The immune response and the evaluation of acquired immunity against gastrointestinal nematodes in cattle: a review

The present review discusses the immune responses to gastrointestinal nematodes in cattle and the different immunological and parasitological parameters used to assess acquired immunity. Measuring acquired immunity to gastrointestinal nematodes in cattle (e.g. for the evaluation of candidate parasite vaccines) is hampered by the limited understanding of bovine immune responses against gastrointestinal parasites. In this paper the available data on protective immunity against gastrointestinal nematodes, and especially Ostertagia ostertagi, in cattle are compared with the current knowledge of protective immune responses against gastrointestinal nematodes in rodent models and small ruminants. In contrast to the immune response in mice, which is controlled by T helper 2 (Th2) lymphocytes and results in mast cell- or goblet cell-mediated expulsion of adult worms, bovine immune responses to O. ostertagi do not show a clear Th2 cytokine profile, nor do they result in rapid expulsion of the parasite. The first manifestation of immunity to O. ostertagi in calves is a reduction of worm fecundity, possibly regulated by the local IgA response. Worm numbers are only reduced after a prolonged period of host-parasite contact, and there are indications that O. ostertagi actively suppresses the host's immune response. Until the mechanisms of protective immunity against O. ostertagi are revealed, the use of immunological parameters to estimate acquired immunity in cattle is based on their correlation with parasitological parameters and on extrapolation from rodent and small ruminant models. Assessing the resistance of calves against a challenge infection by means of parasitological parameters is probably still the most accurate way to measure acquired immunity against gastrointestinal nematodes.

Can an increased intake of metabolizable protein affect the periparturient relaxation in immunity against teladorsagia circumcincta in sheep?

Within the overall hypothesis that the periparturient relaxation of immunity (PPRI) to gastrointestinal parasites has a nutritional basis, we studied whether an increased intake of metabolizable protein (MP) could affect the expression of immunity to Teladorsagia (Ostertagia) circumcincta of twin-bearing and -rearing Greyface ewes. Two sets of iso-energetic diets were formulated to provide either 85% (L-) or 130% (H-) of the MP requirements during the last 6 weeks of pregnancy (-P) and the first 6 weeks of lactation (-L). Thirty ewes were allocated to LP or HP. Six ewes of each group were slaughtered after lambing (S(0)); the remaining 24 ewes of each group were allocated to either LL or HL, thus creating four treatment combinations during lactation (LP/LL, LP/HL, HP/LL and HP/HL; n=12). Six ewes of each treatment combination were slaughtered at Week 3 (S(3)) or at Week 6 (S(6)) in lactation. The HP ewes were heavier at lambing, produced heavier lambs, had lower faecal egg counts (FECs), a lower concentration of plasma pepsinogen, a higher concentration of plasma albumin during late pregnancy and more abomasal globule leukocytes (GLs) but similar worm burdens at S(0) compared to the LP ewes. The LP/HL and HP/HL ewes had a higher calculated milk production than the LP/LL and HP/LL ewes. FECs increased for each treatment combination during the first 3 weeks of lactation. This increase concurred with fewer abomasal GLs at S(3) compared to S(0). The LP/HL and HP/HL ewes had a higher proportion of immature worms and more abomasal GLs than the LP/LL and HP/LL ewes at S(6). The FECs continued to increase during the second 3 weeks of lactation for the LP/LL ewes only, which concurred with an elevated plasma pepsinogen concentration and an absence of abomasal GLs at S(6). The plasma pepsinogen concentration decreased rapidly for the LP/HL ewes during lactation. We conclude that an increased intake of MP could enhance the expression of immunity to T. circumcincta of twin-bearing and -rearing Greyface ewes.

