This article contains the 6 simple steps you can take to get your worming program to a great start. Spring is the best time to kick-start Evidence Based Worming on your horse properties. Most horse managers are, by now, very familiar with the narrative that small Strongyles – Cyathostomins – are becoming resistant to all of our current chemical anthelmintics. But, to date, this awareness is not yet transitioning into a more judicious use of anthelmintics across the industry and we really need to get our skates on!

Download our 6-Step Evidence Based Worming Guide. (sign up required)

My scary story…

In the mid 1980s I acquired a beautiful young warmblood gelding and, unknowingly, also acquired his cargo of resistant worms. No matter what I did, Argell was lethargic, disinterested and he couldn’t maintain good condition.

At the time, I knew nothing about worm ‘resistance’ but I maintained a regular, 6-weekly anthelmintic treatment according to accepted protocol. It always puzzled to me that Argell constantly voided thousands of small, dead Cyathostomins following every treatment. How could this be, given his non-pasture housing and careful health care?

My veterinarian, the late Dr Richard Chapman, soon solved the mystery as he’d been witnessing the progress of worm resistance in horses for some time.

For Argell, it was pure luck that the first of the macrocyclic lactone family, Ivermectin, was just becoming available on the market in a form that could be used safely in horses. Richard acquired and administered Ivermectin and, within a relatively short time, Argell was thriving.

Knowing what I now know about worm resistance, Argell was very lucky but, uh oh… Here we go again!

30 years on from his story, we are again facing the growth of worm resistance. This time, however, the resistance is to the anthelmintic group of drugs that saved his life – the macrocyclic lactones. The significant and concerning difference is that, today, there are no new chemical anthelmintics coming onto the market.

Left unaddressed, worm resistance will continue to advance, having the potential to become an enormous economic, logistical and emotional burden within the horse industry. Significantly, our veterinarians will bear the brunt of this problem and will likely be spending more of their precious time working to save non-thriving horses such as Argell.

Some background

Two excellent articles by Dr Anne Beasley, a parasitologist and researcher from the University of Queensland’s School of Veterinary Science, have appeared in the May 2015 (Resistance on the Rise) and June 2018 (The Big Problem with Small Strongyles) editions of Horses and People.

Read A New Era in Worm Control here.

They are highly recommended as a background to this present article which aims to increase uptake of EBW by providing practical strategies for all horse managers so that, together, we will succeed in slowing – or even halting – the insidious progression of resistance.

Opinions abound about how best to deal with increasing resistance but, in my view, the way forward must be underpinned by good science.

Fortunately, veterinary parasitologists have come up with a practical way to combat resistance but it comes with an alert: It requires a considerable shift in our current mindsets around traditional worm treatment methods.

Evidence Based Worming

EBW is based on the humble faecal egg count (FEC) test that has been around for many years.

This test is proving to be a particularly useful means for combatting resistance when used as an environmental worm management tool as it can identify ‘high egg-shedding’ horses that contribute most to pasture contamination with Cyathostomin eggs.

High egg-shedding horses are those with Cyathostomin egg counts of 200 eggs per gram (EPG) or higher, and they comprise only 16-20% of all horses.

Once identified, these horses are targeted for anthelmintic treatment and, in this way, EBW results in a more judicious use of anthelmintics so they will remain viable into the future.

How does EBW counteract the development of resistant worms?

The less frequently Cyathostomins are exposed to chemicals, the longer resistance takes to develop.
EBW works by looking after the population of non-resistant worms known as ‘refugia’. Now, of course we can’t just ask them “are you a resistant worm? (here… take this lethal concoction…)”, “or a non-resistant worm? (please make yourself at home in my horse…)”. We have to approach our management scientifically.
Basically, it’s a numbers game: the larger the percentage of refugia there are in the environment, the smaller will be the percentage of resistant worms – it must add up to 100%.

Is EBW for all horses?

No. It’s important to know that EBW is aimed at healthy, adult horses, ie, most horses.

What we tend to forget is that horses and their parasitic worm passengers co-evolved and that they may even have a role in boosting the horse immune system. So, if a horse does record some eggs in a FEC test but is obviously very healthy, there is not usually much benefit in an anthelmintic treatment. We must also remember that horses may still have worms even though FECs never spot an egg in a sample!

  • EBW is not for foals and young horses up to 3 years. Foals require special attention to the potentially lethal roundworm (Parascaris equorum) and a FEC may not necessarily identify that these parasites are present. The other reason EBW should not be relied upon for youngsters is that they are in the process of developing their natural immunity to worms which is important for their ongoing health. Young horses will usually have high egg counts and, therefore, should be treated with anthelmintics 3-4 times a year.
  • EBW is not for breeding mares, aged and immuno-compromised horses – these also require special treatment. Although it is still good management to monitor these horses with FECs, this group will benefit from a tailored worming approach. Horses with Cushings Disease come into this category.

