Why delaying fluke treatment can prove costly

Fluke can lead to the rapid loss of condition and sudden death but Zoetis vet Dave Armstrong says farmers frequently get caught out by fluke by not treating it in time.

He says farmers will often delay treatment until nearer to tupping or housing time, without realising that  immature fluke can cause production loss, as well as adult fluke.

Fluke can also lead to black disease (mainly in sheep), which is caused by the bacterium Clostridium novyi.

While this bacterium, ingested from pasture, can remain in the liver for some time with no apparent ill effect, if there is damage to the liver caused by migrating fluke, it can produce death-causing toxins.

All farms that contain wet areas could be at risk of fluke, as it’s those conditions that favour the fluke’s intermediate host – the mud snail.

“Anywhere that is wet, such as around gateways and water troughs, can provide the ideal location for the mud snail,” explains Dr Armstrong.

“If you detect fluke early and treat stock, then you can reduce the risk going forward by stopping the lifecycle before it infects pastures,” he adds.

Fluke facts

  • A single adult can shed about 50,000 eggs per day, chronic infection 100s of fluke
  • Hermaphrodite – can self-fertilise
  • Longevity – live as long as host, continually shedding eggs
  • Snail is intermediate host – spread and amplify infection
  • Fluke may be overwintered in snails as sporocysts – meaning no shedding or maturation
  • Fluke may also overwinter as metacercariae on grass

Detecting fluke on your farm

For sheep, constant management of fluke is necessary because there is often no break in the grazing cycle.

So, with no product persistent against fluke, a sheep treated one day can pick up infection the next if they are grazing infected pasture.

That means farmers need to be aware of what is happening on their farm, says Dr Armstrong.

“Although faecal egg tests will pick up fluke that are 12 weeks old, it won’t pick up immature fluke, which can cause acute disease and sudden death.

“Copro-antigen testing will pick up fluke from about six weeks old, [but] a negative test doesn’t discount the presence of fluke under six weeks of age.”

Dr Armstrong recommends using testing along with other resources, such as abattoir feedback, post mortems on sheep that have died of unknown causes, and general stock health to make a decision on treatment.

Signs and production losses in cattle and sheep

All stages of fluke will cause production loss. Immature fluke cause acute disease whereas adult fluke cause chronic disease.

Acute fluke is most prevalent throughout the autumn and winter and is caused by immature flukes (more than 2,000 in sheep) migrating through the liver. Signs include:

  • Rapid loss of body condition and poor coat quality, despite adequate nutrition
  • Severe depression
  • Inappetence
  • Weakness

In cattle, acute fluke is rare due to them having a larger liver that can tolerate a greater burden.

Chronic fluke is most likely to occur from winter through to the spring and is caused by adult fluke in the bile ducts (up to 500). Each fluke can consume 0.5ml of blood every day. Signs of chronic fluke include:

  • Loss of condition
  • Bottle jaw
  • Reduced fertility
  • Livers trimmed or condemned at abattoir
  • Anaemia
  • Terminal diarrhoea

Fluke treatment in sheep

When selecting treatment options mid-season, farmers need to consider the age of the fluke they are treating for.

Many fluke treatments focus on killing egg-laying adults, which means most immature fluke will still be present. These will continue to cause damage as they migrate through the liver and go on to develop into adult fluke.

Triclabendazole is the only ingredient that is effective against fluke from two weeks old with other products, such as closantel and nitroxynil, effective on fluke more than six weeks old. However, their efficacy at this stage in sheep is only 50-90% effective compared to 99-100% effective when using triclabendazole.

“If you’ve got a mixed worm burden, then using a broad-spectrum combination product, which contains moxidectin and triclabendazole will be effective against worms for up to eight weeks and is also effective against immature fluke.”

Dr Armstrong says other flukicides should then be used in rotation to target the stage of fluke causing the problem.

“It’s important you have a plan in place and make sure you treat for the appropriate risk at that time,” he says.

Fluke treatments

  • Oxyclozanide, albendazole and clorsulon 50-70% effective on fluke between nine-and-a-half weeks old, rising to 80-99% effective on fluke between eleven-and-a-half weeks old to 14 weeks
  • Nitroxynil and closantel 50-90% effective on fluke between six-and-a-half weeks to nine-and-half weeks, rising to 91-99% effective after
  • Triclabendazole is 90-99% effective on fluke between one-and-half weeks old to three-and-a-half weeks old rising to 99-100% effective from three-and-a-half weeks to 14 weeks

Treatment options for cattle

In cattle, a dual-purpose product, which contains moxidectin and triclabendazole has been found to be 90% effective against early immatures, 99.5% against late immatures and 99.9% against adults.

By comparison, products containing closantel killed 26.8% of early immatures, 90% of late immatures and 99.3% of adults.

Like sheep, no fluke treatment has persistency, meaning cattle can become infected with fluke immediately after treatment if they are grazing contaminated pasture.

“This is why it’s important to treat again at housing, regardless of whether you have treated midseason,” advises Dr Armstrong.

Treating at housing cleans out parasites including fluke thereby supporting a healthier transition period. The benefits of this will extend into the new grazing season when the parasite burden on the pasture will be lower as a result.

The WVSC recommends consideration should be given to treatments active against immature fluke.  

Dr Armstrong adds: “Your adviser will help you decide which products to use based on your farm’s situation.”

Fluke lifecycle

Lifecycle of fluke outside the sheep

  • Eggs hatch in spring (more than 10C) to release miracidia (motile) which must penetrate a mud snail within three hours
  • Develop inside snail (sporocyst and redia)
  • Cercariae (motile) emerge from snail
  • Encyst on grass (metacercariae)
  • Infection of a snail with one miracidium can produce more than 600 metacercariae (three or more months)

Lifecyle inside the sheep

  • Once ingested, metacercariae excyst in small intestine and the immature fluke migrate through the gut wall to penetrate the liver
  • The immature fluke tunnel through liver for six to eight weeks before entering bile ducts where they reach maturity
  • Time from infection of cattle or sheep to adult egg-laying fluke is 10-12 weeks
  • Little/no development of immunity

Monitor fluke and worm levels in your area

Farmers can monitor fluke and worm levels in their area by clicking on an interactive map on the Parasite Watch website.

Parasite data from farms involved in the Zoetis Parasite Watch scheme will be updated regularly, which will allow farmers to see if there are spikes in certain parasites throughout the year in their area and enable them to take appropriate action.

To use the map, click on a farm in your area and details of any parasites that have been found as well as when they were detected will be displayed.