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.

Prevention of human fascioliasis: a study on the role of acids detergents and potassium permenganate in clearing salads from metacercariae

Prevention of human fascioliasis could depend on clearing of the leafy salads from the metacercariae. The present work evaluated the role of some chemicals in detaching and killing this infective stage. It was observed that washing in running water for 10 minutes detached only 50% of the metacercariae. Citric acid in the concentration of (10 ml/L) commercial vinegar (120 ml/L), liquid soap (12 ml/L) and KMnO4 (24 mg/L) detached all metacercariae after 10 minutes exposure. The use of vinegar and KMnO4 is recommended: the first is lethal to other parasites in the vegetables, the second destroyed the metacercariae. Vegetable leaves were not softened and remained fresh.

Protection of cattle against Fasciola hepatica infection by vaccination with glutathione S-transferase

Glutathione S-transferase (GST) from the liver fluke Fasciola hepatica was assessed as a vaccine immunogen in cattle in a number of immunological adjuvants. Significant reductions in fluke burdens (49-69%) were only observed in cattle vaccinated with GST in Quil Alsqualene Montanide (SM) and PLG microspheres in SM but there was no correlation between anti-GST IgG titres and protection. In separate experiments, animals vaccinated with GST in Quil AlSM were still significantly protected (48%, P < 0.05) 6 months after boosting and no significant differences in protection were seen when the metacercarial challenge was given over 1 month instead of as a single bolus. Inhibition of GST enzyme activity in vitro by cattle antisera did not correlate with reduced fluke burdens.

Induction of protective immunity in cattle against infection with Fasciola hepatica by vaccination with cathepsin L proteinases and with hemoglobin

Two cathepsin L proteinases, cathepsin L1 and cathepsin L2, secreted by liver flukes may be involved in tissue penetration, nutrition, and protection from immune attack. To ascertain the immunoprophylactic potential of these proteinases, and of another molecule, liver fluke hemoglobin (Hb), we performed vaccine trials in cattle. In the first vaccine trial various doses of cathepsin L1 were tested. The mean protection level obtained was 53.7%. In a second vaccine trial cathepsin L1 and Hb elicited 42.5 and 43.8% protection levels, respectively, while a combination of the two molecules induced a significantly higher level of protection (51.9%). Cathepsin L2 was not examined alone; however, vaccination of cattle with a combination of cathepsin L2 and Hb elicited the highest level of protection (72.4%). The animals that received cathepsin L1-Hb or cathepsin L2-Hb showed reduced liver damage as assessed by serum glutamic dehydrogenase and gamma-glutamyl transferase levels. Furthermore, a reduced viability was observed for fluke eggs recovered from all vaccine groups. This anti-embryonation effect of vaccination was particularly evident in the group that received cathepsin L2-Hb where >98% of the eggs recovered did not embryonate to miracidia. Although all vaccine preparations induced high antibody titers which were boosted following the challenge infection, there was no correlation between antibody titers and protection. The results of these trials demonstrate that cathepsin Ls and Hb could form the basis of a molecular vaccine that would not only reduce parasite burden but would also prevent transmission of liver fluke disease.

Approaches to the control of fasciolosis in ruminants

Approaches to the control of fasciolosis in ruminants are compared for developed countries, and for developing countries with particular reference to regions growing irrigated rice. In all environments problem definition and investigation are based on one or more of observation, abattoir surveys, faecal egg counts, tracer animals and snail studies. In developed countries each husbandman grazes a large number of animals, controls access to pasture and water, and markets high value products. Strategic and tactical treatments control fasciolosis in these circumstances and will continue to do so while anthelmintics remain effective. In many developing countries subsistence families possess small numbers of animals, feed and water sources are shared by many families, and the products are mainly draft power, fertiliser and meat for local consumption. Consequently the agricultural cycle, and the life cycles of the parasite and intermediate host, are closely interrelated and there is some scope for controlling infection by modifying husbandry practices. Anthelmintics are not affordable. Recent observations of a major fasciola resistance gene with substantial dominance, in Indonesian Thin Tail sheep infected with Fasciola gigantica, suggest that parasite control by breed substitution, or cross-breeding and selection, is feasible. Such control would be inexpensive to implement, and sustainable.

