Thread worms and other worms in horses

Thread worms are one of many worms associated with horses.

A horse with worms can go downhill fast; therefore preventative measures should always be used with any new horse arriving at the yard to avoid worm infestation.

When a new horse arrives on a yard, worm it and keep it in the stable for a few days (I keep them in for 5 days) before turning the horse into the field with other horses. 

Here is a list of the different worms, symptoms and treatment available.


These worms can attack a foal in the first few weeks which will cause persistent diarrhoea and loss of appetite.

Thread worms are commonly caught from the foal feeding from his mother, or breaking of the skin.

Large red worms

The most common symptom is intermitant colic and is potentially very dangerous.

A heavy infestation may result in a loss of condition.

Treatment drug used Ivermectin.

Small red worms

Small red worms are very common internal parasites.

Symptoms are weight loss and diarrhoea.

The same chemical is needed as for large red worms (Ivermectin).

Large round worms

Large round worms up to 50cms in length. Symptoms associated with this parasite are, development of a cough, respitary problems, pot belly, dull coat, emaciation.

Anthelmintic drugs are the treatment.


An itchy tail can be a sign of pinworms.

Lift up the tail and inspect the horse’s rear and dock (underneath the tail) as you won’t notice them in the horse’s droppings.

Pinworms won’t show up in worm egg counts either.

Lung worms

These worms are commonly found in donkeys, and mostly with no symptoms.

However, for horses with lungworms a consistent cough will be noticeable, but will not be present in foals and yearlings. (One year old)

Drug used Ivermectin.

Tape worms

If tapeworms are present, they won’t show up in a worm egg count.

Sometimes symptoms are not so obvious. Pyrantel drug is required.

Scroll down from thread worms for more horse worming information.

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