Vaccines against gastrointestinal nematode parasites of ruminants

Vaccines against blood-feeding nematodes of humans and livestock

This paper summarises the progress towards vaccine development against the major blood-feeding nematodes of man and livestock, the hookworms and Haemonchus contortus, respectively. The impact of the diseases and the drivers for vaccine development are summarized as well as the anticipated impact of the host immune response on vaccine design. The performance requirements are discussed and progress towards these objectives using defined larval and adult antigens, many of these being shared between species. Specific examples include the Ancylostoma secreted proteins and homologues in Haemonchus as well as proteases used for digestion of the blood meal. This discussion shows that many of the major vaccine candidates are shared between these blood-feeding species, not only those from the blood-feeding stages but also those expressed by infective L3s in the early stages of infection. Challenges for the future include: exploiting the expanding genome information for antigen discovery, use of different recombinant protein expression systems, formulation with new adjuvants, and novel methods of field testing vaccine efficacy.

Anthelmintic resistance: The state of play revisited

Helminthosis is one of the major constraints in the successful wool and mutton industry throughout the world. Anthelmintic Resistance (AR) is said to have been established when previously effective drug ceases to kill exposed parasitic population at the therapeutically recommended dosages. Anthelmintic resistance is almost cosmopolitan in distribution and it has been reported in almost all species of domestic animals and even in some parasites of human beings. Some of the most important species of parasites of small ruminants in which AR has been reported include: Haemonchus spp., Trichostrongylus spp. Teladorsagia spp., Cooperia spp. Nematodirus spp., and Oesophagostomum spp. All the major groups of anthelmintics have been reported for development of variable degrees of resistance in different species of gastrointestinal nematodes. This paper describes the global scenario of prevalence and methods used for detection of AR in small ruminants. Different mechanisms and contributory factors for the development of AR are discussed. Various options and alternate strategies for the control and/or delay in the onset of AR are suggested in the light of available information.

Nematodirus battus 50 years on--a realistic vaccine candidate?

It has been 50 years since the parasitic nematode of lambs, Nematodirus battus, was first described. This parasite has several interesting features; in particular, it induces a rapid, protective immune response in infected young lambs (< 3 months of age), which is not observed if lambs are infected with other trichostrongyle nematodes. Indeed, protection against most gastrointestinal nematodes only develops once lambs are over five to six months old. In this article I suggest that N. battus offers an opportunity to improve our understanding of protective immune responses in young lambs, and could therefore hold a key to rational anti-nematode vaccine developments, based on natural antigens.

Identification of antigens with potential for immunodiagnosis of Parelaphostrongylus tenuis and Elaphostrongylus cervi infections in red deer (Cervus elaphus elaphus)

Red deer (Cervus elaphus elaphus) were infected experimentally with Parelaphostrongylus tenuis in New Brunswick, Canada, and with Elaphostrongylus cervi in New Zealand. Excretory-secretory (E-S) antigens from adult P. tenuis were evaluated for their serodiagnostic potential in identifying P. tenuis and heterologous E. cervi infections in a Western blot. The antigen recognition profile of sera from animals infected with P. tenuis varied between individuals and with duration of infections, whereas that of pooled sera from animals infected with E. cervi showed less variation. A single molecule of 42-43 kDa was recognized consistently by sera from all animals infected with either P. tenuis or E. cervi. Sera from unexposed control deer and from those with other heterologous nematode infections did not consistently identify this antigen. Serorecognition of the 42-43-kDa antigen by deer infected with P. tenuis resulted in a sensitivity of 99% and a specificity of 85% (> or =1 mo postinfection). Although antibody to this antigen waned with time, the persistence of recognition up to 34 mo postinfection with P. tenuis exemplifies its diagnostic value. The sensitivity and specificity of diagnosis using this molecule were each 100% for identifying deer infected with E. cervi (> or =3 mo postinfection). Two other molecules from E-S of adult P. tenuis, 26-28 and 10-12 kDa, were also diagnostic, although their recognition was not persistent throughout infections. These 2 molecules may prove useful in combination with the 42-43-kDa antigen to help identify all infected animals during all phases of infections. This research represents the first conclusive identification of antigens with real potential for reliable antemortem immunodiagnosis of both P. tenuis infections and heterologous E. cervi infections.

