Antibody responses of sheep to challenge with Trichostrongylus colubriformis and the effect of dexamethasone treatment

Trichostrongylosis: a zoonotic disease of small ruminants

In the present world a significant threat to human health is posed by zoonotic diseases. Helminth parasites of ruminants are one of the most common zoonotic organisms on the planet. Among them, trichostrongylid nematodes of ruminants, found worldwide, parasitize humans in different parts of the world with varying rates of incidence, particularly among rural and tribal communities with poor hygiene, pastoral livelihood and poor access to health services. In the Trichostrongyloidea superfamily, Haemonchus contortus, Teladorsagia circumcincta, Marshallagia marshalli, Nematodirus abnormalis and Trichostrongylus spp. are zoonotic in nature. Species of the genus Trichostrongylus are the most prevalent gastrointestinal nematode parasites of ruminants that transmit to humans. This parasite is prevalent in pastoral communities around the world and causes gastrointestinal complications with hypereosinophilia which is typically treated with anthelmintic therapy. The scientific literature from 1938 to 2022 revealed the occasional incidence of trichostrongylosis throughout the world with abdominal complications and hypereosinophilia as the predominant manifestation in humans. The primary means of transmission of Trichostrongylus to humans was found to be close contact with small ruminants and food contaminated by their faeces. Studies revealed that conventional stool examination methods such as formalin-ethyl acetate concentration or Willi's technique combined with polymerase chain reaction-based approaches are important for the accurate diagnosis of human trichostrongylosis. This review further found that interleukin 33, immunoglobulin E, immunoglobulin G1, immunoglobulin G2, immunoglobulin M, histamine, leukotriene C4, 6-keto prostaglandin F_1α, and thromboxane B₂ are vital in the fight against Trichostrongylus infection with mast cells playing a key role. This review focuses on the prevalence, pathogenicity and immunological aspects of Trichostrongylus spp. in humans.

Immunoglobulins as Biomarkers for Gastrointestinal Nematodes Resistance in Small Ruminants: A systematic review

The rise of anthelmintic resistance worldwide has led to the development of alternative control strategies for gastrointestinal nematodes (GIN) infections, which are one of the main constraints on the health of grazing small ruminants. Presently, breeding schemes rely mainly on fecal egg count (FEC) measurements on infected animals which are time-consuming and requires expertise in parasitology. Identifying and understanding the role of immunoglobulins in the mechanisms of resistance could provide a more efficient and sustainable method of identifying nematode-resistant animals for selection. In this study we review the findings on immunoglobulin response to GIN in the literature published to date (june 2019) and discuss the potential to use immunoglobulins as biomarkers. The literature review revealed 41 studies which measured at least one immunoglobulin: 35 focused on lamb immune response (18 used non-naïve lambs) and 7 on yearlings. In this review we propose a conceptual model summarizing the role of immunoglobulins in resistance to GIN. We highlight the need for more carefully designed and documented studies to allow comparisons across different populations on the immunoglobulin response to GIN infection.

Comparison of the timing of development of immunity in Romney lambs from resistant and resilient selection lines

Variation in the timing of development of immunity to gastro-intestinal nematode parasites was assessed in resistant and resilient Romney selection lines exposed to mixed natural infection. From weaning, at mean 92 days-of-age, animals (n = 53) were sampled for faecal egg count (FEC) expressed as eggs per gram of faeces (epg), saliva for immunoglobulin (IgG and IgA) determination and fasted live weight (LW) every 10 days until 351 days-of-age. Overall, mean back-transformed FEC were consistently low for resistant animals (<200 epg) whereas resilient counterparts' FEC increased with time to reach a peak of 1400 epg at day 230 for females and 1800 epg for males at day 280 before declining to less than 500 epg by day 300, respectively (P < 0.001). Resistant lambs reached a threshold for Trichostrongylus colubriformis L3-specific IgG which was indicative of the presence of immunity earlier at 220.6 ± 8.8 days-of-age compared with resilient-line animals which reach this threshold 40 days later at 263.4 ± 6.9 days-of-age (P < 0.001). In addition, resistant females reached sexual maturity earlier compared with their resilient counterparts viz. 263.5 ± 3.7 c.f. 274.4 ± 3.4 days-of-age, respectively, (P = 0.048). Mean fasted live weight (LW) showed a selection line by time interaction (P < 0.001) which reflected greater LW in the early phase of the study in resilient males but increasing for all groups until day 280 before declining and being similar for all groups from day 330. In summary, differences appear to exist in the timing of immune development between these Romney lines, with resistant animals developing immunity earlier and these resistant-line animals also appear to be more physiologically mature at the same chronological age than resilient animals. These observations have implications on the timing of identification and selection of resistant animals.

Heritabilities of IgA and IgE activities against Teladorsagia and Trichostrongylus L3 larval antigens correlated with traits for faecal worm egg count, health and productivity in Merino sheep

Low faecal worm-egg count (FEC) has been used as a phenotypic trait for breeding sheep for resistance to nematode infection. Application of this trait has resulted in a major reduction in worm-egg output in faeces, but in certain environments some of the resistant sheep develop what is believed to be a hypersensitivity-associated diarrhoea, resulting in soiling of the breech area (dags). To avoid breeding sheep with a propensity to scour, it, therefore, seems logical to combine selection for low FEC with selection for a trait based on the immune response to worm infection. We, therefore, investigated the relationships between immunoglobulin A (IgA) and immunoglobulin E (IgE) activities against L3 larval antigens of Teladorsagia circumcincta and Trichostrongylus colubriformis and measured their inheritance and their genetic relationships with FEC, dag traits and measures of production. Total plasma IgE concentrations were also determined and included in the study. We used 762 progeny from 23 Merino sires selected for resistance to helminths (the ‘Rylington Flock’) and to breech flystrike. Total plasma IgE and IgE activities against T. circumcincta were negatively correlated with liveweight, longissimus dorsi muscle depth, and fleece weight, but positively correlated with severity of dags, softer faeces and back-fat depth. The IgA activity against T. circumcincta was negatively correlated with liveweight and clean fleece weight, but positively correlated with back-fat depth and faecal consistency score. The IgA activity against T. colubriformis was not correlated with any measure, except the breeding value for dags around yearling age. The heritability for the IgA activity against T. circumcincta and T. colubriformis was 0.13 and 0.08 respectively. The heritability for both IgE activity against T. circumcincta and total plasma IgE concentration was 0.39. We conclude that it is feasible to combine FEC and dag traits with an immune trait (in this case, helminth-specific IgE activity and total plasma IgE concentration) and, thus, genetically select sheep for resistance to both helminths and diarrhoea. However, under natural worm-challenge conditions, blood sampling protocols need to developed, on the basis of animal age, season and environment, for optimum estimation of the immune trait and its relationships with other traits.

Immunogenomics of gastrointestinal nematode infection in ruminants - breeding for resistance to produce food sustainably and safely

Gastrointestinal nematode (GIN) infection of ruminants represents a major health and welfare challenge for livestock producers worldwide. The emergence of anthelmintic resistance in important GIN species and the associated animal welfare concerns have stimulated interest in the development of alternative and more sustainable strategies aimed at the effective management of the impact of GINs. These integrative strategies include selective breeding using genetic/genomic tools, grazing management, biological control, nutritional supplementation, vaccination and targeted selective treatment. In this review, the logic of selecting for "resistance" to GIN infection as opposed to "resilience" or "tolerance" is discussed. This is followed by a review of the potential application of immunogenomics to genetic selection for animals that have the capacity to withstand the impact of GIN infection. Advances in relevant genomic technologies are highlighted together with how these tools can be advanced to support the integration of immunogenomic information into ruminant breeding programmes.

