• Have surgery performed and have the lining of the large intestine examined for them.
  • If the horse dies, have a necropsy performed and have the lining of large intestine examined for them.
  • The most rational way to find out is to do a series of fecal egg counts over a year or so. Though not as precise as the first two methods, fecal egg counts will be much less expensive, and invasive.

There are some people who think that fecal egg counts cannot determine the presence of encysted strongyle larvae. It is true that a single fecal egg count cannot determine their presence directly. However, fecal egg counts repeated over time will indicate the likelihood that encysted strongyle larvae are present. Knowing something about the life cycle of small strongyles is critical to understanding why this is so.

We should first understand that every adult strongyle was once an encysted strongyle larva in the lining of the large intestine. Therefore, eggs in the horses stool are indicative of adult strongyles in the intestine that once were encysted. At this point we should review the life cycle of small strongyles.

Strongyle Life Cycle

1. Eggs are laid in large intestine by adult strongyles.

2. Eggs are passed in the horse’s stool.

3. Eggs hatch and develop into 3rd stage infective larvae. As the eggs hatch, they go through 3 larval stages in approximately 1 week under ideal conditions of 65-85 degrees F with moisture. The 3rd stage develops a coating that protects them when eaten by a horse. Infective larvae climb the grass stalk and wait for a passing horse to eat them.

4. The 3rd stage infective larvae are eaten by the horse while grazing and pass into large intestine.

5. Once eaten by the horse, the larvae burrow into the mucosal lining of the large intestine and become encysted. The larvae can remain in cysts for anywhere from two weeks to more than two years. It is not well understood what causes the larvae to leave the cyst (excyst) and become adult strongyle worms.

6. Larvae excyst, become adults, and begin laying eggs, which are then seen in the stool sample.

As the life cycle indicates, every encysted larva began as an egg passed in the stool of a horse and will at some point develop into an egg-laying adult.



Horsemen’s Laboratory owner Dr. John Byrd has extensive experience with racing and breeding horses and oversees Westbrook Boarding Stable. He created Horsemen’s Laboratory in 1992 so that horse owners could better evaluate their worm control programs and make informed decisions about deworming their horses. Dr. Byrd is a member of:

  • American Association of Equine Practitioners (AAEP)
  • American Veterinary Association
  • American Association of Veterinary Parasitologists
  • American Quarter Horse Association

Visit www.horsemenslab.com to learn more about Horsemen’s Laboratory and parasites. You can also order kits by calling 1-800-544-0599.

If you have questions specific to your barn, pastures, or testing program contact Dr. John Byrd by:


Email comments or questions to [email protected]

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