“Worms make sheep and cattle feel sick and as a result suppress appetite.
“A large worm burden can cause gut upset and protein loss resulting in poor growth,” Dowling said.
To reduce worms’ impact farmers need to understand how pasture management can lead to lower worm burdens and understand what effective drenching means.
“About 5% of a worm population lives inside animals. The remaining 95%, comprising eggs and larvae, is in pastures. One wants to reduce the number of larvae in pastures that animals can consume and manage drenching so you don’t create drench resistance,” Dowling says.
Pasture management is key to managing worm loads.
Mixed stock classes grazing and multi-species grazing are critical tools for farmers to combat large worm loads in their pastures.
Research shows the immune system can make life more hostile for worms and is a heritable trait.
Studies show that if Telodorsagia are fed to a 30kg lamb about 30% of those worms can reach adulthood. However, if the same is fed to a ewe then, depending on genetics, only about 2-3% of the worms will reach adulthood.
Worms are host-specific so cattle eating sheep worm larvae do not seem to be affected and vice versa. That is valuable when using different species grazing paddocks to help manage the worm challenge and is also a significant consideration in systems with monocultures of young stock. A farm system with only young cattle will gradually become more wormy, Dowling says.
Ewes can follow lambs on pastures because lambs will eat the newest growth of grass and ewes will clear pasture of worms on lower parts of the grass and create relatively worm-free pastures for lambs that might follow on new growth.
If, in such a grazing system, ewes have a high faecal egg count then the cause needs to be investigated because they could be under some form of stress or potentially have a condition affecting their immune system and their ability to fight off worms. The FEC is a good indicator of the adult worm burden in sheep.
With cattle, calves should follow cows on a specific piece of pasture because cows will create relatively worm-free pastures for calves. FEC is not as reliable in cattle but live weight gain or lack of it is the best indicator of parasitism, Dowling says.
It is a misconception that a drench directly kills worms.
“A drench needs to reach the liver, which metabolises it to form the chemicals which kill the worms,” he says.
“To determine if drenching was effective in a herd do a faecal egg count seven to 14 days after drenching.
“If there are eggs present then they are likely to be from worms not killed by the drench.”
The use of combination drenches is advocated to slow the development of drench resistance because using a single drench means worms can develop resistance faster by facing only one threat. However, a combination drench means worms find it tougher to figure out how to develop resistance to multiple enzymes.
Three other aspects that need to be kept in mind for successful drenching are preventative drenching, quarantining new stock and creating a refugia.
Preventative drenching is a programme of four or five drenches given to lambs or calves at four-weekly intervals. The programme often starts at weaning. After the initial four or five drenches the intervals should be stretched out to prevent over-drenching.
Quarantining new stock is necessary because it prevents the introduction of another farm’s resistant worms. A quarantine drench followed by seclusion of stock from established mobs is necessary. About 10 days after drenching a FEC can be done to determine if drenching was effective and whether the new stock can be moved in with established mobs.
A refugia is a part of a worm population that remains unexposed to drench and completes the life cycle. They are critical because they can breed with drench resistant worms to ensure complete resistance to drench does not develop. On a farm, creating refugia simply means leaving some animals undrenched. The refugia plan for each farm is different and needs careful thought to both be successful and not create clinical parasitism.
