Liver fluke – an essay – by Dr Joe Boray

TO:  WormMail list (recip.undisclosed)   WormMail 2010.03.29.1200Liver fluke – an essay – by Joe Boray

Dr JC Boray is an internationally recognised expert on liver fluke. He recently sent me this essay, with permission to publish it.

I think this is an important document, so have decided to send this to a few people who are not on the WormMail list. (My apologies if you get this twice).

Joe is passionate about liver fluke: I still remember the liver fluke-in-plastic mould attached to the dash of his Volvo.

(I think this fluke was efficacious: I survived Joe’s driving :-).

The timing is right for this essay: April-May in south-eastern Australia is the time for the most important fluke drench of the year.  Read Joe’s essay for more information.



Also see:

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Specialist Veterinary Surgeon (Pathobiology)
Consultant for Parasitology
T: 61 2 88505515

A well planned strategic treatment schedule is essential for the control of fasciolosis to reduce economic loss by liver condemnation, mortality, secondary bacterial infections, interference with fertility, reduced wool, milk and meat production and through the expense of control measures. About 250 million sheep and 300 million cattle are potentially affected by the disease world-wide.  The clinical disease is more often manifested as a chronic anaemia.  However, we have to emphasise the important role of acute and sub-acute fasciolosis which also cause production loss and mortalities.

Completion of the life cycle of Fasciola hepatica and potential infection of sheep and cattle depends mainly on rainfall or the presence of moisture and temperature.  The moisture necessary for the reproduction of snails and the hatching of fluke eggs is usually provided by small permanent creeks, normally fed by springs.  Those creeks running through paddocks and gullies support the amphibious intermediate hosts Lymnaea tomentosa in shallow muddy habitats preferred by the snail.  Similar conditions are created by seepage from canals in the irrigation areas.  In drought some of the springs dry up but some moisture may be still present to maintain the growth of grass which has the infective stage of the fluke.  The sheep and cattle congregate around those gullies and graze the contaminated herbage, resulting in heavy acute infection in the animals.

The second important condition for the completion of life cycle and the survival of metacercariae is suitable temperature.  In winter all stages of fluke development and reproduction of snails cease when the temperature is around 10°C or less, but the snails survive together with the dormant fluke larvae. The snails and the fluke larvae within the snails also survive in the mud during dry periods for about a year. After rain the snails emerge and complete the life cycle of the fluke, producing cercariae, contaminating the pastures with the infective metacercariae, attached to grass.   During drought the sheep and cattle congregate around the moist areas and can obtain heavy infections.

During the winter the metacercariae may survive for some time on the herbage under moist conditions.  The first invasion of herbage with the fluke cysts commences in late Spring when the larval stages of fluke which are obtained from infection of snails in Autumn. The fluke larvae complete the development when the temperature increases (over-wintering infection).   At the same time the snails rapidly multiply under the more suitable conditions and become infected by the fluke larvae (miracidia) which hatch out from the fluke eggs produced by the adult flukes in sheep and cattle.  Within 2–3 months more fluke cysts will invade the herbage during the Summer months (Summer infection) and reach high contamination by the end of Summer and the beginning of Autumn.  Heavy infections may occur during this period but the clinical symptoms of acute and sub-acute fasciolosis are often unnoticed.  The disease produces obvious symptoms in most cases when the parasites reach the chronic adult stage about 2 months later.

Due to progressing anaemia the economic loss is more pronounced, reducing the wool growth and bodyweight in sheep, particularly in younger animals. The clinical symptoms of the acute disease will not be obvious for 2-3 months in the Spring and early Summer period unless the pastures are heavily contaminated and at that time increasing mortalities may occur.

Pathology of Acute Fasciolosis

We should emphasise the pathological effect of acute and sub-acute fasciolosis in sheep and cattle.  In sheep the acute disease is due to a mechanical damage when large numbers of immature flukes migrate through the liver tissues and destroy the functional liver cells.  The inflicted damage to the liver tissues also causes a retardation of growth of the flukes and tissue migration period will be extended
which causes severe sub-acute fasciolosis.  Peracute, acute and sub-acute fasciolosis is caused by the tissue migration of immature flukes. The pathological damage produces cell destruction causing extensive haemorrhage.  Deaths are normally due to profound anaemia resulting from blood loss and the failure of liver function.  However, the role of the excretion of proline and subsequent tissue changes should also be considered (Symons & Boray, 1968, Boray, 1985). Outbreaks may occur with considerable losses when seasonal and climatic conditions result in a large intake of metacercariae during a relatively short period.

