Parasite Management FAQs

Why should I care about controlling parasites in my horse?

Even if you never see a worm around your horses, they can still be heavily infected. At their best, internal parasites eat a portion of the food you feed your horse - meaning you end up spending more money on food! At their worst, parasites can cause extensive internal damage and can impact the immune system, coat health, nutrition, and gastrointestinal tract of your horse. They can even lead to more serious outcomes, such as colic, intestinal ruptures, and death.

So can I just use any dewormer without knowing what parasites my horse has?

We don’t recommend blindly deworming your horse for a couple of reasons. The first is that different dewormers are more effective at killing different types of internal parasites. The second is that if your horse has a large worm burden, killing them all off at once can cause a serious colic - all those rapidly dying worms cause an intestinal blockage.

How do I know which deworming protocol to use?

Importance of Fecal Egg Counts

Gone are the days of rotational deworming, in which horses were dewormed with different products every 2-3 months. This system is no longer recommended because it was helping to develop resistance in the parasite populations, which actually makes our currently available anthelmintics (dewormers) less effective! The drug-resistant parasites reproduce and mutate faster than we can develop new, more effective anthelmintics. We had to make a big change in order to maintain the efficacy of our current anthelmintics.

Based on the American Association of Equine Practitioners (AAEP) recommendations, Henderson Equine supports “targeted deworming”, which generally involves deworming as little as twice yearly (spring and fall), depending on each horse’s need. We can determine the best deworming protocol for each specific horse by performing a Fecal Egg Count (FEC) at least twice yearly (before deworming). The FEC is a quantitative assessment of internal parasite eggs shed by your horse in its manure. A measured volume of manure is mixed with a flotation solution and evaluated on a specially calibrated microscope slide. This allows us to calculate the number of parasite eggs per gram of manure. With this information, we can advise you as to which dewormers are effective on your farm, which horses are high vs low shedders, which interval between dewormers is required (decreasing unnecessary chemical administration), and if parasite resistance is developing on your farm.

To collect a fecal sample: collect 1-2 balls of fresh manure in a plastic baggie.  Remove as much air as possible, and seal the baggie.  Keep refrigerated until you can drop it off at the clinic (or have the veterinarian pick it up during an appointment).  The sample should be no more than 24 hours old when delivered to the veterinarian, and preferably even fresher. Be prepared to tell us the last time you dewormed, and with what product.  Unless we are testing for resistance, the fecal should be at least 4-6 weeks after administration of Panacur (Fenbendazole) or Strongid (Pyrantel), at least 6-8 weeks after administration of Zimectrin (Ivermectin) and at least 12 weeks after administration of Quest (Moxidectin). 

Why do I need to deworm if my horse has a FEC of 0?

Unfortunately, some parasites (like tapeworms and pinworms), are more elusive than others, and they don’t often show up on basic Fecal Egg Counts. The reason for this depends on the parasite, but generally has to do with their natural life cycle and the weight of their eggs/larvae.

Additionally, Fecal Egg Counts only tell us how many eggs are being shed in the manure. We can use that information to extrapolate a parasitic burden, but it’s still not a perfect reflection of the different life stages of parasites within the horse. In other words, just because no worm eggs are being shed in your horse’s manure, doesn’t mean there are no other worms life stages (like larvae) developing inside your horse. Deworming appropriately even in the face of a FEC of 0 ensures we attack these life stages as well.

Why should I test every horse in the same herd?

The difference between high, medium, and low shedders

Horses on the same pasture or management plan may be found to consistently have very different amounts of internal parasite eggs in the manure.  Approximately 20% of horses will carry 80% of the parasites on most farms! Why is this? All horses are exposed to parasites throughout their life.  They develop varying levels of intestinal immunity which allows them to hold adult intestinal parasites in check.  For example, the immune systems of young horses (<3yrs) and aged horses are less able to adapt to and hold intestinal parasites at low levels.

Adult healthy horses consistently carry similar worm burdens through the year, which allows us to customize parasite management plans based on the level of intestinal parasite eggs that they are shedding. Most low shedding horses will end up being treated biannually (early spring and fall), whereas moderate and high shedding horses will need additional deworming and strategic FEC assessments during times of highest parasite risk.

How do I know if my dewormer is effective?

Fecal Egg Count Reduction Assessments

We recommend performing an annual Fecal Egg Count Reduction assessment on at least one horse on your farm, though a larger representative sample is preferred. This entails submitting a fecal sample just prior to deworming and 10-14 days after deworming (depending on the dewormer used). Comparing the Fecal Egg Count results from just prior to deworming to those two weeks after deworming provides a quantitative measure of resistance.  These results will help determine if this deworming product is effective on your property and for your horse(s), and therefore whether it should or should not be used again in the future.

