Parasite control and the FEC

The following information is intended to give you an overview of how strategies for parasite control have changed in recent years and hopefully peak your interest to go on further to learn more.  It is not intended to take the place of an individual parasite control program for your property.

If you have recently had a faecal egg count (FEC) performed for your horse then you are obviously interested in learning about a different way to manage parasites. Information on what your FEC results mean and how you might use them in a parasite control program can be found below.

An egg count may be used to begin to determine the egg shedding potential of a particular horse but can also be used to determine if the anthelmintic (antiparasitic drug) that we have given is effective, this is called an FECRT.  These are both are explained further below.

A lot of the information below came from the AAEP Parasite Control Guidelines which can be read here.

Some interesting information on how they are using egg counts in the US to help guide parasite control programs and pictures of what we see under the microscope when doing an egg count can be found on this website.



It is time to rethink the way we are managing parasites in our horses.

The goal of parasite control is to limit parasite infections so that animals remain healthy, it should be noted that we are not trying to eradicate all parasites from any individual.

To achieve this we must prevent contamination of the environment of the horse with high numbers of eggs and larvae.   Studies have shown that 15-30% of adult horses usually shed 80% of the eggs.  The FEC is currently the best means available to estimate the egg contamination potential of a horse and determine the effectiveness of deworming drugs.

Currently small strongyles and tapeworms are the major parasitic concern of adult horses whilst roundworms remain the most important parasite affecting foals and weanlings.

Adult horses vary greatly in their susceptibility to infection with small strongyles and their levels of egg shedding and so require individual attention to their parasite control needs.

It should be noted that horses less than 3 years old are more susceptible to infection and more at risk of developing disease and so require special attention.  Some general guidelines with points to consider when formulating parasite control programs for young horses as well as for adult horses can be found below.




Strongyle egg shedding

Several horses grazing together and so sharing the same parasite population can demonstrate huge differences in the numbers of strongyle eggs found in their faeces.  These eggs contaminate the pasture and contribute to the reinfection of themselves as well as other horses in the paddock and is termed egg shedding.  Strongyle egg shedding is what we are measuring in a faecal egg count (FEC).  Faecal egg counts can be used to select the moderate and high egg shedders for more regular treatment with an anthelmintic thus reducing the contamination of the pasture.

To determine the egg shedding potential for an individual horse we need to make sure that the effect of the last dewormer given is gone.  This depends on which active ingredient the last drench contained, examples are:

  • Moxidectin – wait 16wks before testing egg count
  • Ivermectin – wait 12 wks
  • BZ (ending in azole) or pyrantel – wait 9 wks

We don’t classify a horses shedding category to be used to formulate a program for that horse based on just one egg count – it is recommended to do several counts over the course of a year.


Current guidelines classify horses into :

low 0-200 epg     (50-75% adult horses)

moderate 200-500 epg     (10-20% adult horses)

high >500 epg .    (15-30% adult horses)


It is currently recommended to give a deworming treatment to all horses with egg counts over 200-250 epg.

Treating all horses over 200 epg reduces the overall egg shedding into the environment by 95% with only needing to treat 50% of the horses.  This has an important result in leaving what is called a refugia (a population of worms not exposed to that drug) which is critical to slowing down the development of resistance in the worm population.

Resistance is the ability of some worms in a population to survive a treatment which would generally be effective.  It is an inherited trait and with continued selection the frequency of resistant genes in a local worm population increases to the point where treatment fails.

It should be noted that it is recommended that all horses over 3 yo receive at least two treatments in a year (usually spring and autumn) with the other treatments throughout the year guided on FEC results.  This should always be in combination with environmental management plan.  You will find more information below…. don’t stop reading yet!




The FEC or egg count

It is important to remember the limitations of the FEC:

  • The count does not reflect the total worm burden of the horse.
  • It does not detect immature or larval stages of parasites.
  • It cannot differentiate between small and large strongyles, the eggs look the same.
  • We are only counting strongyle eggs – not all parasites that may be present.  If ascarid eggs are present (roundworms) then we will count these as well and give a seperate count, this is usually in young horses.  Tapeworm and pinworm infections are often missed by this test and so treatment for these are covered with the two foundation treatments we suggest all adult horses receive a year.

The test determines the shedding status of strongyle eggs of a horse at the time of sampling.

Studies have shown that shedding categories for most horses remain consistent unless they are on the border between categories. So, if we have multiple consecutive tests with egg counts in the same category (low/ mod/ high) we can classify a horse into a shedding category to help develop a parasite control plan for that individual.   There will always be situations which can alter a horses shedding potential and these must be kept in mind.  Examples of such situations would be major changes in their environment or in the health of the horse.  Veterinary care and advice should always be sought in such situations regarding the need for an antiparasitic treatment.