The host immune response to gastrointestinal nematode infection in sheep

Gastrointestinal nematode infection represents a major threat to the health, welfare and productivity of sheep populations worldwide. Infected lambs have a reduced ability to absorb nutrients from the gastrointestinal tract, resulting in morbidity and occasional mortality. The current chemo‐dominant approach to nematode control is considered unsustainable due to the increasing incidence of anthelmintic resistance. In addition, there is growing consumer demand for food products from animals not subjected to chemical treatment. Future mechanisms of nematode control must rely on alternative, sustainable strategies such as vaccination or selective breeding of resistant animals. Such strategies take advantage of the host's natural immune response to nematodes. The ability to resist gastrointestinal nematode infection is considered to be dependent on the development of a protective acquired immune response, although the precise immune mechanisms involved in initiating this process remain to be fully elucidated. In this study, current knowledge on the innate and acquired host immune response to gastrointestinal nematode infection in sheep and the development of immunity is reviewed.

The effects of cold exposure, food allowance and litter size on immunity of periparturient sheep to Teladorsagia circumcincta and Trichostrongylus colubriformis

Sixty twin- and 60 single-bearing ewes were used in a 2 ✕ 2 ✕ 2 factorial design incorporating nutritional and shearing-cold exposure treatment (no. = 15 per subgroup) to study the effects on periparturient immunity against nematode infection. From week –8 to week –4 relative to parturition, animals grazed ryegrass-white clover pastures at allowances designed to provide either 0·8 (low allowance) or 1·2 (high allowance) of estimated metabolizable energy requirement for a ewe carrying 1·5 lambs. During week –4, half the sheep from each reproductive effort and nutritional group were shorn and exposed to artificial wind and rain for 4 h on each of 4 days consecutively. During the same week, all animals were challenged twice with 1·5 ✕ 104 Teladorsagia circumcincta and 1·5 ✕ 104 Trichostrongylus colubriformis infective third stage larvae to supplement the natural infection from pasture. From week –3 to week + 5 relative to parturition the sheep were run together and offered pasture at rates estimated to enable them to meet their nutrient requirement.

Shearing and cold stress reduced ewe body weight (by up to 4·66 kg per sheep), and temporarily reduced the levels of IgA against T. circumcincta (by proportionately 0·24) and T. colubriformis (0·34), and raised faecal egg count (1·6 to 4·4 fold) but did not affect the parasite-specific total antibody levels in the serum. The low allowance during pregnancy was associated with a 6·09-kg decrease in ewe body weight, decrease in the levels of total antibody and of IgA against both T. circumcincta and T. colubriformis, and increased faecal egg count (by 1·5 to 10·6 fold) during pregnancy and lactation. Faecal egg count was consistently higher (up to 8·8 fold) in twin-bearing and rearing ewes than in single-bearing and rearing ewes except during the first 4 weeks of the experiment. A significant effect of litter size on antibody levels was relatively small when compared with short-term changes (reductions) in antibody levels immediately around parturition. There were significant negative correlations between faecal egg count during late pregnancy and ewe body weight in lactation and significant negative correlations between the levels of total antibody against T. colubriformis and faecal egg counts near the end of the experiment. We conclude that litter size is likely to have greater influence on the resistance of ewes to nematode infections in the periparturient period than either pasture allowance or cold stress within the range of parameters used in this work.

Genetic and phenotypic relationships among faecal egg count, anti-nematode antibody level and live weight in Angus cattle

Genetic variation in host resistance to nematode infection was studied in two calf crops of Angus cattle under natural pasture challenge conditions in New Zealand, using faecal egg count (FEC) as a marker trait. Genetic and phenotypic correlations of FEC with anti-nematode antibody (Ab) level and live weights were also estimated. Ab to the third larval stages of Cooperia curticei (Cc), Cooperia oncophora (Co), Ostertagia ostertagi (Oo) and Trichostrongylus colubriformis (Tc) were assayed, along with Ab to the adult stages of Co and Oo. Calves were born in the late winter/early spring of 1995 and 1996 over a 7-week period each year; they were weaned at an average of 4 months of age, and faecal samples for FEC were taken at 10 months of age (1995 crop) and at 7 months of age (1996 crop). Blood samples for Ab were taken from both calf crops at intervals between 4 and 20 months of age, and later in peri-partum heifers and cows (23 to 25, and 36 to 37 months of age). For the two calf crops combined, there were 370 animals by 24 sires. Additional blood samples were taken on related animals from the 1993 and 1994 calf crops (218 extra animals, 19 different extra sires). The heritability of loge (FEC + 100) was 0•32 (s.e. 0•16), and heritabilities of loge Ab between 4 and 9 months of age averaged 0•30, between 11 and 20 months of age 0•22, and peri-partum 0•30. Between-animal repeatabilities of Ab levels from samples taken between 4 and 9 months of age averaged 0•40, between 11 and 20 months 0•48, and peri-partum 0•35. Genetic and phenotypic correlations among loge Abs for various nematode species (in hosts aged 4 to 9 months) averaged 0•82 and 0•61, respectively. The corresponding genetic and phenotypic correlations between loge Ab and loge(FEC + 100) averaged –0•48 and –0•07, respectively, whilst those between loge Ab and yearling weight averaged 0•29 and 0•13, respectively. It was concluded that direct selection to reduce FEC should be feasible if required, but index selection combining increased live weight and reduced FEC would often be preferable; anti-nematode Ab levels were repeatable, and FEC and Ab levels were negatively associated genetically.

Effect of gastro-intestinal nematode infection on sheep performance: a systematic review and meta-analysis

Background

Gastrointestinal nematode (GIN) infections are common in domestic sheep and impact directly and indirectly on the health of infected animals as well as on the associated economic production. In this study, we aim at summarizing the current knowledge on the influence of GIN infections on sheep production by conducting a systematic review. A subsequent meta-analysis of relevant studies was performed to provide an estimate of the effect of GIN infections on weight gain, wool production and milk yield.

Methods

A literature search was performed on the CAB, Pubmed and Web of Science database for the period 1960–2012. Inclusion criteria were: 1) Measurement of at least one production parameter. 2) Comparison between groups of sheep with different nematode burdens. 3) Same conditions regarding all aspects except parasite burden between groups. 4) Quantitative measurements of one or more production traits.

Results

Altogether, 88 studies describing 218 trials were included in this review. The majority of studies (86 %) reported that GIN infections had a negative effect on production but this was reported to be statistically significant in only 43 % of the studies. Meta-analysis indicated that performances of sheep infected with nematodes was 85, 90 and 78 % of the performance in uninfected individuals for weight gain, wool production and milk yield respectively. Our results suggest a possible reporting bias or small study effect for the estimation of the impact of GIN infections on weight gain. Finally, a general linear model provided an estimate for the decrease in weight gain in relation to the increase in faecal egg count of nematodes.

Conclusion

This study underlines the importance of GIN infections for sheep production and highlights the need to improve parasite management in sheep, in particular in face of challenges such as anthelmintic resistance.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-1164-z) contains supplementary material, which is available to authorized users.