Preliminary analysis of Sm14 in distinct fractions of Schistosoma mansoni adult worm extract

In previous studies it was shown that the recombinant molecule, r-Sm14, induces high levels of protection against Schistosoma mansoni infection in two outbred animal models and immune crossprotection against infection by Fasciola hepatica in Swiss outbred mice. r-Sm14 was derived from a living worm extract, called SE, and is being developed as the molecular basis of an anti-helminth bivalent vaccine against the two parasites, for medical and veterinary application. Present data refer to SDS-PAGE and Western Blotting analysis of four different preparations of S. mansoni adult worms focusing Sm14 identification. The extracts correspond to the initial fraction of the SE extraction process, containing products released by living worms (SEi); SE2, reextraction of adult worms in PBS; and SE of separated male and female adult worms. In all extracts it was possible to detect the component of 14 kDa, that was recognized by specific anti-rSm14 antibody raised in rabbits.

The contribution of molecular biology to the development of vaccines against nematode and trematode parasites of domestic ruminants

Rapid developments in molecular biology have had an enormous impact on the prospects for the development of vaccines to control the major nematode and trematode infestations of livestock. Vaccine candidates are purified using conventional protein chemistry techniques but the limitations imposed by the scarcity of parasite material provide an insurmountable barrier for commercial vaccine production by this means. The ability to purify mRNA from different parasite life-cycle stages and to prepare cDNA expression libraries from it has proven central to the identification of immunogenic parasite proteins. Potentially, protective parasite antigens can now be produced in recombinant form in a variety of vectors and this represents a key breakthrough on the road to commercial vaccine production. The contribution of molecular biology to this process is discussed using several examples, particularly in vaccine development against the pathogenic abomasal nematode of sheep and goats, Haemonchus contortus, and the liver fluke of sheep and cattle, Fasciola hepatica. The difficulties of producing recombinant proteins in the correct form, with appropriate post-translational modification and conformation, are discussed as well as emerging means of antigen delivery including DNA vaccination. The opportunities offered by genome and expressed sequence tag analyses programmes for antigen targeting are discussed in association with developing microarray and proteomics technologies which offer the prospect of large scale, rapid antigen screening and identification.

Interactions involving intestinal nematodes of rodents: experimental and field studies

Multiple species infections with parasitic helminths, including nematodes, are common in wild rodent populations. In this paper we first define different types of associations and review experimental evidence for different categories of interactions. We conclude that whilst laboratory experiments have demonstrated unequivocally that both synergistic and antagonistic interactions involving nematodes exist, field work utilizing wild rodents has generally led to the conclusion that interactions between nematode species play no, or at most a minor, role in shaping helminth component communities. Nevertheless, we emphasize that analysis of interactions between parasites in laboratory systems has been fruitful, has made a fundamental contribution to our understanding of the mechanisms underlying host-protective intestinal immune responses, and has provided a rationale for studies on polyparasitism in human beings and domestic animals. Finally, we consider the practical implications for transmission of zoonotic diseases to human communities and to their domestic animals, and we identify the questions that merit research priority.

Genomic and genetic research on bursate nematodes: significance, implications and prospects

Molecular genetic research on parasitic nematodes (order Strongylida) is of major significance for many fundamental and applied areas of medical and veterinary parasitology. The advent of gene technology has led to some progress for this group of nematodes, particularly in studying parasite systematics, drug resistance and population genetics, and in the development of diagnostic assays and the characterisation of potential vaccine and drug targets. This paper gives an account of the molecular biology and genetics of strongylid nematodes, mainly of veterinary socio-economic importance, indicates the implications of such research and gives a perspective on genome research for this important parasite group, in light of recent technological advances and knowledge of the genomes of other metazoan organisms.

