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Horse Care

Types of Wormers


Effective against strongyles, bots, pinworms, roundworms, lungworms, threadworms, stomach hairworms, large mouth stomach worms

> Ivermectin Paste 1.87%
> Bimectin® Paste 1.87%
> Ivercide™ Equine Paste 1.87%


Effective against strongyles, pinworms, roundworms, stomach hairworms, large mouth stomach worms

> Safe-Guard® Equine Paste 10%
> Panacur® Paste 10%


Effective against strongyles, pinworms, roundworms, instestinal threadworms, tapeworms

• Strongid® T Rx


Efficacy varies with product

• EquioPathics® Wrm Clear

Your horse is vulnerable to more than 150 types of internal parasites that can have a serious impact on its health, and every horse is infected by one or more of them. Strongyles (blood worms, red worms), ascarids (roundworms), tapeworms and bots (bot fly larvae) are the most common and the most important. That’s why a parasite prevention and control program is vital to protecting your horse.

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Understanding the parasite life cycle

Horses ingest parasite larvae or eggs while grazing

Implementing a parasite prevention and control program depends on understanding the parasite life cycle. No matter which dewormer is used, sanitation by cleaning up manure and keeping water clean has to be the cornerstone of a good program.

The life cycle begins when an already-infected horse passes eggs and/or larvae in their feces. Other horses then ingest the eggs and larvae while grazing in a pasture. Some worm species, such as strongyles, might even find their way into a water source and become ingested when the horse drinks. Those eggs and larvae then develop into adults within the horse’s digestive system, where they lay more eggs. The larvae of some species migrate to other organs, such as the liver, heart and lungs, before returning to the intestines to lay eggs. Those new eggs and larvae are then passed in the horse’s feces, where the lifecycle begins again.

A parasite control and prevention program includes oral medications called dewormers, although technically they’re known as wormers. Different types of wormers are required to control specific parasites.

Overview of internal parasites in horses


Large and small strongyles begin as eggs in feces and hatch into larvae contaminating the ground, pasture, or paddock. It takes one to two weeks for strongyle eggs to develop into larvae. Strongyle larvae are very active and crawl up onto grass and forage, and sometimes into a water source. They are ingested by horses while they graze or drink.

Large Strongyles
(blood worms, palisade worms, sclerostomes, red worms)
Strongylus vulgaris, one of three types of large strongyles, poses the most serious risk to your horse. S. vulgaris larvae migrate through arterial walls to the cranial mesenteric artery, the primary blood source to your horse’s intestinal tract. A heavy infestation can cause serious damage and possibly even death if the artery ruptures. More commonly, the migrating larvae damage the arterial wall and cause an aneurysm, which leads to the formation of blood clots and can result in severe or fatal colic. The two other species of large strongyles (S edentatus & S equinus) migrate to other areas of the body, including the liver and pancreas.
Wormer: Use an ivermectin-based wormer frequently. Fenbendazole-based wormers may provide control for some mature strongyles.

Small strongyles
(cyathostomins, small red worms)
Once ingested, small strongyle larvae do not migrate out of the intestinal tract. Instead, they encyst themselves to the wall of the large intestine, where they can remain for prolonged periods of time - from a few weeks to as long as two years - before developing to the next stage. While encysted, they are resistant to most wormers. A heavy infestation can cause diarrhea, colic, weakness, and muscle wasting.
Wormer: Use an ivermectin-based wormer frequently. Fenbendazole-based wormers may provide control for some mature strongyles.


Like many other parasites, the roundworm life cycle begins when an infected horse passes eggs in its feces. Other horses then ingest the eggs while eating or drinking contaminated material. The eggs hatch inside the horse’s stomach and the larvae migrate via the bloodstream to the liver and lungs. Larvae are coughed up and swallowed, sending them to the small intestine, where they mature and lay eggs, which are passed out in feces. This entire cycle takes about three months. A heavy roundworm infestation can result in a pot-bellied appearance, colic, slow growth, nutritional deficiencies and rough hair coat. A heavy infestation could rupture the small intestine and cause peritonitis, which is usually fatal.
Wormer: A combination of an ivermectin-based wormer and a pyrantel pamoate wormer is recommended.


Tapeworm eggs are passed in the horse’s feces and then eaten by the oribatid mite, which lives on pasture grass. Horses then ingest the mites when they graze. The tapeworm eggs mature inside the horse in about 6-10 weeks. Adult tapeworms attach themselves to the horse’s intestinal lining and begin feeding. Packets of egg break away from the worm and are passed in the feces. A heavy tapeworm infestation can cause colic, weight loss, rough hair coat and nutritional deficiencies.
Wormer: A combination of an ivermectin-based wormer and a pyrantel pamoate wormer is recommended.

