Horses come into contact with a variety of both internal and external parasites throughout the course of their lifetimes. While having a low worm burden typically has little or no effect on a horse’s health, a high burden of parasites can cause serious health problems if left untreated. Worms that end up in horses come from contaminated pastures. Young horses generally have less natural immunity to worms than older horses, so they tend to carry higher parasite burdens and can contribute more eggs to the pasture. Older horses and those that are stabled continuously are unlikely to pick up many parasites because the worms are unable to complete their life cycle.¹ However, even short periods of grazing can lead to infection, so one should never assume that a horse is free of parasites, whatever its lifestyle.

Types of Worm Infestations in Horses

Small redworms, also known as cyathostomins, are the most common and most dangerous parasite for horses. They reproduce very quickly and have serious consequences to a horse’s health. Adult small redworms feed on intestinal tissue, with large numbers causing harm to the gut wall. They are one of the most common causes of spasmodic colic, particularly in young horses. Encysted small redworms at the larval stage can tunnel into the gut wall where they hibernate. When redworms are hibernating in the gut wall they do not cause problems, but when large numbers emerge, they can damage the gut wall and cause colic, weight loss, diarrhea or even death.¹ Young horses less than six years old are most likely to be affected.

Large redworms (also known as strongyles) are less of a threat than small redworms. Their population and prevalence has decreased over the years, but they still pose serious health consequences. Adult large redworms are found in the large intestine and produce eggs which are passed in the horse’s droppings onto the pasture. The eggs are then eaten by horses while they are grazing. The larvae then hatch and burrow into the walls of the arteries that supply the horse’s intestine, damaging the lining of the blood vessels and causing blockages which stop the blood supply to the intestine. These worms can also cause colic and the rupture of blood vessels.^1,2 Severe damage from large redworms affects the horse’s digestion, causing spasmodic colic. In the very worst cases, the horse may need to have the damaged section of intestine surgically removed. Clinical signs of large redworm infestations can include colic, anemia, weight loss, difficulty maintaining or putting on weight, and a dull or lethargic demeanor. Large redworms can usually be picked up via a fecal egg count. 

Tapeworms can infest horses of any age, but the damage caused to very young and geriatric horses makes them more vulnerable. Adult tapeworms live at the junction between the small and large intestine and release segments containing eggs into the droppings. The eggs are eaten by forage mites on the grazing land and are then picked up by the horse as they graze. The presence of tapeworms around this part of the intestine can cause impaction colic as they block the passage of food. They also irritate the intestine which can lead to spasmodic colic.¹ Tapeworms in foals can prevent normal growth due to malnutrition, and adult tapeworms can cause ulcers in the intestinal wall and may even rupture the intestinal tract. Clinical signs of tapeworm include weight loss, colitis, spasmodic colic and impaction colic. Tapeworm eggs are found in segments, so these will not be picked up on fecal egg counts.

Roundworms, also known as ascarids, typically affect young horses under four years old and are often known as “large roundworms” because they can be up to 30cm in length. Due to their size, roundworms can block the intestine of small foal, causing impaction colic and rupturing of the intestine. This condition can be fatal and may require emergency surgery. Clinical signs of roundworms include coughing, nasal discharge, depression, a rough coat, impaction colic, weight loss or a struggle to maintain or put on weight.¹ A fecal egg counts can confirm roundworm infection.

Ivermectin + Fenbendazol for Worms in Horses

Ivermectin is a drug mixture of two structurally semisynthetic lactones that is used in veterinary medicine as an anthelmintic, boticide, acaricide, and insecticide. Ivermectin has a broad spectrum of activity against intestinal and insect parasites, and kills migrating larvae.² Ivermectin is widely prescribed by veterinarians in all horse deworming programs because of its efficacy.

Ivermectin is known to provide effective broad spectrum control of bots, habronema, ascarids, strongyles, thread worms, pinworms, tape worms, lung worms, and is also effective against the filaria of Onchocerca, larvae of habronema, and arterial larvae stages of strongyles. 

Fenbendazole is a broad spectrum anthelmintic that is used to control a number of common equine parasites, including large strongyles (Strongylus edentatus, S. equinus, S. vulgaris), encysted early third stage (hypobiotic), late third stage and fourth stage cyathostome larvae, small strongyles, pinworms (Oxyuris equi), ascarids (Parascaris equorum), and arteritis caused by fourth stage larvae of Strongylus vulgaris.²

Fenbendazole is considered to be an extremely safe wormer by veterinarians. According to the available literature, it is able to withstand overdoses of 100-200 times the standard dose without causing harm to the horse. For this reason, fenbendazole is often used to clear immature parasites, which are often resistant to worming medications due to their slower metabolism.

Fenbendazole is used as a part of a veterinary reviewed and approved program of parasite control. This program may include a multi-dose regime over a period of days specifically to control of the migrating larvae of S. vulgaris.²

Fenbendazole is known to be effective against:

• Redworms
• Pinworms
• Roundworms
• Stomach hair worms
• Large-mouthed stomach worms²

Swelling and itching reactions after treatment with ivermectin have occurred in horses with heavy infections of neck threadworm microfilariae.² In horses, no side effects for a single dosage of Fenbendazole as high as 454 mg/lb were noted.³ Since equine dewormers have been known to be toxic to dogs and cats, cleaning of barn aisles, wash-racks, etc. is recommended after administration, particularly in operations in which dogs and/or cats are known to be present.

Where to buy Ivermectin + Fenbendazole

Ivermectin + Fenbendazole is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. IVERMECTIN 2% / FENBENDAZOLE 20% (250mL) by NexGen Pharmaceuticals provides a comprehensive anthelmintic/deworming solution for equines of all ages.

Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

¹Nielsen, M., et. al. Handbook of Equine Parasite Control, Second Edition. March 2018, John Wiley & Sons, Inc.

²equimed.com.
³equimed.com.