Vaccination of calves with Haemonchus placei intestinal homogenate

The ability of adult Haemonchus placei intestinal homogenate to confer protection against homologous challenge infection was evaluated. Calves were immunized twice with 100 microg H. placei intestinal protein in 5% dextran-sulfate/PBS (vaccinates) or PBS alone (controls) and were challenged with approximately 3300 infective H. placei larvae. There was no significant difference between groups in the total number of nematodes recovered but significantly fewer (p < 0.001) adult females were recovered from vaccinates. The proportion of fourth-stage larvae in vaccinates was significantly greater (p < or = 0.05) than in controls. Lengths of adult male and female nematodes were significantly shorter (p < 0.001) in vaccinated calves, and the numbers of eggs present in the uteri of female nematodes from vaccinates were significantly decreased (p < 0.001). Counts of nematode eggs per gram of feces (EPG) of vaccinates were significantly less than that for controls on Days 29-49 post-challenge (p < or = 0.05). Vaccinates had significant increases in serum IgG1 and IgG2 log(10) titers (p < or = 0.05) but not in serum IgM. EPG, numbers of females, and size of males and females were negatively correlated with increased mean post-challenge IgG1 and IgG2 titers. Reduction in binding of periodate-treated gut homogenate by immune serum indicated a carbohydrate specific component in the immune response.

The development of vaccines against gastrointestinal nematode parasites, particularly Haemonchus contortus

Many parasitic nematodes are developing resistance to chemical treatment, and the search is on to produce commercially viable molecular vaccines. Much progress has been made with highly protective 'hidden antigens', especially for Haemonchus contortus, and recent work with new 'natural antigens' has yielded promising results. Here, Sue Newton and Ed Munn review the most recent advances in these two main approaches to this problem.

Tropomyosin implicated in host protective responses to microfilariae in onchocerciasis

A cDNA from adult female Onchocerca volvulus encoding the C-terminal portion of a tropomyosin isoform (termed MOv-14) has been shown previously to confer protective immunity in rodent models of onchocerciasis. The full-length sequence (designated Ov-tmy-1) obtained by PCR amplification, codes for a protein of 33 kDa and shares 91% identity with tropomyosins from other nematodes, falling to 57% identity with human alpha-tropomyosin. Ov-TMY-1 migrates with an apparent molecular mass of 42 kDa on SDS/PAGE and is present in all life-cycle stages, as determined by immunoblotting. Immunogold electron microscopy identified antigenic sites within muscle blocks and the cuticle of microfilariae and infective larvae. Anti-MOv14 antibodies were abundant in mice exhibiting serum-transferable protection against microfilariae conferred by vaccination with a PBS-soluble parasite extract. In contrast, little or no MOv14-specific antibody was present in mice inoculated with live microfilariae, in which resistance is mediated by antibody-independent mechanisms. In human infections, there was an inverse correlation between anti-tropomyosin IgG levels and densities of microfilariae in the skin. Seropositivity varied with the relative endemicity of infection. An immunodominant B cell epitope within Ov-TMY-1 (AQLLAEEADRKYD) was mapped to the N terminus of the MOv14 protein by using sera from protectively vaccinated mice. Intriguingly, the sequence coincides with an IgE-binding epitope within shrimp tropomyosin, believed to be responsible for hypersensitivity in individuals exhibiting allergy to shellfish. IgG and IgE antibodies reacting with the O. volvulus epitope were detected in human infections. It is concluded that antibody responses to tropomyosin may be important in limiting microfilarial densities in a proportion of individuals with onchocerciasis and have the potential to mediate hypersensitivity reactions to dead microfilariae, raising the possibility of a link with the immunopathology of infection.

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.

Rational design of nematode vaccines: hidden antigens

Hidden antigens are defined and the general validity of the hidden antigen approach is considered. Approaches to the problem of identifying hidden antigens are offered. The nature of the immune responses induced by injection of hidden antigens and their value in giving protection is considered in the light of the site of the hidden antigen in vivo. Particular attention is given to the value of integral membrane ectoenzymes as protective hidden antigens. The need to generate hidden antigens as recombinant proteins and the possibilities and problems associated with linear, conformational and carbohydrate epitopes are outlined. Finally, concerns about the lack of stimulation of induced immune responses and the risks of resistance developing are addressed.

