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

Structure-activity relationship and target investigation of 2-aryl quinolines with nematocidal activity

Within the context of our anthelmintic discovery program, we recently identified and evaluated a quinoline derivative, called ABX464 or obefazimod, as a nematocidal candidate; synthesised a series of analogues which were assessed for activity against the free-living nematode Caenorhabditis elegans; and predicted compound-target relationships by thermal proteome profiling (TPP) and in silico docking. Here, we logically extended this work and critically evaluated the anthelmintic activity of ABX464 analogues on Haemonchus contortus (barber's pole worm) - a highly pathogenic nematode of ruminant livestock. First, we tested a series of 44 analogues on H. contortus (larvae and adults) to investigate the nematocidal pharmacophore of ABX464, and identified one compound with greater potency than the parent compound and showed moderate activity against a select number of other parasitic nematodes (including Ancylostoma, Heligmosomoides and Strongyloides species). Using TPP and in silico modelling studies, we predicted protein HCON_00074590 (a predicted aldo-keto reductase) as a target candidate for ABX464 in H. contortus. Future work aims to optimise this compound as a nematocidal candidate and investigate its pharmacokinetic properties. Overall, this study presents a first step toward the development of a new nematocide.

Protective efficacy of Barbervax® in Merino weaner sheep trickle infected with five doses of Haemonchus contortus infective larvae

Barbervax® protects sheep against H. contortus infection; however, the level of protection afforded by the vaccine at different levels of larval challenge under field conditions has not been reported. Therefore, this study was designed to investigate the protective efficacy of Barbervax® at increasing doses of trickle infection with H. contortus third stage larvae (L3). Merino lambs (220) were randomly allocated to challenge treatment groups and received Barbervax® vaccinations at approximately 8, 11, 15 and 22 weeks of age or not vaccinated (control group). Animals within each treatment group were subjected to one of five levels of H. contortus L3 trickle infection (0, 300, 600, 1200 and 2400 L3/week in two split doses). Trickle infections started two weeks after the third vaccination (week 0). Worm egg count (WEC), packed cell volume (PCV), antibody titre and bodyweight were measured at week 2 (start of trickle infection), week 5 and then every two weeks for 11 weeks. Vaccinated weaners had a significantly (P < 0.0001) lower faecal WEC than unvaccinated control animals. Vaccination induced significant specific antibody responses that were not influenced by level of trickle infection. Vaccination significantly reduced the number of weaners requiring salvage anthelmintic treatment (9.4% vaccinated weaners versus 33.6% unvaccinated). Vaccine protective index based on WEC was similar at all challenge levels (overall mean> 60%) and at the higher challenge levels did not reduce H. contortus infection to levels that would not require anthelmintic treatment. Therefore, it is suggested that under conditions of high larval challenge the use of Barbervax® should be integrated with other control methods.

Evaluation of protection in grazing lambs immunised with different doses of Haemonchus contortus gut membrane glycoproteins in Southern Brazil

A vaccine containing doses ranging from 2 μg to 50 μg of integral membrane proteins from Haemonchus contortus intestinal cells (H11 and H-gal-GP complex) has been shown to be effective for lambs. A vaccine for H. contortus was tested in two-month old grazing Corriedale lambs during an eight-month trial on the outskirts of Bagé, Rio Grande do Sul, Brazil. The animals were kept in a single paddock and allocated to five similar groups according to weight, sex and faecal egg counts (FEC). Vaccinated lambs received 0, 2, 5, 10 or 50 μg of the same antigen diluted in QuilA adjuvant. Vaccine injections were given at days 0, 21, 49, 91, 133, and 175. Lambs were sampled weekly for FEC, packed cell volume (PCV), and plasma ELISA antibody titre. Lambs with PCVs ≤15 % were drenched with levamisole (7.5 mg/kg body weight) as a salvage treatment. During days 77-98 an artificial challenge was administered to increase parasite transmission. Post-artificial challenge (from day 98-217), the FEC of the vaccinated lambs were 59.3 % lower than those of the control lambs. Antigen dose correlated with changes in PCV (r = 0.387 p-value < 0.001). Vaccinated lambs also had higher PCV than controls (p < 0.001; contrast analysis). Salvage treatment was needed in 16.7 % of vaccinated lambs and 88 % of control lambs.