Much work on the pathology of fasciolosis was carried out in the McMaster Laboratory, CSIRO (Boray, 1967, 1969). Trials were carried out with experimentally infected sheep, which involved a total of 269 animals. It was shown that in sheep with an average fluke burden of 103, clinical disease was not evident until the fluke matured and a large proportion of the sheep was suffering clinical chronic progressive anaemia causing death in some of the sheep in the higher fluke recovery groups.  In two groups of sheep which had an average of 204 flukes after experimental infection, early liver damage causing a reduced liver function was demonstrated with serological tests by detecting a very high level of the enzyme glutamate dehydrogenase (GLDH, see Fig.1 (figure not included -SL)).  In the tests as early as two weeks after infection, the high enzyme levels persisted for 16 to 18 weeks, demonstrating the presence of liver damage, caused by acute and sub-acute fasciolosis.  All of those sheep died of progressive profound anaemia 26 – 36 weeks after the inoculations.

In a group of sheep which had an average of 708 flukes in their liver after infection, the animals had acute and sub-acute clinical fasciolosis, resulting in haemorrhages and severe anaemia.  All the sheep eventually died after 15 to 22 weeks infection.  During decades of field work similar fluke numbers were often found in sheep.  In one occurrence of acute and sub-acute fasciolosis in sheep an average of 1,384 flukes was present in the livers at necropsy.

In further experimental work in merino sheep aged 5 years, serious acute and sub-acute fasciolosis was observed and in a group of 58 sheep an average of 1,535 flukes was present at necropsy.  All those sheep died within 7 to 10 weeks after infection suffering from severe haemorrhages and with the evidence of acute and sub-acute liver damage.

All the above sheep, including those with only around 100 flukes in the liver, had loss of appetite, reduction of weight, anaemia and thus subsequently died indicating the potential pathogenicity in the field if the animals are not treated.

In one experiment a group of sheep was infected with 500 metacercariae each and was treated with triclabendazole at 10 mg/kg ten weeks later to achieve eradication of the fluke. Ten weeks after the treatment the same procedure was repeated to inflict heavy damage in the liver and then remove the fluke. In another group a similar procedure was carried out, but each sheep was infected with 100 metacercariae only. The sheep showed evidence of successful treatment and ten weeks after the second inoculation and treatment all the sheep were inoculated with 500 metacercariae each. One group of sheep remained uninfected and untreated as controls. The high level of liver damage was demonstrated by the increased GLDH levels in the serum. Similar numbers of flukes were present at necropsy 14 weeks after the challenge infection in all the sheep, including the controls. This experiment showed that a previous infection did not generate an immune reaction against the second infection and the serum enzyme test was a useful tool for diagnosing acute fasciolosis in sheep (see Fig.2).

In all the above experiments there was always sufficient liver damage by the flukes to explain deaths.  However, in the majority of cases, secondary pathological lesions, such as peritonitis, pleuritis and traumatic damage in the lungs and pancreas, contributed to the condition.  The primary damage due to the migrating flukes was detected with a single bromsulfalein clearance test as early as three weeks after infection (Symons and Boray, 1968), thus demonstrating the occurrence of liver function failure caused by immature flukes.  Those results were in accordance with results of experimental work in Wales (Sinclair, 1967).

Strategic Control in Sheep

In an endemic area a curative drench has to be given in April/May, when a high level of infection is anticipated. For this treatment a product should be used which is highly effective against both early immature and adult fluke.  The best treatment for this period is an oral dose of triclabendazole and particularly the highly effective formulation of Flukazole S, which has an improved efficacy by the addition of oxfendazole.  The increased efficiency is achieved by a synergistic effect of the two ingredients.