Evaluation of IHA test for fascioliasis in cases of hepatitis C virus infection

Human sera were collected from proven cases of fascioliasis, virus C hepatitis, combined fascioliasis and virus C hepatitis and from normal persons. Antibodies against hepatitis C virus were found to cross react in the indirect haemagglutination test for antibodies against Fasciola. This cross reaction occurred in 2 cases out of 25 cases at titres of 1/320 and 1/640. However, the test showed 100% sensitivity. So, this study could suggest the use of IHA test for human fascioliasis for screening only. This is to be aided by more specific serological test and by other means of investigations like sonography.

Attempted immunisation of sheep against Fasciola hepatica using gamma-irradiated metacercariae

The potential of gamma-irradiated Fasciola hepatica metacercariae to vaccinate sheep against fascioliasis was examined. The effect of the size of the inocula of irradiated metacercariae and the level of gamma-irradiation on the recovery of non-irradiated fluke was assessed following homologous challenge. Groups of Merino wethers were vaccinated with a single infection of either 500 or 2000 metacercariae, previously exposed to either 30, 100 or 400 Gy of gamma-irradiation. No significant reduction of fluke burdens were observed in any group, although a nonsignificant 20% reduction was observed in sheep vaccinated with 2000 metacercariae irradiated with 100 Gy. A second trial was conducted in which groups of sheep were vaccinated with 2 doses, given 4 weeks apart, of 2000 metacercariae, previously irradiated at either 70, 100 or 150 Gy. In both trials parasite viability was severely affected by doses of gamma-irradiation of 30 Gy or greater and no mature flukes were recovered from control sheep given metacercariae attenuated with 70 Gy or greater. A strong humoral immune response to somatic F. hepatica antigens was observed in all sheep. Only sera from sheep receiving 70 Gy irradiated metacercariae recognised the 2 candidate liver fluke vaccine molecules, F. hepatica glutathione S-transferase and cathepsin-L proteases. No reduction was observed in either the number of flukes or the production of fluke eggs in any vaccinated group. Vaccination appeared to affect the development of the challenge fluke population, resulting in reduced hepatic damage during migration, as measured by levels of serum glutamate dehydrogenase, and an increase in mean fluke weight.

Galactose oxidation as a potent vaccine adjuvant strategy. Efficacy in murine models and in protection against a bovine parasitic infection

Potent immunological adjuvants are urgently required to complement subunit protein and peptide vaccines in prophylactic and therapeutic vaccination. Carbonyl-amino condensations, which are essential to the inductive interaction between antigen-presenting cells and T-helper cells, were tested as a target for the enhancement or immune responses to vaccine antigens. Enzymic oxidation of cell surface galactose by the novel adjuvant NAGO, to increase amine-reactive carbonyl groups on lymphocytes and antigen-presenting cells, provided a potent noninflammatory method of enhancing the immunogenicity of viral, bacterial, and protozoal subunit vaccines in mice. In pilot protection studies with a glutathione-S-transferase vaccine against bovine Fasciola hepatica, a formulation containing NAGO as sole adjuvant agent proved very effective in inducing protection. In terms of protection in individual animals, NAGO alone was better than Quil A emulsified in squalene Montanide (five of eight animals were protected better than 80% by NAGO; four of eight animals were protected better than 93% by NAGO; two of eight animals were protected better than 80% by QA/SM). QA/SM (69% mean protection) was, however, more consistent overall than NAGO (65% mean protection). NAGO proved more effective in murine models when combined with muramyl dipeptide, but this combination has yet to be tested in protection studies.

Vaccination against schistosomiasis and fascioliasis with the new recombinant antigen Sm14: potential basis of a multi-valent anti-helminth vaccine?

Molecular cloning of components of protective antigenic preparations have suggested that related parasite fatty acid binding proteins could form the basis of the well documented protective, immune cross reactivity between the parasitic trematode worms Fasciola hepatica and Schistosoma mansoni. We have now confirmed the cross protective potential of parasite fatty acid binding proteins and suggest that it may be possible to produce a single vaccine that would be effective against at least two parasites, F. hepatica and S. mansoni of veterinary and human importance respectively.

