Fasciola hepatica (= Distomum hepaticum), the common liver fluke (also called the sheep liver fluke) is a flatworm belonging to the Trematodes. Its final hosts are sheep, goats, cattle and other domestic and wild mammals, including horses, dogs, cats and humans.

The common liver fluke occurs worldwide but is particularly abundant in humid regions with temperate climate where it can be endemic.

It is one of the most abundant and damaging helminth parasites of grazing ruminants (sheep, goats, cattle). In endemic regions 100% of the animals can be infected. It is particularly harmful, even fatal for sheep. Prevalence and incidence in a particular region depends strongly on ecologic and climatic conditions (e.g. habitats for intermediate hosts and wild mammals, overwintering of the parasites in the environment, etc.) and on livestock management practices (stock density, grazing patterns, etc.).

Adult liver fluke, dorsal view (preserved and stained)Horses and pigs kept outdoors can also be affected, but liver flukes are usually not a problem in housed pigs.

Horses, dogs and cats can be occasionally infected, especially in rural environments, but liver fluke is usually not an issue for these animals.

The disease caused by liver flukes is called fasciolosis, fasciolasis, distomatosis or liver rot.

Are animals infected with Fasciola hepatica contagious for humans?

Usually NOT. If livestock is infected with liver flukes, it is not directly contagious for humans, neither through contact, nor when consuming meat, milk or blood of contaminated animals, nor through the feces. BUT experimental studies suggest that humans may be infected when consuming raw livers contaminated with juvenile liver flukes. The same applies to pets infected with liver flukes. Normally humans become infected through ingestion of water contaminated with infective cercariae (see below under life cycle), not by ingesting adult flukes or their eggs.

You can find additional information in this site on the general biology of parasitic worms and/or flukes.


Final location of Fasciola hepatica

Predilection sites of Fasciola hepatica are the biliary ducts in the liver and the gall bladder.


Anatomy of Fasciola hepatica

Adult liver flukes have a flat body, an oval shape, and are rather large: up to 30 mm long and 15 mm wide. The have a pink-grayish to dark red color. Liver flukes have two suckers, both in the ventral side. The body surface is covered with numerous spines. Liver flukes have no external signs of segmentation. The mouth ends in the pharynx, a muscular tube that allows sucking. The digestive system is blind (i.e. without anus: the only opening is the mouth) and not linear, as in most animals, but branched, ending in several blind ducts (called coeca). Liver flukes are simultaneous hermaphrodites, i.e. they have both male and female reproductive organs.

The eggs of liver flukes are about 80x140 micrometers, with an oval form, operculated (i.e. with a cap-like cover) and of a yellowish to greenish color derived from the host's bile.


Life cycle and biology of Fasciola hepatica

Egg of Fasciola hepatica. © J. Kaufmann / Birkhäuser Verlag

Fasciola hepatica has an indirect life cycle with amphibious snails as intermediate hosts, typically from the genus Lymnaea.

Adult flukes produce eggs in the biliary ducts of their hosts. These eggs reach the gall bladder and are passed to the host's gut when the gall bladder is emptied. They are passively transported to the anus and are expelled with the feces. A single liver fluke can produce up to 25'000 eggs a day! 

Once outside the host, the larvae called miracidia hatch out of the eggs in 7 to 15 days. These larvae can survive for weeks off a host provided there is enough humidity. They die quickly in a dry environment. Miracidia can swim and penetrate actively into the snails where they remain for 4 to 8 weeks and develop successively to sporocysts, rediae and cercariae, the usual larval stages of most fluke species. A single miracidium can asexually produce up to 600 cercariae.

Mature cercariae leave the snail, attach to the vegetation, lose their tail and produce cysts of about 0.2 mm, the so-called metacercariae, which are infective for the final host. Such cysts can survive for months in the vegetation, even under dry conditions, even in hay! Livestock becomes infected by grazing contaminated pastures or hay, i.e. animals kept indoors can also become infected if they are fed contaminated hay.

Snail of the genus Lymnaea, intermediate host of liver flukes. Picture from www.weichtiere.atInside the final host young immature flukes hatch out of the cysts and within a few hours they cross the intestinal wall and get into the peritoneal cavity where they migrate towards the liver, which they reach in about 3 weeks. To reach the biliary ducts they have to cross the hepatic tissue, a particularly harmful process for the host that lasts 6 to 8 weeks. Once in the biliary ducts they complete their development to adult flukes and start producing eggs.

The prepatent period of Fasciola hepatica is 9 to 15 weeks, depending on the host and other factors. The entire life cycle can be completed in a minimum of 16 weeks. Left untreated liver flukes can live up to 20 years on sheep, usually not more than one year in cattle.

Livestock grazing in regions with a high water table or frequently flooded are at high risk of becoming infected with liver fluke. The reason is that the snails acting as intermediate hosts are amphibious and need humid habitats that are periodically submerged or flooded. Relatively small humid habitats (e.g. irrigation channels, ditches, ponds, watering holes, etc.) offer suitable conditions for the development of such snails and keep the surrounding pastures infected. Permanently stabled livestock can also become infected through contaminated hay.