Advances in the discovery and development of anthelmintics by harnessing natural product scaffolds

Widespread resistance to currently-used anthelmintics represents a major obstacle to controlling parasitic nematodes of livestock animals. Given the reliance on anthelmintics in many control regimens, there is a need for the continued discovery and development of new nematocides. Enabling such a focus are: (i) the major chemical diversity of natural products; (ii) the availability of curated, drug-like extract-, fraction- and/or compound-libraries from natural sources; (iii) the utility and practicality of well-established whole-worm bioassays for Haemonchus contortus-an important parasitic nematodes of livestock-to screen natural product libraries; and (iv) the availability of advanced chromatographic (HPLC), spectroscopic (NMR) and spectrometric (MS) techniques for bioassay-guided fractionation and structural elucidation. This context provides a sound basis for the identification and characterisation of anthelmintic candidates from natural sources. This chapter provides a background on the importance and impact of helminth infections/diseases, parasite control and aspects of drug discovery, and reviews recent work focused on (i) screening well-defined compound libraries to establish the methods needed for large-scale screening of natural extract libraries; (ii) discovering plant and marine extracts with nematocidal or nematostatic activity, and purifying bioactive compounds and assessing their potential for further development; and (iii) synthesising analogues of selected purified natural compounds for the identification of possible 'lead' candidates. The chapter describes some lessons learned from this work and proposes future areas of focus for drug discovery. Collectively, the findings from this recent work show potential for selected natural product scaffolds as candidates for future development. Developing such candidates via future chemical optimisation, efficacy and safety evaluations, broad spectrum activity assessments, and target identification represents an exciting prospect and, if successful, could pave the way to subsequent pre-clinical and clinical evaluations.

A simplified Barbervax® vaccination regimen in lambs to evoke immunological protection to Haemonchus contortus

This study assessed the feasibility of altering the pre-weaning vaccination schedule of the commercially available Barbervax® vaccine directed against Haemonchus contortus, to avoid the 2nd priming vaccination which typically falls between lamb marking and weaning. Merino lambs (n = 175) born to maiden ewes, located in the Northern Tablelands, New South Wales, were randomly allocated to treatment groups (n = 35) and subjected to five different vaccination regimes. One group remained as unvaccinated controls and another had the full set of three priming doses. The other three groups were vaccinated only at marking and weaning receiving a double dose of vaccine at marking and/or weaning. The hypothesis tested was that reducing the interval between lamb marking and weaning to 6 weeks, and providing a double dose of vaccine at marking, weaning or both would remove the need for the second priming vaccination between lamb marking and weaning. This priming vaccination in the vaccination protocol necessitates an additional mustering of ewes with young lambs at foot and is a time consuming and costly exercise which increases the risk of mis-mothering. Blood and faecal samples were collected at frequent intervals for worm egg count (WEC), larval differentiation and H. contortus vaccine specific ELISA antibody analysis. Overall, the results supported the hypothesis, and it was found that antibody titres and WEC reductions equivalent to the registered vaccine regimen were achieved by the alternative regimens. This finding requires further investigation under a wider range of conditions. Deviation from the registered vaccination protocol would constitute off-label usage, and at this time and until further evaluations are done these deviations are not recommended.

Trials with the Haemonchus vaccine, Barbervax®, in ewes and lambs in a tropical environment: Nutrient supplementation improves protection in periparturient ewes

Haemonchus contortus is an economic problem in sheep farms worldwide, mainly in the tropics and subtropics. A vaccine against haemonchosis, called Barbervax^®, was evaluated in ewes under two nutritional status, naturally infected with gastrointestinal nematodes. Ewes were divided into four groups: Supplemented Diet - Vaccine; Supplemented Diet - No vaccine; Basal Diet - Vaccine and Basal Diet - No vaccine. Their lambs were divided in Vaccinated and No vaccine. Ewes were immunised six times starting about 1 month of pregnancy with the first three doses at 3 week intervals and the last three shots at 4 week intervals. Supplemented ewes had higher body weight, body score and packed cell volume compared with those fed a basal diet. Both groups of vaccinated ewes showed a similar response in circulating anti-vaccine antibodies but the vaccine had no discernible effect on either body weight, body score and packed cell volume. There was a marked group difference in the number of ewes that received precautionary treatments with anthelmintic. All 14 Basal Diet - No vaccine ewes required treatment. In contrast only 7 ewes, in the Supplemented Diet - Vaccine group required anthelmintic treatment. In the Basal Diet - Vaccine and in the Supplemented Diet - No Vaccine groups, 12 and 13 ewes needed anthelmintic treatment, respectively. Vaccinated lambs showed much higher antibody titres resulting in 80% less Haemonchus spp. egg counts comparing with no vaccine lambs. Taken together these results clearly suggest that in pregnant and lactating ewes a combined protective effect between vaccination and improved nutrition resulted in fewer precautionary anthelmintic treatments. Thus, it was possible to achieve a more sustainable level of control of the haemonchosis, less dependent on anthelmintic drugs.