The second essential treatment (preventive treatment) is very important at the end of the winter or early spring (August/September) to eliminate the flukes surviving in the sheep and reduce the contamination of the pasture before the active period commences with increasing temperatures.  At that time most of the flukes would have reached an advanced immature or adult stage when another product which is effective against those stages could be used with good results. Many products are available for but a product containing closantel would be the most effective for this purpose.
The above products include chemicals which belong to groups different from triclabendazole.  This treatment would achieve an effect of drug rotation, reducing the chances of the development of resistance to triclabendazole.

In endemic areas with high rainfall in Spring an additional treatment in January/February is recommended with drugs highly effective against immature fluke as mentioned above.  At that period a high proportion of the flukes in the liver would be immature.


It has been generally recognised that cattle are more resistant to fasciolosis than sheep (Boray, 1967, 1969, Ross, 1967) due to more intensive tissue reaction in cattle than in sheep and in the bovine hosts a considerable age resistance is present.  It has been concluded that the resistance observed in cattle is due to the combination of an early and a late tissue reaction forming a mechanical barrier against re-infection.  The normally observed preferential migration of young flukes into the ventral lobe produces an effective mechanical resistance and the subsequent hypertrophy of the right lobe facilitates the survival of the host by leaving sufficient undamaged liver tissue. The dystrophic calcification of the bile ducts and the fibrosis proliferating into the parenchyma in chronic cases causes the elimination of the flukes.  Calcification is not present in sheep. Some results of experimental infections carried out in McMaster Laboratory (Boray, 1969) may give reasonable information on the clinical pathology expected to occur in the field.

Severe anaemia was diagnosed in a group of calves aged 6 to 8 months after experimental infections with 1,000 metacercariae each.  One calf died but three calves recovered spontaneously.A group of 4 month old calves which had a mean number of 1,358 flukes in the liver, showed profound anaemia and high egg counts when the flukes reached maturity. In another group of 6 calves aged 6 to 8 months, one calf died, two showed severe anaemia and two of the calves showed no clinical symptoms.  They had a mean number of 1,381 flukes in the liver.  When a group of calves aged 17 months were infected with the same number of metacercariae only a mean number of 620 flukes were recovered from the liver and the calves showed no clinical symptoms of fasciolosis.  In another experiment cattle aged 6 to 8 months or 2 years were infected with 10,000 metacercariae.  All the younger animals showed profound anaemia and all died of sub-acute or chronic fasciolosis and a mean number of 4,671 flukes was recovered at necropsy.  The two year old cattle showed no serious clinical symptoms and a mean number of 512 flukes were recovered from the livers.  It was also shown that calves in good condition showed more resistance than poorer ones.  It was also shown that some breeds of cattle were more or less resilient to fluke. In a comparative experiment, Jersey calves aged 4 months showed more serious clinical symptoms compared to Herefords of the same age after experimental infections resulting in a similar number of flukes recovered at necropsy.

It can be concluded that acute and sub-acute fasciolosis causing serious clinical disease occurs mainly in younger animals with severe anaemia and death occurring on heavily contaminated pasture and more often when susceptible sheep are grazed together with calves. Flukes in sheep normally produce more eggs than in cattle contaminating the pastures. However, there is considerable evidence that even a low infection in young cattle may result in reduced growth rate, reduced pregnancy rate and delayed conception of heifers.  In adult cows milk production is reduced in sub-clinical infections due to Fasciola hepatica.

Strategic Control in Cattle

Strategic liver fluke control is essential to maintain productivity of cattle even if no visual signs of clinical fasciolosis are present but positive egg counts, serological tests or ELISA tests using milk samples confirm the presence of fasciolosis.

The most important treatment should be carried out in April/May when the highest levels of infective metacercariae are present on herbage.  At that time an anthelmintic should be used which is highly effective against early immature and adult flukes.  The most effective product for that purpose is Flukazol C plus Selenium drench, which is a combination of triclabendazole and oxfendazole with synergistic action, or other products applied orally which contain triclabendazole, or Nitromec injection, which is a combination of clorsulon, nitroxynil and ivermectin with high efficacy against early immature and adult flukes and gastrointestinal nematodes. The use of Nitromec has the advantage that it does not include triclabendazole in the formulation and it represents a useful rotation treatment, reducing the chances of producing resistance against triclabendazole. At that time none of the other products, including pour-on treatments, are useful because they are only effective against adult fluke.  In dairies, triclabendazole preparations and Nitromec can be used in young heifers and in dry cows.  In lactating cows the ivermectin + clorsulon Ivomec Plus or VirbamecPlus injectable products can be used which have clearance for lactating cows. However those products are only effective against adult flukes.