Identification of Ostertagia ostertagi specific cells in bovine abomasal lymph nodes

To investigate the contribution of different bovine cell subpopulations in the development of in vitro induced responses by Ostertagia ostertagi third larval antigen extract (L3), bovine abomasal lymph node cell suspensions were depleted of specific cell populations. The depleted cell suspensions were subsequently assayed for their proliferative responses to O. ostertagi L3 antigen extract. Proliferative responses to O. ostertagi L3 antigen extract were restricted to a CD2+ CD4- CD8- cell population and MHC II+ cells different from B-cells were of major importance. Depletion of CD4, CD8, CD4CD8, IgM or CD21 positive cells did not decrease proliferation to L3 antigen extract. Depletion of gammadelta T-cells, which also comprise a subpopulation of CD2+ CD4- CD8- cells, reduced proliferation to L3 antigen extract only in one animal. The results suggest that either gammadelta T-cells could be involved in the proliferation or that another as yet unidentified population is important for proliferation. The precise role of these populations during infection with O. ostertagi and the mechanism by which these cells may influence the host immune response are important issues that remain to be elucidated.

How lambs control infection with Ostertagia circumcincta

Two of the most important questions for immunologists studying nematode infections are what effect does the host response have on the parasite and which components of the host response are responsible for these effects. The number of nematodes and the mean length of adult female Ostertagia circumcincta was measured in over 500 6-7 month old lambs of the Scottish Blackface breed. Quantitative genetic analyses indicated that there was substantial genetic variation among lambs in faecal egg counts and in worm length but the analyses provided no evidence for genetic variation in worm numbers. Separate analyses have shown a strong relationship between decreased worm length and decreased worm fecundity. Therefore, genetic resistance in lambs appears to be due to control of worm growth and not to control of worm numbers. The only immune response that is consistently associated with reduced worm length is the IgA response to fourth-stage larvae. The association is remarkably strong. After allowing for the influence of worm number on worm length (density-dependence) essentially all of the variation among deliberately infected lambs in worm length can be accounted for, in a statistical sense, by variation in the strength and specificity of the local IgA response. Therefore, the IgA mediated suppression of worm growth and fecundity appears to be the major mechanism of resistance to O. circumcincta in lambs.

Cross-reactive antigens of Haemonchus contortus adult worms in Teladorsagia circumcincta infected goats

The adult Haemonchus contortus somatic antigens responsible for cross-reactivity have been analysed using serum samples from goat kids infected and reinfected with Teladorsagia circumcincta. Goat kids infected with T. circumcincta had similar serum ELISA values against somatic antigens of H. contortus as goats infected with H. contortus itself. Immunoblotting confirmed this extensive cross-reactivity particularly in the molecular weight range 105-29 kDa. However, peptides with high (195, 152 and 119 kDa) or low (23 kDa) molecular weight were only faintly recognized by heterologous sera.

Development of a protective immunity against Ostertagia leptospicularis in trickle-infected sheep and parallel changes of serum gastrin, pepsinogen and antibody levels

Nine lambs, approximately 9 months of age were allocated to three groups (A, B, C), with three animals in each. Sheep in Groups A and B were trickle-infected with doses of 1000 third-stage larvae (L3) of Ostertagia leptospicularis (five times per week) over periods of 7.5 and 10.5 weeks, respectively, and were subsequently treated with fenbendazole (7.5 mg/kg). Approximately 3 weeks after anthelmintic treatment, all sheep were challenged with a single dose of 100,000 L3, whereas sheep of Group C received the same dose as a primary infection. Sheep of Groups A and B were almost completely refractory against the challenge infection, as indicated by negative faecal egg counts and adult worm burdens. A relatively high infection level was present in the sheep of Group C. The results indicate that a comparatively short immunization period of 7.5 weeks is sufficient to protect lambs against subsequent larval challenge. During immunization, the pepsinogen-, gastrin- and IgA-responses were similar in the individual sheep. In contrast to parasite-specific IgG1 and IgG2 levels, IgA decreased rapidly after cessation of trickle infection and parallel anthelmintic treatment, and may therefore indicate current exposure to parasite antigen. After challenge, the majority of the immunized sheep exhibited immediate and short-term responses of pepsinogen, gastrin and IgA in the serum. The time course and the level of each of these responses were very similar in the individual sheep, suggesting that the release of pepsinogen, gastrin and IgA into the circulation was influenced by related mechanisms.