The chemical and biological control of Lymnaea truncatula in natural watercress beds in the Limousin region (France)

The snails present in 17 natural watercress beds have been treated during 1 or 2 years by a control associating the use of a molluscicide (CuCl2) and the introduction of a predatory snail, Zonitoides nitidus. Lymnea truncatula was eliminated in only one year of treatment in 14 stations, and in two years in the three others. Elimination of other aquatic pulmonate snails was effected in two years for L. glabra, in 3 years for L. palustris, and in only one year for Physa acuta. Z. nitidus has disappeared from 12 watercress beds during the 4th year after treatment; in the five others, the numbers of this predator increased during 1 or 2 years and then decreased before their disappearance.

Vaccination of sheep with purified cysteine proteinases of Fasciola hepatica decreases worm fecundity

There has been some evidence from several parasite systems that proteinases might have potential as protective antigens against parasitic infection. A cysteine proteinase complex identified in the regurgitant of adult F. hepatica was examined in this context. The thiol-cathepsin-related proteinases of M(r) 28,000 were purified and tested in vaccine trials of sheep infected with liver fluke. Ten animals were immunised with the purified proteinases and developed antibodies to the cysteine proteinases prior to challenge with F. hepatica metacercariae. Infection appeared to cause a boost in antibody response by Week 4 into infection, and antibody levels were generally sustained throughout infection. The cysteine proteinases are not novel antigens, since low-level antibody titres were also detected in nonimmunised controls by late infection. On completion of the trial, there was no difference in worm burden between the two groups. However, faecal egg counts and therefore worm fecundity were significantly decreased.

Experiments on anthelmintic control of Fasciola hepatica in Brazil

Two separate field trials involving naturally infected cattle were carried out on two farms known to have a history of Fasciola hepatica infection. On the first farm, 15 animals per group were allocated as follows: G1, triclabendazole (TCBZ) four times a year; G2, TCBZ twice a year (May and September); G3, untreated control. All groups grazed together and after 3.5 years the animals were slaughtered and their livers examined by federal meat inspectors who condemned 100% of livers in G3 and 8.3% in G2 owing to the presence of lesions of fasciolosis. In G1 no livers were condemned. Significant differences in weight gains were not detected and fluke counts remained at low levels in the treated groups. Also, in the control group, egg counts started to decrease when animals were 2 years old. On the second farm, groups of 20 animals were treated as follows: G1, TCBZ three times a year (May, September and December); G2, TCBZ twice a year (May and September); G3, nitroxynil twice a year (May and September); G4, rafoxanide twice a year (May and September); G5, untreated controls. All animals were weighed and faecal samples examined at approximately 28-day intervals. During the period of the study, larger weight gains were detected in the TCBZ treated groups than in the others. TCBZ treatment kept F. hepatica egg counts at a lower level for longer periods than the other drugs and significant differences in weight gains were only obtained between the group receiving TCBZ three times a year and the control group.

Problem of fascioliasis in animal husbandry in Thailand

One of the most important parasitic diseases in adult cattle and buffalo in Thailand is fascioliasis, caused mainly by Fasciola gigantica. The economic loss from fascioliasis in cattle and buffalo throughout Thailand has been assessed at not less than 100 million Baht. Recent investigations have been shown that the average prevalence of F. gigantica in cattle and buffalo in Thailand was 11.8%. However, the prevalence varies considerably between villages, ranging from 0 to 85%. The prevalence is high in areas surrounding dams or large ponds in which Lymnaea auricularia rubiginosa, the intermediate host of F. gigantica is found. An epidemiological study revealed that the disease has a seasonal pattern from which the following conclusion for control of the disease can be drawn. Strategic liver fluke treatment of all cattle and buffalo which are older than 8 months should be carried out once a year in September. In addition, animals in poor condition should be treated in April to prevent severe losses, especially in high prevalence areas or where strategic treatment was missed. Problems of liver fluke control include the lack of knowledge about the parasite on the part of the farmers and the lack of availability of drug supplies at the village level, both of which are important to allow strategic treatment of animals. To approach these problems, the government had developed "Farmer Self-Help Worm Control Program" in seven provinces in Northeast Thailand which is operated by village farmers trained as program "keymen". This program is very effective and will be extended throughout Thailand in the next Seventh Social and Economic Development Plan.