Production and immunological responses associated with controlled-release-capsule vs 5-drench preventive anthelmintic programmes for parasite control in lambs

AIMS

To determine whether: a) using a controlled-release anthelmintic capsule (CRC) instead of a programme of 5 oral drenches administered at 3-4 week intervals, would delay the development of anti-parasite immunity in lambs; b) the use of ivermectin instead of albendazole, administered either as a CRC or as a programme of 5 oral drenches, would delay the development of anti-parasite immunity in lambs; c) lambs treated with CRCs would have higher liveweight gains than lambs drenched orally 5 times at 3-4 week intervals, and; d) delayed onset of anti-parasite immunity is associated with reduced liveweight gains in the period following anthelmintic treatment.

METHODS

Three field trials were conducted, 1 on a research farm and 2 on commercial sheep farms, in which groups of 30 lambs were treated with either a CRC containing albendazole, a CRC containing ivermectin, 5 oral drenches with albendazole, or 5 oral drenches with ivermectin, administered at 3-4 week intervals. Liveweights and faecal nematode egg counts (FECs) were recorded in all trials. Immunoglobulin-G (IgG) antibody levels to Ostertagia circumcincta and Trichostrongylus colubriformis adult and larval antigens were measured in Trials 1 and 3, and fleece weights and resistance of animals to nematode challenge infection were measured in Trial 1.

RESULTS

CRC-treated lambs had higher levels of antibodies to O. circumcincta infective-stage larvae (L3) than orally drenched lambs in Trial 3, but no other immunological differences due to mechanism of delivery were detected. Antibody levels were lower in lambs treated with ivermectin than albendazole, as a CRC or oral drench in Trial 1, but this was not associated with any measurable effects on FEC or productivity. No significant differences (p>0.05) were detected between drench types (albendazole vs ivermectin) or delivery mechanisms (CRC vs oral drenching) in any of the production parameters measured, in any of the trials. Albendazole-CRCs failed to control FECs in all 3 trials.

CONCLUSIONS

Although some differences between treatments in antibody levels were detected these were not associated with measurable differences in level of parasitism or productivity of lambs. CRC use did not appear to offer substantial gains in productivity over a structured programme of 5 oral drenches administered at 3-4 week intervals.

Cytokine and antibody subclass responses in the intestinal lymph of sheep during repeated experimental infections with the nematode parasite Trichostrongylus colubriformis

The expression of interleukin (IL)-4, IL-5, IL-10, IL-13, TNF-alpha and IFN-gamma genes, and parasite-specific IgM, IgG1, IgG2, IgA and total IgE levels, were monitored daily in intestinal lymph of sheep infected repeatedly with the nematode parasite Trichostrongylus colubriformis. Host genotype had a significant influence on IL-13 gene activity, with resistant-line (R) sheep consistently expressing higher levels of mRNA than susceptible-line (S) sheep. Mean gene expression of IL-13, IL-4 and IFN-gamma did not differ significantly between the first and second nematode challenge. Field-primed R and S as well as field-primed R and naïve S sheep had lower mean gene expression of IL-5 and IL-10, respectively, during the second when compared to primary challenge. Genes for IL-13 and IL-5 were transiently and strongly up-regulated after nematode infection, particularly in animals with previous exposure to nematodes. Genes for TNF-alpha and IFN-gamma were also transiently up-regulated, but to a lesser extent and more typically after primary challenge. Naïve sheep of both genotypes produced relatively little antibody response after primary challenge. A second nematode challenge resulted in large increases in the lymphatic levels of all antibody sub-classes which were significant for adult antigen-specific IgA and larval antigen-specific IgG1. In naïve S line sheep, the larval-specific IgA and IgG2 response appeared delayed when compared to the R line animals. Field-primed R and S line sheep had relatively high lymphatic IgG1 levels prior to experimental infection and these did not change significantly afterwards. These results demonstrate that during nematode infections, the intestinal micro-environment of sheep is transiently skewed towards Th2 cytokine dominance, although IFN-gamma gene expression continues. This response is accompanied by increases of nematode-specific IgG1, IgA, IgG2 and IgM, as well as of total IgE in lymph plasma.

Discovery of quantitative trait loci for resistance to parasitic nematode infection in sheep: I. Analysis of outcross pedigrees

Background

Currently most pastoral farmers rely on anthelmintic drenches to control gastrointestinal parasitic nematodes in sheep. Resistance to anthelmintics is rapidly increasing in nematode populations such that on some farms none of the drench families are now completely effective. It is well established that host resistance to nematode infection is a moderately heritable trait. This study was undertaken to identify regions of the genome, quantitative trait loci (QTL) that contain genes affecting resistance to parasitic nematodes.

Results

Rams obtained from crossing nematode parasite resistant and susceptible selection lines were used to derive five large half-sib families comprising between 348 and 101 offspring per sire. Total offspring comprised 940 lambs. Extensive measurements for a range of parasite burden and immune function traits in all offspring allowed each lamb in each pedigree to be ranked for relative resistance to nematode parasites.

Initially the 22 most resistant and 22 most susceptible progeny from each pedigree were used in a genome scan that used 203 microsatellite markers spread across all sheep autosomes. This study identified 9 chromosomes with regions showing sufficient linkage to warrant the genotyping of all offspring. After genotyping all offspring with markers covering Chromosomes 1, 3, 4, 5, 8, 12, 13, 22 and 23, the telomeric end of chromosome 8 was identified as having a significant QTL for parasite resistance as measured by the number of Trichostrongylus spp. adults in the abomasum and small intestine at the end of the second parasite challenge. Two further QTL for associated immune function traits of total serum IgE and T. colubiformis specific serum IgG, at the end of the second parasite challenge, were identified on chromosome 23.

Conclusion

Despite parasite resistance being a moderately heritable trait, this large study was able to identify only a single significant QTL associated with it. The QTL concerned adult parasite burdens at the end of the second parasite challenge when the lambs were approximately 6 months old. Our failure to discover more QTL suggests that most of the genes controlling this trait are of relatively small effect. The large number of suggestive QTL discovered (more than one per family per trait than would be expected by chance) also supports this conclusion.

The transfer of maternal IgE and other immunoglobulins specific for Trichostrongylus colubriformis larval excretory/secretory product to the neonatal lamb

The transference of immunoglobulins from six New Zealand Romney ewes to their lambs was examined. Immunoglobulin levels were determined in ewe plasma, colostrum and lamb plasma shortly after birth and before the lambs fed, in lamb plasma 2 days after birth, and lamb plasma, ewe plasma and milk 30 days after parturition. Levels of total IgE, and IgE, IgG1, IgG2, IgM, and IgA with specificity for Trichostronglus colubriformis third stage larval secretory/excretory products (TcL3E/S) were determined. Mean levels of total IgE were three times higher in colostrum than in parturient ewe plasma while only trace amounts were detected in milk at 30 days after birth (107.7, 34.3, and 0.2U ml(-1), respectively, differences between means P< or =0.01). Mean total IgE in lamb plasma rose from being undetectable before suckling to levels comparable to those of the ewes by 2 days after birth (21.7U ml(-1)) and then declined to low levels by 30 days (0.4U ml(-1)). Total IgE levels in lamb plasma were significantly correlated with levels in ewe plasma and colostrum (r=0.91, P< or =0.01; r=0.96, P< or =0.003, respectively). The transference of TcL3E/S-specific IgE, IgG1 and IgA was substantial with mean levels of these antibodies in lamb plasma at 2 days comparable to that in parturient ewe plasma (absorbance levels in lamb plasma of 0.283, 0.537, and 0.334, respectively). Proportionally less maternal IgM and IgG2 appeared to be transferred to the lambs (absorbance of 0.112 and 0.081, respectively). Levels of TcL3E/S-specific IgE and IgG1 in lamb plasma at 2 days were significantly correlated with levels in parturient ewe plasma and colostrum (r=0.89 and 0.82, 0.85 and 0.96; all P< or =0.05, respectively). These results indicate that IgE is concentrated in ewe colostrum and that substantial amounts of maternal IgE are transferred to lambs via colostrum. Further, the results suggest that humoral immunity against gastro-intestinal nematode parasites and potentially other parasites in colostrum-fed lambs may approximate that of the ewe. The implications of the transference of humoral immunity through colostrum in ruminants for the passive protection and the development of active immunity against parasites remains to be fully explored.