Susceptibility of Suffolk and Gulf Coast Native suckling lambs to naturally acquired strongylate nematode infection

Three trials compared responses to naturally-acquired strongylate nematode infection between suckling Suffolk and Gulf Coast Native (Native) lambs which grazed together. In Trial 1 (1992), infection in 14 lambs of each breed was monitored from birth to 12 weeks of age using fecal egg count (FEC) and blood packed cell volume (PCV). In Trial 2 (1993), two age-matched lambs of each breed were sacrificed at seven and ten weeks of age to estimate nematode burdens. In Trial 3 (1994), infection in 18 lambs of each breed was monitored (FEC, PCV, white blood cell count, differential leukocyte count, and anti-Haemonchus contortus immunoglobulin level) from birth to 8 weeks of age, at which time six age-matched lambs of each breed were sacrificed to estimate nematode burdens. The remaining 24 lambs were monitored until 12 weeks of age. In both Trials 1 and 3, infection in Native lambs peaked and then declined between 6-10 weeks of age. Infection in Suffolk lambs continued to increase as evidenced by increasing FEC with concomitant reduction in PCV, higher morbidity and mortality (Trial 1), and number of anthelmintic treatments required (Trial 3). In Trials 2 and 3, the principal nematode found at necropsy was H. contortus, and infection level was consistently lower (> 64%) in Native compared with Suffolk lambs. In Trial 3, there was no difference between breeds for WBC, any leukocyte type, or anti-H. contortus immunoglobulin level. These results demonstrated that suckling Native lambs developed resistance to H. contortus infection during their first exposure to infection at an age when they are considered immune incompetent and colostrally transferred anti-H. contortus immunoglobulin did not appear to be involved in the resistance.

Vaccination against worm parasites of animals

The 1990s have seen the culmination of decades of painstaking research with the registration and launch of Tickgard (Hoechst), a recombinant vaccine against Boophilus microplus, and the provisional registration of a Taenia ovis vaccine. Research continues to hold promise for immunological control of Echinococcus, Fasciola, Haemonchus, Trichostrongylus and Ostertagia. Blood-sucking parasites (e.g. ticks and H. contortus) are susceptible to control by vaccines containing 'novel' or 'concealed' antigens where serum antibodies in blood meals attack targets in the gut. Antibodies also provide protection in taeniid models, whereas the protective response to be sought in Fasciola remains unclear. More problematic are formulations and delivery strategies to induce expulsion of gastrointestinal nematodes, using vaccines containing recombinant 'conventional' antigens. The use of computer models to simulate vaccine efficacy in worm control and challenges to the concept of 'hypo-responsiveness' of young lambs will encourage cautious optimism and lively debate as to the prospects for integrated worm control using parasite vaccines. This review covers the aspirations, current success and problems faced by researchers in the parasite arena.

Progress on vaccination against Haemonchus contortus

Control of Haemonchus contortus at present is largely by the use of anthelmintics, assisted in some regions by management programs. Widespread development of resistance, particularly in South Africa and Australia, and concerns associated with the manufacture and use of chemicals have led to increasing interest in vaccination as an alternative means of control. Vaccination strategies basically fall into 2 categories, 'hidden' antigens (usually derived from the gastrointestinal tract of the adult parasite), or 'natural' antigens (those exposed to the immune system of the host during the course of infection, usually derived from the infective larval stage). Particularly promising results have been obtained using the hidden gut antigen H11, or H110D, and more recently with another hidden antigen, H-gal-GP. The use of a natural antigen vaccine, however, would provide advantages such as boosting of the immune response by field challenge. This article will review recent developments in both types of vaccines against H. contortus and consider the advantages and disadvantages of the 2 approaches.

Immunisation of calves with Ostertagia ostertagi fourth stage larval antigens failed to protect calves from infection

Excretory-secretory and somatic antigens of Ostertagia ostertagi fourth stage larvae, emulsified with complete Freund's adjuvant, were intraperitoneally administered to calves on three occasions. Two weeks after the last immunisation all calves were infected with a single dose of 130,000 O. ostertagi third stage larvae. All animals were necropsied 25 days after infection. The immunisation procedure resulted in IgG1 and IgG2 antibodies, however no protective immunity was induced as O. ostertagi worm burdens and worm length were similar in all groups.

