Equine pinworms, or Oxyuris equi, are common parasites that inhabit the horse’s colon. Female pinworms lay their eggs on the perianal skin (around the anus), which can cause intense itching and irritation for the horse. [1]

Pinworm infections are most common in younger horses, but horses of any age can be affected. These parasites are transmitted among horses through the inadvertent ingestion of eggs present in their environment. Eggs from infected horses are shed into the environment, contaminating forage and adhering to fence posts and stall surfaces where horses have been scratching. [2][3]

Although pinworms can be bothersome for horses and their owners, they are relatively harmless and can be effectively treated. If you notice signs of a pinworm infection in your horse, seek assistance from your veterinarian for proper diagnosis and to explore available treatment options.

You can help prevent pinworm infections by keeping your horse’s environment clean and establishing a deworming schedule. Consult with your veterinarian to determine the most appropriate deworming product and frequency based on the parasites prevalent in your area.

Pinworms in Horses

Pinworms are nematode intestinal parasites that belong to the family Oxyuridae. They are predominantly found in the colon and rectum of equids. Pinworm infections, referred to as oxyuriasis or enterobiasis, can cause mild to moderate health issues in affected horses. [4]

The adult female pinworm can grow to 15 cm in length and has a slender, thread-like body with a long, pointed tail that resembles a pinhead. The males are smaller in size, only reaching 1 cm in length.

Adult pinworms inhabit the large intestine of the horse, where they attach themselves to the intestinal mucosa and reproduce. [4]

Oxyuris equi are found throughout the world, especially among groups of horses kept in close proximity with poor cleaning practices.

Life Cycle

The pinworm life cycle begins when a horse ingests an egg from somewhere in its environment. The horse can come into direct contact with the eggs from another infected horse or by consuming feed into which the eggs have fallen. [3]

After being ingested, the pinworm eggs travel through the horse’s digestive system and hatch in the small intestine. The larvae, known as third-stage (L3) larvae, then penetrate the mucosal walls, or lining, of the ventral colon and cecum.

After 3-11 days, the pinworms emerge from the mucosal walls as fourth-stage larvae (L4) and attach themselves to the lining. Approximately 50 days later, they molt and develop into fifth-stage larvae (L5).

Finally, around 100 days later, they reach the adult stage, capable of reproduction. The adult pinworms live and mate within the horse’s dorsal colon.

The male pinworm dies after mating and the female pinworm migrates out of the rectum and anus. [3] The female pinworm deposits up to 60,000 eggs in a large sticky mass in the perianal region.

Over a period of 3-5 days, these eggs develop and become capable of infecting other horses. Subsequently, they detach from the skin and contaminate the surrounding environment or are transferred to surfaces through rubbing or contact.

The pinworm life cycle takes approximately 4.5 – 5 months to complete. However, research suggests that pinworms can reach adulthood sooner, allowing them to spread eggs at a faster rate. [3]

Signs of Infection

Pinworms generally have minimal detrimental effects on horses and rarely cause issues other than irritation. A horse with a pinworm infection may rub their rump on fences, stall doors or trees in their pasture to try and relieve the itch.

This can lead to the following signs of infection: [5]

  • Restless behaviour
  • Irritated hind-end
  • Inflamed skin around the tail head
  • Sores around the anus
  • Hairless patches on the rump

Horses affected by pinworms may have gelatinous streaks of yellow or white substances on their perianal skin, which represents sticky clumps of pinworm eggs.

In rare cases, pinworm infection can lead to a loss of body condition. [5]

In one study foals with pinworms were observed to have poor hair coat quality, loss of body condition and lymphocytosis (increase in white blood cells) indicating systemic disease. [3]

Risk Factors

Equine pinworm infections have a worldwide distribution and can be found wherever horses are present, spanning across different geographic regions.

Pinworm infections are seen most often in younger horses. Foals and yearlings tend to use their tongues and lips to explore their environment, which makes them prone to ingesting eggs. [6]

However, there is a growing trend of adult and geriatric horses developing oxyuriasis. This may be due to genetic adaptation in pinworms that allows them to evade detection by the horse’s immune system. [3]

Older horses also often have waning immunity to intestinal parasites and will be susceptible to the same parasites as young foals. [14]

In most cases, pinworm infections in a herd are limited to only one or two horses, and these individuals may not display any noticeable symptoms.[7]

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Diagnosis

Pinworm infections are sometimes detected during routine physical or rectal examinations performed by veterinarians. Owners may also notice adult pinworms that have been expelled in the feces or protruding from the anus, providing visible evidence of the parasitic infestation. [2]

Unlike other internal parasites that commonly appear in fecal samples, pinworms are seldom found in droppings, making fecal flotation an unreliable diagnostic method. Collecting fecal samples directly from the rectum increases the chances of detecting pinworm eggs. However, other diagnostic tests are less invasive and more reliable.

