by Jill J. Dunkel

If you are not concerned about liver flukes in your livestock operation, you might think again. Liver flukes, which were once thought to be limited to the Southeast and Pacific Northwest, are now being found in many areas of the United States.

The liver fluke, Fasciola hepatica, is an internal parasite that causes damage to liver tissue and bile ducts, often resulting in liver condemnation at slaughter. The parasite begins its life cycle when infected cattle shed liver fluke eggs in their feces. Once the eggs hatch, they rely on an intermediate host – a lymnaeid snail – which thrives only in shallow water in certain soil types. The fluke larvae penetrate the semi-aquatic snail, multiply and mature in the snails, and then are released as tadpole-like forms into a wet environment. The still-immature flukes then attach to vegetation in cysts and are ingested by cattle when they graze. The immature flukes migrate through the intestine and body cavity to the liver, where they mature and begin to shed eggs in eight to twelve weeks. Then the life cycle starts over.

The transmission season of the liver fluke is dependent on the weather, according to Dr. Jack Malone, a veterinary parasitologist at Louisiana State University. The intermediate host, the lamnaeid snail, must have damp or wet conditions to survive. The transmission season is over after two weeks of sustained drought, which results in the snails becoming dormant, he explains. “In Louisiana, the transmission season ends sometime between late May and July, when the ditches dry up and the mud is dry and cracked,” he says.

Approximately three months after the end of the transmission season is the best time to treat for flukes in the southern United states. That’s usually in the fall to winter in most places, he says. According to information from Louisiana State University, liver fluke infections in cattle are most prevalent in late fall and spring.

Cattle usually show few clinical signs of liver fluke infection, other than reduced productivity, according to the College of Veterinary Medicine at the University of Florida. Condemned infected livers at slaughter is often how an infestation is discovered, but the most important losses are seldom associated with fluke infection by livestock owners. Reduced fertility, lighter calves at weaning, slower growth of replacement heifers, and reduced weight gains in the stocker and feedyard phases can be from a fluke infection.

With reduced performance at risk in fluke infected cattle, it definitely pays to treat cattle for flukes. Once cattle are infected, the liver could still be condemned, according to researchers, but decreasing the fluke burden will almost always improve production.

Dr. Ray Kaplan of the University of Georgia says the vast majority of studies done on liver flukes show a benefit in performance when the animals are treated at the right time. Kaplan cautions that the degree of improvement depends on the class of animal and the extent of the fluke burden. “The larger the burden, the greater impact treatment will have on performance,” he says.

Since cattle are routinely transported from one region of the country to another, cattlemen can no longer safely assume they don’t have flukes, researchers suggest. Cattle brought in from unknown regions may pose a threat. Likewise, cattle brought in from fluke-endemic regions should be treated for flukes on arrival. ©