De-worming your horse is an important practice to prevent health complications associated with internal parasites.
Worms, also known as helminths, are large endoparasites which live within horses and other mammals. In high numbers, they can result in colic, weight loss, intestinal impaction and can lead to organ damage.
Deworming medications effectively kill off parasites such as S. vulgaris and other large strongyles, small strongyles (cyathostomins), tapeworms, and ascarids (Parascaris equorum).
While it was previously common to regularly administer paste dewormers to all horses in the spring and fall, concerns about anthelmintic resistance have resulted in modernized deworming strategies.
Fecal egg count (FEC) tests are now used to monitor parasite levels in horses and determine when to administer anthelmintic drugs. Depending on the type of parasite present, different classes of deworming agents will be used.
In this article we will consider some of the common helminths, or worms, which affect horses as well as provide step-by-step instructions for deworming your horse.
Types of Worms Seen in Horses
There are many different types of worms or helminths that can inhabit the horse. Worms are commonly divided into three main groups:
- Round and red worms (also called nematodes) which are most common in horses
- Flukes (also called trematodes) which are rare in horses
- Tapeworms (also called cestodes) which are seen in horses
Red worms, a type of nematode, are the most common worm seen in horses, and can be very damaging.
Red worms are split into two groups: small (also called Cyathostomin spp.) and large (also called Strongylus vulgaris) red worms. 
Small Red Worms
Small red worms (Cyathostomin spp.) are the most common type of worm seen in horses. It is thought that 70-100% of all horses worldwide are infected with a low burden of small red worms. 
Red worm larvae burrow through the gut lining, known as the endothelium, where larvae develop or hibernate for winter (called encysting).
If the red worm larvae hibernate throughout winter, large volumes of larvae burst out of the gut endothelium in spring, stimulated by warming weather. This damages the intestinal endothelium and can lead to diarrhea, weight loss, poor condition, and potentially colic. 
Large Red Worms
Large red worms (Strongylus vulgaris) are less common than their small counterparts but are more damaging to the horse.
Red worm larvae migrate through blood vessels surrounding the horse’s gut, causing bleeding and inflammation.
This damage can lead to rapid and severe weight loss, anaemia (think thousands of worms sucking up to 0.03 ml of blood per day), organ damage, diarrhea, severe inflammation, and colic. Large red worm infection can be fatal. 
Round worms (also known as Parascaris equorum) are a worm found in young horses. Round worm larvae are ingested off pasture and migrate from the gut to the lungs and liver via blood vessels.
In the liver, round worm larvae migrate and consume liver tissue, causing damage.
Once larvae reach the lungs, they are coughed-up and swallowed into the gut where they mature and lay eggs, but not before they cause irreversible lung damage due to migration and consuming lung tissue.
Round worms can cause diarrhea, colic, and a bloated stomach as they mature within the gut. When in the lungs, round worms can cause respiratory symptoms such as coughing, respiratory distress, and nasal discharge.
Round worms tend to infect young horses (usually less than 12 months old) and foals, whereas adult horses have developed immunity towards round worms. 
Tape worms (also known as Anoplocephala spp.) are a type of cestode. Tape worms can grow as long as 20 meters (though are usually around 8 cm in length), and as wide as 1.5 meters.
Tape worms mass together at an important part of the gut where the small and large intestine meet, known as the ileocecal junction. This mass of worms can lead to weight loss, diarrhea, colic, anemia, poor condition, and potentially fatal gut impactions.
Unlike with other worms, horses become infected with tape worms through eating the intermediate host. The intermediate host in the tape worm life cycle is the orbatid mite which is commonly found on grass, hay, and straw.
Tapeworm larvae will infect the mite, waiting for the mite to be eaten with grass, so the larvae can move into the primary host, the horse. Inside the primary host, tapeworm larvae begin developing and maturing, enabling them to lay eggs in horse feces to continue their lifecycle. 
Pin worms (also known as Oxyuris equi) migrate through the gastrointestinal tract to lay eggs around the horses’ anus.
Whilst they cause no direct pathological damage to the horse, the eggs cause intense itching and irritation to the horse around the anal area.
The irritation can be so severe that horses will scratch the area until sores develop, leading to infection and severe skin irritation which can require veterinary intervention. 
Thread worms (also known as Strongyloides Westeri) typically only affect foals. Adult horses develop natural immunity after around six months of age.
In foals, threadworms are passed from the mare’s milk to the foal, and infected foals can show poor condition, colic, anemia, diarrhea, and a general ‘failure to thrive’. 
Worming the mare pre-parturition, and the foal by one month old will help control threadworms in foals.
Neck Thread Worms
Neck thread worms (also known as Onchocerca cervicalis) are different from intestinal thread worms and are commonly seen in warmer climates.
Mature worms are up to 30 cm long and live in the nuchal ligament, which runs from the head and along the neck.
