Not all the same!

Different types of internal parasites affect horses with varying degrees of severity, depending on their life cycle and the organs they target.

Different types of parasites also react differently to deworming agents. 

Ask your veterinarian to help you develop a customized strategic parasite management program for your horse.

Common internal parasites in horses

Know what your horse is up against!

Did you know that there are many different types of parasites that can potentially infect horses? The good news is that just a few of these parasites are of any real concern in North America. You'll find a brief description of them in the chart below.

Internal parasites 101

Parasite Transmission Characteristics
Small strongyles Ingestion of larvae. Considered the primary reason for deworming horses that spend time at pasture.1
Reside in the large intestine.
Larvae can burrow into the intestinal lining and form cysts.
Most dewormers do not kill encysted small strongyle larvae, that can remain dormant for years. 
Clinical signs include weight loss, diarrhea, listlessness, decreased performance, etc.
Large strongyles

Ingestion of larvae.

Larvae migrate from the intestinal tract to the artery that provides blood flow to the intestines. 
The resulting loss of blood supply may result in the death of tissue in a portion of the intestines. 
This can lead to colic and, in extreme cases, death.
Roundworms (ascarids) Ingestion of eggs that are shed in the feces of infected horses. These eggs can remain infective in the environment for up to 10 years, even in extreme conditions. Common in younger horses.
Can grow as large as 50 cm in length and 8 mm in diameter.
The presence of a large number of adult worms can lead to intestinal blockage or rupture.
Larvae can migrate through the liver and lungs, causing signs of respiratory disease and tissue damage, which can leave the horse predisposed to secondary bacterial pneumonia.
Clinical signs include weight loss, diarrhea and colic, among others.
Tapeworms Ingestion, through grazing, of mites that are infected with tapeworm eggs (which are shed in the feces of infected animals). Firmly attach to a specific area of the intestinal tract, at the junction of the small intestine and the caecum.
May lead to colic associated with the telescoping of the small intestine into the caecum.
Pinworms Ingestion of eggs. Resides in the last portion of the colon.
Females deposit their eggs on the skin around the anus.
The slimy substance surrounding the eggs causes skin irritation. As a result, infected horses may have a rough tail head from rubbing the irritated region against objects.
Bots Ingestion of eggs deposited by adult botflies on the hairs of the front legs and on the lips of horses. The eggs hatch when horses lick or bite the area where they were deposited.
Larvae penetrate the lining of the mouth, are eventually swallowed, and find their way to the stomach, where they attach, without causing much damage. Eventually, the larvae detach, pass out in the feces, and mature into botflies.
1. Love S, Murphy D, Mellor D. Pathogenicity of cyathostome infection. Vet Parasitol 1999; 85:113-121.