimulation between the groups. Lambs in the control groups showed strong positive brucellosis card tests and the treatment groups did not. Dexamethasone treatment resulted in depression of the immune response and loss of natural resistance of Native lambs to H. contortus infection. The results of these studies suggest that some aspects of the immune response may underlie the natural resistance of Native sheep to H. contortus infection.     

Wool production of sheep chronically infected with Haemonchus contortus

A chronic infection was established in a group of twenty 18-month-old non-reproductive Merino ewes by oral administration of 3000 infective H. contortus larvae twice weekly for 12 weeks. Their live-weights and wool production were compared with those of 20 uninfected ewes grazing the same pasture. In the infected sheep, faecal egg counts increased over a period of 9 weeks to reach a mean of 5000 eggs per gram, accompanied by small, but significant effects on packed-cell volume and live-weight. The effects of infection on wool growth were also small, and not statistically significant, although there was evidence of a faster seasonal decline in wool growth in infected sheep. It was concluded that mortality induced by acute infection is the most important economic effect of this parasite.     

Association between major histocompatibility complex microsatellites, fecal egg count, blood packed cell volume and blood eosinophilia in Pelibuey sheep infected with Haemonchus contortus

The objective of this study was to assess the correlation among traits associated with resistance or susceptibility to Haemonchus contortus infestation and to evaluate the participation of the ovine major histocompatibility complex (MHC) in Pelibuey sheep, a prevalent breed in tropical and sub-tropical regions in Mexico and elsewhere. Association among the fecal egg count (FEC), blood packed cell volume (PCV), antibody (AB) levels, serum proteins (SP) and blood eosinophil count (EOS) was assessed in 52 lambs experimentally infected with H. contortus, and the participation of the MHC was evaluated using polymorphisms in three microsatellites, located at the class I (OMHC1) and class II (OLADRB1, OLADRB2) regions of the MHC. Spearman correlation analysis among the traits showed a negative association (P<0.01) between FEC and PCV (-0.35), EOS (-0.50), SP (-0.30) and AB (-0.57), and a positive correlation of antibodies with EOS (0.50). The homozygotes for the OMHC1-188 and OLADRB2-282 alleles were associated with a reduction in FEC (-813 and -551, respectively). Conversely, the OMHC1-200 and OMHC1-206 alleles were associated with an increase in FEC (1704 and 1008, respectively). Furthermore, the OLADRB1-482 allele was associated with an increase of 163 EOS by allele copy, while the OMHC1-200 allele showed a reduction of 95 EOS in homozygotes. The associations among microsatellite MHC loci and the remaining variables were not significant. These results reinforce the evidence that MHC polymorphisms have an important role in parasite resistance or susceptibility in Pelibuey sheep and could be used as genetic markers to assist selection and improve parasite resistance to H. contortus.     

A positive association between resistance to ovine footrot and particular lymphocyte antigen types

The distribution of 12 Class I ovine lymphocyte antigens (OLA) was examined in 4 flocks of sheep vaccinated against and/or challenged with Bacteroides nodosus, the transmitting agent of footrot. In a flock of 47 Corriedales in New Zealand, which had been specially bred for resistance to footrot, a higher frequency (70.2%) of OLA type SY6 was found compared with 42.9% in 49 unselected Corriedale sheep (P = 0.001). The serum antibody response of 12 selected Corriedale ewes was compared with that of 12 unselected ewes of the same age after vaccination with a multivalent footrot vaccine and the selected ewes had significantly (P = 0.01) higher agglutinin titres than the unselected ewes, 7 weeks after vaccination. In 3 trials involving 108, 120 and 135 Australian Merinos in Victoria, SYlb was associated with a reduction in the number of feet affected with severe footrot (P = 0.05, P = 0.01, P = 0.02) and in 2 of the trials there was a relationship between SY6 and high vaccinal agglutinin titres. This SY6 effect was evident in the first trial 31 days after primary vaccination (P = 0.05) and again 20 days later after secondary vaccination (P = 0.01). In the second trial, when the sheep were vaccinated 49 days after challenge, an association was again found between SY6 and high agglutinin titres (P = 0.05) after primary but not after secondary vaccination. Exposure of 157 vaccinated Merino rams to B. nodosus during a footrot outbreak in New South Wales also showed an association between low infection and SY6 and SYlb.(ABSTRACT TRUNCATED AT 250 WORDS)