Fascioloides magna is a flatworm parasite native to North America. Its final hosts are deermoose, and related cervids, but it can infect cattle, sheep, goats, horses, buffaloes, etc. if they share the pastures with wild cervids.

Fascioloides magna is also called the giant liver fluke, the large American liver fluke, and the deer fluke.

Adults of Fascioloides magna. Picture from www.michigan.gov

Although native to North America it was introduced into Europe in the 19th century and has been reported in Austria, Italy, Germany, Poland, Czech Republic, Hungary and other Central European countries. In endemic regions in North America prevalence can be very high, with up to 80% of the animals being infected.

This parasite does not affect dogs, cats, swine or poultry.

Are animals infected with Fascioloides magna contagious for humans?

NO. If livestock is infected with stomach flukes, they are not contagious for humans, neither through contact, nor after consuming meat, milk or blood of contaminated animals, nor through the feces. The reason is that Fascioloides magna is not parasitic for humans.

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

Final location of Fascioloides magna

Predilection site of Fascioloides magna is the liver, occasionally the lungs and the intestine.

Anatomy of Fascioloides magna

Adult flukes are large: they reach up to 10 cm in length and 3.5 cm in width. As many fluke species they have an oval shape and a flattened body. They also have two suckers, an oral and a ventral one. They have a reddish-brownish color. The feed on blood. They 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). Ass all liver flukes they are simultaneous hermaphrodites, i.e. they have both male and female reproductive organs.

The eggs are yellow to brown and measure ~90x140 micrometers.

Life cycle and biology of Fascioloides magna

Deer are the natural final hosts of Fascioloides magna

Fascioloides magna has a rather complex indirect life cycle with freshwater snails as intermediate hosts, the most common ones from the genus Fossaria and Stagnicola in North America, and from the genus Galba and Radix in Europe. There are three types of final hosts:

  • Definitive hosts, i.e. the natural final hosts where the flukes complete migration to the liver, mature and produce eggs that are expelled out of the host and re-infest the pastures. Definitive hosts are several deer species.
  • Dead-end hosts, i.e. unusual final hosts where the flukes complete migration to the liver, mature and produce eggs, but the eggs cannot be expelled out of the host because the bile ducts are occluded. As a consequence, the infected dead-end hosts usually do not re-infect the pastures. Dead-end hosts are typically, cattle, buffalo, pig, wild boars, horses, and certain deer species.
  • Aberrant hosts, i.e. unusual final hosts where the flukes follow an aberrant migration until the host dies as a consequence of the infection. The flukes do not mature, i.e. the do not produce eggs and therefore do not re-infect the pastures. Typical aberrant hosts are sheep and goats, but also rabbits.

In definitive hosts, adult flukes are usually encapsulated in the liver tissue. The capsule has a connection to the bile ducts. Mature flukes produce eggs that are collected in the capsule and periodically released to the bile ducts and further to the gall bladder, where they are expelled to the gut in periodic waves, together with the bile. In the gut they are transported outside the host with the feces.

Once outside the host, the eggs produce larvae called miracidia in about 4 to 6 weeks. These larvae hatch out of the eggs and swim in the water until they find a suitable snail. Once they find it they penetrate into it and continue development to sporocysts in various organs of the snail. The sporocysts multiply asexually producing several rediae, and the rediae cercariae. Development inside the snail takes 6 to 10 weeks depending on temperature and the snail species.

Mature cercariae leave the snail and encyst on the vegetation. They are called metacercariae and are infective for final hosts. After a final host ingests the cysts while grazing on infected vegetation, the metacercariae are released in the intestine as juvenile flukes that soon later penetrate the intestinal wall and start migrating in the abdominal cavity towards the liver. Once in the liver they encapsulate in the liver tissue, usually in pairs. Occasionally, migrating flukes end in other organs (e.g. kidneys, lungs) but they will not complete development there.

The prepatent period (i.e. from infection of the final host till eggs production) is about 3 to 6 months depending on the snail species, the final host and the climatic conditions. Inside a final host adult flukes may survive for more than 5 years. 


Harm caused by Fascioloides magna, symptoms and diagnosis

For the natural definitive hosts, i.e. various deer species, infections with Fascioloides magna are mostly benign, without clinical symptoms, although the animals may be weakened (weight loss, lethargy, depression, etc.). Nevertheless, massive infections can be fatal.

For dead-end hosts such as cattle or horses, most infections with Fascioloides magna remain without clinical symptoms. The massive cysts in the liver tissue can impair the liver function. And the livers are condemned at slaughter. 

For aberrant hosts such as sheep and goats, infection with Fascioloides magna is mostly fatal, usually about 6 months after infection, often without previous clinical signs. Excessive fluke migration leads to strong bleeding and peritonitis, heavy damage to the liver tissue and subsequent fibrosis.

In infected cattle, sheep, goats or other non-definitive hosts the eggs are not excreted with the feces. Consequently diagnosis through fecal examination is not possible. It can be confirmed only finding the encapsulated flukes after necropsy.

Prevention of Fascioloides magna

The first preventative measure against Fascioloides magnais is to keep livestock from sharing pastures with deer. If this is not possible, in endemic regions keeping the pastures dry will reduce the suitable habitats for infected snails, e.g.:

  • Ensuring an adequate drainage.
  • Building watering points on solid ground, without puddles.
  • Make unavoidable ditches or channels less attractive to the snails: making the borders steeper and/or cover them with concrete, eliminate the surrounding vegetation, drying them completely out periodically, etc.

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

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

Biological control of giant 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 Fascioloides magna

Active ingredients with efficacy against flukes are usually called flukicides or fasciolicides. The following anthelmintics have reported efficacy against Fascioloides magna in livestock (mainly cattle, sheep, goats):

However, commercial anthelmintic products may not include label claim against Fascioloides magna in some countries. As for other fluke species, migrating immatures 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. Triclabendazole is the active ingredient with the highest efficacy against immature stages.

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

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.

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 are not approved in most countries.

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


Resistance of Fascioloides magna to flukicides

So far there are no reports on resistance of Fascioloides magna to flukicides.

This means that if a flukicide fails to achieve the expected efficacy, chance is very high that it is not due to resistance but either the product was used incorrectly, or it was unsuited for the control of Fascioloides magna. Incorrect use is the most frequent cause for failure of antiparasitic drugs.