Increased expression of interleukin-5 (IL-5), IL-13, and tumor necrosis factor alpha genes in intestinal lymph cells of sheep selected for enhanced resistance to nematodes during infection with Trichostrongylus colubriformis

Cytokine gene expression in cells migrating in afferent and efferent intestinal lymph was monitored for extended time periods in individual sheep experimentally infected with the nematode Trichostrongylus colubriformis. Animals from stable selection lines with increased levels of either genetic resistance (R) or susceptibility (S) to nematode infection were used. Genes for interleukin-5 (IL-5), IL-13, and tumor necrosis factor alpha (TNF-alpha), but not for IL-4, IL-10, or gamma interferon (IFN-gamma), were consistently expressed at higher levels in both afferent and efferent lymph cells of R sheep than in S sheep. However, only minor differences were observed in the surface phenotypes and antigenic and mitogenic responsiveness of cells in intestinal lymph between animals from the two selection lines. The IL-4 and IL-10 genes were expressed at higher levels in afferent lymph cells than in efferent lymph cells throughout the course of the nematode infection in animals of both genotypes, while the proinflammatory TNF-alpha gene was relatively highly expressed in both lymph types. These relationships notwithstanding, expression of the IL-10 and TNF-alpha genes declined significantly in afferent lymph cells but not in efferent lymph cells during infection. Collectively, the results showed that R-line sheep developed a strong polarization toward a Th2-type cytokine profile in immune cells migrating in lymph from sites where the immune response to nematodes was initiated, although the IFN-gamma gene was also expressed at moderate levels. Genes or alleles that predispose an animal to develop this type of response appear to have segregated with the R selection line and may contribute to the increased resistance of these animals.

Total and nematode-specific IgE responses in intestinal lymph of genetically resistant and susceptible sheep during infection with Trichostrongylus colubriformis

Total and antigen-specific IgE responses in afferent (AIL) and efferent (EIL) intestinal lymph of sheep with a nematode resistant (R) or susceptible (S) genotype during challenge infection with the intestinal nematode parasite Trichostrongylus colubriformis were examined. Within each sheep line, lambs with a nematode naive or nematode field-primed pre-challenge status were used. Total IgE level in AIL and EIL was dependent on nematode infection and was further influenced by genotype or the immune phenotype (nematode immune mean FEC+/-SDM=77+/-179 or non-immune mean FEC+/-SDM=4016+/-4318) of the animal. During T. colubriformis challenge immune animals had higher levels of total IgE in lymph than non-immune sheep, R line sheep had higher concentrations of total IgE than S line sheep, and field-primed animals had higher total IgE levels than nematode naive animals. Concentrations of total IgE were consistently higher in AIL than EIL or serum and were higher in lymph draining the proximal than the distal jejunum demonstrating that polyclonal IgE in AIL was largely derived from the intestinal mucosa of the anatomical compartment where the nematodes reside. The consistently higher concentration of total IgE in AIL was dependent on phenotype or genotype and in S genotype sheep also on the pre-challenge status. Concentrations of nematode specific IgE were significantly higher in EIL than AIL indicating a preference for the production of IgE reacting with excretory secretory products of the infective T. colubriformis larvae in the regional lymph node.

Long-term collection and characterization of afferent lymph from the ovine small intestine

Reliable methods for long-term collection of afferent lymph draining from the small intestine of sheep are described and validated. The procedure was used successfully in normal sheep, in animals infected experimentally with the parasitic intestinal nematode Trichostrongylus colubriformis and in animals infected naturally with Mycobacterium avium subsp. paratuberculosis, the causative agent of Johne's disease. Our approach enabled afferent lymph draining from the small intestine to be collected continuously for up to 4 months, without any detrimental effects on the animals. Based on cytokine gene expression profiles of afferent intestinal lymph cells, the two infections induced contrasting regional immune responses, namely, Th2-type immunity in the case of T. colubriformis infection and Th1-type immunity in natural cases of Johne's disease. Some immune parameters differed markedly between the two disease models, highlighting the potential value of this approach to gain real-time insights into distinctive host-pathogen interactions as they occur in vivo within the regional immune system of the gastrointestinal tract.

The effect of anthelmintic capsules on the egg output and larval viability of drug-resistant parasites

Challenge with an equal mix of drug-resistant and drug-susceptible larvae of Teladorsagia circumcincta resulted in infections in groups of lambs (n = 6) either untreated or given controlled-release capsules, containing either albendazole or ivermectin. Lambs treated with albendazole capsules contained similar numbers of adult worms at necropsy to the other groups but had no detectable faecal egg count. Animals treated with ivermectin capsules had similar worm burdens and faecal egg counts to the control group but the worms had significantly higher numbers of eggs in utero. These results provide evidence for suppression of egg production by both anthelmintic treatments. The observation that albendazole caused a significant reduction in the developmental success of parasite eggs also has implications for the use of faecal egg count as an indicator for pasture contamination with resistant parasites. In two further groups of lambs, either untreated or given albendazole capsules, treatment caused a significant reduction in egg count and adult worm burden of Trichostrongylus colubriformis. No significant effects were observed on in utero egg counts or egg viability and the apparent effect on the number of eggs produced in faeces per adult female was not significant (p = 0.077). There was, therefore, no evidence that albendazole controlled-release capsules caused suppression of egg output in this species.

The effect of pasture species on parasitism and performance of lambs grazing one of three grass-white clover pasture swards

Lamb parasitism and performance were compared on three pasture treatments: browntop/white clover, Yorkshire fog/white clover, and ryegrass/white clover swards maintained at a sward height of 5cm in each of two years by using continuous variable stocking management. In year 1, lamb performance (both suppressively drenched (SD) and trigger drenched (TD)) was similar on ryegrass and browntop swards but was poorer (P<0.05) on Yorkshire fog swards. In year 2, lamb performance (both SD and TD) was markedly better on ryegrass than on Yorkshire fog or browntop swards. This coincided with an increase in white clover content in the ryegrass/white clover swards and a decrease in white clover content in the browntop/white clover swards. Lamb performance was related to the white clover content of the swards (R2=85.4, P<0.001 and R2=77.5, P<0.001 for SD lambs and TD lambs, respectively). Differences among pasture treatments in faecal egg count (FEC) of TD lambs were greater in year 2 than in year 1 when no significant pasture effects were observed. In year 2, FEC was lowest in lambs which grazed ryegrass/white clover, intermediate in those which grazed Yorkshire fog/white clover and highest in lambs which grazed browntop/white clover, despite herbage allowance (kg herbage mass/kg lamb mass) being greater on the browntop/white clover and Yorkshire fog/white clover swards than the ryegrass/white clover swards. Circulating antibodies to gastrointestinal parasites were measured at the end of year 2. Antibody titres were highest (P<0.05) in the plasma of lambs which grazed ryegrass/white clover swards, and lowest in lambs which grazed browntop/white clover swards due to differences in levels of antibody to (P<0.05) Haemonchus contortus, Ostertagia circumcincta larvae and adults, but not Trichostrongylus colubriformis larvae or adults. These findings indicate that pasture composition, especially, the proportion of white clover, can help alleviate the production losses due to gastrointestinal parasitism. This information can provide a quick and readily adoptable practice for farmers who aim to reduce drench reliance and could become a key component of any integrated parasite control programme.