(Note: Various triclabendazole products, including some pour-on formulations, are registered by the APVMA (  in Australia as being effective against all three stages of fluke (adults, immatures, early immatures).  Note that the definition of ‘effective’ may vary in different contexts. Nitromec is not registered in Australia for use in dairy cattle. Not all triclabendazole products are registered for use in dairy cattle. It is important to read and follow the (APVMA-) approved product labels. -SL 20100331).

The second important treatment (preventive treatment) is recommended to be carried out in August/September to eliminate the flukes surviving in the cattle after the Summer/Autumn period.  This time most of the fluke have reached adult stage and combination treatments such as Virbamec Plus, Ivomec Plus or other products effective against adult flukes can be used.  Another triclabendazole treatment and particularly a Pour-On application of the drug should be avoided. Delayed absorption of the drug from these formulations may contribute to the development of drug resistance.  The use of an alternative drug, such as Nitromec would achieve chemical rotation and reduce the chances for the development of resistance to triclabendazole.

In endemic properties with a history of heavy infections, a third treatment may be necessary in January/February, particularly when Spring and early Summer rain would stimulate the reproduction of snails or dry periods when the animals congregate in moist pastures.  This treatment is highly recommended for young cattle which are more susceptible to infection and likely to develop clinical fasciolosis.  During this period a high contamination of the herbage is expected through the “overwintering” larval stages of the parasite when the temperature increases in late Spring.  Increased numbers of cercariae will also complete their development in the snails which were infected in early Summer.  This treatment in January/February should be carried out with an oral drench preparation of triclabendazole or as an injection with Nitromec which are highly effective against immature flukes.  Dairy heifers can be treated until 4 weeks before their first calving.

Triclabendazole resistance in sheep and cattle is present in the Goulburn Valley irrigation area around Echuca, Pyramid Hill and Shepparton.  In those areas an alternative product should be used such as an injectable application of the combination products containing ivermectin and clorsulon, which is suitable for both dry and lactating cows but they re only effective against adult flukes. The new combination product, Nitromec with high efficacy against early immature flukes and adults can be used for calves, beef cattle and dry cows.


Recently pour-on products were released to the market and the problems of the treatment with those products will be discussed below.

Triclabendazole showed high efficacy against both early immature and adult Fasciola hepatica in sheep (Boray et al., 1983) and in cattle (Boray, 1982).  The drug is a halogenated benzimidazole derivative, but the presence of chloride atoms and a thiomethyl group and the absence of a carbamate moiety clearly distinguishes it from all other benzimidazole compounds.  Its spectrum of activity is unusual.  Triclabendazole is very specific for F. hepatica, F. gigantica and Fascioloides magna.  It lacks activity against nematodes and cestodes and other trematodes as well.

Triclabendazole is metabolised by the liver into two active forms, triclabendazole sulphoxide and triclabendazole sulphone. The first metabolite is more effective than the second against Fasciola sp. It is clear that the rate at which these metabolites are produced and in what concentration, will determine their efficacy against liver fluke. To achieve high efficacy against early immature liver fluke a high concentration of those metabolites are required to act against immature fluke, which are migrating in liver tissues.  The adult flukes in the bile ducts are killed by those metabolites as they are excreted into the bile.

Triclabendazole given orally as a drench is absorbed from the gastrointestinal tract and quickly transported to the liver via the portal blood flow, which drains directly into the liver, achieving a high concentration of the drug, which will be quickly metabolised.  Triclabendazole given either parenterally or as a pour-on will travel through the entire vascular system before it can be metabolised in the liver.  The delayed absorption will produce a dilution effect and will reduce the concentrations of active triclabendazole metabolites in the liver, resulting in lower efficacy for early immature flukes aged 2 to 4 weeks.  It is reasonable to say that oral dosing of triclabendazole will produce a greater concentration of the metabolites in a shorter time because the drug has direct access to the liver from the gastrointestinal tract via the portal system.

