Fasciola: kinetics and quality of humoral responses to fatty acid binding protein and cathepsin l following delivery as DNA vaccines in mice

The humoral response to DNA vaccination of mice with two important Fasciola antigens has been investigated. Both F. gigantica fatty acid binding protein (FABP) and F. hepatica cathepsin L5 (FhCatL5) were shown to be expressed in COS 7 cells and induced a humoral response when delivered as secretory constructs in mice. FABP induced an IgGl dominant response, with significant IgE, IgG2a, and IgG2b responses also present, indicating a mixed Th1/Th2 response. The total Ig response peaked at 1:24,500 and antibody titers were sustained for at least 32 weeks. In contrast, the delivery of FABP as a nonsecreted construct did not result in the induction of a measurable humoral response. FhCatL5 was delivered as a secretory construct, with secretion mediated by the native F. hepatica signal sequence, which was shown to operate in COS 7 cells. The humoral response peaked at 1:2000 at week 8 and was sustained for at least 20 weeks. Antibody isotype analysis demonstrated a Th2-like response, which was qualitatively different from that obtained for FABP with an IgE dominant response, and lower titers to IgG1 and IgG3. The results demonstrate that Fasciola antigens can be delivered as DNA vaccines, but that the quality of the response varies between antigens and is influenced by the method of vaccine delivery.

Molluscicidal activity of Lawsonia inermis and its binary and tertiary combinations with other plant derived molluscicides

Molluscicidal activity of leaf, bark and seed of Lawsonia inermis against Lymnaea acuminata and Indoplanorbis exustus was studied. Highest toxicity was observed in the seed of Lawsonia inermis. Toxicity of binary (1:1) and tertiary (1:1:1) combinations of the essential oil of cedar (Cedrus deodara Roxh) and neem (Azadirachta indica A. Juss), powder from bulb of garlic (Allium sativum Linn), and oleoresin extracted from rhizome of ginger (Zingiber officinale Rosc) with Lawsonia inermis and Embelia ribes fruit powder were studied against L. acuminata and I. exustus. L. inermis seed powder in combination with Cedrus deodara oil and Azadirachta indica oil was more toxic than their individual components and other combinations.

[Natural infestation of Lymnaea truncatula Muller by Fasciola hepatica in the Tozeur oasis in southwest Tunisia]

Natural infestation of Lymnaea truncatula by Fasciola hepatica was studied at various sites in the traditional oasis of Tozeur 8 times between September 1997 and August 1998. Infestation of snails was documented in five of the eight sites with a mean level of 26.1 p. 100. Level of infestation varied depending on the site. It was highest in secondary irrigation canals (32.8 p. 100), seguias, (34.8 p. 100), and secondary drainage canals (34.8 p. 100). Lower levels were observed in the main drainage canal, (15.2 p. 100). Two infestation periods were noted at two sites in relation with spreading of manure by farmers in autumn and spring. Perennial infestation due to the permanent presence of animal hosts was noted at one. Infestation of drainage canals is enhanced by incoming parasite eggs from upstream irrigation canals. Infestation was age dependent reaching up to 100 p. 100 in older snails of 5.5 mm. Environmental factors such as climatic conditions and human intervention are important to sustaining snail infestation in the oasis of Tozeur.

Cathepsin L proteinases as vaccines against infection with Fasciola hepatica (liver fluke) in ruminants

Fasciolosis, caused by trematodes of the genus Fasciola, is one of the most important diseases of farmed ruminants in temperate and tropical zones. The appearance of Fasciola hepatica populations that are resistant to common flukicidal drugs means that new methods of treatment will soon be required. The future prospect for the development of anti-liver fluke vaccines is optimistic and given their consumer acceptability and environmental friendliness, offer the best way forward. Cathepsin L proteases (Fhe CL 1 and Fhe CL 2), secreted by liver flukes at all stages of their development in the mammalian host, are believed to play important roles in facilitating parasite migration (tissue degradation), feeding and immuno-evasion. The authors consider them prime targets for which new vaccines can be developed. Vaccine studies in cattle and sheep have shown that protection levels of up to 72 and 79 per cent, respectively, can be obtained with immunisations of cathepsin Ls in Freunds' adjuvant. The vaccine also exhibited high anti-embryonation/anti-fecundity effects on parasites that survived in vaccinated animals and thus could have a major impact on the transmission of disease to the intermediate host. While natural infections in sheep and cattle appear to elicit non-protective Th2 immune responses, the authors' studies indicate that the protection induced by vaccination involves elements of a Th1 response.

