By Bryony Sands, UVM Extension
Sheep and goat farmers in the Northeast have reported 20% losses from herds this winter due to liver fluke, a trematode or flatworm that infests the liver, causing damage and a deadly secondary bacterial infection known as black disease.
Farmers have reported sudden death in animals without clinical signs, while some animals displayed 6 to 12 hours of reduced appetite and isolation from the herd, or a rough hair coat and abortion several weeks prior to death. Field necropsies showed damaged, misshapen livers containing lesions, black areas and necrosis (tissue death), as well as the presence of liver fluke and their tracks. It is likely that wet conditions in recent years have contributed to the rise in liver fluke and black disease cases in small ruminants. Liver fluke thrive in waterlogged and wet areas due to relying on snails as an intermediate host. The presence of freshwater snails on a farm is a risk factor for the transmission of liver fluke, and even the slime trials on vegetation can be a source of infection.
There are two species of liver fluke of main concern to small ruminant producers. Fasciola hepatica, the common liver fluke, causes chronic disease. Ruminants are the main host. Fluke eggs are passed out in ruminant feces and hatch in water. The free-living stage swims through the water and finds a freshwater snail. It continues to develop inside the snail, then leaves and swims onto partly submerged vegetation. It encysts on the vegetation waiting to be eaten by a ruminant, where it migrates through the liver causing the tissue damage and disease observed above.
Fascioloides magna, or the American deer fluke, is a natural parasite of deer that can enter sheep and goats as abnormal hosts. Disease in small ruminants is acute and usually fatal. The lifecycle is the same as for the common liver fluke, including the aquatic stages and freshwater snail as an intermediate host. However, it is spread by deer and when the fluke enters a sheep or goat, it is a dead-end host. This means that while the fluke do migrate through the liver causing fatal damage, they are not able to mature and shed eggs. This makes diagnosis difficult because there are no fluke eggs present in the feces of the animal to indicate infection. As they migrate through the liver, fluke are known to track clostridial species of bacteria, which release liver-destroying toxins, resulting in necrosis (tissue death) and black disease.
Prevention and treatment of liver fluke involves restricting access to wet areas whenever possible. For example, fencing off or only grazing those areas when it is dry. Usual worming treatments are not effective against liver fluke. Ask your veterinarian for specific dewormers that are effective against fluke (there are several), and if you suspect your animals have been exposed, consider vaccinating for black disease. There are several vaccinations against clostridial species available.
Remember that liver fluke fecal egg counts can only sometimes detect F. hepatica, and never F. magna, so while fecal testing can be a great way to monitor for this parasite, it cannot be totally relied upon.
For more information about liver fluke and other small ruminant parasites, visit https://go.uvm.edu/parasite-management
