Frequently Asked Questions

Equine Worm Control SpecialistsFrequently Asked Questions about Worm Control in Horses

Fecal Testing Procedures

Why is it important to have a horse's stool checked?

Monitoring egg counts is the best test to assess the adequacy of a worm control program in live horses.

Does stool sample examinations detect all species of worms?

No, however the procedure is the most accurate test available for detecting the presence of adult round worms and strongyles in the digestive tract. These 2 species of worms are the most common cause of problems. Strongyles are the most common cause of colic. Strongyles are also the worms that most deworming schedules are based on.

What worms are not detected by their eggs appearing in the stool of horses?

The most common 2 worms not detected in the stool sample examination are Bots and Pinworms. Bots are the larval form of the bot fly that lays its eggs on the hair of the horse. These eggs are easily seen and if present the horse should be dewormed at least twice a year with a boticide. Pinworms lay their eggs around the rectal opening and cause the horse to rub its tail.

Are there any other worms not seen in the stool?

Yes, tapeworms.
Tapeworms are very difficult to diagnosis because their eggs are contained in packets, which are actually segments of the adult worms that break off and are passed. Most de-worming medications are also ineffective at the dosage level commonly used. Habronema is another worm not commonly diagnosed by a stool sample examination. This worm is carried by flies and may cause ulcers on the skin as well as in the stomach of horses.

How fresh of a sample do I need to get and how soon do I need to get it out?

We recommend to get a sample that has just been excreted or at least in the last 24 hours. Then fill the container completely full and snap the lid closed. We do also recommend to try to get the sample in the mail as soon as possible or at least within 24 hours. If you need to wait till the next day please leave the sample at room temperature (do not place in the refrigerator).

How are tapeworms and habronema diagnosed?

By clinical signs, history, and location. Tapeworms have been reported in certain part of the United States. However, HORSEMEN’S LABORATORY has found several horses with individual tapeworm eggs in their stool. Habronema have certain areas of the country where they are much more common than others do. They are also often found in biopsies taken from chronic non-healing skin ulcers.

Do all horses have worms?

Horses kept in a clean environment, such as clean well bedded box stalls, will often remain free of worms once effectively de-wormed until exposed to infected larva or eggs. Horses kept in pastures in herds are the most likely to suffer from worm infections, especially the young animals less than 2 years of age.

How does one know that these horses kept in clean box stalls are not infected with worms?

Worm eggs are rarely found in their stool samples. Secondly very little or no damage, due to parasites is observed in the digestive tract of these horses when they die and an examination or necropsy is performed.

Why don't horses kept in clean well bedded box stalls have as high an incidence of worm infection as those in pastures?

This type of care greatly interferes with the life cycle of most worms, especially round worms and strongyles. The only way horses can be infected with roundworms is to eat the eggs. The only way a horse can become infected with strongyles is to eat the third stage larva that take 7 days to develop after the egg hatches. The larva also needs a moist environment in which to live until it is eaten. When the stall is cleaned most of the eggs are removed before they hatch or reach the infected stage thereby reducing the horse’s exposure to infection. Also the drying effect of sawdust, shaving, straw or other dry bedding dries out the larvae and they die before becoming infected . When a horse is kept in a box stall it only has access to it’s own stool or manure and if there are no eggs being passed it will be impossible for it to become infected.

Many veterinarians say that a stool sample evaluation is a poor indicator of parasitic disease. Is this true?

Eggs in the stool of a horse do not tell one whether the worms producing those eggs are impairing the function of that horse. Just as no eggs in the stool of a horse does not tell one that the horse’s function has not been impaired by past infections with worms. However, the presence of worm eggs in the stool of a horse does tell one that adult worms are living in the intestines of that horse and that there is or likely will be impairment to the function of this horse and others that are exposed to the eggs in this stool if they become infected. No eggs in the stool of a horse only indicate that there are no adult laying eggs in the intestines of that horse at that time. No eggs in the stool do indicate that this horse will not be responsible for re-infecting it or other horses. However, this horse may have impaired function due to past infection or due to migrating larva. Only consistent periodical stool sample evaluations started when the horse is very young can give one an indication of possible impairment of function in a horse due to past worm infections. Therefore, the answer is yes if we were to base the evaluation on a single examination and define disease as an alteration of a living body that impairs its function. However, if one looks at the results of several examinations over the life of the horse it is the best indicator we have of parasitic disease in the live horse.