FECRT – egg count reduction test

This can tell us if a particular anthelmintic is working on your property for strongyles or ascarids (ie. if there is resistance).  An egg count is performed before and 14 days after administration of the anthelmintic in question and a reduction of less than 95% may indicate resistance.




Current reccommendations for parasite control – General points


Adult horses

  • In well managed horses (not severely neglected or wild) 99% of strongyle eggs seen are from small strongyles.
  • All horses should receive a foundation of two treatments a year.  These treatments will target the large strongyles, tapeworms, bots etc.  All further treatment should be focused on the control of small strongyles.  So further treatments should be targeted at horses with high strongyle contamination potential (moderate and high shedders).
  • Remember – we cannot determine a horses shedding category with just one FEC, a consistent level over several consecutive tests would allow us to determine the category for that horse given a fairly stable grazing environment.
  • Low shedding horses with their naturally strong immunity to small strongyles may need no other treatments other than the foundation two a year because the two treatments have covered the needs of the other parasites and they are protected naturally from the small strongyles by their immune state.   Repeated treatment of these horses every 2-3 mths will not improve their health but will promote drug resistance.
  • Moderate and high shedders will need a third or fourth treatment for small strongyles during the year, resulting in treatment of these horses every 3-4 mths.
  • Treatments should be focused during seasons of peak transmission when refugia are highest (usually spring and autumn) to slow resistance development.
  • It is often recommended that moxidectin be used not more than once a year unless under veterinary advice to slow the development of resistance.  Autumn is usually the recommended time to give a moxidectin/praziquantal drench to target both encysted small strongyles and tapeworm.


Foals, weanlings and yearlings

Remember, young horses are more susceptible to parasite infection and development of disease and so are treated differently.   We do not do targeted treatment based on FEC’s in very young horses.

General advice is that horses should receive 4 treatments in the first year of life.

For example:

  • first at 2-3 mths old targeting ascarids (roundworms) – a BZ is most appropriate (fenbendazole, oxfenbendazole, oxibendazole)
  • second just before weaning (4-6 mths old)
  • Do a FEC around weaning (before given treatment) to determine if you have primarily strongyle or an ascarid infection
  • 9 mths old – base treatment on the parasitic infection from FEC results.  We often treat for tapeworm as well at this dose.
  • 12 mths – again based on treatment, often strongyles are targeted.


Weaned foals should be turned out on to the cleanest pastures with the lowest parasite burdens.


Young horses – 2 and 3 yo’s

Continue to treat these horses as though they were ‘high’ shedders.  This means three to four yearly treatments.




Environmental control must not be forgotten

  • Strongyle larvae on pasture will die in hot weather (>40 degrees) after a few days but in cooler weather may survive months.  In temperatures of 10-25 degrees larvae can develop in 2-3 wks and survive for a few weeks to a few months.  In warmer weather, between 25-33 degrees, larvae can develop in just 4 days and still survive for a few weeks.


  • Removing manure from the pasture can significantly reduce the eggs and larvae available for reinfection.  You can see from the above information, this would ideally be done regularly in our warmer weather here in the Northern Rivers (at least once to twice a week).


  • Composting this manure is very important to remove the larvae (>40 degrees C for one to two weeks) if you intend to spread it back onto the pastures.


  • New arrivals into a stable group of worm controlled horses should have an FEC and a larvicidal dewormer (moxidectin/praziquantal) before turning out with resident horses.


  • Keep in mind that heavy stocking rates will challenge even the best parasite control program.


  • Cross grazing with other cattle and sheep can be helpful to reduce the larvae on the pasture.  Common parasites do not spread between these species.




Anthelmintics (anti-parasitic drugs)

Some tips on which active ingredient is suggested to be used for which worms:

Ivermectin and Moxidectin are the foundation for strongyle control

Praziquantal or high dose pyrantel for tapeworms

BZ’s (oxfendazole/fenbendazole) when treating roundworm (ascarids) in young horses (esp foals)

Ivermectin or BZ’s for pinworm

Ivermectin or Moxidectin for bots


It is always suggested to check efficacy of the active ingredient against strongyles or ascarids on your property with a FECRT if you have any concerns about resistance.


Below are some commonly available products and their active ingredients:

Strategy T – Oxfendazole + Pyrantel

Equimec – Ivermectin

Equimax – Abamectin + Praziquantal

Equimax Elevation – Ivermectin + Praziquantal + Pyrantel

Equest plus tape – Moxidectin + Praziquantal

Ammo – Abamectin + Morantel (like pyrantel)


Current recommendations for rotation of drenches are to rotate the anthelmintic family (ectins vs azoles) every 2-4 treatments, it is important to note that if using straight BZ’s (azole) as a rotational drench then it is worth doing some FECRT’s to ensure that the strongyle worms on your property are not showing resistant to these.



























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