Adult bot flies lay eggs on the horse's hairs


(botfly larvae, Gasterophilus nasalis, Gasterophilus intestinalis)
Adult bot flies attack the horse and lay eggs on the hairs, mainly around the legs, chest, neck, throat and mouth. After about a week, the eggs hatch and larvae enter the horse’s mouth, either through migration or when the horse bites or chews its legs or other areas. The larvae then burrow into the tongue and gums. After three weeks, the larvae emerge and are swallowed. They attach themselves to the horse’s stomach lining, where they’ll stay until spring or summer, when they detach and are passed out of the horse in feces. The bots then mature into adult flies in the ground. Most bots do not cause serious injury to the horse, but a heavy infestation could lead to gastric ulcers.
Wormer: Use an ivermectin-based wormer.


(Dictyocaulus arnfieldi)
Lungworm infection begins when infective larvae are ingested by the horse. The larvae migrate through the lymph nodes to the lungs, where they mature and lay eggs. The eggs hatch and new larvae are coughed up, swallowed, and passed out of the horse in feces. The entire life cycle takes around 28 days. Donkeys are more typically infected with lungworms, but horses that are pastured with donkeys can become infected. Lungworms can cause parasitic bronchitis, which results in difficulty breathing, severe coughing and loss of appetite. Other complications can include pneumonia, pulmonary edema and secondary bacterial infections.
Wormer: Use an ivermectin-based wormer frequently.

A pinworm infestation may cause such irritation that a horse will rub himself raw


(Oxyuris equi)
Adult pinworms inhabit a horse’s intestines. When the horse is asleep or relaxed, female pinworms crawl out of the rectum and lay their eggs in a sticky film on the perianal area, then crawl back into rectum. The sticky film is very irritating and can cause tail rubbing to the point of hair loss. When the horse rubs, residue is left on fences or stables walls, helping to spread the infection. Pinworms do not cause internal damage like other worms. However the irritation and itching can cause a horse to rub herself raw, which can result in infection and other complications.
Wormer: Be sure to use disposable materials to clean the area under the horse’s tail to help stop the spread of infection. Ivermectin and Pyrantel based wormers are recommended.

Habronema spp

(Habronema muscae, H microstoma, Draschia megastoma, stomach worms)
Horses become infected with stomach worms by ingesting house or stable flies that contain infective larvae. They may also ingest infective larvae that have emerged around the mouth. If the larvae are deposited in open wounds, broken skin or around the eyes, they can cause a condition sometimes called “summer sores.” These are lesions (granulomas) resulting from irritation that cause severe irritation and disfigurement. Larvae that are swallowed can cause tumor-like lesions in the stomach and present as mild gastritis.
Wormer: Use an ivermectin-based wormer frequently.


(Trichostrongylus axei, Stomach Hair Worms)
Hairworms generally infect horses that are mingled with or rotated on pastures with ruminants. Hairworms develop in the stomach and lay eggs, and the eggs are subsequently passed out of the horse in feces. A heavy infestation can cause chronic gastritis, diarrhea and weight loss.
Wormer: Use an ivermectin-based wormer frequently.

Intestinal Threadworms

(Strongyloides, Strongyloides westeri)
Threadworm infections are mainly found in the small intestines of foals. Larvae is ingested through the mare’s milk, or sometimes through infected bedding. A threadworm infestation can result in diarrhea in foals.
Wormer: Use an ivermectin-based wormer within 24 hours postpartum to prevent transmission of larvae to foals.

Horses infected with neck threadworms will sometimes roll and scratch to the point of hair loss

Neck Threadworms

(Onchocerca cervicalis, Onchocerca Microfilariae, parasitic filarial worm)
Adult neck threadworms live in the horse’s nuchal ligament. The larvae live in the tissue right under the horse’s skin. They are ingested by biting flies (culicoid flies) and after about three weeks develop into the infective stage inside the fly. When the fly bites another horse, the infective larvae enter the horse’s system, and migrate to the nuchal ligament and also the flexor tendons and suspensory ligaments. Larvae can cause severe itching and inflammation. Some horses will roll and scratch to the point of causing sores and hair loss. Other complications can include uveitis, swelling around ligaments and tendons, lameness and blindness if treatment is delayed.
Wormer: Use an ivermectin-based wormer frequently.

The information on this page is provided as an information resource only, and is not to be used or relied on for diagnostic or treatment purposes, and should not be used as a substitute for professional veterinary treatment.