Blood leukocyte responses in rats vaccinated with cDNA encoding glutathioinie-S-transferase of Fasciola hepatica

Changes in blood leucocyte levels were investigated in Spraque-Dowley rats vaccinated with cDNA or protein of glutathione S-transferase (GST) of F. hepatica and subsequently challenged with metacercariae of the liver fluke. The analysis of the leucocyte responses measured in vaccinated rats suggests that the form of antigen used for vaccination influenced dynamics of white blood cell response to the fluke infection. The most clear differences were observed in neutrophil and eosinophil levels. The weakest reaction of these cells to the challenge infection was observed in rats vaccinated twice with cDNA. In contrast, in rats which received the first antigen dose as cDNA and the second vaccination with GST protein, both neutrophil and eosinophil responses were much higher, especially at 5 and 9 WAI.

Fasciola hepatica: a review of the economic impact in cattle and considerations for control

To understand the rationale behind recommended control programs for liver flukes in cattle, the economic impact and the complex life cycle of Fasciola hepatica need to be appreciated. Fluke-infected cattle rarely demonstrate clinical disease, but subclinical impairment of feed efficiency, growth, and fertility can have an important impact on productivity. Although the precise benefit from controlling liver flukes is unknown due to the inherent variability between studies, most investigations demonstrate a considerable economic advantage to treatment. In addition, recent studies have demonstrated that liver flukes may cause a decrease in host fertility by altering normal metabolism and/or balance of sex hormones. This review focuses on these issues and how to best use available drugs to implement optimal fluke control programs.

Resistance of Fasciola hepatica against triclabendazole in cattle and sheep in The netherlands

In the winter of 1998/1999, sheep on a farm in the province of North Holland, The Netherlands, died from subacute and chronic liver fluke disease despite four previous treatments with triclabendazole (TCBZ). Faecal examinations of sheep and cattle on the farm showed high number of liver fluke eggs. In a randomised clinical trial, the fluke egg output was monitored weekly for 3 weeks in sheep which were treated with TCBZ or with closantel; in dairy cows treated with TCBZ or with clorsulon; and in heifers treated with TCBZ or clorsulon. The results showed a significant reduction of 99.7, 98.1 and 99.2%, respectively, in fluke egg output at 21 days in all non-TCBZ treated animals. TCBZ treatment produced percentage decreases of 15.3, 4.3 and 36.6%, respectively. These results are highly indicative of the presence of TCBZ-resistant Fasciola hepatica in sheep and cattle on this farm.

Successful DNA immunisation of rats against fasciolosis

The liver fluke Fasciola hepatica contributes to great economic and health losses in the cattle industry in many countries, including Poland. Unfortunately, no vaccine against fasciolosis is commercially available. We have designed a DNA vaccine and tested it in rats. Groups of male or female rats received one intramuscular injection of 50 microg of a pcDNA 3.1 vector carrying cDNA encoding for a cysteine proteinase of F. hepatica. The plasmid was diluted in saline containing 0.05% bupivacaine. Control rats were injected with empty plasmid or not injected at all. All rats were challenged with 45 metacercariae of the fluke on day 28 of the experiment. Seven weeks after the challenge infection fluke burdens were evaluated in vaccinated and control rats. Male rats vaccinated with cysteine proteinase cDNA revealed 100% protection against F. hepatica infection. Females immunised in the same way exhibited the reduction of fluke burden by 74%.