The effect of feeding sulla (Hedysarum coronarium) or lucerne (Medicago sativa) on lamb parasite burdens and development of immunity to gastrointestinal nematodes

Two experiments investigated the efficacy of the legume Hedysarum coronarium (sulla), which contains condensed tannins (CTs), for reducing gastrointestinal nematode infections relative to lucerne. Experiment 1 was aimed to show whether the lower faecal egg count (FEC) and larval establishment previously reported in lambs grazing sulla were due to direct effects of the forage on Ostertagia circumcincta and Trichostrongylus colubriformis or were mediated through an enhanced immune response. Experiment 2 evaluated the impact of feeding sulla relative to feeding lucerne (Medicago sativa), before, at, or after larval challenge on subsequent FECs and nematode burdens. In experiment 1, 64 Romney lambs were fed either freshly cut lucerne or sulla (32 lambs per herbage) for the duration of the trial. Within each herbage there were four treatment groups (n=8 per group). Initial levels of immunity were assessed in uninfected (UN) lambs which were maintained parasite-free until challenged with 15,000 O. circumcincta and 15,000 T. colubriformis larvae on day 63, and slaughtered on day 81. The other three treatment groups were trickle-infected with each of 5000 O. circumcincta and 5000 T. colubriformis larvae three times per week from day 1 to 35. Non-steroid infected (CONTROL) and steroid-treated (STER) groups were treated with anthelmintic on day 49 and challenged with 15,000 O. circumcincta and 15,000 T. colubriformis on day 63 and slaughtered on day 81. The STER lambs were given dexamethasone trimethylacetate from day 49 to 81 to determine effects of immunity on parasite infection. From day 35 an establishment group (EST) on each herbage was fed a common pelleted lucerne diet and slaughtered on day 56 to determine nematode establishment during trickle-infection. Diet did not affect FECs but feeding lucerne increased (P<0.05) numbers of T. colubriformis in CONTROL lambs compared to those fed sulla. O. circumcincta numbers were lower (P<0.05) in UN lambs fed sulla than lucerne. The sulla diet was associated with higher (P<0.05) antibody titres against secretory-excretory antigens to adult O. circumcincta and to adult and larval T. colubriformis, so there appeared to be some immunogenic response to the sulla diet but effects upon T. colubriformis numbers were not significant. The second experiment involved 48 Romney lambs grazing conventional pasture which were infected with 10,000 each of O. circumcincta and T. colubriformis larvae either 7 days before, 7 days after, or at the time they commenced grazing either sulla or lucerne. Lambs which grazed sulla had lower (P<0.05) FEC and lower (P<0.05) O. circumcincta burdens than lambs which grazed lucerne but timing of infection had no effect on FEC or worm burdens. T. colubriformis numbers were not affected by treatment or herbage. In conclusion, the sulla diet resulted in lower O. circumcincta numbers compared to lucerne outdoors and some evidence of an immunogenic response was obtained indoors. However, neither the herbage nor the immunogenic response reduced T. colubriformis numbers in either experiment.

Variation among Merino sheep in susceptibilty to lice (Bovicola ovis) and association with susceptibility to trichostrongylid gastrointestinal parasites

Sheep of two bloodlines of Merino were artificially infested with equal numbers of lice (Bovicola ovis) and the resulting louse populations were monitored over the following 20 months. The sheep were shorn 6 and 17 months after infestation and, for analysis, the louse counts considered in 3 years separated by shearings. Nematode faecal egg counts (FECs) were assessed on faecal samples collected on five occasions, three times following natural challenge and twice after artificial challenge with 40,000 trichostrongyloid larvae (84% Trichostrongylus vitrinus). In addition, blood samples were collected and measured for B. ovis-specific immunoglobulins (predominantly IgG), B. ovis-specific IgE and serum total IgE. Bloodlines differed significantly in the size of louse populations at the end of year 2, FEC after both natural and artificial challenge and in serum levels of all three antibodies (p<0.05). There were also large variations in louse counts and FEC among sheep within bloodlines. Louse counts at inspections after louse populations had been allowed to build up were highly repeatable, both between and within years. However, correlations with counts at inspections soon after initial infestation and following shearing were lower. FEC after natural challenge was correlated with louse counts in year 2 (r=0.45, p<0.01) and year 3 (r=0.38, p<0.05), but the correlation with counts in year 1 was not significant (r=0.25, p>0.05). FEC following artificial challenge was significantly correlated with louse counts in year 3 (r=0.36, p<0.05), but not in year 2 (r=0.25, p>0.05) or year 1 (r=0.04, p>0.05). Louse counts in the 3 years were significantly correlated with anti-B. ovis antibody concentration (r=0.60, 0.48, 0.36), but not with levels of either anti-B. ovis or total serum IgE. These results suggest that sheep with greater resistance to gastrointestinal parasites also tend to be less susceptible to lice. Whether this is due to interaction of the effects of the parasites or to correlation in underlying resistance mechanisms requires clarification.

Segregation of B lymphocytes into stationary apoptotic and migratory proliferating subpopulations in agglomerate cultures with ileal epithelium

The B lymphocyte-epithelial cell interactions that define the microenvironment of the ileal Peyer's patch, the primary B lymphocyte organ of the fetal lamb, have been replicated in tissue culture. Mixed suspensions of ileal epithelial cells, lymphocytes and fibroblasts from fetuses of 63-103 days of gestation organized into macroscopically visible agglomerates within 72 h. These agglomerates contained translucent spherical cavities and were enclosed within a marginal cell layer and surrounded by an expanding corona of emigrating cells. The lining of the cavities and the marginal layer consisted of well-differentiated, polarized columnar ileal epithelial cells. One population of B lymphocytes in the initial mixed suspension differentiated into two discrete populations reproducing the characteristics of intact fetal ileal Peyer's patches. B cells apposed to follicle-associated epithelium (FAE) within agglomerates underwent apoptosis. The other population of emigrant B cells proliferated and expressed the BAQ44A differentiation marker. Differentiation of ileal epithelial cells into FAE, typical of Peyer's patches, was markedly accelerated. The mutually inductive influences of intestinal epithelial cells and B lymphocytes in these agglomerates replicate normal mid-gestational fetal development of the mucosal immune system and afford new opportunities for its further investigation.

