Anthelmintics: When Should I Use a Wormer?

What are wormers?

An anthelmintic, or wormer, is a drug which treats infections of parasitic worms. These could be nematodes — roundworms like Ascaradia galli in chickens, cestodes — tapeworms like Moniezia expansa in sheep, or trematodes — flukes like Calicophoron daubneyi in various livestock species. Anthelmintics are widely used across the globe, but misuse is leading to the growing issue of resistance.

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 and have had a positive result.

If worm egg counts are not part of your normal operations, you can use them as a one-off test if you suspect worms. While this can be valuable in some situations, regular testing will help to catch infections early. This can prevent productivity losses, or even worse, animal loss.

Testing at times of the year when worm burdens are expected to increase can be very beneficial. This could be when they are put out to pasture, or when introduced to new land. If you know how many worms and what species are infecting your animals, you can use targeted treatments to manage them. At RRL we are experts at conducting Worm Egg Counts. We also perform Worm Egg Count Reduction tests to see if your wormer has been effective. We provide Rumen and Liver Fluke testing as well as lungworm testing, as these can be very dangerous for livestock.

Resistance to worming agents

Always test animals before you decide to worm your flock or herd. Worming a flock which has a low burden is unnecessary and over-worming can lead to resistance. It may worsen your problems if your animals catch develop a resistant strain of parasite, and if everyone over-worms their animals, eventually anthelmintics will not be effective at all. While this may seem far off now, already resistant populations of worms are appearing all over the world.

Once you have your results, consult with your vet to find the most suitable wormer for the species of worms found. Follow their advice for most effective treatment, and get a follow up Worm Egg Count Reduction test to make sure your efforts weren’t in vain.

Do I have to use wormers on my animals?

Farmers who own organic animals or small holdings may not want to use wormers due to withdrawal periods and associated costs. It might be that you are not seeing any real problem from infections and so don’t need to worm.

Almost all animals go through life with some worms at low levels. Low levels of infection don’t always cause obvious signs of discomfort, although usually, animals 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 is usually an associated economic cost. However very high burdens, or certain worms even at lower levels, for example lungworm, can cause discomfort and mortality in animals. It is up to you to do some investigation into your personal situation and whether you would like to use a wormer as part of your worm management.

Alternatives to worming

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

Options include:

  • Weaning young animals early on and moving naïve animals to ‘clean’ pastures. This is a pasture which animals haven’t grazed on in a while. This generally easier to do with roaming animals, like sheep or cattle. It can be more difficult and potentially costly with chickens, as they 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. Also 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 age groups so they are all at same level of infection. This also protects them from infection 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, there are many online resources you can find. Like this guide SCOPS has produced for sheep. Alternatively, your livestock vet will be able to give advice on up-to-date treatments and best practice.


Holden-Dye & Walker, WormBook: The Online Review of C. elegans Biology. Pasadena (CA): WormBook; 2005-2018.