A study of a restricted programme of strategic dosing against Fasciola hepatica with triclabendazole

A restricted programme of strategic dosing with triclabendazole was used to treat a sheep flock with severe clinical fascioliasis. After five years the percentage of ewes passing fluke eggs was reduced from 49 per cent to less than 1 per cent, without the reappearance of clinical fascioliasis. It is suggested that this programme offers an efficient and practical means of controlling fascioliasis.

Comparative efficacy of closantel and triclabendazole against Fasciola hepatica in experimentally infected sheep

The effect of closantel (10 mg/kg orally) and triclabendazole (10 mg/kg orally) on the reappearance of a patent infection of Fasciola hepatica was studied in experimentally infected sheep. The treatments resulted in the interruption of faecal egg output for 11 weeks with triclabendazole and 13 weeks with closantel. Necropsy of untreated control animals revealed a mean burden of 360 flukes with a mean (+/- se) surface area of 171 +/- 64.3 mm2, whereas the fluke burdens in the closantel and triclabendazole-treated animals 14 weeks after treatment were 61 (83 per cent reduction) and 21 (94 per cent reduction), respectively. The surface areas of the flukes in the triclabendazole-group were comparable with the untreated controls (141 +/- 51.8 mm2), but the flukes in the closantel group were markedly smaller (43.1 +/- 26.9 mm2). It is concluded that closantel has, in epidemiological terms, a potency comparable with that of triclabendazole, despite its slightly lower efficacy against the very immature stages.

[Epidemiology and prevention of distomatosis in France]

Only the large liver fluke Fasciola hepatica is frequently encountered in France. The small fluke is exceptional, owing to its mode of transmission. Opithorchiasis is observed among refugees from South-East Asia. The prevention of all types of distomatosis rests on food control.

Glutathione S-transferases in Fasciola hepatica

Glutathione S-transferases (GST's) are widespread in the tissues of the liver fluke, Fasciola hepatica, and consist of multiple isozymes. Following purification to apparent homogeneity by affinity chromatography on glutathione agarose, fluke GST's were shown to comprise 2 components with molecular weights of about 25,000. Fluke GST's were immunogenic to rats, but when used as a vaccine conferred no protection on the animals against a challenge infection with F. hepatica metacercariae.

Report from the PHLS Communicable Disease Surveillance Centre

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Protective effect of irradiated metacercariae of Fasciola gigantica and irradiated cercariae of Schistosoma bovis against fascioliasis in goats

Sensitisation of goats for eight weeks with metacercariae of Fasciola gigantica gamma-irradiated at 3 kr resulted in significant resistance to an homologous challenge with normal metacercariae. However, serum sorbitol dehydrogenase assay suggested that, whereas little damage was produced by immunising infections, considerable hepatic damage was caused by flukes remaining from challenge infections. On the other hand sensitisation with cercariae of Schistosoma bovis gamma-irradiated at 3 kr did not stimulate any resistance against heterologous challenge with F. gigantica.

[Control of fascioliasis in heifers in Czechoslovakia 1970-1985]

A programme for the control of fascioliasis caused by F. hepatica in cattle was launched in 1970 and continued until 1985. The animals were subjected to helminth-control treatment and preventive measures were taken in the herds. The anti-fasciolics Bilevon M and to some extent also Bromsalan pulv. were administered by the veterinary service on the basis of state subsidies in 1973 to 1979. The preparation Helmisan (Spofa) has been in use since 1980; its production is subsidized by the agricultural enterprises. In the period from 1972 ot 1979, up to 213 thousand helminth-control treatments (both therapeutic and preventive) were performed every year in the cattle. All treatments reduced the occurrence of Fasciola hepatica and pathological changes in the liver of the killed cattle (from 5.8% recorded in 1971 to 0.9 in 1980, on an national average). As a result, 29 districts are free from fascioliasis although the disease originally occurred in all districts of the Czech Socialist Republic. After ten years of purposeful use of antifasciolics, the findings were free from living flukes and the cattle only showed pathological changes after the action of F. hepatica. Since 1980 the extensity of the findings of F. hepatica in slaughtered cattle has ranged within one percent, though living flukes have recently occurred again in the liver of certain animals. In the enzootically affected regions and districts of the Czech Socialist Republic the fascioliasis control programme succeeded in reducing the number of invaded cattle herds.