Variation in immune responsiveness of sheep to the antigens of intestinal nematodes and blowfly larvae

The total and IgG1 antibody responses to the intestinal nematode parasites Haemonchus contortus and Trichostrongylus colubriformis were measured in the serum of 160 lambs, 4 months of age. These antibodies had developed as the result of natural exposure to the parasites on pasture. Three sires were examined and strong sire effects on half-sib progeny were found. Plotting of ELISA antibody results in two dimensions revealed clustering of responses within sire groups. Bimodal antibody distributions were also observed within sire groups and the whole population for T. colubriformis. A bimodal distribution of antibodies to H. contortus was found for one sire group but not for the whole population. The injection of blowfly larvae (Lucilia cuprina) extract into 42/160 lambs at a later age (12 months) was followed by increased antibodies to L. cuprina and an apparent increase in antibodies to T. colubriformis. A bimodal distribution for antibodies to L. cuprina was found in one sire group and in the whole population. These bimodal distributions of antibodies to L. cuprina did not coincide with the distribution of antibodies to T. colubriformis or H. contortus, measured on the same serum samples. It was concluded that high and low responder sire groups could be differentiated in lamb populations for all three parasites. These effects persisted during lamb maturation and appeared to be genetic effects. Finally, cross-reacting antibodies between L. cuprina and T. colubriformis appear to be stimulated by injection of L. cuprina antigens.

The immunogenetics of resistance to Trichostrongylus colubriformis and Haemonchus contortus parasites in sheep

Three possible immunogenetic markers for resistance to intestinal parasites in sheep have been studied. Allotypes of the major histocompatibility complex (MHC) of the sheep have been investigated as markers, using serological typing or gene probes, for associations between allotypes and resistance to parasites in selected high responder and low responder lines of sheep. Only the serologically-determined class I ovine leucocyte antigen (OLA) types SY 1a and SY 1b have been found to be consistently associated with increased resistance to Trichostrongylus colubriformis, but this association has not extended to the immunologically distinct Haemonchus contortus parasite. Gene probes of the sheep DRB, DQB and DQA MHC class II loci have detected animals with increased susceptibility to T. colubriformis. Eosinophilia was investigated as a marker and found to be associated with increased resistance to parasites in lines of Australian Merinos and New Zealand Romneys selected for resistance on the basis of low faecal egg count. Blood eosinophilia was distinct from eosinophil infiltration of the gut which was poorly associated with resistance. The mechanism of parasite resistance appeared to involve the release of vasoactive amines and leukotrienes into intestinal mucus, since the selected high responder sheep to T. colubriformis and H. contortus had significantly increased amounts of these agents in their gut mucus, compared with selected low responder or random-bred sheep. Antibodies to T. colubriformis and H. contortus have also been used as markers to select high responder sire groups of lambs in contact with the parasites, for the first time, at weaning. This assay had the advantage of detecting distinct antigens for the two parasites, which would allow resistance to the species of parasite to be selected in the lambs. Vaccines have been developed against H. contortus using 'novel' gut antigens from the parasite, but variable responsiveness of the host sheep seemed to result in varying degrees of protection which were stimulated by these vaccines.

Genetic variation in parasitic nematodes and its implications

An absolute pre-requisite for a genetic response to a selective pressure is genetic variation within the population under selection. Helminth populations are clearly able to respond to selective pressures and must, therefore, be genetically heterogeneous. While not quite tautological, this is at best indirect evidence for the existence of genetic variation but there are few examples of well documented helminth phenotypic variation with a proven genetic basis. Isozyme analysis has provided more direct evidence for variation but attempts to link this variation to responses to selection or to identify the forces maintaining that variation have been largely unsuccessful. Thus there is a clear need for new techniques. The recent application of PCR and direct sequencing technology to the study of helminth genetics has allowed the genotypes of individual worms to be determined and the first direct measurements of allele frequencies to be made in this group of organisms. In addition, the application of genetic and molecular data from Caenorhabditis elegans is a potentially rich source of new markers. These techniques do not require that the genetic basis of the phenotype in question be known since a large number of loci can be examined and selection detected through changes in the frequency of anonymous linked marker loci. Phenotypes with complex genetic bases can, therefore, be analysed. I have applied these techniques to the study of anthelmintic resistance genetics and others have applied them to the genetics of inhibited development in Ostertagia. Other phenotypes that are of great interest are the potential for selection of resistance to vaccination and the use of genetically resistant hosts. The ease with which helminths have countered all classes of anthelminitics and the apparently high levels of polymorphism in helminth populations suggest that immunological control methods may also prove to be vulnerable to the adaptive capabilities of the parasite. Evidence from a mouse-helminth model system has already provided evidence that worms can meet the challenge.