The recognition of molecules from fourth-stage larvae of Ostertagia circumcincta by IgA from infected sheep

The recognition of antigens from fourth-stage Ostertagia circumcincta by plasma IgA was studied in a group of 30, ten-month-old Scottish Blackface sheep which had been naturally, then deliberately infected. Western blotting revealed 49 bands that were recognized by antibody from one or more of the 30 sheep. There was a heterogeneous pattern of antigen recognition amongst the sheep. No individual recognized all the 49 bands and only one of the 49 bands was recognized by all sheep. Two antigens with approximate molecular weights 87,000 Da and 129,000 Da were significantly associated with a reduction in mean adult worm lengths. The observed variation in recognition of these two antigens on fourth-stage larval preparations accounted, in a statistical sense, for nearly 40% of the total variation in worm length. In addition to the variation in antibody mediated recognition of these two parasite molecules, three other components have been implicated in regulating worm length. They are a 37,000 Da band from adult worms, the amount of fourth-stage larval specific IgA in the abomasal mucosa and the density-dependent influence of adult worm burden. Together, these components and their interactions accounted for over 90% of the observed variation in worm length. These results indicate that the parasite-specific IgA response, or something extremely closely associated with it, is the major immunological mechanism controlling worm length.

Mechanisms underlying resistance to nematode infection

Lambs show considerable genetic variation in faecal egg count following natural, predominantly Ostertagia circumcinta infection. This genetic variation is acquired and not innate. Worm length is positively associated with worm fecundity. The genetic variation in faecal egg count is a consequence of genetic variation in worm length and hence worm fecundity, and not of genetic variation in worm burdens. In contrast to lambs, mature sheep may be able to regulate both fecundity and worm numbers. In lambs, three factors account for the majority of the variation in worm length: the strength of the local IgA response against fourth-stage larvae, the specificity of this response against four molecules in particular, and the density-dependent influence of worm number.

Characterization of protective immune responses in local lymphoid tissues after drug-attenuated infections with Ostertagia ostertagi in calves

In the present study, cell-surface markers and cytokine gene expression of lymphocytes from the local lymph nodes were studied 9 days after primary infection with Ostertagia ostertagi in previously naive calves or in calves previously immunized with multiple, chemically attenuated infections. Changes in lymphocyte populations were assessed by flow cytometry utilizing monoclonal antibodies specific for bovine cell-surface markers. Changes observed in the percentages of lymphoid populations after challenge were similar in animals immunized by either three or five drug-attenuated infections. In both immunized groups, the CD4+/CD8+ ratio was significantly higher than in naive animals after challenge infections. In addition, both immunized groups showed significantly lower levels of Ig-bearing cells upon experimental challenge when compared to animals with a primary experimental infection. No differences were observed in the number of gammadelta or interleukin 2 receptor (IL-2R) positive cells. The levels of mRNA for IL-4, IL-10, IL-15, IFN-gamma and TGF-beta1 were examined by competitive RT-PCR. After challenge, the levels of these cytokines were lower in animals immunized by five drug-attenuated infections, and in the case of IL-4 and TGF-beta1, these differences were statistically significant. These results indicate that animals exhibiting protection from reinfection with O. ostertagi do not show a shift to higher percentages of Ig+ cells characteristic of a primary infection. In addition, protected animals appear to show a decreased IL4 and TGF-beta1 response upon challenge when compared to non-immune animals.

Stochastic effects in a model of nematode infection in ruminants

We illustrate the importance of stochastic effects in population models of biological systems and demonstrate a number of analytic and simulation-based approaches that can usefully be applied to such models. In so doing, we compare the stochastic approach to the more usual deterministic one. The model studied represents the gastrointestinal infection of ruminants by nematodes when the hosts maintain a fixed density. The incorporation of a feedback mechanism, which accounts for the immune response of the infected animals, results in a highly nonlinear model; similar forms of nonlinearity are a feature of many plausible models in population biology. In the absence of an analytic solution to the full stochastic model we explore a number of approximations and compare them to simulations of the full stochastic process. We explore three modes of behaviour of the system. In the endemic regime the stochastic system fluctuates widely around the non-zero fixed points of the deterministic model. In the managed regime, where the system is subject to external periodic perturbation, stochastic effects are negligible. Finally, we find that in a regime in which the deterministic model predicts the long-term persistence of oscillations the stochastic model shows that extinction can occur. Of the approximation procedures we consider, the Normal approximation to the full stochastic process is the most generally applicable, and it is also the most accurate in the light of simulation results. Local linearization provides reasonably accurate prediction of the variance-covariance structure, and a transfer function approach allows calculation of the time-lagged auto- and cross-correlations in the endemic regime. Linearization of the stochastic updates themselves results in poor prediction of the population variances.

