Liver Flukes

Liver Flukes

Causative Agent

Parasitic disease of ruminants caused by infection with a flatworm, particularly the giant liver fluke, Fascioloides magna.

Click on a photo to enlarge.

Tissue damage in the liver of a moose infected with Fascioloides magna

Two adult F. magna removed from a capsule (host-derived tissue) in liver, white-tailed deer

Distribution

Geographic:

In North America, F. magna can be found in the southeastern United States, the Great Lakes basin, Labrador and northern Quebec, the Pacific Northwest including Vancouver Island, the Rocky Mountains of BC/AB and isolated pockets in central Saskatchewan.
F. magna infections are most common around wetlands where large numbers of susceptible definitive hosts congregate for extensive periods with a suitable population of snails.

Seasonality:

Infections generally occur in late summer and fall. Seasonal changes in moisture and temperature will affect the abundance and activity of the intermediate host (snail).

Hosts and Life Cycle

Hosts:

3 types of definitive hosts and 1 type of intermediate host.
- definitive hosts such as moose, elk, white-tailed deer, mule deer, and caribou, harbour the adult flukes.
- intermediate host are aquatic snails that carry the larval stages of the life cycle.

Life Cycle:

Adult worms occupy the liver of infected definitive hosts.
Eggs are swept into the bile collecting system within the liver, enter the small intestine and leave the host along with the feces where they hatch if they are in contact with aerated water.
A free-swimming life stage emerges from the egg, which then penetrates the tissues of a suitable snail host (intermediate host) where multiplication of the parasite occurs.
A more developed life-stage emerges from the snail and encysts on aquatic vegetation where it can remain for prolonged periods. Contaminated vegetation is then fed upon by a suitable herbivorous definitive host.
Ingested larval flukes penetrate the intestinal wall and migrate within the abdominal cavity towards the liver, where they slowly develop into adults.
Flukes are generally found in groups of 2 or more within the liver.
Adult flukes are generally observed more often in older animals.
If larval flukes are ingested by “dead end” hosts such as moose or domestic cattle, they are unable to complete their development because they are walled off in scar tissue or encapsulated by tissues of the host.
Flukes are also not able to complete development if ingested by an “aberrant” host such as domestic sheep and goats. These hosts may die from extensive tissue damage that occurs during migration of the fluke in the liver.

Signs and Symptoms

Adult flukes are purple-gray in colour, flat, elongate, oval, and look like "bloodsuckers" or "leeches". When found while cutting open or slicing liver, they resemble a blood clot and are surrounded by a thick black-grey discharge. The flukes vary in size from 15-30mm wide by 30-100mm long by 2-5mm thick.
Livers of infected animals may be enlarged.
In definitive hosts, flukes are found within thin-walled capsules containing 2 or more adult flukes. Changes in liver structure may occur in response to tracts and damage caused by migrating flukes. Liver damage is usually minor in definitive hosts but increases with age and number of flukes present.
In aberrant hosts, flukes are not contained in a capsule and liver damage in the form of hemorrhage and dead tissue is extensive.
In dead-end hosts, flukes are contained in thick-walled capsules with areas of black pigmentation (digested blood) visible throughout the liver.
Animals infected with the adult fluke may be healthy or be in poor condition, appearing drowsy, depressed, with poor appetite.
Distended abdomens have been observed in infected Elk.

Meat Edible?

Except for infected liver tissue, the meat from infected animals is suitable for human consumption.

Human Health Concerns and Risk Reduction

Humans are not at risk.
Transfer of the fluke to domestic sheep and cattle herds can lead to economic losses due to mortality of animals and condemnation of infected livers. Despite the endemic status of the giant liver fluke in areas of BC, there does not appear to be a significant problem in the livestock industry.

Samples for Diagnosis

Infection can be verified through examination of the liver for adult flukes or on the basis of finding eggs in fecal material.