Worm Control in Cattle

Maintaining Thrive and Avoiding Anthelmintic Resistance on Farm – The Do’s and Don’ts

Controlling lung and gut worms on farms is essential. However, it is also a fact that gut worm populations on many farms have been developing resistance to the products used to control them. Resistance is defined as when less than 95% of the gut worm population is killed by a dosing product. Research carried out by Teagasc on 17 dairy calf-to-beef farms showed resistance in the gut worm population on 100% of the farms to ivermectins (clear), with 60% resistant to albendazoles (white), and 18% resistant to levamisoles (yellow).

Minimising Anthelmintic Resistance

The key to reducing the chance of worms developing resistance to a product on your farm is to minimise their exposure to that product and using the product as per the manufacturer’s instruction. This can be done by reducing the worm burden and following simple protocols at dosing. Here are some simple strategies:

1. Leader follower system

The typical worms that need to be controlled in cattle in the summer months are lung worms and gut worms. Both build up on pastures as the grazing season progresses. Older animals build up immunity to lung and gut worms over time and therefore shed less larvae, however, younger animals are more naive and shed a large number of larvae. This is why the leader follower system can help suckler farmers to reduce the worm burden on farm. While the calf is on the cow, the cow is reducing the burden; once weaned the cows should follow younger animals to eat the worms and kill them, this reduces the larvae on pasture

2. Mixed grazing

Sheep and cattle generally host different worm species. Grazing sheep and cattle together on the same pastures reduces overall worm burdens, which in turn reduces the number of treatments required for both species, thus reducing the farms worm populations exposure to products.

3. Graze younger animals on low-risk pastures

In order for animals to develop immunity, they need to have a level of exposure to lung and gut worms, therefore, some exposure is good. If that exposure can be kept low for longer, then you reduce the build up of worm in the animals, maintain performance and reduce the need to treat until later in the year. Low risk pastures are:

• Reseeded paddocks: Land may have been ploughed – burying the larvae. It has not been grazed for 6-8 weeks, reducing the burden
• Pastures grazed by other species
• Clean silage aftermath: Areas where silage has been cut and removed, reducing larval presence.

4. Monitoring animal performance and faecal egg counts

• If animals are thriving as expected— for example, if your suckler calves normally gain  1.0 kg/head/day and they are on target, they probably do not need dosing.
• From June onwards, take faecal samples to monitor stomach worm egg build-up. Once egg counts rise above a certain threshold (200 eggs per gram), treatment should be considered.
• Be careful with lungworm!

In the case of lung worm, you cannot depend on faecal sampling. It is the larvae that is picked up off pasture, that passed into the lung that cause damage, and only when they mature to adultsdo they lay eggs, these eggs are then coughed up and swallowed, they hatch in the intestine, produce larvae which is then passed out in the dung. Therefore, significant lung damage has already occurred before larvae will be detected in the dung which will ultimately affect thrive.

5. Proper dosing practices

When dosing, regardless of the product used, ensure to:

• Dose according to the correct weight; underdosing increases the risk of resistance developing.
• Use products that are in date and have been properly stored.
• Calibrate your dosing gun regularly.
• Shake the container well before use.
• Administer the dose as prescribed on the datasheet.

How to check if the product has worked

1. Conduct a Faecal Egg Count Reduction Test (FECRT): This involves collecting individual faecal samples from 10 -15 animals (once you know they have a high worm count), dosing them with your product of choice, then if using a levamisole re-test 7 days later, if using an ivermectin or albendazole re-test 14 days later.

2. Re-test the exact same animals. Compare the egg counts pre and post the dose, if the count does not reduce by >95% you are developing resistance.

Note: Currently there is a project being coordinated by Teagasc to do a Faecal Egg Count Reduction Test, test for free, its called the Managing Anthelmintic Resistance in Cattle Sustainably (MARCS) Or

1. Use a pooled faecal egg sample. Test a group of animals, dose them and then retest (as above for different products).  This option is not as accurate as option 1.

Product choice

The best product is the one that kills the worms on your farm. Use products that you know work on your farm, based on the tests above. Using an effective product is more important that rotating between all products.

Paddock choice after dosing

Move animals back to the paddock they came from or onto another paddock that is being grazed over the grazing season. It’s not advisable to move them to a “clean” pasture. A clean pasture is considered to be one not grazed this year, one that was used for silage or a reseed paddock as discussed previously. The science behind this is to control the rate at which resistance develops Figure 1 explains the concept. If animals are returned to pastures previously grazed as in good refugia, the resistant worm are mixed with susceptible worms left on the pasture and are thus diluted, so resistant worms breed with susceptible worms and resistance develops more slowly If cattle were put on a clean pasture after treatment and only passing resistant worm onto a pasture with very few susceptible worms remaining on the pasture, the resistant worms will breed together and become dominant and resistance will build much faster in the worm population, this is show in the poor refugia example.