Pathogenicity and immunogenicity of different isolates of Teladorsagia circumcincta

The effects on liveweight gain and development of immunity were studied in lambs trickle infected for 8 weeks with either a benzimidazole-resistant isolate (Moredun ovine resistant isolate, MORI), a multiple benzimidazole + ivermectin-resistant isolate (Moredun caprine resistant isolate, MCRI) or an unselected susceptible isolate (Moredun ovine susceptible isolate, MOSI) of Teladorsagia circumcincta. Plasma pepsinogen concentrations of infected groups were significantly elevated compared to an uninfected control group (P < 0.001) by day 14. The liveweight gains varied markedly but there were no statistical differences between the infected and uninfected control groups at any point in time during the study. Lambs infected with the MORI had significantly lower faecal consistency scores than the other challenged groups on days 7 and 14 (P < 0.05) but from day 21 onwards, faecal consistencies were similar in all of the groups. There was a notable difference in the pre-patent periods of the different isolates with the MOSI producing positive faecal egg counts (FECs) by day 14 of the study. The FECs remained reasonably low once infections had reached patency and there were no further differences between the groups. Following administration of anthelmintic to remove residual worms from the trickle infection, no differences between the infected groups in terms of worm burden or mucosal mast cell numbers were evident as a consequence of a single challenge infection. The changes in genetic code associated with enhanced resistance against anthelmintics do not appear to have resulted in any fundamental alteration of the pathogenicity and immunogenicity of these three isolates of Teladorsagia.

Interpreting comparative experiments on parasite burdens in animals using randomization and canonical variates

Comparative experiments on parasite burdens present difficulties for modelling and interpretation: data tend to have highly skewed distributions, some standard methods of testing for effects have low power, and there is a need to make allowance for dependencies among the various measurements. Randomization testing and canonical variates analysis are advocated as a means of minimising these problems.

Quantification of cytokine gene expression in lamina propria lymphocytes of cattle following infection with Ostertagia ostertagi

Changes in cell surface markers and cytokine transcription were analyzed in lamina propria lymphocytes from control animals (noninfected calves) and calves after a single high but nonprotective primary infection with Ostertagia ostertagi. Flow cytometry of cells recovered from the lamina propria showed an increase in the percentages of IgM+, WC1+, and IL-2R+-bearing cells 10 days after infection; however, 2 mo after infection, cell staining was comparable to preinfection levels. Transcription levels of interleukins IL-2, IL-4, IL-10, and interferon (IFN)-gamma mRNA were measured using a competitive reverse transcription-polymerase chain reaction. Results indicated elevated levels of IL-4 and IFN-gamma in the infected animals at 10 days and at 60 days after infection. Transcription of IL-10 also increased; however, this change was not observed until 60 days postinfection.

Immunity development against Ostertagia ostertagi and other gastrointestinal nematodes in cattle

The immunological response to gastrointestinal nematodes is complex and it will take time and financial support to dissect the different components of protective immunity in ruminants. The present review briefly summarises different aspects and manifestations of protective immunity in cattle against gastrointestinal nematodes, especially Ostertagia ostertagi, and the factors that may interact with the development of this immunity.

