Horses come into contact with a variety of internal and external parasites throughout their lifetimes, and among these are worms. While having a low worm burden will have little or no effect on a horse’s health, a high parasite burden can cause serious health problems if left untreated. Worms are among the most common of horse ailments. 

The life cycle of worms includes egg and larval stages. The eggs of most worms are ingested from infected pastures and develop inside the horse’s gut or lungs where they have the potential to cause disease. Eggs produced by adult worms are then shed in the horse’s feces. This increases the existing worm burdens on pastures and can thereby infect (or re-infect) other horses.¹

Worms Affecting Equines

Small Redworms (Cyathostomes)

Small redworms are the most common internal parasite of horses. They produce larvae that hibernate in the gut wall during the winter and emerge in large numbers as adults in the spring, causing severe damage to the intestines.¹Cyathostomes can cause weight loss, diarrhea and colic with potentially fatal consequences, particularly at the time of emergence.¹

Large Redworms (Strongyles)

Large redworms (e.g., Strongylus vulgaris, S. edentatus and S. equinus) are among the most dangerous internal parasites of horses.¹ Their life cycle includes a larval stage during which they migrate in the blood vessels and can cause anemia and blood clots. They can eat through the lining of the gut wall and travel through the blood vessels of the gut, causing significant bleeding. Severe cases can lead to the death of the horse.

Roundworms (Ascarids)

The roundworm most commonly affecting horses is Parascaris equorum.¹ The adult worm is large, measuring up to 50 cm in length and 6 mm wide. These can be particularly serious in young foals, as they can cause impactions (blockages of the intestine) that may require surgery. Roundworm larvae can migrate through the gut wall, to the liver and then the lungs. The larvae are coughed up and swallowed where they mature to egg-laying adults within the gut. 

Pinworms 

Pinworms (Oxyuris equi) can damage a horse’s bowel prior to laying eggs around the outside of the anus, causing intense itching and irritation.^2,3 Female pinworms crawl out of the horse's rectum, deposit eggs on the perianal region and crawl back into the rectum. The infective pinworm eggs are ingested orally; once in the colon, the larvae develop through various stages before becoming sexually mature. 

Threadworms

Threadworms generally remain dormant in adult horses but can transfer to newborn foals through the mare’s milk.¹ Given their underdeveloped immune response, this leaves the foal susceptible to diarrhea and anemia, and its growth may also be affected. Natural immunity to threadworms usually develops by 6 months of age.¹

Tapeworms (Cestodes)

The equine tapeworm, Anoplocephala perfoliata, is present in most places where horses have pasture access.² Thus, the majority of horses harboring tapeworms tolerate them without any clinical signs. The presence of tapeworms depends on climatic conditions favoring the orbibatid mite, an intermediate host.^1,2

Lungworms 

Lungworm is caused by the parasitic roundworm Dictyocaulus arnfieldi and causes lower respiratory tract issues in horses. This often results in bronchitis or pneumonia evidenced by severe coughing. Thus, lungworm can be difficult to distinguish from other respiratory ailments. Lungworms are ubiquitous in pastures frequented by donkeys, which are the lungworm’s natural host.¹ 

Ivermectin for the Treatment of Worms in Equines 

Ivermectin is an antiparasitic that is FDA-approved for horses. It is known to provide effective broad spectrum control of:

• Small strongyles, including those resistant to some benzimidazole class compounds (adults and fourth-stage larvae): Cyathostomum spp., Cylicocyclus spp., Cylicostephanus spp., Cylicodontophorus spp., Gyalocephalus spp., Triodontophorus spp. (adults)
• Intestinal threadworms (adults): Strongyloides westeri
• Pinworms (adult and fourth stage larvae): Oxyuris equi
• Ascarids (adult and third and fourth stage larvae): Parascaris equorum
• Hairworms (adult): Trichostrongylus axei
• Large-mouthed stomach worms (adult): Habronema muscae
• Neck threadworms (microfilariae): Onchocerca spp.
• Bots (oral and gastric stages): Gasterophilus intestinalis, G. nasalis³

Swelling and itching reactions after treatment with ivermectin have occurred in horses with heavy infections of neck threadworm microfilariae.¹ 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

Ivermectin is available in the U.S. through pharmaceutical manufacturers and through veterinary custom compounding companies. 

Ivermectin carries several potential drug interactions. 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.

¹vcahospitals.com.

²aaep.org.

³Merck Veterinary Manual.