Identification and localization of a schistosome-associated fucosyllactose determinant expressed by Fasciola hepatica

A Biomphalaria alexandrina-derived lectin (BaSII), of proven specificity to a Schistosoma mansoni-associated fucosyllactose [(Fuc alpha1-2) Gal beta1-4 Glc] determinant, was employed to investigate the putative antigenic cross-reactivity between Schistosoma mansoni and Fasciola hepatica in terms of this structurally-defined oligosaccharide sequence. BaSII affinity column chromatography of extracts of adult worms metabolically radiolabelled with 35S-methionine and analysis by two-dimensional gels established the expression of the fucosyllactose determinant in multiple copies among heterogeneous, acidic glycoproteins synthesized by adult Fasciola hepatica. Direct fluorescence microscopy revealed that determinant-bearing glycoproteins were localized to the external glycocalyx and perikarya of the tegument as well as the epithelial lining of the intestinal caeca and vitelline ducts and glands. Determinant expression was also evident in embryonated cells of eggs and miracidia as well as the intermediate cellular wall of encysted metacercariae, suggesting its conservation during the course of development of the parasite. Based on the structural relatedness of the cross-reactive fucosyllactose determinant to the antigenic mammalian blood group H trisaccharide, our observations may have implications in serodiagnosis and immunoprophylaxis of schistosomiasis/fascioliasis.

A strategic dosing scheme for the control of fasciolosis in cattle and sheep in Ireland

This study examined the effectiveness of a strategic dosing scheme in lowering the incidence of fasciolosis on a mixed dry-stock farm and in maintaining the reduced incidence following a reduction in dosing intensity. Two neighbouring farms with a history of chronic fluke disease were selected, the strategic dosing scheme being implemented on one (the trial farm) while the other (the control farm) continued to treat according to its normal practice. The strategic dosing scheme was designed to suppress the faecal egg output of Fasciola hepatica at critical times of the year in order to limit infection of the intermediate host snail population and thus reduce the subsequent contamination of the pasture with metacercariae. On the trial farm cattle and sheep were treated three times per year for the first 2 years at approximately 8 week intervals, starting in March of each year. A fourth treatment was given when the cattle were housed and out-wintered sheep received an additional treatment in January. In Years 3 and 4 the dosing intensity was reduced. By the end of Year 2, data from faecal egg counts, tracer-sheep fluke burdens and snail infection levels indicated that the treatment strategy had succeeded in suppressing the fluke population and eliminating the occurrence of clinical fasciolosis. The decrease in dosing intensity in Years 3 and 4 maintained both stock and snail infections at low levels and there was no re-emergence of the disease.

Fasciola hepatica: an antigen fraction derived from newly excysted juveniles, containing an immunoreactive 32-kDa protein, induces strong protective immunity in rats

Crude antigens of adult Fasciola hepatica and of newly excysted juveniles (NEJ) and a low-molecular-weight fraction of antigen from NEJs were tested for inducing protective immunity in rats. Two routes of vaccination were applied. The results showed that intraperitoneal vaccination induced significantly better protection (P <0.05) than intramuscular vaccination. Intraperitoneal vaccination with antigens from NEJs induced more effective protection: after challenge infection, rats that were so vaccinated had 92.6% (+/-2.5% SEM) fewer parasites in their liver and 57.3% (+/-13.3% SEM) fewer parasites penetrating the gut wall than control rats. Rats that were vaccinated with a low-molecular-weight fraction of antigen from NEJs were also highly protected against a challenge. F. hepatica antigens that are immunoreactive were identified on immunoblots, using sera collected from highly protected rats that had been vaccinated with NEJ antigens and also sera from cattle and rats that were experimentally infected with F. hepatica. The low-molecular-weight fraction of antigen from NEJs contained an immunodominant 32-kDa protein that was recognized by serum antibodies of vaccinated rats and immune cattle. This 32-kDa protein was not detected in partially purified antigens from adult flukes. We conclude that antigens of NEJs of F. hepatica, when injected intraperitoneally in rats, are highly protective. In particular, the 32-kDa protein contained in these antigens may be highly valuable for the development of an effective vaccine against F. hepatica.