Effects of gastrointestinal nematode infection on the ruminant immune system

Gastrointestinal (GI) nematodes of ruminants evoke a wide variety of immune responses in their hosts. In terms of specific immune responses directed against parasite antigens, the resulting immune responses may vary from those that give strong protection from reinfection after a relatively light exposure (e.g. Oesophagostomum radiatum) to responses that are very weak and delayed in their onset (e.g. Ostertagia ostertagi). The nature of these protective immune responses has been covered in another section of the workshop and the purpose of this section will be to explore the nature of changes that occur in the immune system of infected animals and to discuss the effect of GI nematode infections upon the overall immunoresponsiveness of the host. The discussion will focus primarily on Ostertagia ostertagi because this parasite has received the most attention in published studies. The interaction of Ostertagia and the host immune system presents what appears to be an interesting contradiction. Protective immunity directed against the parasite is slow to arise and when compared to some of the other GI nematodes, is relatively weak. Although responses that reduce egg output in the feces or increase the number of larvae undergoing inhibition may occur after a relatively brief exposure (3-4 months), immune responses which reduce the number of parasites that can establish in the host are not evident until the animal's second year. Additionally, even older animals that have spent several seasons on infected pastures will have low numbers of Ostertagia in their abomasa, indicating that sterilizing immune responses against the parasite are uncommon. In spite of this apparent lack of specific protective immune responses, infections with Ostertagia induce profound changes in the host immune system. These changes include a tremendous expansion of both the number of lymphocytes in the local lymph nodes and the number of lymphoid cells in the mucosa of the abomasum. This expansion in cell numbers involves a shift away from a predominant classic T cell population (CD2 and CD3 positive), to a population where T cell percentages are decreased and B cells (immunoglobulin-bearing) and gamma-delta cells are increased. At the same time the expression of messenger RNAs for T cell cytokines (IL2, IL4, IL10 and gamma-interferon) is changed to that of increased expression of IL4 and IL10 and decreased expression of IL2 and perhaps of gamma-interferon. The reasons for these changes remain to be elucidated, but it is evident that the lack of protective immune responses is not the result of a poor exposure of the host to parasite products, or to the stomach being an immunoprivileged site. In fact, a superficial look at the responses elicited indicates that Ostertagia induces responses (the so-called TH2 mediated responses) that are widely considered to be the type of responses necessary for protection against GI nematodes. There are many factors that could lead to this apparent lack of immunity in the face of a strong stimulation of immune responses including: (1) the elicitation of suboptimal responses; (2) the failure of the abomasum to function as an efficient effector organ; (3) active evasion of the functional immune response by the parasite; and (4) that these classic responses are not protective in this particular ruminant-parasite system and that novel protective mechanisms may be required. The strong stimulation of the host gut immune system by Ostertagia and perhaps by other GI nematode infections, raises questions about the potential effects of such infections on the overall well-being of the host. A number of authors have indicated that Ostertagia infections may diminish the host's ability to mount subsequent immune responses to antigenic challenges such as vaccination against other infectious organisms. In addition, recent studies have indicated that infections with GI nematodes may result in increased circulatory levels of stress-related hormo

The genetic basis of resistance to Ostertagia circumcincta in lambs

The relationship between Ostertagia (Teladorsagia) circumcincta and sheep is one of the best understood host-parasite relationships in any species. The key components of resistance have been quantified, the extent of genetic control has been established for lambs, and methods now exist to breed lambs which will be both more resistant to worms and more productive than unselected lambs. A major gene for resistance has been identified within or around the major histocompatibility complex, and this gene appears to be the strongest yet identified for resistance to any parasite species. The most important mechanisms of resistance are local IgA responses which regulate worm fecundity and immediate hypersensitivity responses which regulate worm burdens. IgA responses develop before effective immediate hypersensitivity responses. Good simulation models now exist to predict, for example, outbreaks of disease and the response of sheep to selection. The challenge now is to use our improved understanding of the population biology to develop even better simulation models and to produce expert systems based on these models which can be used by veterinarians and others to determine optimal procedures for individual farms to control disease and reduce sub-clinical economic losses.

