Worming Horses: How to slow dewormer resistance by up to 300%

Worming Horses: How to slow dewormer resistance by up to 300%


In a four to six times a year interval de-worming regimen, horse worms are continuously exposed to de-worming medications so it should come as no surprise that they are constantly forced to genetically adapt to the deworming drugs to survive.

Best practice guidelines have been designed to address the horse health and welfare problem of de-wormer drug resistance in worms.

Current guidelines move far away from the traditional, calendar-based interval deworming strategy because it is the major cause of the health and welfare problem of de-wormer resistance. 

The modern surveillance faecal egg count (FEC) worm control approach has been recommended by experts for many years. Recently Belgium horse scientists used model calculations to confirm the approach is effective in delaying the development of drug resistance.


 The Study

The long-term study clearly indicates that the up-to-date best practice targeted (or selective) de-worming protocol for adult horses (over 3 years) will slow the development of drug resistance in equine strongyle worms.

Thomas Geurden, DVM, at Zoetis and his fellow researchers in Belgium and at the Gluck Equine Research Center, studied Belgium draft horses living at pasture on two horse farms that practiced the old intensive, calendar-based de-worming. The horses there had been treated routinely either four times a year with moxidectin or six times a year with ivermectin.

Based on the results targeted de-worming pays off in the long term and reduces the risk of dewormer resistance many times over.

In this 3 year long-term study researchers found that targeted worm control slowed the rate of drug resistance in intestinal worms by 200 to 300%

Researchers also found treating the entire herd four to six times a year leads to de-wormer resistance in horse worms 2-3 times faster than less frequent de-worming driven by faecal egg count tests.

Furthermore, there was no increase in parasitic worm-related disease over the three-year period in either group.

Research scientists were able to convince the owners that deworming all horses four to six times a year was not effective for worm control, and that faecal egg count tests support a tailored worm control program to reduce treatment frequency with ivermectin or moxidectin and the odds for resistance.

You can read more about the study. “Three-year study to evaluate an anthelmintic treatment regimen with reduced treatment frequency in horses on two study sites in Belgium,” Veterinary Parasitology, October 2021


 So, does my horse need egg counts?

Surveillance deworming calls for less frequent treatments and the use of quality faecal egg counts (FECs) to determine treatment intervals for each adult horse.

When faecal egg counts are used for small strongyle worm surveillance, the main purpose is to find out the level of egg contamination from an animal. The approach targets the horses that shed most of the worm eggs in their manure onto pasture. A horse that spreads most of worm eggs reinfects itself and infects other horses with worms.

Eggs counts can not tell you how many worms are inside your horse. You can swipe through to find out why egg counts do not correlate with the number of worms in your horse - here

Targeted (or selective) worm control reduces treatments and selection for dewormer drug resistance. 

Recommendations for targeted worm control is currently being promoted only for mature horses (over 3 years) and is based on faecal egg count(FEC) analysis of all horses on the property. All horses should benefit from a foundation de-worming treatment twice a year but horses with consistent high egg counts are targeted for an additional treatment. The aim is to reduce the worm egg output on pasture. 

Several studies have shown that adult horses tend to maintain roughly the same level of egg output. Horses that consistently have very low or even 0 eggs counts  often maintain this level for most of their lives but sometimes they can shift to a high egg shedding category due to stress. Horses with high egg counts generally do not shift to low shedders. Surveillance egg counts monitor egg shedding levels for a tailored worm control strategy.


Targeted Preventative Care

Did you know most horses are low egg shedders - about 60% , while around 20 to 30% are moderate. A minor portion of a herd (10 to 30%) are high egg shedders and they are responsible for more pasture contamination with worm eggs. Treatment for horses with high egg counts effectively provides overall reduction of egg shedding by the herd and lessens de-wormer resistance.

Have you got a low, moderate or high egg shedding horse? You can find out with the Super Surveillance Faecal egg count test. 