The most common method to diagnose pinworm infections involves using a tongue depressor to scrape the area around the horse’s anus for pinworm eggs. Clear, sticky tape can also be applied to the perianal skin to collect eggs.

These samples are then examined under a microscope. Equine pinworm eggs are easily differentiated from other parasite eggs under a microscope. [1][8]

Differential Diagnosis

Several conditions can cause irritation, itchiness, and patches of hair loss in horses. Additional tests may be conducted by your veterinarian to rule out the following skin conditions prior to making a definitive diagnosis: [2]

Treatment

If your horse is diagnosed with with pinworms, they should be isolated from the herd during treatment to reduce the risk of environmental contamination and further transmission.

Anthelmintics (Dewormers)

Anthelmintics, also known as dewormers, are a class of medications specifically designed to eliminate parasitic worms. Various anthelmintics have proven efficacy against pinworms, including: [1]

  • Macrocyclic lactones
  • Benzimidazoles
  • Fenbendazole
  • Pyrantel salts

To ensure the safety of the horse while effectively targeting the parasites, dewormers inhibit metabolic processes unique to the parasites. These medications are administered orally, either in the form of a paste or gel using a syringe or as a daily feed-through product. [9]

It is very important to know how much your horse weighs prior to deworming. This will ensure your horse receives the proper dose of anthelmintic and help prevent anthelmintic resistance.

Anthelmintic Resistance

Anthelmintic resistance refers to the phenomenon where parasites become less susceptible or completely resistant to anthelmintic medications. This reduces the effectiveness of deworming treatments in controlling or eliminating the parasites. [8][10]

There is no research on resistance to anthelmintics in equine pinworms and it is difficult to assess resistance in these worms due to their unique egg-laying processes. Due to the infrequent presence of pinworm eggs in fecal samples, fecal egg counts (FECs) are generally not useful for determining infections. [3]

Case reports have shown anthelmintic resistance in equine pinworms. [11] Pinworms, being exposed to broad-spectrum anthelmintics during routine deworming for other parasites, gradually develop reduced susceptibility to these medications.

The growing frequency of pinworm infections in mature horses serves as evidence of the increasing anthelmintic resistance. [3] In the past, these infections were primarily observed in foals and young horses. However, as pinworms have adapted, they are now capable of infecting a wider range of hosts, including adult horses already on a deworming program.

Macrocyclic lactones

The commonly used macrocyclic lactones in horses are the medications ivermectin and moxidectin.

Macrocyclic lactones, specifically avermectins and milbemycins, are known for their efficacy against various parasites inhabiting the equine digestive tract. This includes large strongyles, ascarids, and pinworms. [5][12]

Macrocyclic lactones exert their effects on parasites by targeting and binding to glutamate-gated chloride channel receptors in the cells. This leads to an influx of chloride ions into the cells, resulting in paralysis of the parasites and, ultimately, death. [9]

These broad-spectrum dewormers are approved for use in horses and are effective even at low doses. A single dose can provide prolonged protection against larval and adult parasites.

However, there have been recent reports of anthelmintic resistance against macrocyclic lactones, even with more frequent use and increased doses. [3]

It is worth noting that ivermectin and moxidectin were not initially recognized by regulatory agencies for effectiveness against adult pinworms.

Benzimidazoles

Benzimidazoles are considered the most effective anthelmintics for combating pinworms. They can kill pinworm eggs, are effective at low dosages, and carry a low risk of side effects. [3][13]

Benzimidazoles inhibit cellular transport and metabolism in parasites, effectively depleting their energy levels and preventing the excretion of waste products.

These dewormers can eliminate 90 – 100% of mature pinworms. However, clearing the body of third and fourth stage larvae may require repeated administration or higher doses, as these larvae embed themselves in the intestinal walls.

If pinworms exhibit resistance to macrocyclic lactones, treatment with a benzimidazole dewormer may be used as an alternative approach.

Pyrantel salts

Pyrantel salts (Tetrahydropyrimidines) are widely used compounds found in many deworming formulas and products. Pyrantel salts include:

  • Pyrantel embonate
  • Pyrantel hydrochloride
  • Pyrantel pamoate
  • Pyrantel tartrate

These salts kill several species of parasitic worms by inducing excessing muscle contractions. [3][9]

The efficacy of pyrantel salts against pinworms is variable, but pyrantel pamoate is believed to be more than 90% effective against adult pinworms. While anecdotal reports of anthelmintic resistance exist, there is a limited evidence to evaluate these claims. [8]

Pyrantel salts are commonly used in combination with dewormers from other drug classes, such as ivermectin, to enhance their efficacy. Discuss this combination with your veterinarian, particularly for problematic infestations that seem to recur with regularity. Combining benzimidazole or pyrantel pamoate with a 2- or 3- month course of daily pyrantel tartrate can kill the late stage larvae as they mature in the horse before they can begin laying eggs and contaminating the environment again.