The immune response of sheep to larval challenge with Ostertagia circumcincta and O. ostertagi

One of three groups of sheep was challenged twice-weekly with infective-stage larvae (L3) of the sheep parasite O. circumcincta, another with the cattle parasite O. ostertagi while the third received no larval challenge. Positive faecal egg counts (FEC) and a rise in plasma pepsinogen levels were observed only in those animals given O. circumcincta. Anti-O. circumcincta L3 IgG titres were rapidly elevated during parasite challenge with either O. circumcincta or O. ostertagi. Throughout the experiment, no rise in anti-adult IgG titres or eosinophil numbers was observed in peripheral blood in any group. On evidence of self-cure of the trickle-infection, determined by a reduction in FEC, all groups were drenched and challenged with 15,000 O. circumcincta L3. No effect of previous challenge on parasite establishment or FEC was observed, although egg viability was significantly reduced in both groups given prior challenge. Significant differences in adult female worm length were observed between groups. Those recovered from animals previously challenged with O. circumcincta were shorter than from those given O. ostertagi which were in turn shorter than those from previously unchallenged animals. In utero egg counts were significantly lower in worms from animals previously challenged with O. circumcincta than in those from unchallenged control animals. The results indicate that a level of immunity to O. circumcincta can be conferred by exposure to O. ostertagi.

Biological parameters of Trichostrongylus colubriformis in Meriones unguiculatus

In a series of three experiments, 64 jirds, Meriones unguiculatus, were infected with 700 infective larvae of Trichostrongylus colubriformis for the study of several biological parameters of this laboratory host-parasite model. In jirds the third stage larvae of T. colubriformis were shown to reach the fourth larval stage by the Day 6 post-infection (PI). By Day 10 PI, all the worms harvested had reached the immature adult stage. All immature adult stages of T. colubriformis developed to sexually mature adult stage by Day 12 PI when the female worms showed developing eggs in their uteri. Developed eggs were observed in the uteri of females on Days 13 and 14 PI. The first eggs of T. colubriformis appeared in the faeces of jirds on Day 13 PI. The peaks of egg production were recorded between Day 21 and Day 31 PI. Immunosuppression of jirds infected with 700 L3, by administration of dexamethasone from Day 57 to Day 94 PI led to increased faecal egg count when compared to untreated controls. All the worms were located in the small intestine. The jirds of dexamethasone treated group harboured higher number of adult worms than those of the untreated group. The number of adult worms was significantly higher in the first part of the small intestine than in the three other similar parts of the small intestine. The coefficient of correlation between the faecal egg count and worm number on the day of necropsy of jirds ranged between r = 0.58 and r = 0.89. Patent infections in jirds were maintained till the end of experiment on Day 100 PI, indicating that in this host and unlike other laboratory hosts, T. colubriformis is responsible for long lasting infections similar to what happens in domestic ruminants. The results of the present study suggest that the jird is a suitable laboratory model to study various aspects of this host-parasite relationship.

The effect of continuous drug exposure on the immune response to Trichostrongylus colubriformis in sheep

The effect of albendazole (ABZ)-capsule (CRC) administration on parasite establishment and immunity to ABZ-resistant (RES) and -susceptible (SUS) T. colubriformis was measured in Romney lambs. During 12 weeks of twice-weekly dosing with 3000 parasite larvae (L3), eggs were observed in faeces from CRC-treated and untreated lambs given RES L3, but not CRC-treated lambs given SUS L3. Following the period of trickle challenge all lambs were drenched and, 1 week later, dosed with 20000 SUS L3. Resulting worm burdens were higher in control lambs than in those previously treated with CRCs and challenged with SUS, which in turn were higher than those in the CRC-treated or -untreated lambs previously challenged with RES L3. During the period of trickle challenge, the number of peripheral eosinophils and titres of anti-L3 and anti-adult antibody were raised only in those groups given RES L3. There was no effect of CRC administration. Following drench and challenge, antibody titres and eosinophil numbers increased in the control animals but not in those groups which had received previous trickle infection. The results demonstrate that the larval challenge alone resulted in incomplete though substantial protection against subsequent parasite challenge. The use of CRCs may potentially impact on subsequent animal performance and selection for anthelmintic resistance through a reduced level of immunity.

Serum IgE responses during primary and challenge infections of sheep with Trichostrongylus colubriformis

Serum IgE responses during primary or challenge infections with Trichostrongylus colubriformis were examined by measuring total IgE and parasite-specific IgE by ELISA. Three- to four-month-old lambs reared nematode-free received a single primary infection of 30,000 T. colubriformis-infective larvae (TcL3). Infections were terminated after 100 days by anthelmintic treatment, at which time faecal egg counts had fallen to low levels. Total serum IgE increased slowly from 20 days p.i. until anthelmintic treatment. The specific IgE response to T. colubriformis adult antigen peaked at 20-27 days p.i. before returning to near baseline levels. A further slight increase occurred between 56 and 100 days p.i. The animals were then divided equally into two groups. A first series of challenge infections consisting of either weekly 10,000 TcL3 infections (Group A) or a single 30,000 TcL3 infection (Group B) produced low faecal egg counts. Total and specific IgE to adult and L3-ES antigens increased rapidly over a 21-day period, then remained elevated irrespective of challenge regime. Termination of primary and the first challenge infections with anthelmintic resulted in a rapid decline in serum IgE responses but not other T. colubriformis-specific Ig responses. A second series of challenge infections consisting of repeated 10,000 (Group A) or 20,000 TcL3 infections (Group B) resulted in all IgE responses peaking within 7-8 days p.i. Marked softening of faeces coincided with peaks of specific and total IgE responses during challenge infections. The results of this study showed that infection of sheep with T. colubriformis leads to elevated levels of total and parasite-specific IgE in serum. IgE responses following challenge suggest involvement of this antibody isotype in protection from T. colubriformis infections.

Phenotypic markers for selection of nematode-resistant sheep

Nematode-resistance of an animal can be defined as an enhanced natural ability, relative to its peers, to both prevent establishment of larval nematodes and evict any that do establish. These parameters are not measurable in a practical sense and consequently nematode-resistance has usually been defined in terms of low faecal nematode egg counts (FEC). Studies in New Zealand and Australia have demonstrated that nematode-resistance, as measured by FEC, has a heritability of about 0.3 in Romney and Merino sheep. However as a selection trait FEC has practical limitations and its use may incur production penalties through withholding drench treatment for prolonged periods or from a need for artificial challenge. FEC is influenced by the level and composition of a natural nematode challenge and especially the expression of the immune response. Thus immunological parameters which reflect the underlying genetic resistance could potentially be used as phenotypic markers. Ideally, a useful phenotypic marker would be easy to sample and its assay would be inexpensive and able to be automated, in addition to being strongly correlated with nematode-resistance. Results from several New Zealand trials have indicated that antibody levels (particularly IgG1) to excretory/secretory antigens of L3 nematodes such as Trichostrongylus colubriformis may meet these criteria. Levels of antibody against L3 antigens are also independent of on-farm drenching strategies. Blood eosinophil count has also been considered for use as a selection parameter but a high degree of sample variability reduces its potential. Other immunological parameters associated with nematode-resistance which have potential as phenotypic markers include serum nematode-specific IgE and products of mucosal mast cells such as proteinases. It is likely that as the critical immune responses of sheep to nematodes become more clearly defined, new immunological parameters with potential for use as phenotypic markers will be found. The definition of these immune responses will also assist in the identification and characterization of genetic markers.