  • Use roundworm vs strongyle results to guide dewormer selection for foals and yearlings

    Collect faecal sample using Kit contents

  • Send sample in Express Post bag included

    Send sample in Priority Paid mailer bag included

  • Receive digital image and test results straight to your inbox with exclusive What Do My Results Mean? Guide

    Receive digital image and test results straight to your inbox with
    exclusive What Do My Results Mean? Guide

  • Apply best practice recommendations to make positive changes to your worm control approach

    Apply best practice recommendations to make positive changes to your worm control approach


It`s the precision of the faecal egg count (FEC) test that counts

Understanding faecal egg count (FEC) methods can help you make a better and smarter choice for you, your horse or herd. There are many available egg counting techniques and modifications. It can be challenging to choose the best.

Here`s what you need to know

Diagnostic tests are never perfect. Faecal egg count testing is subject to many sources of variability and errors are introduced by differences in test protocols, the technique or training. The commonly used old McMaster manual counting microscope is faster, simpler, and more convenient but is particularly susceptible to these variables, resulting in its generally accepted poor performance. 


Manual Microscope Counting?

Here are just a few examples of major drawbacks associated with manual methods.

“Modified” microscope tests that are not scientifically validated for their performance. For example - eliminating the filtration step to save time and improve convenience. 

Systemically overestimating all counts due to using a Whitlock, rather than a McMaster slide without adjusting the other steps.

Counting only one of the two grids on a McMaster slide.

Over or under counting due to incorrect sample preparation ratio (poo and flotation solution) without adjusting other manual math calculations.

Always ask

Always ask about the protocols in place for diagnostic testing quality assurance. Does the person performing the egg counts routinely validate their technique against findings of an established lab. Ask for validation data if a `modified` technique is used - you want proof changing the method doesn`t affect results.

Then there`s


Inadequate training

Sore eyes

Bad counting day


And rushing eggs counts significantly reduces both accuracy and precision by 50-60%, while the commonly used practice of cutting corners by only counting one grid has a similar effect on precision. Learn more about the study here

You`ll be pleased to know the technology that we use is not subject to such sources of variability.


        For worry-free FEC results

Give your horse the power of precision and accuracy with WormGuide. 

At WormGuide, we believe that you should know exactly what technique is used along with it`s scientific validation so there’s no question about the results — transparency is key. 

You have direct access to all the facts you need about the AI automated system that removes the major drawback of  human-error associated with manual methods.

Parasight System has been validated by numerous labs and major research universities and published in several peer-reviewed papers.

The ultimate satisfaction comes from knowing that our egg count technique is by far the most precise technique available. And that means reliability.

WormGuide has deliberately chosen to replace the manual microscope counting method to help horse owners, managers and veterinarians meet the demand for better egg count testing for the current worm control guidelines.

Are you ready to build a preventative up-to-date program where FEC surveillance is performed to slow the development of resistant worms by up to 300%? 

Build surveillance worm control that works


Slusarewicz, M., Slusarewicz, P., and M.K. Nielsen. 2019. The effect of counting duration on quantitative fecal egg count test performance. Vet. Parasitol. X: doi 10.1016/j.vpoa.2019.100020.
Scare, J.A., P. Slusarewicz, M.L. Noel, K.M. Wielgus, and M.K. Nielsen. 2017. Evaluation of accuracy and precision of a smartphone based automated parasite egg counting system in comparison to the McMaster and Mini-FLOTAC methods. Vet. Parasitol. 247:85-92.
 Cain, J.L., Slusarewicz, P., Rutledge, M.H., McVey, M.R., Wielgus, K.M., Zynda, H.M., Wehling, L.M., Scare, J.A., Steuer, A.E., and M.K. Nielsen. 2020. Diagnostic performance of McMaster, Wisconsin, and automated egg counting techniques for enumeration of equine strongyle eggs in fecal samples. Vet. Parasitol. In Press. 


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