An elaborated SIR model for haemonchosis in sheep in South Africa under a targeted selective anthelmintic treatment regime

Infection with the abomasal nematode Haemonchus contortus is responsible for considerable production loss in small ruminants globally, and especially in warm, summer-rainfall regions. Previous attempts to predict infection levels have followed the traditional framework for macroparasite models, i.e. tracking parasite population sizes as a function of host and climatic factors. Targeted treatment strategies, in which patho-physiological indices are used to identify the individuals most affected by parasites, could provide a foundation for alternative, incidence-based epidemiological models. In this paper, an elaboration of the classic susceptible-infected-recovered (SIR) model framework for microparasites was adapted to haemonchosis and used to predict disease in Merino sheep on a commercial farm in South Africa. Incidence was monitored over a single grazing season using the FAMACHA scoring system for conjunctival mucosal coloration, which indicates high burdens of H. contortus, and used to fit the model by estimating transmission parameters. The model predicted force of infection (FOI) between sequential FAMACHA monitoring events in groups of dry, pregnant and lactating ewes, and related FOI to factors including climate (temperature, rainfall and rainfall entropy), using a random effects model with reproductive status group as the cluster variable. Temperature and rainfall in the seven days prior to monitoring significantly predicted the interval FOI (p≤0.002), while rainfall entropy did not (p=0.289). Differences across the three groups accounted for approximately 90% of the variability in the interval FOI over the period of investigation. Maintained FOI during targeted treatment of cases of haemonchosis suggests strong underlying transmission from sub-clinically infected animals, and/or limited impact on pre-existing pasture contamination by removal of clinical worm burdens later in the grazing season. The model has the potential to contribute to the sustainable management of H. contortus by predicting periods of heightened risk, and hence to focus and optimise limited resources for monitoring and treatment. SIR-type model frameworks are an alternative to classic abundance-based compartmental models of macroparasite epidemiology, and could be useful where incidence data are available. Significant challenges remain, however, in the ability to calibrate such models to field data at spatial and temporal scales that are useful for decision support at farm level.

Vaccination of goats against Haemonchus contortus with the gut membrane proteins H11/H-gal-GP

Forty goats, aged from 2 to 5 months were subjected to two different immunization protocols with a vaccine containing Haemonchus contortus gut membrane proteins H11/H-gal-GP to evaluate protection against H. contortus on pre-contaminated pastures. Goats were allocated to four groups of ten, three of them received their first vaccination before turnout. One group (V4) was then vaccinated at 4-week-intervals whereas another two groups (V6 and V6SEP) were vaccinated at 6-week-intervals. A control group (CTRL) remained unvaccinated. In May, after the second vaccination, all goats were turned out on pastures which had been previously contaminated with H. contortus eggs by seeder sheep for a period of six weeks. Goats of groups V4, V6 and CTRL were grazed together, whereas V6SEP was kept separately at an identical stocking rate. Clinical (PCV, FAMACHA, body weight), parasitological (faecal egg count, FEC) and serological (antibody titres) parameters were measured fortnightly. All goats were stabled in October, drenched with levamisole and two weeks later infected with 5000 L3 of H. contortus and slaughtered four weeks later for determination of abomasal worm burdens. Group mean FEC peaked 42-56days after turnout. Significantly lower FEC were observed in V6SEP vs CTRL between D 28 and 70 (p<0.01). Mean egg output of all groups decreased substantially and fluctuated at low levels until the end of the grazing period (D 154). Goats responded to vaccination with increasing antibody titres peaking after every booster. Mean worm burdens deriving from experimental infections were reduced by 89, 65 and 47% in groups V4, V6 and V6SEP, respectively, compared with the controls. The difference was significant for V4 (p<0.01). Antibody titres measured 14days before slaughter did not correlate statistically with the worm burdens. It was concluded that the vaccination protocol did not result in sufficient protection on pasture, as antibody titres were still low at the time the goats were exposed to larval contamination on pasture after turnout.