How else can we control parasites on farm?

In our area, the highest risk for parasite eggs on pasture is May/June through October. Consider the following list of techniques for Pasture Management:

• Don't overstock your pastures (over grazed pastures increase ingestion of parasite eggs)

• Regularly dispose of manure and don't spread onto pastures currently being grazed

• Drag/harrow paddocks to break up manure piles and leave open for 2-3 weeks

• Use feeders for hay or grain rather than feeding off the ground

• Keep foals and weanlings separate from yearlings to help prevent ascarid infections

• If you have other species of livestock (sheep, cattle, chickens), rotate your pastures between species to interrupt the life cycles of the parasites

• Remove bot eggs from your horse’s coat to prevent them from eating them

How Does Henderson Equine Perform a Fecal Egg Count?

FECs can be performed manually, by preparing a suspension of the manure and evaluating it systematically with a microscope. However, we have advanced to use OvaCyte, an AI-driven instrument. This technology is faster and more reliable (less room for human error). It can detect parasite eggs and larvae within multiple layers of the manure suspension, meaning it can detect multiple species at one time. The OvaCyte is easily able to distinguish between different species of parasites as well as eliminate artifact (false positives). It also saves images of the parasite eggs so that we can confirm the results.

Common Equine Internal Parasites

 Small Strongyles (Cyathostomes) - Have become an important group of intestinal parasites in horses.  Larvae burrow into the lining of the intestine and remain dormant or "encysted" for several months before completing their life cycle.  During this time the larvae are resistant to most dewormers.

Small strongyle larvae can cause severe damage to the lining of the intestine, especially when large numbers of larvae emerge from the encysted stage all at once.  Colic and diarrhea are common in heavily infected horses.  These parasites also cause weight loss, slowed growth in young horses, poor coat condition and lethargy, or lack of energy.  While lighter infections are not obvious, it is common for a horse's general health and performance to improve after treatment for these parasites.

The early and late larval stages (before and after they burrow into the lining of the intestine) and the adult parasites are susceptible to several dewormers.  But currently there are only two types of dewormers that are effective against the encysted larval stage.

Large Strongyles - As larvae they penetrate the lining of the bowel and migrate along the blood vessels that supply the intestines.  Even small numbers of these larvae can cause extensive damage.  Infection with large strongyles can cause unthriftiness, weight loss, poor growth in young horses, anemia (low number of red blood cells) and colic.  In most cases, colic caused by these parasites is relatively mild, but severe infections can result in loss of blood supply to a portion of the intestine, leading to severe and potentially fatal colic.  These intestinal parasites have been controlled for the most part by our routine deworming protocols.

Roundworms (Ascarids) - Greatest concern for horses under 6mo of age.  Healthy older horses have immunity but may still shed eggs (mostly eliminated by modern deworming programs).  Adults cluster in the small intestine causing impaction, often with colic, can result in rupture and death

Most often a problem in young horses (especially foals, weanlings, and yearlings).  Adult roundworms are several inches long and almost the width of a pencil; in large numbers they can cause blockage (or impaction) of the intestine.  In addition, roundworm larvae migrate through the internal organs until they reach the lungs.  They are then coughed up and swallowed back into the digestive tract to complete their lifecycle.  Large infections can lead to damage to the liver or lungs due to migration of these larval forms.  

Roundworm infection in young horses can cause coughing, poor body condition and growth, rough coat, pot belly, and colic. Colic is most likely in older foals (over 3months of age) that are heavily parasitized with roundworms when dewormed for the first time.

Tapeworms - Found in virtually all grazing animals.  Contribute to colic by causing inflammation, ulceration and bowel obstruction.  The tapeworm lifecycle involves a tiny mite as an intermediate host, and horses are at risk of developing tapeworm infection when they eat this mite in grass, hay or grain.   Praziquantel has been demonstrated to be highly effective against tapeworms.  

Pinworms - Lay eggs on the skin around the horse's anus.  The irritation they cause makes the horse repeatedly rub its tail. The eggs can sometimes be found by placing a piece of scotch tape on the skin around the horse's anus and then viewing under a microscope.

Bots - Not usually a cause of major health problems, although they can damage the lining of the stomach where they attach.  They may also cause small areas of ulceration in the mouth where the larvae burrow into the tissue for a time after the eggs are taken into the mouth.