The PCR typing of MHC-DRB genes in the sheep using primers for an intronic microsatellite: application to nematode parasite resistance

The strong association between polymorphisms in an intronic microsatellite and the coding sequences for (BoLA)-DRB3 genes, previously described for demonstrating alleles of class II major histocompatibility complex (MHC) in the cow, was examined in sheep to see if similar polymorphisms could be demonstrated in the DRB region of the MHC. The bovine primes LA53 and LA54, previously used to amplify the bovine DRB3 microsatellites, were used with DNA from Australian sheep, eight DRB alleles were identified by length polymorphisms of polymerase chain reaction (PCR) products amplified from the DRB microsatellite region. Incomplete amplification of both alleles was sometimes found for sheep DNA samples using bovine primers, so a modified primer (LA53b) was used, and found to amplify the microsatellite next to intron 2 of the MHC more reliably than the LA53 primer. Two additional primers (LA31 and LA32), used in amplification of the exon 2 region of bovine DRB3, were used in the sheep, and the PCR products were analysed by single-stranded conformation polymorphism (SSCP). These primers successfully amplified the variable region of the ovine DRB region coded by exon 2, and the SSCP technique demonstrated polymorphisms with sheep DNA. Family studies demonstrated the segregation of alleles, by amplification both of intronic microsatellites and of the exon 2 variable region. Close correspondence was found between the two regions for several alleles, suggesting that the intronic microsatellites were closely linked to DRB-variable region alleles. Three families of Merino sheep with different antibody responses to intestinal nematode parasites were examined. The sire group with the highest antibody levels possessed two microsatellite alleles of closely similar length (alleles 3 and 4) inherited from the sire and present in high frequency in the lambs. In contrast, the other two sires did not possess these two alleles and the alleles were in low frequency in their progeny. Further studies are required in unrelated sheep to confirm whether these two alleles are associated with resistance to nematode parasites.

Prospects for development of genetic markers for resistance to gastrointestinal parasite infection in sheep

Selection of sheep for resistance to internal parasites represents a viable option for future parasite control. Many phenotypic measures are available for determining the level of infection in individual sheep, although no phenotypic markers are available which allow prediction of an individual's resistance status. Genetic markers are therefore the best way to incorporate parasite resistance into selection programmes. With the recent development of genetic maps, several experiments are underway to search for markers linked to parasite-resistance genes in sheep. It can be predicted confidently that markers associated with resistance will be discovered within 12 months. Markers useful as selection criteria will be available within 5 years, although considerable quantitative genetic analysis needs to be done to find the best way to utilise marker information in selection programmes. In future, methods for differential DNA analysis or mRNA expression will lead to isolation of the genes involved.

Sequential cellular and humoral responses in the abomasal mucosa and blood of Romney sheep dosed with Trichostrongylus axei

Abomasal cannulae were surgically placed in 7 2-year-old New Zealand Romney sheep which had been maintained parasite-free from birth. Four of these sheep were randomly selected and dosed orally with 10,000 infective Trichostrongylus axei larvae per week for 8 weeks, while the remaining 3 sheep served as uninfected controls. Abomasal biopsy, blood and faecal samples were obtained from all sheep at regular intervals from 5 days before and until 58 days after the first infection. The sheep were then killed, worm burdens assessed and abomasal and small intestinal samples collected Faecal egg counts of all 4 dosed sheep were low and only one (No. 701) had a substantial worm burden (8400) post mortem. Overall, levels of mucosal mast cells/globule leukocytes, eosinophils, T19+ cells and larval migration inhibitory activity increased significantly in the abomasal mucosa of the dosed sheep compared to the controls. The CD4+:CD8+ cell ratio in the abomasal mucosa of the dosed sheep also increased compared to that of the controls (P = 0.06). In blood, T. axei-specific antibody (total and IgG1) and eosinophil numbers increased significantly in the dosed sheep. Mucosal cells staining for IgE (IgE+), and blood and mucosal eosinophils showed the earliest substantive increases in number followed by increases in specific serum antibody levels, numbers of mucosal cells fluorescing under UV light (UVf) and T19+ cells. The difference in the IgE+ and UVf cell responses indicated that expansion of globule leukocyte numbers lagged behind that of mucosal mast cells. The results supported the concept of CD4+ T cell help in the abomasal mucosa and defined the sequential expression of components of the immunological responses potentially mediating resistance to T. axei. In sheep No. 701, persistence of adult worms was associated with lower mucosal IgE+ cell and eosinophil responses compared with the other dosed sheep.

Immune responsiveness of nematode-resistant or susceptible Romney line-bred sheep to continuous infection with Trichostrongylus axei

Two divergent lines of Romney sheep have been selected on the basis of differences in faecal egg counts (FEC) to natural poly-generic parasite challenge in New Zealand. However, it is not known if the expression of resistance or susceptibility extends to parasitic nematodes that were not a major part of the selection challenge. To examine this, the immune response to infection with Trichostrongylus axei was examined in these sheep lines. Changes in the proportions of CD5+, CD4+, CD8+, T19+ and CD45R+ lymphocytes and parasite specific antibody titres in peripheral blood were monitored each week in six resistant and six susceptible line lambs that were maintained indoors in pens during the course of 14 weekly infections with 10,000 T. axei larvae. No difference in FEC was observed. Similarly, no significant difference in T. axei specific antibody titre between sheep lines was seen although antibody titre increased steadily from Week 4. Significant increases in the proportions of CD5+, CD4+, CD8+ and T19+ cells occurred in both resistant and susceptible line lambs during Weeks 1-4 of infection. Following peak levels, proportions of CD5+, CD4+ and CD8+ cells fell with the rate of decline of CD5+ and CD4+ cells significantly greater in the resistant line lambs. Proportions of CD45R+ cells showed significant changes with time that were the inverse of those of CD5+, CD4+ and CD8+ cells. Susceptible line lambs showed higher proportions of CD5+ and lower proportions of CD45R+ cells than resistant lambs before infection with T. axei. Overall, during infection, these differences were maintained while CD4+ and T19+ levels were higher in the susceptible line lambs.

Nematode burdens and immunological responses following natural challenge in Romney lambs selectively bred for low or high faecal worm egg count

Breeding lines of Romney sheep, selected as lambs for consistently low or high faecal nematode egg count (FEC) following periods of natural challenge, have been maintained at Wallaceville for some years. In order to determine the extent to which FECs in low and high genotypes reflected their ability to resist the establishment of gastro-intestinal nematode burdens, we investigated the infection status and immune responses in 8- to 9-month-old progeny of selected rams from low and high FEC breeding lines following a period of grazing without anthelmintic treatment in autumn/early winter. In each of the 2 years of the study, outcross male progeny of the two lowest FEC (LFEC) (i.e. most 'resistant') and two highest FEC (HFEC) (i.e. most 'susceptible') rams from the divergent lines were slaughtered shortly after autumn/early winter FECs had been analysed. Post-mortem worm counts and examination of intestinal histology were then undertaken. Blood samples collected before slaughter in the second year of the study were assayed to measure serum levels of Trichostrongylus colubriformis-specific antibody and immunoglobulins (IgG1 and IgM), and numbers of circulating eosinophils. Overall, correlations between pre-slaughter FEC and total trichostrongyle burdens in the lambs proved to be very high (0.91 and 0.85, respectively, for the 2 years studied). In the first year, LFEC lambs, which were shedding only 28.6% as many strongyle eggs as their HFEC counterparts at slaughter, were found to harbour 37.6% as many adult trichostrongyle worms, while in the second year, LFEC lambs, which were shedding 16.1% as many strongyle eggs as their HFEC counterparts at slaughter, were found to harbour 33.5% as many adult trichostrongyle worms. Results, particularly in the second year, confirmed that significantly fewer worms of most of the important abomasal and small intestinal nematode species which infest lambs in New Zealand (i.e. Haemonchus contortus, Ostertagia circumcincta, Cooperia curticei, Nematodirus spathiger, T. colubriformis, and Trichostrongylus vitrinus) had established in the LFEC genotypes than in their HFEC counterparts. In addition, in utero egg counts of female intestinal Trichostrongylus spp. were significantly lower in LFEC lambs than in their HFEC counterparts, indicating a reduction in fecundity of those worms which did establish. There was also some evidence of an effect of host response on the developmental composition of burdens in the case of some worm species. In relation to host responses, numbers of globule leucocytes/mucosal mast cells in the intestinal mucosa were significantly higher (P < 0.01) in LFEC lambs than in HFEC lambs in both years of the study. Numbers of connective tissue type mast cells and eosinophils in the intestinal mucosa were also significantly higher in LFEC lambs but only in the second year of the study (P < 0.01 and P < 0.05, respectively). Numbers of circulating eosinophils did not differ significantly between the genotypes. T. colubriformis-specific antibodies, IgG1 and IgM to both L3 and adult worm antigens were all significantly higher (P < 0.01 or P < 0.05) in LFEC lambs than in HFEC lambs.