Diagnosis, Treatment and Management of Haemonchus contortus in Small Ruminants

Haemonchus contortus is a highly pathogenic, blood-feeding nematode of small ruminants, and a significant cause of mortalities worldwide. Haemonchosis is a particularly significant threat in tropical, subtropical and warm temperate regions, where warm and moist conditions favour the free-living stages, but periodic outbreaks occur more widely during periods of transient environmental favourability. The clinical diagnosis of haemonchosis is based mostly on the detection of anaemia in association with a characteristic epidemiological picture, and confirmed at postmortem by the finding of large numbers of H. contortus in the abomasum. The detection of impending haemonchosis relies chiefly on periodic monitoring for anaemia, including through the 'FAMACHA' conjunctival-colour index, or through faecal worm egg counts and other laboratory procedures. A range of anthelmintics for use against H. contortus is available, but in most endemic situations anthelmintic resistance significantly limits the available treatment options. Effective preventative programmes vary depending on environments and enterprise types, and according to the scale of the haemonchosis risk and the local epidemiology of infections, but should aim to prevent disease outbreaks while maintaining anthelmintic efficacy. Appropriate strategies include animal management programmes to avoid excessive H. contortus challenge, genetic and nutritional approaches to enhance resistance and resilience to infection, and the monitoring of H. contortus infection on an individual animal or flock basis. Specific strategies to manage anthelmintic resistance centre on the appropriate use of effective anthelmintics, and refugia-based treatment schedules. Alternative approaches, such as biological control, may also prove useful, and vaccination against H. contortus appears to have significant potential in control programmes.

Understanding Haemonchus contortus Better Through Genomics and Transcriptomics

Parasitic roundworms (nematodes) cause substantial mortality and morbidity in animals globally. The barber's pole worm, Haemonchus contortus, is one of the most economically significant parasitic nematodes of small ruminants worldwide. Although this and related nematodes can be controlled relatively well using anthelmintics, resistance against most drugs in common use has become a major problem. Until recently, almost nothing was known about the molecular biology of H. contortus on a global scale. This chapter gives a brief background on H. contortus and haemonchosis, immune responses, vaccine research, chemotherapeutics and current problems associated with drug resistance. It also describes progress in transcriptomics before the availability of H. contortus genomes and the challenges associated with such work. It then reviews major progress on the two draft genomes and developmental transcriptomes of H. contortus, and summarizes their implications for the molecular biology of this worm in both the free-living and the parasitic stages of its life cycle. The chapter concludes by considering how genomics and transcriptomics can accelerate research on Haemonchus and related parasites, and can enable the development of new interventions against haemonchosis.

Comparative immunoprophylactic efficacy of Haemonchus contortus recombinant enolase (rHcENO) and Con A purified native glycoproteins in sheep

Haemonchus contortus is the most economically important blood feeding nematode parasite of sheep and goats all over the world. Enolase in helminth parasites is a multi-functional enzyme which involves in glycolysis and host tissue invasion. In this study, the recombinant H. contortus enolase (rHcENO) was evaluated for its immunoprophylactic efficacy in sheep along with Con A purified native glycoproteins in a vaccine challenge trial. Group I and Group II experimental sheep were immunized thrice with rHcENO and Con A purified native glycoproteins along with Montanide ISA 61 VG adjuvant. The animals were challenged with 5000 L3 stage active H. contortus larvae after 21 days of third immunization. A significant increase in the IgG titre was observed in rHcENO and Con A purified native glycoproteins immunized animals as compared to the control animals. Immunoprotective efficacy of Con A purified native glycoproteins was comparatively higher than rHcENO antigen.

