Developed by Ethan Alan Ward Stone (Veterinary Parasitology MPS Program, Cornell University, April 2025) with guidance from Mary C. Smith, DVM; Araceli Lucio-Forster, PhD; and Manigandan Lejeune, PhD, DipACVM.

Description

Fascioloides magna is a large, parasitic trematode (leaf-shaped flatworm; fluke) capable of infecting and killing livestock in North America and Europe. It is commonly known as the giant liver fluke because it is large, reaching up to 4 inches long, and lives in the host's liver. The life cycle of this parasite is complex, requiring an aquatic snail host and a cervid host. Cervids, including white-tailed deer, elk, black-tailed deer, mule deer, red deer, caribou, and fallow deer, are the normal or definitive hosts of F. magna and typically experience little to no clinical signs. The term definitive host in veterinary parasitology describes the animal where the parasite matures and completes its reproductive cycle, producing offspring. In the case of F. magna, the offspring comes in the form of eggs, which are released by the mature parasite residing in the liver of the infected cervid host. The eggs exit the liver via the bile ducts, ultimately reaching the small intestine to be shed into the environment within the feces, thus contributing to environmental contamination.

Domestic livestock, such as sheep, goats, and cattle, are not considered normal or definitive hosts for F. magna; instead, they are classified as abnormal hosts. Infections in these livestock animals demonstrate two notable differences compared to those in normal or definitive cervid hosts. Firstly, infections in livestock hosts tend to be significantly more pathogenic and often result in fatal outcomes, particularly in sheep and goats. Secondly, the parasite cannot complete its reproductive cycle within the liver of the livestock hosts, meaning that eggs are not excreted in the feces, and these animals do not contribute to further environmental contamination. Understanding these host differences is critical for effective prevention and control.

Life Cycle of Fascioloides Magna

1. Cervid Host

Infection of the cervid definitive host

Wild cervids, such as white-tailed deer, elk, and caribou, are the normal or definitive hosts for this parasite. These animals become infected when they ingest semi-aquatic plants that harbor the attached infective larvae. Once infected, the parasite matures into its adult fluke stage within the cervid's liver. Typically, infected cervids exhibit few or no clinical signs.

2. Fecal Eggs

Eggs released into the environment with feces

Roughly six months after infection, adult flukes release their eggs into the bile ducts of the liver, which are then passed into the environment through the feces.

3. (1) Free-Swimming

Free-swimming larvae hatch from eggs

For the eggs to hatch, they must first reach a water source (i.e. pond, wetland). Upon entering the water, the eggs develop for a period before hatching into free-swimming larvae.

4. Snail Host

Larvae infect snail host

The free-swimming larvae locate and infect an aquatic snail intermediate host, where they undergo clonal amplification (asexual multiplication) and develop into hundreds to thousands of new free-swimming larvae.

5. (2) Free-Swimming

Free-swimming larvae exit the snail host

The free-swimming larval stage exit the snail intermediate host in search of semi-aquatic vegetation.

6. Infective Stage

Infective stage on aquatic vegetation

The free-swimming larvae attach to semi-aquatic vegetation along the edges of water sources. If ingested, this larval stage will infect definitive hosts, such as cervids, and abnormal hosts, including livestock.

Three key points

1. This life cycle requires three components: a snail intermediate host, a cervid definitive host, and a water source.
2. Only cervid definitive hosts shed the parasite's eggs in feces, supporting ongoing transmission cycles.
3. There is only one route of infection: ingestion of aquatic plants harboring the attached infective larvae.

Infections in Definitive Hosts (Wild Cervids)

1. Infected via ingestion of aquatic and semi-aquatic vegetation with the fluke larval stage.
2. Upon ingestion, the larvae penetrate the gut wall to migrate to the deer’s liver and eventually grow into adult flukes. Infected deer may present as “poor doers” or remain relatively healthy.
3. Approximately six months after infection, the adult flukes release eggs into the liver's bile ducts. These eggs are ultimately passed with the deer's feces, contaminating the environment.
4. Given adequate moisture and a water source (e.g., a wetland, pond, stream, lake, etc.), the eggs hatch into an aquatic larval stage, which infects the snail intermediate host.
5. The infected snail harbors and amplifies the parasite before releasing a new larval stage into the water. This stage swims and attaches to aquatic or semi- aquatic vegetation, where it awaits ingestion by a deer host, thereby continuing the life cycle.