Serum mast cell proteinase responses of sheep to challenge with Trichostrongylus colubriformis and the effect of dexamethasone treatment

Eight-month-old random bred Romney wethered lambs were reared nematode-free in pens and assigned to 4 groups of 5 lambs. Lambs in 2 groups were dosed orally, twice a week, with 5000 Trichostrongylus colubriformis infective larvae (L3) for the duration of the experiment. These 2 groups were treated weekly with dexamethasone (0.5 mg kg(-1) body-weight), one between days -7 and 70, the other between days 77 and 147. A third group was dosed with L3 until anthelmintic treatment on day 133. A fourth group remained uninfected throughout and served as a control group. Nematode eggs in sheep faeces (FEC) were monitored at weekly intervals. Serum samples were taken twice a week and assayed for sheep mast cell proteinase (SMCP). Serum levels of SMCP in uninfected control sheep were 459 +/- 190 pg ml(-1). Twenty-eight days after nematode dosing commenced, SMCP levels were significantly above control sheep levels and after 49 days reached a plateau level of 1154 +/- 364 pg ml(-1). The SMCP response persisted even after cessation of dosing, and SMCP levels remained significantly above control levels to the end of the experiment (day 213). Dexamethasone treatment prevented elevation of SMCP and resulted in a rapid reduction of extent SMCP levels in resistant sheep. Overall, serum levels of SMCP were significantly correlated (P<0.001) with specific anti-T. colubriformis L3 antibody in serum (r = 0.601, d.f. = 78), blood eosinophils (r = 0.609, d.f. = 78) and log(FEC+15) (r = -0.521, d.f. = 78). These results show that serum levels of SMCP correlate with other indicators of parasitism and may have potential use as a non-invasive indicator of gastrointestinal mast cell responses to nematode infection.

Drug-abbreviated infections and development of immunity against Trichostrongylus colubriformis in sheep

A protective immune response without liveweight loss can be induced in sheep against T. colubriformis but results depend on the anthelmintic used and duration of immunizing infections. More than 90% protection was achieved in sheep immunized by three 15- or 7-day oxfendazole abbreviated infections or three 21-day nonabbreviated infections. Only 41% protection was induced by 3-day oxfendazole abbreviated infections. Significantly higher worm burden and faecal egg counts were present after challenge in sheep immunized by 7-day levamizole abbreviated infections compared to 7-day oxfendazole abbreviated infection. Liveweight gains of sheep immunized by 15- and 7-day abbreviated infections were not significantly different than non infected controls. Liveweight loss seemed to be associated with high activity of mucus peroxidase and high numbers of eosinophils in the intestinal lumen. High parasite numbers seemed to be associated with low activity of alkaline phosphatase in mucus. Mucus peroxidase, arylsulphatase, larval migration inhibition of mucus, mucus or serum antibody against L3 excretory/secretory antigen or somatic L3, L4 and adult antigen were not associated with protection.

Genetic factors affecting antibody responses to four species of nematode parasite in Romney ewe lambs

Serum levels of antibodies (Ab) and immunoglobulin G1 (IgG1) to the larval (L3) stage of the internal parasites Cooperia curticei and Trichostrongylus colubriformis and levels of Ab to the L3 stages of Haemonchus contortus and Ostertagia circumcincta were determined in 1432 Romney ewe lambs which were born on one farm in 1990 and 1991 and were the progeny of 63 rams. The objectives were to estimate heritabilities of, and genetic correlations among, the serum concentrations in newly weaned lambs under commercial conditions and to estimate genetic correlations of Ab and IgG1 with production traits. Lambs were exposed to a natural parasite challenge on pasture, following an anthelmintic drench at weaning. Blood and faecal samples from 4- to 6-month-old lambs were then taken when the mean faecal nematode egg count of a monitor group reached 800-1500 eggs g-1. Heritabilities for the serum levels of the four Abs ranged from 0.25 +/- 0.05 to 0.37 +/- 0.06. Heritabilities for the level of IgG1 developed against C. curticei was 0.19 +/- 0.04 and against T. colubriformis, 0.18 +/- 0.05. Genetic correlations between Abs for the 4 species were high, averaging 0.84, and between the two IgG1s it was 0.82. The genetic correlations between Ab or IgG1 levels and weight or gain traits were negative (for the 6 significant values out of 18), with yearling fleece weight positive (for the 2 significant values out of 6), whilst those with loge faecal egg count were all negative (average -0.15).(ABSTRACT TRUNCATED AT 250 WORDS)

The immune responsiveness of Romney sheep selected for resistance or susceptibility to gastrointestinal nematodes: lymphocyte blastogenic activity, eosinophilia and total white blood cell counts

Blastogenic activity, eosinophil and total white blood cell counts (TWBC) were examined over a period of 14 weeks in Romney lambs, genetically resistant or susceptible to gastrointestinal nematodes. The lambs were infected with 5000 infective Trichostrongylus colubriformis larvae twice weekly. Compared to preinfection levels, the blastogenic activity of unstimulated lymphocytes in lambs of both lines peaked at week 3, and was significantly higher in resistant than in susceptible lambs. These changes may have been due to in vivo polyclonal activation. Lymphocytes from susceptible sheep responded more strongly to Con A, PHA and PWM than cells from resistant sheep. Counts per minute (c.p.m) for Con A- and PHA-stimulated lymphocytes increased in both lines of sheep from week 2 to week 7 and then returned to initial levels. An increase in c.p.m. in PWM-stimulated cell cultures was observed from weeks 3 to 5 in both groups. The blastogenic activity for LPS-stimulated cultures was significantly higher for resistant than susceptible sheep at weeks 3 and 4. No significant correlations between the decline in faecal egg counts (FEC) and the blastogenic activity was observed. Eosinophil counts in peripheral blood began to increase one week earlier in resistant than in susceptible sheep. No significant correlation between FEC and eosinophil counts was observed in resistant lambs, whereas in susceptible lambs a significant correlation was found between FEC and eosinophil counts at some sampling times. TWBC in resistant lambs steadily increased with infections whereas susceptible lambs showed a decrease until week 5 and then steadily increased. There was no significant correlation between the decline in FEC and TWBC.(ABSTRACT TRUNCATED AT 250 WORDS)