author/source: Jessica Mizzell

All About The Worms: A Guide to Internal Parasites

Equine Parasitesby Jessica Mizzell 

Internal parasites are an inevitable problem that all stable managers and horse owners alike have to deal with.  Most horses will have some sort of parasite load at any given point, so it is important for individuals involved in a horse's care to be educated on all aspects of the topic - from types, risk factors, prevention, and treatment.

The most common types of internal parasites found in horses, and a brief description of their lifecycle and effects are as follows: 

  • Threadworms: These can be ingested through environment, or through skin lesions.  They can also be passed through a mare’s milk to the foal.  Effects include damaged organs, potbelly, and diarrhea.
  • Large strongyles: Eggs are laid in the horse’s intestine by females.  They exit via manure and hatch on the ground.  When larvae is ingested, they can move through a horse’s intestinal wall, making their way to the blood stream, compromising organs, and further making it back into the gastrointestinal tract.  The cycle is then repeated.  Symptoms include stunted growth, dull/stringy coat, weight loss, and colic. 
  • Small strongyles: Depending on climate, eggs can hatch on the ground from manure; this can happen anywhere from months to a matter of days.  When inside the horse’s system, larvae can encyst on the wall of the cecum or colon.  If the encysted larvae erupt, the horse can become very ill.
  • Ascarids: These are found primarily in foals.  Eggs are very resilient and can live for years, sticking to anything they come across.  Once ingested, the larvae move through the gut wall and into the liver and lungs.  They cause gastrointestinal and respiratory issues.
  • Pinworms: These live in the horse’s colon and females lay eggs in the rectum and perianal.  They can then fall to the ground and be ingested, but are not ejected through manure as most types of worms are. Pinworms thrive in moist climates, and infected horses may rub their tails from itchiness and discomfort. 
  • Tape worms:  These are found in the ileocecal valve, and in extreme cases can cause severe colic. Eggs are ingested by mites first, and then by the horses. 

Overall, a dull and stringy coat, weight loss, lack of appetite, lethargy, diarrhea, decreased performance, pot belly, and colic can be symptoms of a high parasite burden in your horse, and should be investigated if a horse is displaying these problems.

When it comes to controlling the parasite load in your horse’s system, protocols have changed a lot in the last decade.  There are some outdated practices that are used today, which can unfortunately do more harm than good.  I briefly boarded at a barn that required bi-monthly deworming; I think this practice is more common than we would like to think, so my goal is to bring more awareness to the subject.  Understanding protocols and methods to prevent the load from becoming a health risk to the horse is a crucial part of animal husbandry. Horses can acquire parasites very easily by ingesting parasite eggs in their environment and feed, so here are some tried and true ways to mitigate the risk without causing more damage:  

  1. Cleaning turnout areas: cleaning paddocks and fields of manure will decrease the chance of eggs hatching once they have left the horse's system. This should be done at least every few days, if possible, as it will help to dispose of parasite eggs in manure before they hatch into the pasture where horses can ingest them.  Furthermore, meticulous stall and water/feed bucket cleaning is another good way to keep worms at bay, as some parasites can live on many surfaces.
  2. Manure management: Composting is a manure management practice that will not allow parasite eggs or larvae to thrive.  Utilizing this practice at the recommended temperatures of between 135 to 160 degrees Fahrenheit will kill any eggs, and also make good use of the waste!  Otherwise, keeping dumpsters or manure piles far from the barn can also help to lower the risk of worms getting back into your horse's system as the manure will not be in the general vicinity of where the horse resides.
  3. Resting grass fields and paddocks: If pastures cannot be cleaned on a regular basis, resting and rotating fields is another way to decrease the chances of your horse ingesting parasite larvae.  Most eggs and larvae will not survive for months on end, depending on type and climate.  If you have access to other species such as sheep or cows, putting them on rested pasture can be beneficial, as the types of parasites that affect horses will not affect these species, giving the pasture time to recoup and eggs/larvae to die off or be ingested by the other animals.
  4. Fecal Egg Counts: Preforming a fecal egg count on your horse is a simple and inexpensive way to tell what your horse's parasite load is. The test measures the amount of most types of parasite eggs per gram in your horse’s manure. A low shedder typically has less than 200 eggs per gram. A moderate shedder has between 200 and 500 eggs per gram. A high shedder has over 500 eggs per gram.  You can do this yourself with a microscope (I have done this in labs during school - it is definitely an acquired skill and takes practice!), have your vet perform the test, or buy a kit from an equine store to send to a testing site.  It is recommended to do a fecal egg count at least twice yearly, usually in the spring and fall, and deworm (or not) based on the results.  Your vet can be a helpful advisor in selecting which dewormer to use based on count and type of parasite. Checking egg count before deworming is an important factor in curbing resistance to dewormers which ultimately leads to a higher parasite burden. Outdated protocols such as utilizing daily dewormer and deworming on a rotational basis every other month are very harmful to our horse’s system and can have detrimental consequences if done irresponsibly. 
  5. Deworming based on Fecal Egg Counts: Depending on the type and parasite load determined, you may need to deworm your horse.  There are many different kinds of anthelmintics or dewormers which target specific types of parasites with their active chemical ingredient. Yes, these are chemicals we are putting in our horses’ systems, so care must be taken!  Each anthelmintic targets a different cycle in a parasite's lifecycle.  These include:
  • Ivermectin: targets threadworms, ascarids, pinworms, large strongyles, small strongyles, and tapeworms.  Good for adult worms and migrating larvae.
  • Oxibendazole: targets threadworms, ascarids, pinworms, large strongyles, and small strongyles.  Good for adult worms.
  • Moxidectin: targets threadworms, ascarids, pinworms, large strongyles, small strongyles, and tapeworms.  Good for adult worms, and encysted larvae, but not migrating larvae.
  • Fenbendazole: targets ascarids, pinworms, large strongyles, and small strongyles.  Good for adult worms and encysted larvae with 5-day treatment plan.
  • Pyrantel Pamoate: targets ascarids, pinworms, large strongyles, small strongyles, and tapeworms.  Good for adult worms.
  • Pyrantel Tartrate: targets ascarides,  large strongyles, and small strongyles.  Prevents larvae migration.
  • Praziquantel: targets tapeworms. 

As mentioned before, it is important to consult with your vet for deworming protocols. With the primary goal of keeping our four-legged friends in top health, your vet should be the principal source to guide horse owners of all backgrounds to the best practices for each individual horse. 




Preston, Lisa. The Ultimate Guide to Horse Feed, Supplements, and Nutrition. Skyhorse Publishing, 2016, Pg. 103-114.