Most importantly, veterinary advice always takes precedence over FECs where there is doubt about a horse’s wellbeing.

It doesn’t have to be confusing!

The key to effective EBW is to sneak a peek at the Cyathostomin lifecycle in order to target their vulnerabilities. Their strengths lie in their ability to lay thousands of eggs daily when conditions are just right for providing the greatest chance of survival for their offspring. They love –

  • mild-warm, moist conditions (regardless of season) conducive to good pasture growth
  • high stock density which gives a greater chance for their infective offspring to be ingested
  • a lack of cross-grazing with other species because they can’t complete their lifecycles inside a cow or a sheep
  • poor pasture hygiene – lots of horse manure provides great shelter for developing worms
  • pastures that are never spelled/rested
  • inappropriate anthelmintic usage leading to resistance.

Effective, integrated pasture hygiene will address most of these situations, but EBW can strike the lethal blow. It’s time to get on board with EBW the easy way!


Step 1: Establish your baseline.

Obtain a FEC for each horse that you are responsible for. The best time to do this for the first time is in spring after the weather warms up a bit and there has been a bit of rain. This is when Cyathostomin egg counts will likely be at their highest.

Step 2: Treat the high-shedders.

Horses with a result of 200 EPG (eggs per gram) or higher, should be treated with an anthelmintic. (Spoiler Alert: if you have been routinely treating horses for a long time, it will require all of your self-control not to treat your horses when they register egg counts less than 200 EPG!)

Step 3: Set up a record for each horse.

A simple Excel spreadsheet is perfect for this. (Hint: name your paddocks and match these this with your horse results – this will alert you that a particular paddock might have a worm issue)

Step 4: Re-test high shedders at the end of the treatment efficacy period.

This will assist you to monitor what is happening with their egg counts, so you can treat them again if necessary. For various reasons, some horses will always be high-shedders.

Step 5: Establish a FEC routine.

Low-shedding horses (less than 200 EPG) in the herd will need less frequent FECs; high shedders will need more frequent FECs. You will build up a picture of the shedding status of each of your horses over time.

Step 6: Treat all horses once a year (usually in late autumn) for other parasites.

It is important not to forget that even low egg-shedders will need the occasional targeted treatment to remove tapeworms and bot larvae.

How often should I test?

Initially, aim for 4 times a year,

  • more frequently during the warmer, wetter conditions; less frequently when it’s dry and cold, or dry and hot.
  • test the high egg-shedders more frequently; low shedders, less frequently.

Here is a simple strategy

  • Gear up in Spring
  • Exercise restraint in Summer
  • Treat for other parasites in late Autumn
  • Have a break in Winter

The best way to remember is:

“When the grass is growing actively, the worms are on the move.”

The simplicity of this easy-to-remember sentence has broad applications because it encompasses:

  • seasonal variation
  • tropical climates
  • irrigated pastures
  • pasture around damp courses such as dams, rivers and streams.

What happens to the worms in drought?

From a horse worm perspective, there is one small piece of good news associated with drought – the chance of ingestion of infective L3 Cyathostomin larvae will be quite low at this time. The refugia populations we want to look after will also be under pressure at this time so, during drought, it is important to only treat horses when it is absolutely necessary.

When the drought does end, this is the time to be worm-vigilant as Cyathostomins have an impressive evolutionary trick – they can encyst themselves (burrow) within the lining of the horses’ digestive tract and can stay this way for long periods – 2 years or more – emerging when environmental conditions improve.

Even during a drought, there is one equine parasite that cannot be forgotten. The eggs of the foal roundworm, P. equorum, are extremely resilient and can survive in dry environments for years. You must always assume their presence and treat your young horses accordingly.

Who does FEC testing?

FECs can be undertaken by your veterinarian, the NSW Department of Primary Industries and private providers, such as myself.

Shop around, prices vary greatly, particularly if your vet sends the samples to a diagnostic laboratory.
They can also be undertaken by you, the horse owner, with the right equipment, access to a microscope and a small amount of training.

Undertaking FECs for your own horses will put you into the driver’s seat of Cyathostomin management, result in the use of less chemicals and save you money on unnecessary anthelmintic treatments over time.

Evidence Based Worming has released a stress-free, do-it-yourself guide to undertaking faecal egg counts for horses – Count Your Eggs Before They Hatch which you can purchase here.

Further information:

  • Count Your Eggs Before They Hatch
  • Handbook of Equine Parasite Control (Martin K. Nielsen and Craig R. Reinemeyer) 2018
  • American Association of Equine Practitioners (AAEP) Parasite Control Guidelines