Volume VIII Number 4 August 2000

Preventive Program for Liver Flukes Important as Parasite Infections Spread Across the Country

by Jill J. Dunkel

Liver flukes, parasites once thought to be limited to specific geographies, are proving to be a much more widespread and serious problem in cattle. According to a new survey of veterinary diagnostic laboratories and veterinary schools, the cattle liver fluke has been diagnosed in cattle in 24 states -- including four new states where infections have not been found previously: Arizona, North Dakota, Minnesota and Michigan.

Historically, fluke-infected cattle have been associated with warm, wet regions and have been thought limited to areas such as the Southeast or Pacific Northwest.

"However, cattle are routinely transported from one region of the country to another," says Jay Brown, D.V.M., director of large animal veterinary professional services at Merial. "That's why producers can no longer assume that because they are not located in an endemic area, their animals are not at risk for liver fluke infection.

"Cattle brought in from unknown areas may pose a threat because, if infected, they can spread liver fluke eggs in their feces, and if conditions are right for both flukes and their intermediate host, the fluke will thrive on the farm. This intermediate host is the lymnaeid snail, found throughout the United States. Fluke transmission occurs in warm, wet conditions such as the banks of ditches and streams, edges of ponds and in puddles," he says.

Since the parasite is spreading, producers not living in traditionally fluke-infected areas need to be informed about management techniques that can keep liver flukes under control.

"Producers need a good preventive program," says Dr. Jack Malone, a veterinary parasitologist at Louisiana State University. "Implementing a strategic treatment plan will keep pasture contamination down."

To develop such a plan, a producer must understand the life cycle of the liver fluke. Initally, the process begins when cattle shed liver fluke eggs in their feces. Once the eggs hatch, the larvae penetrate the skin of the lymnaeid snail, which serves as the intermediate host. The immature flukes undergo several stages of development in the snail, 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 while they graze. Immature flukes migrate for two months in the liver tissue, then enter the bile ducts, and mature rapidly to egg-laying adult forms a month later.

Understanding the life cycle is critical because ingested flukes can only be killed after they have been in the cattle for at least eight weeks, at the earliest. Flukicide products are most effective against the mature stages, which is about 12 to 14 weeks after the end of the transmission season, Malone says.
Condemned liver due to liver fluke infection.
"If a cow/calf operator is treating at the wrong time, he's wasting his money."

The transmission season is very dependent on the weather, according to Brown and Malone. The intermediate host, the lymnaeid 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. "In Louisiana, the transmission season ends sometime between late May and July, when the ditches dry up and the mud is dry and cracked," Malone says.

Approximately three months after the end of the transmission season is the best time to treat for flukes in the southern United States, Malone says. "That is usually in the fall to winter in most places."

Liver flukes have a single transmission season, so a yearly strategically-timed treatment for the cow/calf producer will help keep pasture contamination down.

Stocker operators, however, should seriously consider treating calves for flukes if the cattle originate from fluke-infested areas, according to Malone. "Back- grounders have to potential to gain the most from treating flukes," he says. "That is where flukes will cause the most problems."

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