Health Care

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Worming Recommendations For Adult Horses

Rotational and calendar-based Worming strategies are outdated and no longer recommended. Instead, owners are now encouraged to work with their veterinarians to implement diagnostic driven worming plans. The overall aim of a diagnostic driven program is to reduce the risk of parasite related disease in our horses.

In order to achieve this, owners need to:

  1. Make sure the wormers we are using are effective
  2. Avoid unnecessary worming
  3. Reduce the risk of parasite transmission in the environment

Are my worming treatments working?

Unfortunately, due to the indiscriminate use of wormers in the past three decades, equine worms are now widely resistant to commonly used wormers. Resistance refers to the parasite’s ability to survive a worming treatment. The overuse of wormers means that we have been very successful at killing the susceptible worms, but what’s left behind are the resistant worms. These resistant worms then reproduce which leads to more resistant worms developing in your horse.

No horse property is free of drug-resistant parasites and no worming product is free of resistant issues. The only way to know if the wormer is effective in your horse, is through resistance testing.

Resistance testing refers to the percent reduction in parasite eggs between a sample collected just before worming and another sample collected 14 days post-worming. It’s really simple to do and your vet will be able to guide you with the appropriate reduction percentages for each worming product.

Avoid unnecessary worming

The majority of worming treatments are administered unnecessarily. We won’t ever beat worms and we shouldn’t be trying to. Horses have evolved alongside their worms for centuries and there’s even some evidence to suggest they might be beneficial. Studies in other species have found that parasites are important in the development of immunity and prevention of auto-immune and allergic diseases in the host 1,2. A very recent study also found that reducing the frequency of worming is not associated with any negative health risks to the horse3.

Most adult horses, on well managed properties are considered low risk for parasite-related disease and should not require more than one worming treatment per year. Strongyles (cyathostomins) are arguably the most important cause of parasitic disease and they should be the focus of any worming strategy in mature horses.

Adult horses (more than six years old) develop robust (but variable) immunity to cyathostomins and we frequently forget they can tolerate relatively large burdens of cyathostomins without developing clinical disease 4. Ivermectin and moxidectin are generally the most effective against strongyle parasites. Other wormers (such as pyrantel) may also be effective, but should only be used if it’s shown to be effective on your property.

How to worm adult horses:

  1. Perform a faecal egg count (FEC) three to four times per year.
  2. Based on the FEC result, determine whether your horse is a low or high shedder.
    1. Low: 0-500 eggs per gram (EPG). No additional treatment is required.
    2. High: >500epg. Consider an additional 1-2 treatments through the grazing season. High Shedders may require treatment up to 4 times per year. Treatment with Ultimum (moxidectin) may be recommended for horses with suspected encysted small strongyle larvae.
  3. Moxidectin should NOT be used more than once per year, unless you are specifically told to do so by your veterinarian.
  4. Ask your vet to perform a faecal egg reduction test (FECRT) to make sure the wormer you are using is effective against the parasites on your farm. Cases of ivermectin and moxidectin resistance have been reported in Australia and a FECRT should be performed annually with a different class of wormer tested each year.

Adult Worming Recommendations

Reduce the risk of parasite transmission in the environment

Minimising pasture contamination and implementing these 10 simple tips will help you reduce the need for frequent worming treatments on your property.

  1. Reduce stocking density.
  2. Maintain consistent horse populations.
  3. Use FEC to identify animals that persistently shed parasite eggs onto pasture.
  4. Poo-pick paddocks at least twice weekly. Ideally everyday!
  5. Rest and rotate pasture (especially when stocking densities are high).
  6. Avoid harrowing pasture (this will only spread infective larvae across your paddocks).
  7. Keep manure piles away from grazing areas.
  8. Compost manure and frequently turn compost piles. Composting manure is very effective in killing parasite eggs. It takes approximately one week at core temperatures of 40C or above to kill all larvae. This needs to be done BEFORE the composted manure can safely be spread onto grazing areas.
  9. Co-graze with ruminants.
  10. Quarantined/new horses that are below 2 years of age should be given 2 separate worming products on arrival i.e. Ultimum (moxidectin + praziquantel and Strategy-T (Oxfendazole + pyrantel) one after the other. For horses over 2 years of age treat with Ultimum or as recommended by your veterinarian. Quarantine them for at least 3-5 days to allow for any worm eggs to be deposited in the yard. Collect all manure and dispose of manure rather than spread it on your paddocks.


The rise in resistant parasites is an alarming trend and there is no one-size-fits-all approach to Worming your horse. Owners should consult with their veterinarian to develop a diagnostic driven Worming plan. The ultimate aim of these plans will be to develop a strategy which prevents the development of parasite-related disease whilst minimising the use of wormers.


  1. Furze, T H, ME S. Amelioration of Influenza-Induced Pathology in Mice by Coinfection with Trichinella spiralis. Infection and Immunity. 2006 Mar 1;
  2. Maizels RM, Yazdanbakhsh M. Immune Regulation by helminth parasites: cellular and molecular mechanisms. Nat Rev Immunol. 2003;3(9):733–44.
  3. Nielsen MK, Gee EK, Hansen A, Waghorn T, Bell J, Leathwick DM. Monitoring equine ascarid and cyathostomin parasites: Evaluating health parameters under different treatment regimens. Equine Vet J. 2020;
  4. Nielsen MK. Sustainable equine parasite control: Perspectives and research needs. Vet Parasitol. 2012;185(1):32–44.

About the author

Dr Tania Sundra BSc.(Hons) BVMS MANZCVS (Equine Medicine)

Avon Ridge Equine Veterinary Services, Western Australia

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