Anthelmintics: when should I use a wormer?

What are Anthelmintics?

Anthelmintics or wormers are widely used across the globe, and a lot of the time can be misused causing resistance issues. Anthelmintics are drugs used to treat infection with nematodes (roundworms like Ascaradia galli in chickens), cestodes (tapeworms like Moniezia expansa in sheep) and trematodes (flukes like Calicophoron daubneyi in livestock).

How do I know which wormer to use?

Firstly, how do you know your animals need to be wormed? Have you noticed symptoms or seen worms in faeces? Or perhaps as part of your management scheme, you get your animals tested regularly?

This is probably your best solution, paired with testing at times of the year when worm burdens are expected to increase (when animals are put out to pasture, for example). At RRL we provide Worm Egg Counts, and Worm Egg Count Reduction tests to see if your wormer has been effective. We also provide Rumen and Liver fluke testing as well as lungworm testing. Before you decide to worm your flock, please get them tested. Worming a flock which has a low burden is unnecessary and overworming can lead to resistance and may worsen your problems.

Once you have your results, consult with your vet to find the most suitable wormer for the species of worms found. Ensure to follow their advice for most effective treatment, and get a follow up Worm Egg Count Reduction test to ensure your efforts were not in vain.

Do I have to use wormers on my animals?

Farmers who own organic animals or smaller holdings may not want to use wormers due to withdrawal periods and associated costs, as well as not seeing any real problem from infections. It is likely that animals with low levels of infection may live quite happily with no obvious signs of discomfort (of course apart from income loss as they will eat more to put on or maintain the same weight). Chickens can quite happily survive with some worm infections their entire lives but there will be an economic cost. However very high burdens or certain worms, for example lungworm, at lower levels can cause discomfort and mortality animals. It is up to you to do some investigation into your personal situation whether you would like to use a wormer as part of your worm management.

There are alternatives to using wormers. Mostly these alternatives are management strategies and will not work for all types of animals or in all situations.

Options are but not limited to:

  • Weaning young animals early on and moving naive animals to ‘clean’ pastures (a pasture which has not been used for animals in a while). This is more easily done with roaming animals like sheep or cattle but more difficult and potentially costly with chickens who need houses.
  • Mixed grazing will reduce number of animals available to become infected/propagate the parasite as many worms are species specific.
  • These are only options if you are a farmer that has a lot of land and can leave an area empty for a while. If you are more confined with space you may not have these options available. And they are not foolproof, they need to be well planned, monitored and managed.
  • Leaving land for a season to ensure there are no hosts or intermediate hosts for the worms to replicate in.
  • Keeping animals in their birth age groups so all at same level of infection/can’t be infected by others.
  • Using adults to graze infected pastures to act as “hoovers” and thereby reduce parasite numbers on pasture.

If you decide to use wormers as part of your animal management, you can utilise lots of online resources like this guide SCOPS has produced for sheep.


Holden-Dye L, Walker RJ. Anthelmintic drugs and nematicides: studies in Caenorhabditis elegans. In: WormBook: The Online Review of C. elegans Biology [Internet]. Pasadena (CA): WormBook; 2005-2018. Available from:,FECs)%20in%20determining%20treatment%20requirements.