Vaccination against gastrointestinal nematodes of sheep using purified secretory acetylcholinesterase from Trichostrongylus colubriformis--an initial pilot study

Purified secretory acetylcholinesterase (sAChE) from Trichostrongylus colubriformis was used as a candidate vaccine against mixed T. colubriformis, Haemonchus contortus and C. oncophora challenge infections of sheep. Cross species protection was achieved with an average reduction in worm burden of all species of 31%, rising to 58% in individual cases. There was no consistent reduction in faecal egg counts and increases in anti-T. colubriformis sAChE IgG antibody levels following vaccination were modest. We suggest that improved antigen delivery systems will result in increased host protection.

Parasites, animal production and sustainable development

Ecologically sustainable development is aimed at reducing environmental degradation while enabling economic development with equity between the developed and developing worlds and between generations. Parasite control in livestock can both contribute to, and take advantage of, sustainable agriculture. This will tend towards less intensive, lower input, diversified crop and animal production with less risk of parasite-induced losses and greater opportunities for integrated control including the exploitation of grazing management. Chemotherapy will continue to play a part but the most serious problem is resistance in the target species. Except for a few isolated issues, currently used parasiticides are relatively minor contaminants of the food supply or the environment. Nevertheless, the compounds of the future will need to be narrow-spectrum, non-persistent and rapidly degraded, with convenience in the hands of the user reduced in importance. Environmentally friendly alternatives to chemotherapy, including genetic resistance of hosts, vaccines, and biological control, show considerable promise and must be pursued. Sustainable systems pose optimisation problems and more attention must be given to systems research, models and products to aid decisions. If governments are serious about sustainable development, greater support will be needed for longer-term patient, multi-disciplinary research.

An association between successful vaccination against the bovine nodular worm, Oesophagostomum radiatum and induction of eosinophilia

Immunization of 48 cattle with somatic antigen extracts of the nodular worm, Oesophagostomum radiatum, reduced faecal egg counts (FEC) by 51% and worm burden by 47%. The antibody titre of each animal was correlated to both worm burden (r = -0.40, P < 0.005) and FEC (r = -0.37, P < 0.01). The vaccinated cattle showed wide variation in their response to vaccination with worm burdens after challenge infection varying from 0 to 4060. Of these 48 vaccinated animals, eight developed sterile immunity (worm burden = 0, FEC = 0) whereas a further 10 had no protective response to immunization (worm burden > 1,000, FEC > 100). A comparison of these two sub-groups showed that the group with sterile immunity had a higher antibody titre (P < 0.05). In addition, the group with sterile immunity developed a blood eosinophilia between 21 and 29 days after challenge infection. Vaccinated, unprotected calves and control, unvaccinated calves did not develop eosinophilia.

Parasite enzymes and the control of roundworm and fluke infestation in domestic animals

The potential application of parasite enzymes to the serodiagnosis and control of veterinary helminthiases is reviewed. Consideration is given to the use of secreted enzymes as potential antihelminth vaccine components, in the search for novel anthelmintic agents and as serodiagnostic targets. The discussion focuses on recent advances in the definition of the molecular and functional properties of helminth enzymes and the application of this information to the development of novel anthelmintics as well as vaccines. Enzymes included are acetylcholinesterases, enzymes of polyamine and carbohydrate metabolism, proteases and detoxifying activities such as superoxide dismutases and glutathione S-transferases.

The major histocompatibility complex and peptide vaccines in domestic animals

Three hypotheses are suggested to explain the phenomenon of low responsiveness in domestic animals after injection of peptide vaccines. The first hypothesis proposes involvement of MHC haplotype and the special case in livestock breeding, where inheritance of the sire's haplotype can be closely examined by injection of antigen into a large number of paternal half-sib progeny. The second hypothesis examines the effect of repeated antigen injections in overcoming age and MHC haplotype effects and distinguishing these effects from those caused by deficiencies in the T cell repertoire. The third hypothesis concerns non-MHC effects that influence the expression of MHC haplotype effects and enable the host to mount an effective immune response. It is suggested that the antigen recognition signal from T cell receptor/MHC interaction is amplified to a varying extent in animal populations. Deficiency in this amplification through myeloid cell or cytokine responses may be yet another factor limiting immune responsiveness.