Humans and carnivores such as dogs and cats are infected mainly ingesting aquatic plants or drinking water contaminated with cercariae. There is also evidence that consumption of raw liver contaminated with juvenile flukes can be contagious for humans.


Harm caused by Fasciola hepatica

Bile ducts damaged by liver fluke infection. © J. Kaufmann / Birkhäuser VerlagLiver flukes are very harmful for their hosts, particularly for sheep. Young immature flukes migrating through the liver tissues and crossing the wall of the bile ducts cause the major harm. This process destroys the tissues and causes bleeding. The spines in the surface of the flukes irritate the tissues that become inflamed. All this leads to cell death and fibrosis, i.e. the formation of excessive connective tissue that replaces the dead liver cells, which impairs the normal functioning of the liver. Affected livers increase in size and become fragile. Some flukes can become encapsulated in the liver tissues and build cysts as large as walnuts. The bile ducts are also damaged: they become thickened and can be calcified and even obstructed. Infections with secondary bacteria can also happen, mainly due to the general weakness of the host that debilitates its own immune system. In addition, flukes produce own toxins that impair the normal function of the liver.

The bottom line is that the numerous vital physiological processes that run in the liver are disturbed and the affected animals become sick in a degree that depends on the number of flukes that infect them. Infections with liver flukes are usually more harming for sheep (fatalities are not rare) than for cattle and other livestock. Besides fatalities the major economic damage is the consequence of reduced weight gains of young animals (up to 30% less, even after slight infections) and the condemnation of livers at slaughter. Light infection can already reduce milk production in dairy cattle.

Worlwide annual economic loss due to fasciolosis has been estimated to be over US$ 3 billion!


Symptoms and diagnosis of Fasciola hepatica infections

There are no really typical and easily recognizable symptoms of a liver fluke infection in livestock or other animals. The major symptoms are related with the inflammation of the liver (hepatitis) and of the bile ducts (cholangitis) that can be also due to other disorders. Other vital organs are usually not affected. Infections with a few dozen adult flukes may not cause clinical signs other than general weakness and reduced productivity.

Chronic fasciolosis is the more common form in sheep, goats and cattle. It develops along the gradual establishment of adult flukes in the bile ducts. It is characterized by the progressive development of such symptoms as anemia (reduced number of red blood cells), edema (local swellings due to excess fluid) often as "bottle jaw", digestive disturbances (diarrhea, constipation etc.), and cachexia (wasting, i.e. weight loss, fatigue, weakness, loss of appetite, etc.).

Acute fasciolosis is seldom in cattle but can occur in sheep. It is caused by the sudden migration of many immature flukes through the liver, which leads to complete organ failure. It can develop in healthy animals that may be killed in a few days.

Detection of eggs in the feces confirms the diagnosis. However, since the eggs are passed to the intestine only when the gall bladder is emptied, a negative fecal egg count is not conclusive, i.e. there can be false negatives. It is also important to distinguish Fasciola eggs from Paramphistomum eggs that have a similar aspect.


Prevention and non-chemical control of Fasciola hepatica infections

Typical habitat of liver fluke vector snails. Picture from www.ansgeulaiche.co.ukFasciola hepatica can infect virtually all mammal species, wild and domestic. Consequently it is almost impossible to eradicate it from a given property in endemic regions with favorable conditions. Therefore, where it is known to occur, preventive measures are a must to reduce the snail populations, the infection of pastures with infective stages, or the access to livestock to highly infested pastures.

Vector snails are amphibious and live both in water (e.g. streams, lakes, pools, swamps, marshes, irrigation channels, ditches, ponds, watering holes, etc.) as well in the humid vegetation around such places. These snails are enormously prolific: a single snail can produce more than 100'000 snails within one year.

Whatever measures help keeping the pastures dry have to be encouraged, e.g.:

  • Ensuring an adequate drainage.
  • Building watering points on solid ground, without puddles.
  • Make unavoidable ditches or channels less attractive to the snails: make the borders steeper and/or cover them with concrete, eliminate the surrounding vegetation, drying them completely out periodically, etc.
  • Avoid even very small water points that support the snails, e.g. hardened footprints (of shoes or car tires).

If permanent humid environments cannot be eliminated, they have to be fenced to prevent livestock from grazing there.

To avoid: livestock grazing in flooded pasture harboring potential fluke vectors. Picture from pa.water.usgs.gov-projects-microweb-blue_marsh

Rotational grazing is highly recommended to diminish fluke infestations. Simultaneous grazing of sheep and cattle is not advisable. Alternate grazing (e.g. sheep and cattle, sheep and horses, etc.) won't help, because liver flukes can infest all livestock.

Livestock infected with liver flukes can develop a certain level of natural immunity and become resistant, especially cattle. Animals with chronic fasciolosis may recover spontaneously. However, such natural resistance is usually associated with hepatic fibrosis, i.e. a partial impairment of the liver function that will certainly result in a reduced productivity.