Immune responses of cattle to experimental anti-Fasciola hepatica vaccines

Fasciola hepatica infection of cattle and sheep is an important cause of clinical disease and production losses, and is controlled at present by a combination of chemotherapy and management measures. However, the prospects for the control of F. hepatica infection by vaccination are good, and we have previously shown substantial protection of cattle against experimental challenge infection following immunisation with a combination of the purified fluke-derived enzymes cathepsin L1 (CATL 1), cathepsin L2 (CATL 2) and fluke-derived Hb fraction (FHB). This and other recent studies have also demonstrated fundamental differences between protective and non-protective immune responses to liver fluke infection. In this present study we have further analysed the response of animals to liver fluke challenge following experimental vaccination. Calves were vaccinated with either CATL 2 plus FHB, or CATL 1 plus CATL 2. Partial protection against challenge infection was achieved in both vaccinated groups, with the greatest level of protection (55 per cent reduction in fluke burdens) recorded in the group vaccinated with CATL 1 plus CATL 2. This latter group also showed the greater level of lymphocyte proliferation and the greater production of gamma-INF in response to stimulation with fluke antigen in vitro following challenge. These results are significant in our attempts to characterise the elements within the immune response to vaccination which are protective.

Vaccination with cathepsin L proteinases and with leucine aminopeptidase induces high levels of protection against fascioliasis in sheep

The potential of different parasite proteinases for use as vaccine candidates against fascioliasis in sheep was studied by vaccinating animals with the cathepsin L proteinases CL1 and CL2 and with leucine aminopeptidase (LAP) purified from adult flukes. In the first trial, sheep were immunized with CL1 or CL2 and the mean protection levels obtained were 33 and 34%, respectively. Furthermore, a significant reduction in egg output was observed in sheep vaccinated either with CL1 (71%) or with CL2 (81%). The second trial was performed to determine the protective potential of the two cathepsin L proteinases assayed together, as well as in combination with LAP, and of LAP alone. The combination of CL1 and CL2 induced higher levels of protection (60%) than those produced when these enzymes were administered separately. Those sheep that received the cocktail vaccine including CL1, CL2, and LAP were significantly protected (78%) against metacercarial challenge, but vaccination with LAP alone elicited the highest level of protection (89%). All vaccine preparations induced high immunoglobulin G titers which were boosted after the challenge infection, but no correlations between antibody titers and worm burdens were found. However, the sera of those animals vaccinated with LAP contained LAP-neutralizing antibodies. Reduced liver damage, as assessed by the level of the liver enzyme gamma-glutamyl transferase, was observed in the groups vaccinated with CL1, CL2, and LAP or with LAP alone.

A geographic information system forecast model for strategic control of fasciolosis in Ethiopia

A geographic information system (GIS) forecast model based on moisture and thermal regime was developed to assess the risk of Fasciola hepatica, a temperate species, and its tropical counterpart, Fasciola gigantica, in Ethiopia. Agroecological map zones and corresponding environmental features that control the distribution and abundance of the disease and its snail intermediate hosts were imported from the Food and Agriculture Organization (FAO) Crop Production System Zones (CPSZ) database on east Africa and used to construct a GIS using ATLAS GIS 3.0 software. Base temperatures of 10 degrees C and 16 degrees C were used for F. hepatica and F. gigantica, respectively, to calculate growing degree days in a previously developed climate forecast system that was modified to allow use of monthly climate data values. The model was validated by comparison of risk indices and environmental features to available survey data on fasciolosis. Monthly Fasciola risk indices of four climatic regions in Ethiopia were used to project infection transmission patterns under varying climatic conditions and strategic chemotherapeutic fasciolosis control schemes. Varying degrees of F. hepatica risk occurred in most parts of the country and distinct regional F. hepatica transmission patterns could be identified. In the humid west, cercariae-shedding was predicted to occur from May to October. In the south it occurred from April to May and September to October, depending on the annual abundance of rain. In the north-central and central regions, risk was highest during heavy summer rains and pasture contamination with metacercariae was predicted to occur during August-September, except in wet years, when it may start as early as July and extend up to October. At cooler sites above altitude of 2800 m, completion of an infection cycle may require more than a year. Fasciola gigantica risk was present in the western, southern and north-central regions of the country at altitudes of 1440-2560 m. However, a transmission cycle could be completed in a single year only at elevations below 1700 m. The greatest risk of F. gigantica infection was in the humid western region. Regional strategic chemotherapy schemes of two or three treatments per year were developed. Results suggest that the model can be extrapolated to all CPSZ in the country and adapted for use in control of other vector-borne diseases of economic and public health importance.