Vaccination of sheep and cattle against haemonchosis

Vaccines against gastrointestinal nematodes are one potential option for the control of parasitic gastroenteritis in ruminants. Excretory/secretory (E/S) and hidden antigens are being studied as candidates for vaccines against Haemonchus spp., which is a major parasite in cattle and small ruminants that are raised in warm climates. Protection has been observed after vaccination with some E/S proteases, particularly cysteine proteases and with some glycans that are abundant on the surfaces and in the secretory products of helminths. However, the most promising results are being obtained with glycoprotein antigens extracted from the microvillar surfaces of the Haemonchus contortus intestinal cells. These antigens are called 'hidden' because they are not exposed to the host's immune system during infection. Thus far, recombinant forms of these antigens have not been usefully protective. However, because only 5 μg of antigen is required per dose, production of a native antigen vaccine from adult parasites has been found to be practical and commercially viable. Trials indicate that a vaccine made from one particular isolate will cross-protect against geographically distant isolates.

Fasciola hepatica vaccine: we may not be there yet but we're on the right road

Major advances have been made in identifying potential vaccine molecules for the control of fasciolosis in livestock but we have yet to reach the level of efficacy required for commercialisation. The pathogenesis of fasciolosis is associated with liver damage that is inflicted by migrating and feeding immature flukes as well as host inflammatory immune responses to parasite-secreted molecules and tissue damage alarm signals. Immune suppression/modulation by the parasites prevents the development of protective immune responses as evidenced by the lack of immunity observed in naturally and experimentally infected animals. In our opinion, future efforts need to focus on understanding how parasites invade and penetrate the tissues of their hosts and how they potentiate and control the ensuing immune responses, particularly in the first days of infection. Emerging 'omics' data employed in an unbiased approach are helping us understand liver fluke biology and, in parallel with new immunological data, to identify molecules that are essential to parasite development and accessible to vaccine-induced immune responses.

Impact of next-generation technologies on exploring socioeconomically important parasites and developing new interventions

High-throughput molecular and computer technologies have become instrumental for systems biological explorations of pathogens, including parasites. For instance, investigations of the transcriptomes of different developmental stages of parasitic nematodes give insights into gene expression, regulation and function in a parasite, which is a significant step to understanding their biology, as well as interactions with their host(s) and disease. This chapter (1) gives a background on some key parasitic nematodes of socioeconomic importance, (2) describes sequencing and bioinformatic technologies for large-scale studies of the transcriptomes and genomes of these parasites, (3) provides some recent examples of applications and (4) emphasizes the prospects of fundamental biological explorations of parasites using these technologies for the development of new interventions to combat parasitic diseases.

Immune responses associated with resistance to haemonchosis in sheep

This paper examines the known immunological and genetic factors associated with sheep resistance to infection by Haemonchus contortus. Such resistance is an inheritable genetic trait (h², 0.22–0.63) associated with certain sheep breeds. Resistant sheep do not completely reject the disease; they only harbor fewer parasites than susceptible sheep and therefore have a lower fecal egg count. Protective immune response to haemonchosis is an expression of genetic resistance. Genes associated with resistance and susceptibility are described. Genetically resistant sheep have nonspecific mechanisms that block the initial colonization by Haemonchus contortus larvae. These sheep also have an efficacious Th2 type response (e.g., increases in blood and tissue eosinophils, specific IgE class antibodies, mast cells, IL-5, IL-13, and TNFα) that protects them against the infection; in contrast, susceptible sheep do not efficiently establish this type of immune response. Finally, the main reported antigens of H. contortus were reviewed.

Experiences with integrated concepts for the control of Haemonchus contortus in sheep and goats in the United States

The generally warm, moist environmental conditions in the southern United States (U.S.) are ideal for survival and growth of the egg and larval stages of Haemonchus contortus and other gastrointestinal nematodes (GIN) of sheep and goats. Consequently, infection with GIN is the greatest threat to economic small ruminant production in this region. With anthelmintic resistance now reaching epidemic proportions in small ruminants in the U.S., non-chemical control alternatives are critically needed. The Southern Consortium for Small Ruminant Parasite Control (SCSRPC) was formed in response to this crisis and over the last decade has successfully validated the use of several novel control technologies, including FAMACHA(©) for the implementation of targeted selective treatments (TST), copper oxide wire particles (COWP), nematode-trapping fungi, and grazing or feeding hay of the high-tannin perennial legume sericea lespedeza [Lespedeza cuneata (Dum.-Cours. G. Don)]. Producer attitudes toward GIN control in the U.S. have been shifting away from exclusive dependence upon anthelmintics toward more sustainable, integrated systems of parasite control. Some novel control technologies have been readily adopted by producers in combination with appropriate diagnostic tools, such as FAMACHA(©). Others techniques are still being developed, and will be available for producer use as they are validated. Although new drugs will likely be available to U.S. goat and sheep producers in the future, these will also be subject to development of anthelmintic resistance. Therefore, the adoption and implementation of sustainable GIN control principles will remain important. With emerging markets for grass-fed or organic livestock, there will continue to be a critical need for research and outreach on development and on-farm application of integrated GIN control systems for small ruminants in the U.S. for the foreseeable future.

