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Pinworms In Horses – A Pain In The Bum

Arrrrgh, could it be pinworms? Your horse has scratched his bottom so much that the head of his tail is bald! This is a disaster! The likely problem in a horse with an itchy bottom is pinworms. Pinworms in horses is a very common problem and without appropriate pinworm treatment, it will cause pain and discomfort for your horse.

In this article, I will explain:

  • what causes pinworms in horses,
  • treatment for pinworms and
  • how to get rid of pinworms in the horse and the environment –  for good!


What Is The Pinworm Lifecycle?

The lifecycle of pinworms in horses (Oxyuris equi) is a little different to other worms. It’s important to understand the lifecycle so that appropriate pinworm treatment can be given.

The key differences with the life cycle are:

  • It’s direct, meaning that there are no intermediate hosts and no migration through other organs
  • It’s long, it takes worms 5 months to develop from egg to adult to reproduce again unlike other worms whose life cycle is around 3 weeks.
  • Adults live in the rectum of the horse while other worms live in the colon or small intestine.
  • Adult females exit the rectum and lay eggs around the anus of the horse. In contrast, other worms lay their eggs within the intestines and pass into the environment via faeces.

After fertilization, the female migrates to the anus of the horse and lays her eggs in a gelatinous material in clumps on the skin around the anus.

After laying her eggs the females pass out of the anus and dies.

Larval development takes place inside the egg and is quite rapid, taking only 3 to 5 days.

Pinworms in horses life cycle

Pinworms in horses life cycle. Photo credit: Pony Club Victoria.














The third stage larva (egg + L3) is the infective stage. These infect the horse when the horse licks them off a surface and ingests them.

Once ingested, the larvae (L3) travel to the small intestine where they hatch and then pass into the large intestine.

In the large intestine, the larvae enter crypts of the caecum and colon then moult into fourth stage larvae (L4). This takes about 8-10 days after being ingested.

A final moult into immature adults takes place about 45 to 60 days after infection. The adults mature for a further 100 days until they reproduce.

It can take anywhere from 139-156 days for females to lay their eggs after infection.

Signs Of Pinworms In Horses

Video Credit: roos_worldd

The classical symptom that horses with pinworms have is that they love to scratch their bottom.

Often horses scratch so badly that they remove the hair from the base of their tail. It can be nasty!

The main symptoms of pinworms in horses:

  • intensely itchy rump, tail head
  • broken hairs, bare hairless patches of skin
  • inflamed skin with possible infection around the rump and tail head
  • restless
  • loss of condition
  • dull staring coat

Tail rubbing is a sign of pinworm in horsesA rubbed tail is a clue that your horse might have pinworms

pinworms can cause extreme itch and result in skin soresSevere itch can cause skin lesions

adult pinworm on perianal region of a horseAdult worms can sometimes be seen in the perianal region

pinworm eggs are laid in a sticky yellow gelatinous fluidFemale pinworms lay eggs in a yellow, sticky, gelatinous substance.

Diagnosis Of Pinworms In Horses

As we mentioned above, pinworm eggs are not found in faeces like other worm eggs.  Therefore, we need to check for these worms by other means!

The symptoms described above gives us a good hint that the horse might have pinworms.

Check around the anus and look for the gelatinous substance. Use sticky tape to remove this substance and send to your vet for them to look under the microscope for evidence of eggs.

Sometimes you will see the female worm around the anus or in a dung pile. (See picture above).

How To Get Rid Of Pinworms: Treatment

The key to pinworm treatment is to combine appropriate management and hygiene with an effective parasitic wormer.

Management & Hygiene

Pinworm infections are spread when your horse ingests eggs that have contaminated surfaces.

Once contaminated, they can be tough to eradicate. You need to ensure you practice good hygiene.

It is therefore vitally important to disinfect and clean any area that the contaminated horse has contact with: fence posts, stable doors, covers, feed/water bins and grooming equipment.

Clean your horses’ perianal, tail and dock area using disposable wipes, not sponges. Throw out the wipe after cleaning. In fact, burn it.

Adult females usually lay their eggs at night, so try to wash your horses’ bottom in the morning.

You can try applying vaseline or petroleum jelly around the perianal area. This will aid cleaning the eggs off your horse.

What Wormer Kills Pinworms In Horses?

Unfortunately, anthelmintic resistance is a major issue with many worm species and pinworms isn’t exempt.

For many properties ivermectin just isn’t effective in eliminating pinworm.

The most effective anthelmintic active ingredients that you need to use to eradicate pinworm are pyrantel and fenbendazole.

The number of treatments really depends on individual circumstances. Contact Your Vet Online to help you with this.

Have you had a problem with pinworm on your property? In the comments section below, tell us what worked best for you to eradicate this problem. 

how to get rid of pinworms in horses

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