As adult neck thread worms mature and reproduce within the nuchal ligament, they release microscopic larvae, known as microfilaria, onto the horse’s skin. Microfilaria can lead to skin irritation on the face, neck, shoulders, and stomach.
Neck thread worm infection, also known as subcutaneous filariasis, is spread by blood-sucking biting insects, such as the biting midge. 
Lung worm (also known as Dictyocaulus arnfieldi) is commonly seen in donkeys and can spread to horses who share their pasture with donkeys.
Lung worms develop within lungs, causing respiratory irritation, nasal discharge, and coughing as worms migrate, feed, and develop within lung tissue.
Unlike horses who quickly show respiratory symptoms, donkeys can be infected with a large burden of lung worms and show little-to-no symptoms. 
Bot flies (also known as Gastrophilus) are not typical worms but flies that use the horse as part of their life cycle.
Bot flies lay a yellow, sticky substance on horses’ coats which contains their eggs, typically during summer months. When horses’ groom one another, they ingest the eggs accidentally.
Bot fly larvae hatch in the mouth and migrate to the stomach where they adhere to the squamous stomach lining using mouth hooks and mature over 10 months, causing severe irritation.
Once mature, bot fly larvae are excreted out in feces where they pupate into flies and begin to lay eggs on horse coats. 
How Do Horses Get Worms?
The lifecycle of the worm depends on its specific type. In general, horses acquire worms through consuming grass pasture contaminated with worm eggs, shed from infected horse feces.
Once the eggs have been ingested with the grass, the worms hatch into larvae and migrate to their type-specific area to develop and reproduce. Worms normally develop within the gastrointestinal tract or the lungs.
Once there, worms can cause significant, and sometimes irreversible, damage to the organ. 
Mature eggs laid by the worms are excreted within feces onto grass pasture and the cycle continues.
Some worm eggs are extremely resistant when excreted onto pasture in faeces. They can resist temperature changes and mechanical disruption such as harrowing. 
How Do I Know If My Horse Has Worms?
Typical signs that your horse may have worms include:
- Weight loss
- Poor condition
- Poor performance
- Bloated stomach
- Itching of head, neck, or anus
- Coughing, nasal discharge, or respiratory irritation
- Failure to thrive in foals
- No symptoms at all!
Level of worm infection before symptoms become apparent is affected by:
- Age (older horses are less susceptible until geriatric)
- Crowding effect (too many worms for them to thrive)
What are De-Wormers?
Dewormers are chemical substances known as anthelmintic drugs. Anthelmintics are used as there are no vaccines available to protect horses against worms, although vaccine trials have been undertaken.
The aim of anthelmintics is to: 
- Control worms at differing life stages (egg, larvae, and mature adult)
- Eliminate worms within the host (the horse) and on the pasture and intermediate host (if applicable)
- Prevent re-infection after anthelmintic treatment
There are currently 8-9 differing anthelmintic chemical groups depending on your location, including: 
- Benzimidazoles e.g., Panacur
- Imidothiazoles e.g., Levacide
- Avermectins/Mibemlycins e.g., Moxidectin and Ivermectin
- Tetrahydropyrimidines e.g., Pyrantel
These groups have different ways of killing worms and target different species. 
Please note: Equine dewormers are not designed for use in non-equine species and can be toxic to livestock, cats, dogs, and humans. Keep dewormers out of reach from animals, and children; carefully dispose of used dewormers. Appropriate parasite control for your livestock and pets should be discussed with your veterinarian. 
Why Should I De-Worm My Horse?
De-worming your horse with an appropriate anthelmintic is vital to help prevent the symptoms associated with worms. Some of these symptoms, including colic, can be fatal.
Mares should also be de-wormed during the last 4-weeks of pregnancy to help reduce the chance of the foal developing round worms and threadworms. Significant worm infections in foals can be fatal.
Best Deworming Schedule and Medications
Horses should be dewormed with an appropriate anthelmintic, at an appropriate time of year. The following summarizes the treatment for many of the common worms. 
Please consult your veterinarian if you think your horse may have thread worms or neck thread worms.
Small Red Worms and Round Worms
Time of Year: Late autumn and early winter. Can be administered at other times if dose missed or failed
Worm Treated: Small red worm (encysted), lung worm, bots, round worm (pyrantel only)
Anthelmintic Group: Avermectins, Mibemlycins, Tetrahydropyrimidines
Anthelmintic Agent: Ivermectin, Moxidectin e.g., Equest, Strongid P, Equimax, Noromectin, Pyrantel
Additional Considerations: Pyrantel can be given if horse less than four years old as it also treats round worm.
Worm egg counts can help monitor red and round worm burdens.
Fly repellent can deter bot flies and blood-sucking midges (neck thread worm). A bot knife can remove bot eggs to help prevent re-infection.
Time of Year: Late autumn and early winter. Can be administered at other times if dose missed or failed
Worm Treated: Tape worm
Anthelmintic Group: Tetrahydropyrimidines
Anthelmintic Agent: Praziquantel e.g., Equest Pramox, Pyrantel (double dose), Equimax
Additional Considerations: Generally mixed with Ivermectin, so can include red and round worm treatment (see above).