Strategic control of fasciolosis in the inter-Andean valley of Cajamarca, Peru

Fasciolosis is recognised as a major problem in dairy cattle in Cajamarca, Peru. The infection has an annual cycle, with the major period of infection from January to March. A control programme, involving two doses of the fasciolicide triclabendazole aimed at reducing the passage of Fasciola hepatica eggs on to the pasture, together with the use of the molluscicide, niclosamide, was evaluated against traditional treatment programmes. The double treatment regimen did not significantly reduce the overall parasite burden, as measured by faecal egg counts, but did control the parasites to a level similar to that achieved by traditional programmes averaging 3-7 treatments per year. The cows' eosinophil counts and serum liver enzyme activities were significantly lower than in animals treated by the traditional programmes. The use of niclosamide in addition to the triclabendazole treatments gave no clear advantage, although there was a significant reduction in the number of intermediate host snails after the application of the molluscicide.

[Molluscacidal activity of Paraiso (Melia azedarach L.) (Meliaceae) on Lymnaea cubensis, host snail of fascioliasis]

INTRODUCTION

Fasciolosis is a source of serious economic loss in various regions of Brazil and when control measures of its host (molluscs) are not taken under favorable ecological conditions, isolated cases of human Fasciolosis may occur. Among the alternative measures for its control is the use of vegetable extracts and the purpose of this project is the assessment of the use of juice extracted from the fruit and seeds of the Paraiso plant (Melia azedarach L.) in the control of Lymnaea cubensis, the main vector of Fasciolosis in Cuba.

MATERIAL AND METHOD

Various concentrations of the juice extracted from the Paraiso fruit (Melia azedarach L.) were tested to determine the average and maximum lethal doses (DL50 and DL90' respectively) used in a computerized PROBIT-LOG program. Seven experimental series were undertaken, 72 molluscs being used in each of them. Three groups of ten molluscs were tested to determine the effect on cardiac frequency, two of them being treated with CL50 = 0.88627 and CL90 = 1.7641, respectively, the third being used as control.

RESULTS AND CONCLUSIONS

A considerable effect of both doses on the cardiac frequency of the mollusc studied was observed. The results were encouraging, for they demonstrated the potential use of this plant in the control of undesirable molluscs.

Fasciola gigantica: studies of the tegument as a basis for the developments of immunodiagnosis and vaccine

The tegument of bile-dwelling Fasciola gigantica is the interfacing layer that helps the parasite to maintain its homeostasis, and evade the hostile environment, including the host's immune attacks. The tegument is a syncytial layer about 10 mm thick, that is formed by the fusion of cytoplasmic processes of tegument cells, whose soma lie underneath the two muscle layers. The surface of the tegument is highly folded and invaginated into numerous ridges, pits and spines, which help to increase the surface area of the tegument for the absorption and exchanging of molecules, as well as for attachment. The outer membrane covering the tegument is a trilaminate sheet about 12 nm thick, and coated with a carbohydrate-rich glycocalyx layer that also bears high negative charges. Some host molecules may also be adsorbed onto this layer. These unique characteristics enable the parasite to evade the antibody-dependent cell-mediated cytotoxicity (ADCC) reaction exerted by the host. The outer membrane and glycocalyx is continuously replaced by the reserved membrane synthesized and stored in secretory granules of tegument cells, that are transported via cell processes towards the tegument by microtubules. The basal membrane of the tegument is trilaminate and invaginated to form membrane infoldings with closely aligned mitochondria. The tegument cytoskeleton is composed of a highly cross-linked network of 4-6 nm knobby microtrabecular fibers, bundles of intermediate filaments, microtubules that splay out from the tegument cells' processes. Major proteins of the cytoskeleton are actin, paramyosin and tubulin. The flukes' antigens that can elicit strong immunological responses in animal hosts are synthesized and released mainly from the tegument and the cecum. The majority of antigens derived from the surface membrane and the tegument are of MW 97, 66, 58, 54, 47 and 14 kDa, while those released from the cecum are cysteine proteases of MW 27, 26 kDa. Monoclonal antibodies have been raised against some of these antigens, and have been employed in immunodiagnosis of the infection. From the protection conferred to animal models and the in vitro killing assays of young parasites by specific antibodies, candidate vaccines could be selected from these antigens, such as, an antioxidant enzyme, glutathione-S-transferase, the digestive enzyme cysteine proteases, the surface-tegument proteins, such as fatty acid binding protein (14 kDa), membrane proteins (at 66 kDa), as well as muscle protein paramyosin, and hemoprotein. Ongoing research have been directed at deciphering the genetic codes and the syntheses of some of these antigens by recombinant DNA technology.