Attempts to immunize sheep against Haemonchus contortus using a cocktail of recombinant proteases derived from the protective antigen, H-gal-GP

The objective of this study was to determine whether an antigen cocktail containing recombinantly expressed versions of most of the protective proteases of H-gal-GP, a known protective antigen from Haemonchus contortus, would confer any protection to lambs in a vaccine-challenge trial. Haemonchus contortus metalloendopeptidases, MEP1, MEP3 and MEP4, were expressed as soluble recombinant proteins in insect cells, but attempts to express the H. contortus aspartyl proteases, PEP1 and PEP2, by the same techniques were not successful. Recombinant H. contortus PEP1 was therefore expressed in Escherichia coli and refolded. Groups of sheep were immunized thrice with either native H-gal-GP, a cocktail of recombinantly expressed proteins (rMEP1, rMEP3, rMEP4 and rPep1), or adjuvant only (QuilA in PBS). All sheep were challenged with 5000 infective larvae 1 week after the final vaccination. High levels of serum antibodies that recognized H-gal-GP were detected in both the native antigen and recombinant cocktail-immunized groups by the time of challenge, but protective immunity was only observed in the group immunized with native H-gal-GP.

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

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

Immunological aspects of nematode parasite control in sheep

Gastrointestinal nematode parasitism is arguably the most serious constraint affecting sheep production worldwide. Economic losses are caused by decreased production, the costs of prophylaxis and treatment, and the death of the infected animals. The nematode of particular concern is Haemonchus contortus, which can cause severe blood loss resulting in anemia, anorexia, depression, loss of condition, and eventual death. The control of nematode parasites traditionally relies on anthelmintic treatment. The evolution of anthelmintic resistance in nematode populations threatens the success of drug treatment programs. Alternative strategies for control of nematode infections are being developed, and one approach is to take advantage of the host's natural or acquired immune responses, which can be used in selection programs to increase the level of resistance in the population. Vaccination can also be used to stimulate or boost the host's acquired immunity. The induction of protective resistance is dependent on the pattern of cytokine gene expression induced during infection by two defined CD4+ T-helper cell subsets, which have been designated as Th1 or Th2. Intracellular parasites most often invoke a Th1-type response, and helminth parasites a Th2-type response. Breeds of sheep resistant to infection have developed resistance over a much longer term of host-parasite relationship than genetically selected resistant lines. The immune components involved in these different responses and types of host-parasite relationships will be reviewed. The potential for using vaccines has been investigated, with variable results, for several decades. The few successes and potential new antigen candidates will also be reviewed.

Haemonchus contortus: characterization of the baculovirus expressed form of aminopeptidase H11

Recombinant form of Haemonchus contortus aminopeptidase H11, an intestinal membrane glycoprotein considered to be in its native form the most promising vaccine candidate, was produced in insect cells, characterised and tested in pilot vaccination-challenge trial on sheep. The sequence of the cloned gene, obtained by RT PCR isolated from adult worms, showed 97% identity to the highly immunogenic H11 clone, described by Graham et al., (database accession number AJ249941.1). A 1305 bp fragment of H11 was expressed in E. coli and used to raise a specific antiserum, which recognized recombinant forms of H11 and 110 kDa protein from H. contortus extract. H11 was expressed by baculovirus recombinants in insect cells in full length and as a fusion protein with H. contortus glutathione S-transferase (GST). The baculovirus produced recombinant antigens were used without adjuvants to immunize sheep, which resulted in 30% (full length H11) and 20% (GST-H11) reduction of worm burden. These animal experiments indicated that, although the protection induced by in vitro produced protein is lower than in case of H11 isolated from worms, recombinant forms of aminopeptidase may be considered as antigens for the control of haemonchosis.

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.

