by: Dr. J.L. Goelz
Liver flukes are a regional nemesis to the sheep industry in the U.S. In parts of the Gulf Coast region, Great Lakes and Pacific Northwest flukes, are a threat to grazing sheep. Flukes have a unique life cycle which has an intermediate host, a snail, that is necessary to complete its life cycle. Therefore; low lying, damp pastures with standing water and/or land subject to flooding are potential fluke sources. Additionally, one species, the giant liver fluke of white tail deer (Fascioloides magna) can result in death with just one fluke.
Immature flukes are ingested as sheep graze. The fluke migrate throughout the sheep, eventually making it to the liver. In the process, the flukes cause damage to tissues and provide areas where clostridia bacteria can survive. Non-specific signs of ill thrift, poor performance, and unexpected death are the only clinical signs. Generally, these signs are common with other diseases and often the diagnosis is only clear on a post-mortem exam. Once flukes are identified in a location, fluke treatment and prevention should be instituted to the entire flock.
Treatment and control in potential fluke areas are aimed at either preventing fluke infestations or minimizing damage within the sheep. There are only two fluke pharmaceutical products available in the U.S., Ivermectin plus and Valbazen. Ivermectin by itself is not effective against liver flukes, but ivermectin plus contains an added drug, clorsulon, which increases the coverage to adult liver flukes. There is not a product in the U.S. that is effective against the immature stages of liver flukes. In fluke areas in the South, treatment in summer and fall is best to avoid fluke trouble. In the North, late fall winter treatment is appropriate.
While the regional limitation of flukes is dependent on the intermediate host, snails, I have been surprised to find flukes in feeder lambs from areas I would not expect. Livers should always be examined in post-mortem exams for the presence of flukes, regardless of the geographic location.