Partial characterization of proteolytic enzymes in different developmental stages of Ostertagia ostertagi

Proteolytic enzymes present in extracts of third (L3) and fourth (L4) stage larvae and adults of the cattle nematode Ostertagia ostertagi were defined on the basis of pH optima and proteinase inhibitor sensitivity in spectrophotometric assays using azocasein and elastin-orcein as protein substrates. Evidence that different classes of proteinases are expressed in a stage specific manner was provided by the contrasting pH optima and inhibitor sensitivities shown by the enzymes in the different parasite stages. Stage specificity was confirmed by gelatin-substrate analysis. In addition, proteolytic activity was sought in the excretory/secretory products (ES) of the L4 following simple in vitro culture. Contrasting pH and inhibitor sensitivities as well as gelatin-substrate analysis showed that different proteinases were present in somatic L4 extracts and L4 ES products. The secreted proteinases may be useful targets for serodiagnosis or vaccination.

Production of vaccines against gastrointestinal nematodes of livestock

Three international collaborations involving Australian research scientists are currently developing vaccines against Haemonchus, Trichostrongylus and Ostertagia parasites using recombinant DNA technology. The variety of protective antigens identified can be classified as 'conventional' (stimulate naturally acquired immunity) or 'novel'/'convert'/'concealed' (protective once immunity is induced by vaccination). To date, the most gratifying progress has resulted in 60-90% protection against Haemonchus and other blood-sucking parasites (e.g. ticks) using novel antigens, where high titres of serum antibody ingested by feeding worms leads to their demise. A great deal of research effort is unravelling the complexity of naturally acquired immunity so that conventional antigens, which may be the principal means of removing 'mucosal-browsing' parasites, may be formulated and delivered to achieve optimal efficacy. This work reveals that to remove early stages of parasites before they take up residence, deliberate induction of hypersensitivity responses akin to asthma, may be a desirable goal for vaccines and that the two models have much in common.

Towards sustainable nematode parasite control of livestock

Farmers worldwide have come to expect, and rely almost exclusively on, broad-spectrum anthelmintics to effectively control nematode parasites amongst their livestock. However, the threats of resistance, residues and ecotoxicity are of increasing concern to the future of chemotherapy. It is imperative that sustainable parasite control schemes be developed and implemented which will integrate a range of techniques to minimise anthelmintic use and still maintain high levels of profitability of the farming enterprise. At present, these need to focus on the better use of existing drugs to maximise their effectiveness and minimise the selection for resistance and impact on the environment. New drugs should also be used according to these principles. In the future it is expected that other non-chemotherapeutic options will become available, e.g. helminth vaccines, resistant hosts, biological control, nematode growth regulators, which will revolutionize the current thinking on nematode parasite control of livestock.

Control strategies to prevent resistance

Anthelmintics will continue to play a central role in nematode control programs of livestock for the foreseeable future. However, to prevent the emergence of resistance, anthelmintics need to be either so effective that there are no survivors following treatment or treatment should be so infrequent that those that survive make little or no contribution to subsequent generations of the parasite populations. In this regard, the record for the sheep, horse and particularly the goat industry is not good, whereas the second factor is almost certainly responsible for the lack of a significant resistance problem in the cattle industry. In Australia, where high levels of resistance prevail, control programs designed to limit the increase and spread of resistance have been promoted for some time. However, their development, and certainly their implementation, has tended to be reactive rather than proactive--attempting to deal with the problem when it threatens future productivity, rather than preventing it in the first instance. Regions and/or livestock industries with relatively less of a resistance problem should heed these developments and endeavour to establish a responsible ethos amongst users of anthelmintics. In concert with the general philosophy of Integrated Pest Management (IPM), parasite control schemes should focus on the planned integration of a range of techniques, rather than relying almost entirely on anthelmintics. By these means, sustainable control strategies to prevent resistance could be achieved.