Effects of immunization of Pelibuey lambs with Oestrus ovis digestive tract protein extracts on larval establishment and development

Larval midgut proteins of hematophagous parasites contain strong antigens that can be used for host immunization. This concept has been applied for immunization of Pelibuey sheep against Oestrus ovis L. (Diptera: Oestridae). The aim of this study was to examine the effect of immunization on larval establishment (LE) and development. Immunized lambs (I, n = 6) received two injections of crude gut membrane protein extracts (GMPE) from third instar larvae with Freund's incomplete adjuvant (FIA) on days 0 (Day of first immunization) and 21 (0.4 and 0.45 mg GMPE/lamb, respectively). The control group (C, n = 5) received physiological saline with FIA. Lambs were challenged with first instars on Day 29 (20 larvae) and Day 43 (25 larvae). Blood samples were collected biweekly and IgG titers were analyzed by ELISA. All lambs were slaughtered on Day 90 and number of larvae recovered, larval stage and larval weight were recorded at necropsy. No significant effect of immunization on LE (C = 28.9%; I = 31.0% P > 0.05) was observed. Antibody titers were higher in the immunized group on Day 28 (P < 0.05), but subsequently similar in both groups. Larval physiological age and weight were also significantly (P < 0.05) affected by immunization. Immunization of Pelibuey lambs with GMPE did not affect LE but did delay O. ovis larval development.

Developments and hurdles in generating vaccines for controlling helminth parasites of grazing ruminants

As a direct consequence of rising drug resistance among common nematodes of grazing animals, efforts toward state-of-the-art vaccine development have clearly intensified in recent years, fuelled primarily by the advent of newer technologies in gene discovery, by advancements in antigen identification, characterisation and production. In this regard, it is appropriate to review progress that has been made in generating helminth vaccines and in particular, vaccines against common nematodes of production animals for consumption. In like manner, it is prudent to evaluate barriers that have hindered progress in the past and continue to present obstacles that must be solved when utilizing and depending on host immunity to attenuate parasitic infections.

Failure to protect goats following vaccination with soluble proteins of Sarcoptes scabiei: Evidence for a role for IgE antibody in protection

Developing an anti-scabies vaccine is thought to be a feasible alternative to chemical control, since animals which have recovered from sarcoptic mange become resistant against mite reinfestation. The purpose of this study was to evaluate the protective value of immune responses developed in animals after immunisation with soluble mite proteins. Soluble proteins from Sarcoptes scabiei were extracted then subjected to ion exchange chromatography, and proteins from the column were eluted step-wise with 0%, 10%, 25% and 50% of 1 M solution of NaCl in a Tris buffer. Each protein fraction was concentrated and dialysed against PBS. To evaluate the immunogenicity of the fractions, 36 goats were allocated into six groups, group1 goats were unvaccinated, group 2 were vaccinated with intact soluble mite proteins, and groups 3-6 were vaccinated respectively with the fractionated proteins. Vaccinations were conducted four times with 1 mg protein/dose and 4-week intervals between vaccinations. One week after the last vaccination, all goats were challenged with approximately 2000 live mites on the auricles and infestations were allowed to progress for 6 weeks. The severity of lesions caused by the infestation was assessed throughout the study. The challenge caused mange or encrustation dermatitis in all animals and no differences in severity of lesions were observed between vaccinated and unvaccinated control goats. Vaccination with each fraction of the mite proteins invoked high levels of scabies-specific IgG in the serum of all animals but failed to induce specific IgE as determined by Elisa. In contrast, goats challenged experimentally with a primary or repeated mite challenge developed strong serum IgE and IgG antibody responses to Sarcoptes antigens. The latter animals were shown in a previous study to be resistant to reinfestation. The lack of immune protection in the vaccinated animals may be attributed to the absence of protective levels of IgE antibody, and the present findings indicate that allergens and IgE antibody is important in immunity to S. scabiei infection.

Characterization of proteinases in different isolates of adult Haemonchus contortus

A high degree of intra- and inter-geographical variation has been demonstrated previously in the excretory/secretory proteinases released by adult Haemonchus contortus. Proteinase activity has also been associated with host-protective ‘hidden’ antigens isolated from the gut of adult H. contortus. If similar geographical strain variation also exists within the gut-associated proteinases, this will have important implications for the development of a globally effective vaccine. The proteinases active in integral-membrane protein extracts from 3 different strains of adult H. contortus were characterized on the basis of their pH optima and molecular size. Although enzyme activity was detected over a wide pH range, the majority of proteinase activity was detected at acidic pH. Differences in specific activity and size of enzymes were observed between the 3 different parasite strains at different pH values. A high degree of conservation in reactive peptides was observed when protein extracts were probed with antisera raised to the protective hidden gut-antigen complexes isolated from the Moredun strain of H. contortus, or to bacterially expressed subcomponents thereof. Therefore, despite the observed differences in membrane-bound proteinase profiles, the similarity of the immunogenic response against these hidden antigens may be sufficient to prove protective against different geographical isolates of H. contortus.

