Liver Flukes

Liver flukes (Fascioloides magna) are a large trematode (or flatworm) whose normal definitive host is the white-tailed deer. Moose, elk, mule deer, sheep, and cattle can also be infected and are considered dead-end hosts, meaning the flukes are unable to complete their lifecycle in this hosts.
The liver in the right image came from a moose that died of brainworm even though it had multiple capsules within the liver. In general, mammals can tolerate a surprising amount of liver damage and still function normally.

As the name suggests, liver flukes are found in the liver. Flukes are found in capsules in the liver (often with more than one fluke per capsule) and are typically one to three inches in length and ½ - one inch wide.

On the right is an image of a liver fluke taken out of a capsule in a moose liver.

Occasionally, liver flukes can migrate outside of the liver and be found encapsulated in other tissue, such as muscles lining the abdominal wall.
Liver Flukes

A liver fluke
In deer, infection is diagnosed using a fecal sedimentation technique to look for eggs in the feces. This diagnostic technique does not work for moose or elk as they do not shed eggs. In moose and elk (as well as deer), infection can be identified by examining the liver for the capsules (with flukes inside). Once cut, the capsules typically release a brown fluid, which is also an indication of infection.

The life cycle of the liver fluke is quite complex. Eggs are produced by flukes located inside the liver of a white-tailed deer. They are passed from the liver to the bile ducts and into the intestinal tract, where they are subsequently shed in the feces. These eggs will only hatch in moist conditions, and produce free-swimming larvae called a miracidia. An aquatic snail is required for the life cycle to continue. Miracidia penetrate an aquatic snail, where several generations of replication occur. Another larval stage, called cercaria, is then formed and leaves the snail. These cercaria are free-swimming and attach to vegetation, at which point they become dormant cysts termed metacercariae. Metacercariae are the stage which is consumed by the deer (or moose, elk, etc.). Once ingested, they excyst and migrate to the liver. Development to the mature form of the fluke is delayed until they contact another immature fluke at which point they pair-up and encapsulate.
Liver flukes generally do not cause illness in deer. However, in moose, liver fibrosis results from extensive migration through the liver, which may cause overall poor body condition.

Even so, presence of flukes is not fatal to a moose. The liver on the right with multiple capsules came from a hunter-harvested moose that was seemingly healthy, with reasonable fat reserves and good coat condition..

One complication that can occur is potentially fatal secondary bacterial infections that have been identified in moose infected with liver flukes.

The liver on the right shows both liver flukes and a secondary infection indicated by the presence of yellowish pus. The cause of death of the moose was septicemia most likely caused by liver flukes. While liver flukes may not kill a moose directly, the effects of a damaged liver can weaken the moose and predispose it to other illnesses.
Cyst closeup

Fluke with secondary infection
Following a study of moose in northwestern MN, researchers concluded that liver flukes were likely a major pathogen responsible for moose deaths and were considered the greatest single source of mortality in the study animals (Murray et al., 2006). Temperature and rainfall/moisture play a significant role in the speed of fluke development within the snail as well as the rate of snail reproduction.  If the temperatures rise and moisture levels increase, there will likely be a higher level of infected snails on the landscape.  This, coupled with increased deer numbers, has put moose at a higher risk for infection. Liver flukes, while aesthetically unpleasing, pose no health risk to humans. Below are images of 2 moose livers that are not infected with liver flukes.
Healthy Liver 1 Healthy Liver 2

Further Reading

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