Saliva test for tapeworm is available.
Time of Year: Anytime
Worm Treated: Pin worm
Anthelmintic Group: Tetrahydropyrimidines, Benzimidazoles
Anthelmintic Agent: Double dose Pyrantel e.g., Strongid P, or 5-day course of fenbendazole e.g., Panacur 5 Day Guard
Additional Considerations: Only administer if Pin worms are confirmed by veterinarian.
Disinfect equipment and stable/field area to prevent re-infection.
How Do I Deworm My Horse?
Dewormers are usually given as a paste in a dosing syringe. Panacur 5 Day Guard is unusual in that it is a liquid.
To dose your horse correctly, a weigh bridge or scale should be used to get an accurate bodyweight.
Many feed companies, nutritionists, and veterinarians offer the use of a horse weigh bridge. If no weigh bridge is available, a less accurate weight tape can be used to give an approximate weight.
Testing for Worms
Fecal egg counts (FECs) are useful to assess the level of red worms and round worms in your horse. An FEC represents the number of worms present in your horse based on the amount of parasite egg shedding in feces.
A FEC of less than 200 eggs per gram indicates that your horse does not need anthelmintic treatment for red or round worms. Another FEC count should be repeated in 6-8 weeks to monitor the worm burden. 
An ELISA saliva test to analyse for tape worm twice yearly (spring and autumn) is also advisable as tapeworms show high resistance to anthelmintics. 
Preventing Worm Infections
Rotating fields after deworming and poo picking can also help to reduce worm infections as it helps prevent re-infection from contaminated pasture.
All horses who share a pasture should be dewormed at the same time and then moved to the new pasture. If possible, stable your horses for 48-hours post-deworming to reduce new pasture contamination.
Spring Deworming Practices
Fecal egg count for red worms and round worms. Saliva test for tape worms.
Summer Deworming Practices
Fecal egg count for red worms and round worms.
Watch out for bot fly eggs. Eggs can be removed with a bot knife to prevent infection.
Fly repellent can reduce bot fly and neck thread worm infections (spread by blood sucking flies).
Autumn Deworming Practices
Fecal egg count for red worms and round worms. Saliva test for tape worms.
Late Autumn/Early Winter De-Worming Practices
Deworm as indicated above and depending on risk factors (e.g., age, number of horses on pasture, presence of donkeys, climate etc.)
Rotate fields after deworming. Deworm herd as a group before rotating fields. Keep stabled for 48 hours post-worming, if possible, to reduce pasture re-infection.
There is widespread concern regarding anthelmintic resistance in horses due to common use of the same deworming agents. Over time, medications can become ineffective as parasites evolve and develop resistance to their effects.
In horses, the greatest concern is small red worm resistance (particularly benzimidazole and avermectin resistance).
Resistance is worse within the Southern hemisphere as the warmer climate speeds up worm maturation and reproduction.
Worm resistance usually occurs due to under-dosing wormers, enabling strong worms to survive and pass on their survival genes. Within the lab, worm resistance has been created within 8 generations of worm reproduction. 
Resistance to all known deworming medications could mean in years to come that anthelmintics are no longer able to treat worm problems in your horse. This could increase the mortality rate of worm infections and raise the risk of serious complications. 
What Can I Do About Anthelmintic Resistance?
To help reduce resistance in your horse or herd, there are many things you can do: 
1) Dose with an anthelmintic to your horse’s exact weight (use a weigh bridge) to prevent under worming (over worming can be dangerous to the horse too).
2) Quarantine all new horses to the pasture until 48-hours after worming to prevent contamination of pasture.
3) Monitor worms through worm egg counts (red worm and round worm) and saliva test (tapeworm) to ensure minimal worming.
4) Rotate pasture after worming the whole herd with the same anthelmintic group to help prevent pasture contamination and re-infection.
5) Rotate anthelmintic group annually or more frequently if not doing fecal egg counts and tapeworm saliva tests.
6) Maintain good nutrition and dietary protein content. Support your horse’s gut health and immune system.
There is currently research being undertaken into alternative natural therapies, botanical wormers, and worm vaccines; however, none have yet been proven safe or effective.
There are a few key points to take away about effectively deworming your horse:
- High worm burdens are extremely damaging to your horse.
- Round worm and thread worm are commonly only seen in young horses (less than 4 years old) and foals.
- Deworming the pregnant mare is important to help protect the foal.
- Deworming with the right dewormer, at the right time is crucial.
- Anthelmintic resistance is a serious concern. Weigh horses before deworming for an accurate dose and monitor worms by worm egg counts and saliva tests to help prevent over-worming.
- If unsure, speak to your veterinarian, nutritionist, or suitably qualified person.
Is Your Horse's Diet Missing Anything?
Identify gaps in your horse's nutrition program to optimize their well-being.
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