The nature and prospects for gut membrane proteins as vaccine candidates for Haemonchus contortus and other ruminant trichostrongyloids

Substantial progress has been made in the last decade in identifying several antigens from Haemonchus contortus which, in their native form, stimulate useful levels of protective immunity (70-95% reductions in faecal egg output) in the ovine host. Much work has focussed on proteins/protein complexes expressed on the surface of the worm gut which are exposed to the blood meal, and, hence, antibody ingested with it. The antigens generally, but not in all cases, show protease activity and antibody is thought to mediate protective immunity by blocking the activity of enzymes involved in digestion within the worm. This review summarises the protective efficacy, as well as the biochemical and molecular properties, of the principal candidate antigens which are expressed in the gut of these parasites. Of course, such antigens will have to be expressed as recombinant proteins to be sufficiently cost-effective for use in a commercial vaccine and the current status of recombinant antigen expression is discussed with particular reference to conformation and glycosylation. There is a need for continued antigen definition even in the confines of gut antigens and potential targets can be selected from the rapidly expanding genome/EST datasets on the basis of predicted functional homology. Gene knockout technologies such as RNA interference have the potential to provide high throughput, rapid and inexpensive methods to define whether the protein product of a particular gene would be a suitable vaccine candidate.

Abomasal lymph node responses to Haemonchus contortus intestinal antigens established in kid goats by infection or immunization with intestinal antigens

Immune responses to Haemonchus contortus intestinal antigens were evaluated using abomasal lymph node (ALN) lymphocytes from kid goats protected against challenge infection by immunization with parasite intestinal antigen, and from kids that were challenged after immunization with ovalbumin. ALN lymphocytes from the intestinal antigen-immunized group produced significantly higher antibody levels against intestinal antigens than the ovalbumin group, supporting the theory that immunization contributed to that ALN response. In contrast, intestinal lysates and membrane enriched preparations from intestinal cells stimulated significant proliferation of ALN lymphocytes in both groups. The proliferation was antigen-dependent, since intestinal antigens failed to stimulate proliferation in ALN lymphocytes from unimmunized and uninfected kids. For both the intestinal antigen and ovalbumin immunized groups, CD4+ T lymphocytes predominated in ALN lymphocytes that were stimulated to proliferate by intestinal antigens. The results indicate that H. contortus infection alone can induce ALN lymphocyte responses to intestinal antigens. In contrast to ALN lymphocyte responses, serum antibody against intestinal antigens was generally low to undetectable in ovalbumin-immunized kids following infection. Abomasal mucus from an H. contortus infected lamb was probed with a monoclonal antibody that binds to a periodate sensitive determinant on numerous H. contortus intestinal membrane and secreted proteins. Numerous bands of reactivity were detected, indicating that multiple parasite intestinal antigens were released into abomasal mucus during infection. The results, challenge the general concept that H. contortus intestinal antigens are 'hidden' from the host immune system during an infection. On the contrary, parasite intestinal proteins may be relatively abundant antigens presented to the host during infection. In addition, ALN T lymphocytes appear to provide a more sensitive measure than serum antibody to detect presentation of these antigens to the host immune system.

Vaccination against gastrointestinal nematode parasites of ruminants using gut-expressed antigens

To date, proteins isolated from the surface of the gut of gastrointestinal nematodes, particularly Haemonchus contortus, have generally proved to be useful protective antigens and several are being progressed towards recombinant protein-based vaccines. This paper describes the properties of some of the most promising antigens and summarises their performance in laboratory and field based trials. The antigens described include contortin, H11, H-gal-GP, GPI and cysteine proteinases. In addition, the discussion addresses the utility of selected antigens to protect against co-infecting nematode species such as Teladorsagia circumcincta and against related nematode infections such as Ostertagia ostertagi in cattle.