B+LNZ Wormwise Programme Manager Ginny Dodunski urges farmers to think twice before drenching for fluke this autumn and consider their specific farm situation.
After a summer where most of New Zealand has grown good feed, the temptation for farmers in liver fluke areas to reach for a fluke drench "just in case" may be strong - but this is a season where your farm's individual situation really matters. Here's what to consider.
Summer conditions matter
Liver fluke challenges tend to be worse following dry summers, when sheep are forced to graze hard down into swampy areas and waterways - the very habitats favoured by the mud snails that carry the infective stage of fluke.
Mythbuster – it’s not the snail itself that animals ingest to become infected with fluke. The infective stage is actually a cyst that forms on grass blades after a mobile stage of the fluke life cycle that has wriggled out of the snail’s body (ugh!) to settle on the grass blades. These cysts can last for many months on pasture but are susceptible to freezing and drying.
So, your ewes actually have to graze grass near or in waterways, to become infected. They don’t pick up fluke directly from drinking water.
After a well-fed summer with good pasture cover, your ewes are likely to have grazed away from those wet margins, meaning fluke exposure is likely to be lower than in a drought year. This is good news - but it doesn't mean fluke can be ignored entirely on farms where it has been a problem before.
Does your farm actually have a fluke problem?
Many areas of New Zealand have the climatic conditions to support mud snail populations, but not every farm in those regions has a liver fluke problem. The key is to find out what's happening on your property.
The best starting point is simple and low-cost
- When you process tail-end or cull ewes this autumn, open up the underside of their livers and have a look. Adult liver fluke are flat, grey, leaf-shaped parasites roughly the size of your thumbnail - quite visible in the bile ducts when present.
- If you still have cull ewes to send for slaughter, you can ask for a fluke check at the works. If you are having liver trace elements tested at the same time, your veterinarian can add a request for a fluke check to the submission form.
- If your lightest, most susceptible ewes are not carrying fluke by early winter, it is very unlikely that liver fluke is present in numbers that warrant treatment across your flock.
Why ewes in particular?
A healthy, well-functioning liver is essential for ewes rearing multiples through late pregnancy and lactation. Liver fluke damage - caused as the parasite migrates through liver tissue to reach the bile ducts - can undermine ewe performance at exactly the time she needs to be firing on all cylinders. Ewes that have concurrent liver damage from facial eczema, can be doubly affected.
Unlike gut worms, most adult sheep do not develop meaningful immunity to liver fluke - all ages remain susceptible throughout their lives. Little is documented about what accounts for the individual variation seen within a ewe flock, and between farms in the same district for levels of fluke infection.
Cattle can develop some immunity to liver fluke. It is common for ‘fluke’ to be listed in slaughter defect reports, based on scarring from fluke which are no longer resident in the liver of two and three – year – old cattle. This is especially true of trade cattle that were born on a farm with fluke and finished elsewhere.
A large slaughter study of Scottish cattle highlighted that while livers from some farms had high levels of fluke scarring, days from birth to slaughter on these farms could be shorter than on farms where livers had less fluke scarring, meaning that nutrition and management probably have a large role to play in animals’ ability to tolerate fluke infection.
If fluke is confirmed onyour farm
In an ideal world, any accumulated fluke would be removed with one treatment, once temperatures in the snails’ habitat are consistently 10 degrees C or less. Snail activity ceases at this point and further infection of grass stops. For northern parts of New Zealand, this means the fluke life cycle ‘almost never’ stops, making a one-treatment protocol more difficult.
Going into winter, or maybe at scanning, treating to remove an accumulated fluke load can makes sense, as this eases the burden on ewes ahead of a nutritionally demanding period. Certainly, heavily pregnant multiple ewes do not like sharing their liver with fluke.
Important – use the right product
Most standard worm drenches have no effect on liver fluke. Two main products are used for sheep in New Zealand:
- Triclabendazole - kills all life stages
- Closantel - kills fluke from eight weeks of age. Note that all products registered in New Zealand also contain abamectin; the impact of using abamectin regularly in ewes needs to be considered through the lens of promoting resistance in other worm species.
Albendazole, still a component of some oral combination sheep drenches, can kill flukes three months and older. Hence lambs on a regular drenching programme may not need specific fluke treatment if the product/s used contain albendazole.
Avoid relying on the same fluke product every time. Triclabendazole-resistant fluke has already been confirmed on at least one New Zealand sheep farm, and resistance is common overseas.
The bottom line
After a good summer, fluke challenge in your ewe flock is likely to be lower than after a drought - but the only way to know for sure is to monitor. Check those cull ewe livers, talk to your vet, and treat only if there is genuine evidence of a problem on your property. Unnecessary fluke treatments waste money, and crucially increase the risk of resistance developing to the limited range of flukicides we have available.