March 31, 2026, 8:49 am | Read time: 4 minutes
Worms are not uncommon in horses—and often go unnoticed for a long time. In the digestive tract, they can cause significant damage. PETBOOK reveals why worms are so insidious and what owners should watch out for.
When Parasites Disrupt Digestion
In a horse’s body, digestion usually occurs quietly and reliably. The gastrointestinal tract ensures that food is broken down and nutrients are absorbed. When this finely tuned system is thrown off balance, it often has noticeable effects on the entire organism. One possible cause is parasites in the digestive tract. Worms settle in the stomach and intestines and attach to the mucous membrane—the sensitive inner lining. There, they deprive the horse of nutrients.
Depending on the type of worm, the infestation may be limited to the intestines or affect other areas of the body. Some larvae migrate through blood vessels to organs like the liver or lungs during their development. They can damage tissue and cause inflammation. In the intestines, the mucous membrane is attacked, which can impair digestion. Nutrients are absorbed less effectively, affecting weight, performance, and the horse’s overall health.1
Causes of Worm Infestation in Horses
The infection usually begins unnoticed: Horses ingest worm eggs or larvae while eating. This occurs most frequently in the pasture, but infection can also happen in the stable through contaminated feed, water, or contact with feces. Some parasites are transmitted by intermediate hosts. Tapeworms, for example, enter the body via moss mites.2
Additionally, many worm eggs are extremely resilient. They can survive in the environment for months, creating a continuous cycle: Infected horses excrete eggs, which develop further and are ingested by other animals.
Recognizing Worm Infestation in Horses
A worm infestation often develops gradually. The symptoms are nonspecific and can be easily misinterpreted in daily life.
Typical signs include:
- dull, lackluster coat
- weight loss or lack of weight gain
- reduced appetite
- diarrhea or constipation
- recurring colic
- decreased performance
- fever
Depending on the type of worm, additional symptoms may occur. If larvae migrate through the lungs, coughing can develop. Pinworms cause severe itching around the anus, leading horses to rub their tails. Notably, even a more severe infestation can remain undetected for a long time.
How Worms Are Diagnosed in Horses
The primary method for detecting a worm infestation is examining fecal samples. Eggs, larvae, or other developmental stages of the parasites are identified under a microscope.
For certain types of worms, such as large strongyles, further laboratory tests are necessary. For pinworms, a tape test can be conducted in the anal area to secure eggs.
Since individual samples are not always conclusive, multiple samples or pooled samples from a group are often examined. Additionally, special tests, such as those from saliva or blood, can be used, especially for tapeworms.
How Worms Are Treated in Horses
Treatment is usually carried out with anthelmintics, which are medications that kill worms. The choice of active ingredients depends on the type of parasite.
Today, there is increasing emphasis on using medications selectively and not excessively. The reason is the development of resistance: Some worms are already less sensitive to certain active ingredients. Therefore, treatment ideally relies on prior fecal examinations.
There are two basic approaches:
- Strategic Deworming: Treatment according to a tailored plan for the entire herd, combined with regular monitoring
- Selective Deworming: Treatment only for horses with a confirmed significant infestation
For certain parasites, such as tapeworms, the entire herd is often treated once a positive result is obtained.
Recognizing and Treating Lungworm Infestation in Cats
Why Colic in Horses Can Become a Major Threat
What Is the Prognosis?
Worms in horses are often treatable. Severe illnesses have become less common today, as effective medications are available.
However, a severe or untreated infestation can have serious consequences, including inflammation in the intestines, damage to internal organs, or intestinal blockage. Such complications can be life-threatening. The prognosis largely depends on how early the infestation is detected and treated.
Can Horse Owners Prevent Worm Infestation?
Prevention is a key aspect of managing parasites. The goal is to break the cycle of infection.
Important measures include:
- regular fecal examinations
- an individually tailored deworming plan
- consistent hygiene
In the pasture, feces should be removed regularly, ideally every two to three days. Adjusting stocking density and rotating pastures can also reduce risk.
In the stable, daily mucking out and cleaning and disinfecting stalls, feed, and water areas are crucial. Since horses can infect each other, the entire herd should always be considered. Thoughtful management helps limit the spread of parasites and maintain the effectiveness of medications in the long term.