Infections in Abnormal Hosts (Domestic Livestock)

1. Infected via ingestion of aquatic and semi-aquatic vegetation with the fluke larval stage.
2. In sheep and goats, the parasite is confused upon infection, resulting in extensive wandering migration by immature flukes in the liver, body cavity, and lungs. This can cause tremendous tissue damage, with an often fatal outcome.
3. In cattle, flukes often become encapsulated (enclosed by fibrous tissue) in the liver, in which case, the infection is not fatal.
4. Importantly, eggs will not be released in the feces of any domestic livestock (no further environmental contamination).

Clinical Signs in Infected Livestock

In commercial flocks, the first indication of F. magna infection is often the sudden death of animals that appeared healthy the day before—most commonly observed during the fall and winter months. While the severity of disease varies by species, the liver is the primary target organ due to extensive migration by immature flukes, making early detection and treatment difficult. This migration causes significant liver damage, including inflammation, fibrosis, and tissue necrosis.

Clinical signs may include:

• Weight loss
• Anemia
• Hypoproteinemia leading to "bottle jaw" (fluid, swelling under the jaw)
• Jaundice (yellowing of mucous membranes)
• bdominal distension (from liver enlargement)
• Reduced milk production in dairy cattle

Not all infected livestock will display clinical signs, and the infection will not be detected until the liver is examined during slaughter or necropsy. No diagnostic tests to detect infection in live animals (that are not shedding eggs) are yet available.

Prevention and Control

1. Wildlife, specifically deer, should be kept away from livestock water sources and the areas where they graze to prevent infection.
2. Restrictive fencing and removing emergent vegetation in water sources can also help reduce the infection risk. In some instances, drainage of non-essential water sources may be considered an option.
3. Treatment of wetlands with chemicals to kill snails will cause significant environmental damage and is not recommended.

Treatment Recommendations

Treating infections caused by F. magna can be challenging due to the flukes' location in the liver. Unlike other flukes that inhabit the bile ducts, these reside within the liver tissue, making it difficult to deliver sufficient medication. In addition, immature flukes are difficult to kill. For these reasons, we recommend that a licensed veterinarian oversee all treatments.

• Albendazole (Valbazen®) is the only approved treatment for flukes in sheep and goats in the United States, and the approved dose of 7.5 mg/kg (3 mL per 100 pounds) in sheep and 10 mg/kg (4 mL per 100 pounds) in goats and cattle may not kill all the adult Fascioloides flukes. The drug is not effective against immature flukes under 12 weeks of age. It should not be given to small ruminants during the first month of pregnancy. The meat withdrawal for sheep and goats is 7 days, but it should not be given to animals that provide milk for human consumption. If a higher dose is needed, the producer must involve the herd veterinarian, who can provide extended withdrawal periods after consultation with FARAD.
• Vaccination against clostridial diseases (such as the 7- or 8-way vaccine) is advised to prevent deadly bacterial infections. Migrating flukes cause so much damage to the liver that clostridial spores can germinate, producing a toxin that kills rapidly. This vaccine should be repeated annually on farms where flukes have been a problem before the time of year when death losses begin.

Necropsy

Livers of infected sheep and goats will be found to have black tracts where the flukes have migrated, and often, there will be areas of dead liver tissue. One or more flukes may be found and can be 2 inches long or longer.

Sever tissue damage in goat liver due to F. magma infection.

Adult F. magma specimen.

References

1. Howell, A. K., & Williams, D. J. (2020). The epidemiology and control of liver flukes in cattle and sheep. Veterinary Clinics: Food Animal Practice, 36(1), 109–123.
2. Juhász, A., & Stothard, J. R. (2023). The giant liver fluke in Europe: A review of Fascioloides magna within cervids and livestock with considerations on expanding snail- fluke transmission risk. Advances in Parasitology, 119, 223-257.
3. Králová-Hromadová, I., Juhásová, L., & Bazsalovicsová, E. (2016). The giant liver fluke, Fascioloides magna: past, present and future research (p. 106). Heidelberg, Germany: Springer International Publishing.
4. Pybus MJ (2001). Liver flukes. pp 121–149 in Parasitic Diseases of Wild Mammals, 2nd Edition (Samuel WM et al., eds.) Iowa State University Press, Ames.