Cytokine profile induced by a primary infection with Ostertagia ostertagi in cattle

Changes that occur in the local draining lymph nodes including, changes in cell surface markers and cytokine gene expression were studied over the first 4 weeks of a primary, Ostertagia ostertagi infection of the abomasum. Cells recovered from the abomasal lymph nodes (ABLN) after infection showed a decrease in the percentage of CD3+ cells, and an increase in the percentage of IgM+ cells and cells bearing the TcR1 marker. These changes were coincident with an increase in the proportion of activated cells (II-2R). Analysis of mitogen-stimulated ABLN cells by RNase protection assay (RPA) showed a dramatic reduction in IL-2 and IFN-gamma transcription after infection. In addition, analysis of unstimulated ABLN cells by competitive RT-PCR showed a similar decrease in demonstrable levels of IL-2 mRNA, but IL-10, IL-4 and IFN-gamma mRNA levels were elevated.

Isolation and phenotypic characterization of abomasal mucosal lymphocytes in the course of a primary Ostertagia ostertagi infection in calves

Isolation and characterization of surface marker phenotypes of abomasal intraepithelial (IEL), lamina propria (LPL) and abomasal lymph node lymphocytes (ABLN) from uninfected calves were conducted, and the dynamics of change in these populations during the course of a primary Ostertagia ostertagi infection were defined. To obtain viable IEL and LPL from the abomasal mucosa of cattle, a modified isolation method was developed. The phenotypic characterization of abomasal lymphocytes was accomplished by indirect immunofluorescence staining. In uninfected animals, numbers of T cells exceeded the number of immunoglobulin-bearing cells in IEL, LPL and ABLN. The predominant T cell type in IEL and LPL was CD8+ cells, while the CD4+ T cell predominated in ABLN. Levels of activated cells and T cell receptor-1 gamma delta T cells were higher in IEL and LPL compared to ABLN. Within 3 weeks of infection, the number of lymphocytes recovered from the abomasal lamina propira and the mass of the ABLN was dramatically increased when compared to uninfected animals. Laser flow cytometric analysis demonstrated increased levels of immunoglobulin-bearing cells, gamma delta T cells, and activated T cells in IEL, LPL and ABLN in the infected animals. The greatest changes in LPL and ABLN took place during the first days of infection, and these changes were apparent throughout the 28 days covered by the experiment.

Induction and suppression of lymphocyte proliferation by antigen extracts of Ostertagia ostertagi

To obtain an insight into the responses of T-cells of cattle to Ostertagia ostertagi, the responses of peripheral blood and lymph node lymphocytes to O. ostertagi antigen extracts were determined in both exposed and naive calves. The lymphocyte responses induced by O. ostertagi antigen extracts of the third (L3) and fourth (L4) larval stages, as well as adult worms, were analysed. Although peripheral blood lymphocyte responses were very low or absent, abomasal lymph node lymphocytes of exposed animals showed a strong response to the L3 antigen extract. No such response was observed in naive calves or in mesenteric lymph node cells of exposed calves. L4 and adult worm antigen extracts suppressed the proliferative responses induced by the L3 antigen extract. Whether or not this suppressive effect plays a role in the slow rate at which protective immunity develops against O. ostertagi is under further investigation.

The influence of a Cooperia oncophora priming on a concurrent challenge with Ostertagia ostertagi and C. oncophora in calves

The development of immunity to Ostertagia ostertagi and Cooperia oncophora and interactions between these species were investigated in experimentally infected calves. Parasitological, serological and histological parameters were used for assessing immune responses. No conclusive evidence of an effect of C. oncophora on the course of an O. ostertagi infection in calves could be shown. Following a challenge with C. oncophora and O. ostertagi of C. oncophora primed calves, no significant reductions in establishment rate, faecal egg counts, worm length or the percentage of early fourth stage larvae could be demonstrated. Results also confirmed earlier work showing the very different degrees of immunity conferred following immunisation with either C. oncophora or O. ostertagi. While a protective immunity was generated in the case of C. oncophora, continuous infection of calves with 420000 L3 of O. ostertagi during almost 5 months induced immune reactions which affected growth and fecundity of the worms but not the establishment rate.