Control of gastrointestinal nematodes in Australia in the 21st century

Because the Australian livestock industries rely overwhelmingly on year-round grazing of pastures, nematode parasites are of major concern. Nematode parasitism is recognized as the cause of the most economically important diseases of sheep, while in cattle, the relative importance of nematode parasitism will probably increase following the eradication of diseases such as brucellosis and tuberculosis. The extraordinarily high prevalence of resistance to benzimidazoles, levamisole and morantel in nematodes parasitising sheep has stimulated the formulation and, to a surprisingly large extent, adoption by farmers of strategic control programmes aimed at preserving the useful life of ivermectin. Even so, it is highly likely that we will enter the 21st century with ivermectin resistance. Current research aimed at control in the next century is progressing in three broad directions. First, towards better control using existing anthelmintics through modifications to pharmacokinetics, together with integrated management aided by computer simulation models of nematode population biology. Secondly, towards immunological control arising from a better understanding of the ruminant immune response to helminths, particularly in young animals, and finally, a coordinated investigation of genetic resistance, within breeds, of sheep to nematode parasitism. The appearance in the 21st century of novel anthelmintic compounds, most probably as a result of research by the international pharmaceutical industry, may well reduce the sense of urgency driving current Australian research. Nevertheless, success in one or more of these three lines of endeavour will mean a longer and more profitable life for the next generation of anthelmintics marketed in Australia.

Current and future prospects for control of ostertagiasis in northern Europe--examples from Denmark

This review primarily discusses the status and prospects for control of bovine ostertagiasis in northern Europe, with examples from Denmark. There are different ongoing developments in agricultural systems and practices, and methods and possibilities for practical control depend on the intensity and specialisation of these; the modern dairy farm remains at highest risk of parasitism, owing to increasing stocking densities and limited natural control elements at hand. Epidemiology and course of infections are significantly influenced by the gradual build-up of acquired immunity, which usually contributes to prevent loss-producing effects in second season and older animals. It may be of doubtful value to exaggerate worm control in first season animals, because this may reduce development of immunity with the risk of translocating parasite problems from the young to the older, economically more important age categories of animals. A number of reasons for adopting an overall consideration on worm control and performance throughout adolescence is emphasised. Control by management relies on a fairly detailed insight into local transmission factors of Ostertagia ostertagi and related trichostrongyles. No doubt future investigations will provide important additional knowledge in this area. Anthelmintics will continue to constitute a major control measure, but it is unlikely that there will be any acceleration in the rate of commercial release of new compounds. However, ongoing modifications and new formulations of existing anthelmintics will continue to be produced, and implementation at the farm level of the proper use of anthelmintics and other control measures will be one of the important tasks of the coming century. Until now, the development of anthelmintic resistance in cattle has been negligible, but it may possibly pose a potential risk over the coming decades. With regard to some new anthelmintics that have environmental concerns related to their faecal excretion, this should be carefully examined in the future. Control in the form of vaccination or biological control by microfungi or others would be attractive alternatives that should be given a high research priority. Yet, at present it is not easy to predict which of these may lead to feasible, practical control.

Development of a vaccine against haemonchus contortus

Haemonchus contortus is an economically important nematode parasite of sheep and the occurrence worldwide of strains resistant to anthelmintic chemicals has emphasized the need to develop a vaccine against it. Here, Ed Munn describes the approach to this problem adopted in his laboratory. The principle developed should be applicable to other gastrointestinal parasites.

Protection in lambs immunised with Haemonchus contortus gut membrane proteins

Substantial protection against Haemonchus contortus was induced in lambs by hyperimmunising them with an integral membrane extract of intestines dissected from adult worms. The procedure was effective in young growing animals but did not afford any cross protection against either Ostertagia circumcincta or Nematodirus battus. A serum transfer experiment suggested that the effector mechanism was serum antibody which bound to the brush border membrane of the parasites intestinal cells. Sheep which had acquired immunity to H contortus by previous exposure to a trickle infection did not recognise the gut membrane proteins, which suggested that these are normally hidden from the host.

Prospects for biological control of nematode parasites of ruminants

Control of nematode parasites in ruminants is directed almost exclusively at the parasitic stage in the host using conventional anthelmintic therapy. However, to complete their life cycle, parasites have to develop through a series of free-living stages on pasture. It is within this environment that there is a vast range of cohabiting micro-organisms, some of which are known to infect or prey on nematodes. These include bacteria, protozoa, viruses and fungi. To date, most interest has been focused on those organisms producing chemical toxins which have been developed as anthelmintics, e.g. Streptomyces/avermectins. Yet the potential of predacious and nematoxic fungi and bacteria for the biological control of nematode parasites of livestock remains largely untapped. These organisms may well offer a cheaper and more sustainable approach to reducing the impact of parasitism in livestock.