Correlation of specific antibody titre and avidity with protection in cattle immunized against Fasciola hepatica

Cattle produce specific serum antibody mainly of the IgG1 isotype in response to infection with the liver fluke, Fasciola hepatica. In these animals a positive correlation between fluke-specific serum IgG1 levels and fluke-burden in non-immunized infected animals was observed. In contrast, immunization of cattle with a combination of the fluke-derived antigens cathepsin L2 (CL2) and fluke haemoglobin (FHb) in Freund's complete/incomplete adjuvant (FCA/FLA) induced a specific antibody response involving IgG2, as well as IgG1. These immunized animals also exhibited very high (72%) levels of protection against a subsequent challenge infection. When the vaccine was administered in FIA alone the specific antibody response, while still involving IgG1 and IgG2, was of lower magnitude (10-fold and 100-fold, respectively) and no significant reduction in fluke burden was observed following challenge. Nevertheless, in these animals, a strong IgG2 response was associated with low fluke burdens. These results provide further evidence of the non-protective nature of specific immune responses in cattle following F. hepatica infection, and demonstrate that vaccination can induce a qualitatively different, and protective, response.

Pharmacokinetics and efficacy of long-term low-level administration of triclabendazole in urea molasses blocks against induced bovine and bubaline fasciolosis

The pharmacokinetics and flukicidal efficacy of triclabendazole delivered in low doses on a daily basis through urea molasses blocks were studied in cattle and buffaloes experimentally infected with Fasciola gigantica. The observations were compared with single therapeutic doses at 12.0 and 24.0 mg/kg body weight in cattle and buffaloes, respectively, prior to becoming experimentally infected. After receipt of triclabendazole by cattle and buffaloes at 0.83 and 3.0 mg/kg body weight, respectively, on a daily basis, both the animal species exhibited equilibrium between its absorption and disposition of its metabolites in plasma on day 4 and remained almost unchanged thereafter. The continuous subtherapeutic dosing of triclabendazole through urea molasses block at 0.83 mg/kg body weight in cattle and 3.0 mg/kg body weight in buffalo proved to be efficacious against mature liver flukes.

Crystallization, structural determination and analysis of a novel parasite vaccine candidate: Fasciola hepatica glutathione S-transferase

Glutathione S-transferases (GSTs) represent the major class of detoxifying enzymes from parasitic helminths. As a result, they are candidates for chemotherapeutic and vaccine design. Indeed, GSTs from Fasciola hepatica have been found to be effective for vaccinating sheep and cattle against fasciolosis. This helminth contains at least seven GST isoforms, of which four have been cloned. The cloned isoforms (Fh51, Fh47, Fh7 and Fh1) all belong to the mu class of GSTs, share greater than 71% sequence identity, yet display distinct substrate specificities. Crystals of Fh47 were obtained using the hanging drop vapour diffusion technique. The crystals belong to space group I4122, with one monomer in the asymmetric unit, which corresponds to a very high solvent content of approximately 75%. The physiological dimer is generated via a crystallographic 2-fold rotation. The three-dimensional structure of Fh47 was solved by molecular replacement using the Schistosoma japonicum glutathione S-transferase (Sj26) crystal structure as a search model. The structure adopts the canonical GST fold comprising two domains: an N-terminal glutathione-binding domain, consisting of a four-stranded beta-sheet and three helices whilst the C-terminal domain is entirely alpha-helical. The presence of Phe19 in Fh47 results in a 6 degrees interdomain rotation in comparison to Sj26, where the equivalent residue is a leucine. Homology models of Fh51, Fh7 and Fh1, based on the Fh47 crystal structure, reveal critical differences in the residues lining the xenobiotic binding site, particularly at residue positions 9, 106 and 204. In addition, differences amongst the isoforms in the non-substrate binding site were noted, which may explain the observed differential binding of large ligands. The major immunogenic epitopes of Fh47 were surprisingly found not to reside on the most solvent-exposed regions of the molecule.