Keeping livestock healthy and well fed diminishes the harm caused by liver flukes and favors the development of the previously mentioned natural immunity.

There are so far no vaccines against the common liver fluke. To learn more about vaccines against parasites of livestock and pets click here.

Biological control of liver flukes (i.e. using their natural enemies) is so far not feasible.

You may be interested in an article in this site on medicinal plants against external and internal parasites.


Chemical control of Fasciola hepatica infections

Active ingredients with efficacy against flukes are usually called flukicides or fasciolicides. The following anthelmintics are used nowadays against liver flukes:

As previously mentioned migrating immature flukes are the most harmful, therefore efficacy against these stages is essential. Products without efficacy against immature flukes will not protect livestock from migrating flukes and need to be administered more frequently. The tables below summarize the efficacy of these flukicidal active ingredients (at their usual therapeutic doses) against the various stages of Fasciola hepatica  in sheep and cattle.

    SHEEP  
    Age in weeks  
   

Early imm.

 Immatures  Adults  
Active ingredients    1  2  3  4  5  6  7  8   9  10 11 12  
Albendazole                            
Clorsulon                        
Closantel                        
Nitroxinil                        
Oxyclozanide                            
Rafoxanide                      
Triclabendazole                 
    CATTLE  
    Age in weeks  
   

 Early imm.

 Immatures  Adults  
Active ingredients    1   2   3   4   5   6  7  8  9 10 11 12  
Albendazole                            
Clorsulon                        
Closantel                        
Nitroxinil                       
Oxyclozanide                          
Rafoxanide                      
Triclabendazole                 

Neither macrocyclic lactones (e.g. ivermectin) nor levamisole are effective against liver flukes. 

Several flukicides (e.g. bithionol, brotianide, diamfenetide, disophenol, hexachloroethane, carbon tetrachloride) used in the last century have been abandoned and replaced by more effective and less toxic flukicides.

In many situations livestock needs to be protected not only against flukes but also against roundworms, less frequently against tapeworms as well. From all the flukicides previously mentioned only albendazole is a broad-spectrum wormer effective also against most roundworms and tapeworms, but its flukicidal efficacy is limited. For this reason mixtures are often used, typically ivermectin + clorsulon, ivermectin + nitroxiniltriclabendazole + levamisole, etc.

For use on livestock these active ingredients (alone or in mixtures), are mostly available as drenches for oral delivery, as injectables or as slow-release boluses. Solid formulations (e.g. tablets, pills, etc, feed additives, etc.) are rather seldom for livestock.

There are very few commercial flukicides approved for use on pig, poultry, dogs and cats, simply because flukes are seldom a problem for them.

In endemic regions strategic use (i.e. preventative or prophylactic use) of flukicides combined with the previously mentioned preventive measures is highly recommended. Delaying the treatments until the first symptoms become evident (i.e. therapeutic or curative use) is often too late: most harm has been already done and probably the pastures have been already contaminated with fluke eggs. Best timing for such preventative treatments depend strongly on local climatic and ecologic conditions that drive the snail population.

Chemical control of the snails with molluscicides (i.e. snail killers) such as copper sulphatesodium pentachlorophenate, niclosamide, etc. can make sense for very specific purposes, e.g. for treating places where livestock congregates (water holes, feeding areas, salt licks, shade trees, etc) to keep them free of snails. However, trying to eradicate snails from a property is hopeless and useless. It is virtually impossible to treat every place where they can survive and they reproduce extremely quickly. Cleaned pastures would become re-infested very fast. In addition, it would be too expensive and very harmful for the environment. In fact, such molluscicides (mainly niclosamide) are approved only in a few countries as an aid in the prevention of human schistosomiasis (also called bilharziosis or human snail fever).

Ask your veterinary doctor! If available, follow more specific national or regional recommendations or regulations for liver fluke control.


 


Resistance of Fasciola hepatica to flukicides

There are numerous reports on liver fluke populations resistant to triclabendazole and albendazole (both benzimidazoles) in numerous countries. The first cases were reported in 1995 in Australia, later on it has been also reported for many other countries on sheep (e.g. Argentina, Ireland, Netherlands, Spain, UK) and on cattle (e.g. Argentina, Netherlands). Since than it seems to be quite widespread in these countries and most likely already present in other regions, even if no specific cases have been reported so far.

There are also reports on liver fluke populations on sheep resistant to rafoxanide and closantel (both salicylanilides), probably with cross-resistance to nitroxinil, and also to clorsulon. However resistance to these compounds seems not to be as widespread as resistance to benzimidazoles.

A practical consequence of this is that if a particular product does not achieve the expected control of Fasciola hepatica, it is reasonable to consider a resistance problem, especially if that product has been used for several consecutive years. However, experience shows that many cases of product failure are due to incorrect use and not to resistance.

To prevent or at least delay the development of resistance it is highly recommended to periodically change the chemical class of the product used, before resistance is suspected, e.g. every one or two years. This is usually called rotation.

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