The mode of action of triclabendazole was reviewed by Fairweather & Boray (1999).  Triclabendazole has a multiple action against the flukes.  It seems to affect the energy-producing pathways resulting in a decrease of motility.  Triclabendazole also damages the reproductive system of F. hepatica reducing egg production and growth of the fluke.  The most significant effect of the drug is the inhibition of protein synthesis which in turn, produces morphological damage to the integument of flukes.  Most of the experimental data conform with a microtubule-disrupting action and disruption of protein synthesis. Additional studies showed that triclabendazole is also capable of uncoupling oxidative phosphorylation and this action was greater by the sulphoxide than that of the parent compound.

During the development of triclabendazole in the second half of the 1970s, the sheep and cattle experiments were carried out in the CIBA-GEIGY Research Centre in Australia, supervised by Dr. Boray, who was then Director of Research and Development in the company.  It was shown that at comparative dose rates the oral formulations were superior to the injectable or pour-on formulations.

The pour-on formulation produced by CIBA-GEIGY in Basel used an excellent solvent, resulting in very good absorption from the surface of the skin.  However, even at drastically increased dose rates of up to 30 mg/kg, high efficacy was only achieved against adult flukes.  Experience from trials with other pour-on products, which were carried out by J.C.Boray at the Elizabeth Macarthur Agricultural Institute, NSW Agriculture in the late 1980s, has shown that the age and breed of cattle and the season when the treatments were carried out greatly influenced the efficacy of a pour-on formulation. The absorption of the drug may be impaired by the dense hair growth during winter, particularly on beef cattle.

When the choice of drug for the treatment of cattle is triclabendazole, it can be concluded that the best results will be achieved by using the products, which are formulated for oral application or use another combination product, Nitromec injectable, which has no triclabendazole in its formulation, reducing the chances of producing resistance.


Boray, J.C., (1967), Studies on experimental Fasciola hepatica infections, with particular reference to acute fascioliasis in sheep. Annals of Tropical Medicine and Parasitology, 61: 439-450.

Boray, J.C. (1967), The effect of host reaction to experimental infections in sheep and cattle. In Veterinary Medical Review ed.E.J.L.Soulsby, N.G.Elvert, Marburg, pp 84-96, 1967.

Boray, J.C. (1969). Experimental fascioliasis in Australia, In Advances in Parasitology (ed. Ben Dawes). Vol. 7, pp. 95-210, Academic Press, London and New York, 1969.

Boray, J. C. (1982) “Chemotherapy of Fasciolosis”, New South Wales Veterinary Proceedings, 18:  42-47.

Boray, J.C. (1985), Flukes of Domestic Animals, (in: Parasites, Pests and Predators, ed:. S.M.Gaafar, pp 179-218. Elsevier, Amsterdam- Oxford-New York-Tokyo, 1985

Boray, J. C., Crowfoot, P. D., Strong, M. B., Allison J. R., Schellenbaum, M., von Orelli, M. and Sarasin G. (1983) Treatment of Immature and Mature Fasciola hepatica Infections in Sheep with Triclabendazole, Veterinary Record., 113: 315-317.

Fairweather, I and Boray, J. C. (1999).  Fasciolicides:  Efficacy, Actions, Resistance and its Management, The Veterinary Journal, 158: 81-11.

Hutchinson,G.W.,  Dawson, K, Fitzgibbon, C.C. and Martin , P.J. (2009). Efficacy of an injectable combination anthelmintic (nitroxynil + clorsulon + ivermectin) against early immature Fasciola hepatica compared to triclabendazole combination flukicides given orally or topically to cattle.  Veterinary Parasitology, 162(3-4) 278-284.

Ross, J.G. (1967), A comparison of the resistance status of hosts to infection with Fasciola hepatica, In: Veterinary Medical Review, Ed.: E.J.L Soulsby, N.G. Elvert, Marburg, pp 96-105, 1967

Sinclair, K.B. (1967), Pathogenesis of Fasciola and other liver flukes, Helminthological Abstracts, 36(2): 115-134.

Symons, L.E.A. and Boray, J.C. (1968), The anaemia of acute and chronic fascioliasis, Zeitschr. Tropenmed. Parasitol. 19:451-472.

A 120: Fasciolosis Essay updated 17.03.2010

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