Heterogeneity in the recognition of Ostertagia circumcincta antigens by serum antibody from mature, infected sheep

The recognition of parasite molecules from third-stage and adult Ostertagia circumcincta by serum antibody was studied in a group of matched, mature Scottish Blackface sheep that had been naturally and then deliberately infected. A total of 20 molecules was recognized in somatic extracts from third-stage larvae and 31 molecules in somatic extracts from adult parasites. However, no sheep recognized all immunogenic molecules and no molecule was recognized by all sheep. There was no obvious relationship between recognition of any parasite antigen and polymorphism at class I loci or at the DRBI class II locus of the major histocompatibility complex in these outbred animals. Only 15 molecules from third-stage larvae were present at a frequency suitable for statistical analysis and recognition of three of these 15 molecules was associated with differences in worm burdens. Recognition of two of five molecules from adult parasites was associated with differences in worm length. These results indicate that variation in the recognition of specific, identifiable parasite molecules may be partly responsible for variation among sheep in resistance to O. circumcincta.

The death rate of Ostertagia circumcincta and Trichostrongylus colubriformis in lactating ewes: implications for anthelmintic resistance

Lactating adult Romney ewes were infected, 4 weeks post-lambing, with benzimidazole (bz) resistant strains of Ostertagia circumcincta and Trichostrongylus colubriformis. Commencing 4 weeks after the initial infection the ewes were subjected to challenge 3 times weekly with 5000 L3 of bz-susceptible strains of both parasite species. At weekly intervals over the following 6 weeks, groups of ewes were drenched with a bz anthelmintic (oxfendazole) to remove bz-susceptible parasites and slaughtered to determine adult worm burdens of the bz-resistant parasites. The O. circumcincta infection declined exponentially with a mean daily death rate of 10.6% day-1 and no worms were recovered after 4 weeks or more of challenge. The T. colubriformis infection did not decline significantly over the 6 weeks of continuous challenge, indicating that the death rate could not be distinguished from zero. The upper 95% confidence limit for the death rate of T. colubriformis was 4.9%. The implications of these death rates on selection for drug resistance following ewe drenching during the post-partum period are discussed with selection pressure likely to be greater for T. colubriformis than for O. circumcincta.

The presence of an early L4 population in relation to the acquired resistance of calves naturally infected with Ostertagia ostertagi

The aim of the present study was to determine if the presence of inhibited L4 Ostertagia ostertagi populations during the housing period influenced the level of acquired immunity in calves at the start of the second grazing season. Four groups (A-D) of first season grazing calves were turned out onto a heavily infected pasture for five months, while the fifth group (E) was kept as uninfected controls. At housing all group A calves were necropsied for worm counts. A mean total of 78915 Ostertagia worms were found, of which 85% were in the early L4 stage. The calves of group C were dewormed with ivermectin at housing, while groups B and D remained untreated. After the calves were housed, egg counts, pepsinogen and antibody levels all declined in groups B, C and D, but during the second half of the housing period pepsinogen and antibody levels increased again in groups B and D. The effect of housing on the resident worm population was evaluated by slaughtering all group B calves for worm counts at the end of the housing period. A mean burden of 57,950 Ostertagia was present, with 78% as early L4 larvae. The remaining three groups (C, D and E) were treated with oxfendazole and turned out on the same pasture as the previous year for a challenge period of four weeks. In the control calves (group E) the egg output rose steeply, while egg counts remained low in groups C and D. In addition the worm burdens of the previously infected groups C and D were significantly reduced (75-90%), and the percentage of inhibited L4 larvae was significantly increased compared to the control group E. No significant difference was observed between the calves that were treated at housing (group C) and the untreated animals of group D. It was concluded that the continued presence of an inhibited L4 population during housing was not essential for maintenance of immunity.

Immunisation of sheep by drug-abbreviated infections of Ostertagia circumcincta and Trichostrongylus colubriformis against field challenge of gastro-intestinal nematodes

A very high level of protection was achieved against homologous (up to 97%) and heterologous (up to 87%) infections in 12-month-old Romney sheep immunised with oxfendazole-abbreviated infections of Ostertagia circumcincta and Trichostrongylus colubriformis. No significant protection occurred following ivermectin-abbreviated infections. None of the immunised sheep showed an increase in antibody level against excretory-secretory antigen of T. colubriformis infective larvae. The immunisation procedures did not cause a decrease in wool production, or liveweight gains compared with non-immunised controls.