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Does my horse have worms?


Horses have evolved over millions of years with their parasites that they carry. In the wild, horses avoid accumulating large worm burdens by rarely grazing the same place repeatedly, instead ranging for 20-40km per day. But unfortunately, by confining our horses to fenced areas of land, large worm burdens can accumulate.

Different species of intestinal worms harm our horses in different ways. Some species damage organs and blood vessels as they migrate to complete their life cycles. Some species can cause obstructions and ulcerations within the horse’s intestinal tract, and others can cause intense irritation as they lay their eggs.

But whether your horse is mostly confined, lightly grazed, or full-time on pasture, understanding your horse’s level of intestinal worm burden is important in managing their overall healthcare.


Signs of worm infestation

Horse parasites can cause extensive damage to your horse’s digestive system before you even recognize a problem. Therefore, it is important to have the knowledge of different signs and symptoms that could indicate that your horse might have a worm burden.  

Here are some signs that your horse may have intestinal parasites:

  • They seem lethargic or depressed with decreased stamina.
  • There may be a change in their hair texture. Having worms can result in a dull or rough coat.
  • Loss of weight and body condition with or without appetite loss.
  • Your horse can develop a pot belly. This is especially seen in young horses.
  • Your horse is colicky. 
  • Your horse has irregular bowel movements, especially diarrhea. 
  • Coughing and/or nasal discharge.  
  • Tail rubbing and hair loss.  
  • Resistance to the bit due to mouth lesions.  
  • Summer sores

The importance of faecal examinations

Unfortunately, we cannot “see” inside a horse, but an easy, non-invasive way to determine the possibility of a worm burden, is to examine the manure for parasites (more specifically, the parasite eggs.) One of the most under-utilised tools in an effective parasite control program is regular faecal examination, which merely involves collecting and taking fresh faecal samples to your veterinarian for laboratory analysis.

At the laboratory, the faecal sample is measured and mixed with a solution to prepare the sample for examination under a microscope using a McMaster Slide. A Faecal Egg Count (FEC) test is a quantitative assessment and identification of how many parasite eggs a horse is shedding at the time of the test. An FEC test primarily identifies small strongyles (redworms) and ascarids (roundworms). The number of parasite eggs visible and counted is reported on an eggs per gram (EPG) basis. The parasite EPG calculation is often referred to as a horse’s parasite load or worm burden.

Mcmaster Slide

McMaster slide being filled with faecal flotation mixture (Source:

Fecal Worm Egg
Mcmaster Slide

Faecal worm egg count done by using a microscope and a McMaster slide (Source:

In adult horses, serial FEC tests, determining the number of eggs per gram in manure (epg), helps to:

  • estimate individual worm burdens; 
  • identify horses with moderate or heavy parasite burdens that needs to be dewormed; 
  • identify the specific horses in the herd that are long-term parasite reservoirs and require more frequent deworming; and 
  • determine which specific deworming products are effective against parasites on that farm.

On the basis of FEC tests, horses are categorized as low shedders (0-200 epg), moderate shedders (200-500 epg), or high shedders (> 500 epg). Interestingly, within a herd of horses, a minority of animals harbour the majority of worms, usually based around the 20/80 rule, where 70-90% of adult horses fall in the low-shedding category. The objective of FEC tests is to identify the high egg shedding horses so that they can be targeted for treatment.  This will greatly reduce the number of worm eggs being shed on the pasture. The low shedding horses tend to require much less treatment.  By doing several scheduled FEC tests, a worming protocol can be designed to effectively protect the whole herd.   

It must be noted though, that this rule applies to adult horses over 4 years old. In younger horses, egg shedding can fluctuate as the horse’s immunity develops, and therefore trying to identify high egg shedding horses in any horse under 4 years old is not reliable.

The most common internal parasites are shed in seasonal patterns, so ideally, two FEC test cycles should be performed annually. One set of tests is done in spring and another set in autumn. Each set consists of a FEC test before deworming treatment and another FEC test 14 days after deworming. The before-treatment test identifies your horse’s worm burden and shedder category. The after-treatment test helps determine the efficacy of the anthelmintic (how well your dewormer is working).  

New horses should have an FEC on arrival at the premises, and be dewormed if warranted, before being turned out with resident horses.

How to collect fecal samples

Firstly, you need to determine with certainty that the faecal matter has come from your horse and not another if you keep your horse with others.  If necessary, isolate your horse in an area where you know you can collect a sample without a doubt.  

The manure sample must be fresh, so try to collect the sample shortly after your horse has passed it.  Parasitic matter deteriorate with time and temperature change, which leads to less reliable test results.  

Avoid collecting a contaminated sample (including dirt, shavings, grass, etc).  Select the sample from the top or middle of the manure pile and avoid taking the sample close to the ground.  Ideally, the sample size should be larger than a golf ball but smaller than a tennis ball.  The sample can be collected in a plastic bag.  Elimination of air from the bag helps to preserve the sample and prevents eggs from hatching.  Refrigerate the faecal sample up to delivery to the veterinary clinic.  Do not freeze the sample.

Fecal Sample



Feecal egg count tests with periodic deworming of your horse is only one element of an effective control program and environmental management is also important. The most effective environmental control is the removal of manure from pastures – twice-weekly collection is recommended. Composting of the collected manure and soiled bedding generates enough heat to kill parasite larvae and eggs. Parasite exposure can be further reduced by avoiding overgrazing, reducing stocking density, rotating and resting pastures and grazing other types of livestock on rested pastures. 

Your horse will thank you!

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