Evaluation of antigens of Fasciola gigantica as vaccines against tropical fasciolosis in cattle

Vaccine trials were conducted in Brahman cross cattle evaluating the efficacy of 4 native antigens purified from adult Fasciola gigantica flukes, and 1 recombinant F. gigantica antigen, as vaccines against tropical fasciolosis. The antigens tested were native glutathione S-transferase, cathepsin L, paramyosin, fatty acid binding protein (FABP), and a recombinant FABP expressed in E. coli, and were formulated in 1 or more of several adjuvants (Quil A, Squalene Montanide 80, MF59-100, Auspharm, NAGO, polylactoglycolide microspheres, Algammulin, DEAE, Freund's). Vaccination induced low, moderate or high antibody titres to the various antigens which were dependent on the adjuvant. Low but significant reductions in fluke burdens (31%, P < 0.026) and fluke wet weight (36%, P < 0.041) were only observed in cattle vaccinated with the native FABP in Freund's adjuvant. There was no correlation between total antibody titres to FABP and protection. The protection observed in cattle vaccinated with native FABP of F. gigantica supports the notion that this class of proteins is a useful target for protection of animals against Fasciola and extends the efficacy of FABPs to the tropical liver fluke. This is the first report of vaccination of cattle against F. gigantica with a purified protein.

Immunological approaches for the control of fasciolosis

The immunological relationship between liver flukes and their mammalian hosts is being unravelled by in vivo and in vitro studies. Vaccine studies in cattle and sheep with purified antigens (fatty acid binding protein, FABP; glutathione S-transferase, GST; cathepsin L, CatL; hemoglobin) have shown that high reductions in worm burdens (31-72%) and egg production (69-98%) can be achieved, raising the realistic possibility that immunological control of Fasciola infection is a commercially achievable goal. Combination vaccines may also be feasible since a cocktail of CatL and hemoglobin elicits a significant 72% protection in cattle. Analysis of immune responses to Fasciola during infection in ruminants suggests that chronic infection correlates with a type 2 helper T cell response, implying that type 1 helper T cell responses are down-regulated in fasciolosis. Recent results studying the resistance of Indonesian Thin Tail (ITT) sheep to F. gigantica have shown that this breed exhibits high innate (or rapidly acquired) resistance to infection and acquires a higher level of resistance after a primary challenge. Initial studies suggest that the resistance of ITT sheep to F. gigantica may be determined by a major gene. Merino sheep also acquire resistance to F. gigantica. In contrast, ITT and Merino sheep do not exhibit resistance to F. hepatica. These results suggest that there are fundamental differences between these two species of Fasciola in the biology of their interaction with the sheep immune system. In vitro studies on immune mechanisms of killing of juvenile fluke have shown that juvenile larvae of F. hepatica are susceptible to antibody-dependent killing by activated rat macrophages in vitro which is mediated by nitric oxide. Future studies on the immune effector mechanisms expressed by resistant sheep which control infection by F. gigantica will lead to new knowledge which may allow the design of more effective vaccines for fasciolosis.

Fasciola hepatica: vaccination of rabbits with native and recombinant antigens related to fatty acid binding proteins

The current study was designed to compare the immunogenic and immunoprophylactic properties of native (nFh12) and recombinant (rFh15) antigens from Fasciola hepatica in rabbits infected with the fluke. Levels of specific anti-nFh12 and anti-rFh15 antibodies were significantly higher in the rabbits vaccinated twice compared with non-vaccinated infection controls. A reduction of 40% in worm burdens was found in rabbits immunized with nFh12 and infected 4 weeks after the second immunization. The recombinant vaccine induced lesser levels of protection than the native one, suggesting that both molecules may have slight differences either in immunogenicity or in their configuration. Further biochemical studies are required to define these differences. The mean length of flukes recovered was always smaller in all vaccinated rabbits. In addition, infected control rabbits had higher gamma glutamil transferase (GGT) levels than immunized rabbits. Lastly, gross anatomic observation always showed fewer liver lesions in all vaccinated rabbits than in controls. This finding clearly supports the possibility of vaccination regimes in fasciolosis.