It takes two months for flukes to migrate into the cow’s liver and mature, at which time they are most susceptible to drugs. Thus, treatment for flukes must be administered five months after the eggs hatch from manure and end up in the cow, Faries says. If this life cycle is occurring from April through August, treat for cattle liver flukes in September. “If it was too cold in April or we didn’t have rain, it will be a little later,” he adds. 

In some climates, if there’s sufficient water and warmth, there may be another cycle in the fall. “When treating cattle for both worms and flukes, it can be difficult to find the proper time, since in our climate (Texas), it’s usually best to treat for stomach worms in early summer after spring rains. People often use a dewormer that kills both, and feel they’ve accomplished their goal, but didn’t actually kill the flukes,” Faries says. If climate is mild or there are warm water springs in a pasture, flukes are possible any time of year.

Craig says late fall is generally the best time in the South to treat for flukes. “The drugs presently available in North America, used at prescribed dosage, only kill adult flukes in the liver. They’ve already done their damage – during migration through the liver. We don’t have an effective drug to kill them during that period,” Craig explains.

You need to know the life cycle of the flukes in your area, and how long it takes for immature stages to become adults. “This could be in late winter or spring in the Northwest where there are wet winters,” Hawkins says.

“Target the time of year when there’s the most transmission, and treat at least eight weeks later. This would be September-November in the Southeast, and March-May in the Northwest. You’d get less infection during the other months. Pay special attention to replacement heifers and young bulls to ensure they’re gaining and/or producing to their potential, as well as to save on feed costs” he says.

Amoung the products that kill liver flukes are Ivomec® Plus (Merial); Valbazen® (Pfizer) and Noromectin® PLUS (Norbrook).

“To get deer flukes, however, you must use 2-4 times the recommended dose for cattle flukes; even then, you don’t always get a good kill. Deer flukes are more resistant to the drugs. Cattle are an abnormal host, treating them as foreign and walling them off. After they become adults, they pair up in the liver and the cow walls them off with a fibrous connective-tissue capsule, like an abscess,” Hawkins says.

The cyst becomes a mass of expanding necrotic tissue. “Because of the fibrous capsule around them, drugs can’t touch them. Cattle are a dead-end host; after the flukes get walled off they can’t pass eggs (and we can’t check feces for eggs, for diagnosis). All we kill, when treating deer flukes, are the migrating immatures,” explains Hawkins.

“In our studies in Minnesota, we often found very little normal liver tissue left, in calves with deer flukes,” says Bert Stromberg, University of Minnesota professor of parasitology. “We recommend treating calves at the end of summer – into the fall – as deer flukes start maturing. It would be nice to have something that will kill immature stages more effectively, but we have to treat with what we have, and hope to knock numbers down. The only way to control deer flukes is to control deer, and that’s difficult or impossible.”