Supportive Treatment

To provide relief and alleviate symptoms, regularly clean your horse’s perianal region. The gelatinous substance deposited by the female pinworms can cause itchiness and irritation. Scrubbing this region helps to make the horse more comfortable during treatment, helping to reduce inflammation hair loss and. [7]

Horses may develop sores in the tail, rump, and anal area due to excessive rubbing. These sores are susceptible to bacterial infection and should be managed by clipping the affected area and regularly washing with antiseptic wash. [2]

If a secondary skin infection develops, horses may require oral antibiotics to effectively treat and clear the infection. [1]

Some horse owners apply petroleum jelly or another soothing lubricant to the perianal area of the horse to prevent the eggs from sticking to the skin. This can create a barrier that reduces irritation and makes it easier to clean the tissue during treatment.

To prevent the transmission of pinworm eggs between horses, the materials used to clean the area should be washed thoroughly with soap and hot water or disposed of properly

Prevention

Preventing the spread of parasitic infections starts with good biosecurity measures at your horse barn.

Pinworm eggs, and other parasites are easily transmitted between horses on grooming supplies, shared tack (i.e. tail wraps), and common surfaces. An itchy horse will rub its tail on fence posts or stall walls, causing the eggs to stick to these surfaces or fall to the ground where other horses may ingest them. [7]

The best strategies to prevent your horse from being affected by pinworms and other internal parasites include:

  • Avoid sharing grooming equipment and tack to minimize transmission of parasites between horses.
  • Avoid overcrowding in barns, pastures and paddocks to reduce the risk of parasite transmission.
  • Maintain a clean and hygienic environment, including regularly cleaning water troughs, feeders, and stalls.
  • Thoroughly wash your hands and clean your clothes and boots after handling your horse.
  • Minimize exposure to parasites by keeping infected horses separated from the herd until treatment is completed.
  • Quarantine new horses until they have been cleared by a veterinarian to prevent introducing parasites to the existing herd.
  • Keep horses away from areas where eggs may be prevalent, such as contaminated pastures or areas with high pinworm populations. Eggs survive in the environment for 8 to 10 weeks.
  • Practice good pasture management, including regular manure removal and rotational grazing.
  • Consider using targeted deworming treatments based on fecal egg counts and individual horse needs.
  • Monitor your horse’s behavior and look for signs of itching or rubbing, which can indicate a pinworm infestation. Promptly address any signs of irritation to prevent further spread.
  • Provide a well-balanced diet to support your horse’s overall health and immune system

Summary

  • Adult equine pinworms infect the horse’s colon and lay eggs on the skin around the anus, causing irritation.
  • Infected horses may have itchy, inflamed skin around the rump, anus and tail head from scratching.
  • Pinworm infections are diagnosed based on clinical signs, a scrape test or by using clear sticky tape. A fecal egg count test is not helpful for diagnosing pinworms.
  • Horses should be kept in a clean environment and put on a regular deworming schedule to reduce the likelihood of parasite infection.
  • Pinworm infection can be treated using a combination of anthelmintics (dewormers) and supportive care.
  • If you suspect your horse has a pinworm infection, contact your veterinarian for a physical examination.

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References

  1. Nielsen, M. K. Oxyuris equi Infection in Horses. Merck Veterinary Manual. 2019.
  2. DiPietro, J., Lloyd, S., Senter D. Oxyuriasis. Vetlexicon. Accessed at June 4th, 2023.
  3. Reinemeyer, C. R. Review of the biology and control of Oxyuris equi. Equine Vet Educ. 2014.
  4. Sellon, DC., Long, MT. Equine Infectious Diseases, 2nd Edition. Saunders. 2014.
  5. Wolf, D. et al. Oxyuris equi: lack of efficacy in treatment with macrocyclic lactones. Vet Parasitol. 2014. View Summary
  6. Reinemeyer, C. R. & Nielsen, M. K. Control of helminth parasites in juvenile horses. Equine Vet Educ. 2016.
  7. Nielsen, M. K. et al. Internal Parasite Control Guidelines. American Association of Equine Practitioners. 2019.
  8. Waldridge, B. Pinworms in Horses. Kentucky Equine Research. 2012.
  9. Vercruysse, J. & Claerebout, E. Anthelmintics. Merck Veterinary Manual. 2014.
  10. Reinemeyer, C. R. et al. Efficacy of pyrantel pamoate and ivermectin paste formulations against naturally acquired Oxyuris equi infections in horses. Vet Parasitol. 2010. View Summary
  11. Reinemeyer, C.R. Anthelmintic resistance in non-strongylid parasites of horses. Vet Parasitol. 2012.View Summary
  12. Vercruysse, J. & Claerebout, E. Macrocylic Lactones. Merck Veterinary Manual. 2014.
  13. Vercruysse, J. & Claerebout, E. Benzimidazoles. Merck Veterinary Manual. 2014.
  14. Klei, T.R. and Chapman, M.R. Immunity in equine cyathostome infections. Vet Parasitol. 1999. View Summary