Are there stages of parasites that may be developing in the horse that are not detected by fecal egg counts on a horse's stool?

There are stages of developing round worms and strongyles that may be present. These stages may or may not be in the intestine. Larval stages of roundworms migrate through several areas and organs of the horse as they develop toward adulthood. Strongyle larva also may be encysted in the intestinal wall or migrating through tissues of the body. During these migrating or encysted stages no eggs are produced therefore these stages cannot be detected even though this condition is one of the more serious phases of parasitic disease. However, if stool samples were done at this time on a horse so infected and it’s herd mates eggs would likely be found in their stools, because there are generally all stages of developing worms in a horse. Also the infected horse must be consuming large number of eggs or infective larva to become heavily infected with these immature stages of worms. Consequently, it would appear that if stool sample examinations are run periodically high egg counts must appear in a horse or it’s herd mates before a serious infection may take place. Therefore early detection of horses passing high numbers of eggs is important in the prevention of serious immature parasitic disease.

Is there a better method of evaluating a worm control program in horses than by doing periodical fecal egg counts?

This is the most practical testing procedure on a live horse. One could biopsy the intestinal mucosa in several places during a surgical procedure to determine the number of encysted larva. This procedure would be very costly, could compromise the horse’s health, and entails much risk.

What phase of raising horses can best use the information gained from doing stool sample examinations?

This information is valuable to all phase of raising and training horses. For instance many broodmare owners have asked if they should de-worm their mares just before they foal or just after they foal. If they test their mares regularly throughout their pregnancy and no eggs are found they most likely need not be de-worm at all. If there are eggs present the owners could schedule the de-worming accordingly. Checking the foals regularly throughout their life will assist owners in protecting them from infection and also give them valuable evidence that they have either been kept free of parasite infection or were treated very soon after an infection occurred to minimize the damage done by the infection.

How often do you recommend testing a horse's stool?

We send our clients containers to recheck their horses every 3 months unless there is a change in the horse’s environment. Such as when a horse goes from a box stall to a pasture situation, especially if there are other horses in the pasture or there have been other horses in the pasture recently. When a horse tests positive we recommend that the horse be de-wormed and rechecked in 2-3 weeks. There should be a 90% drop in the number of eggs seen in the stool.

Is the 90% reduction significant?

If there is not a 90% reduction in the number of eggs it would indicate one of two things has occurred. The worms were not sensitive to the de-wormer or the horse did not receive an adequate dose of the de-wormer. This means that the horse is infected with a strain of worms that are drug resistant, or the horse did not swallow the full dose of de-wormer. Either way one would not be aware of the situation had they not done the stool examination following the de-worming.

Are there any negative aspects to doing stool sample examinations?

There is no danger to the horse, because the procedure is completely non-invasive. However, one must understand that while doing fecal egg counts are critical to developing the most efficient worm control program, this procedure is only one of the tools in protecting horses from the effects of internal parasites.


Does Horsemen's Laboratory provide horse owners any other services?

Yes, we also are available for consultation when someone has a special problem with internal parasites. We also provide speakers for horse owner groups and clubs on many topics pertaining to horse health.

How long has HORSEMEN'S LABORATORY been in business?

I started checking most horses I was deworming in the spring of 1991. However HORSEMEN’S LABORATORY was established in Jan of 1993.

Do insurance companies accept stool sample findings as evidence that horses are not harboring worms?

Most insurance companies we’ve spoken to gave the opinion that they would rather have the horse checked and if the sample was negative for eggs the horse would not need to be exposed to the de-wormer or the procedure of de-worming.

How does the cost of doing stool sample evaluation compare to the cost of a paste de-wormer?

The cost is very similar because the cost of a stool sample at “HORSEMEN’S LABORATORY” varies from $18 to $15 or less depending on the number of stool samples an owner intends to run through out the year. However, there is really no comparison in the value received for the money invested. A stool sample evaluation gives the owner information on how well their worm control program is working. This information is a valuable aid in assisting owners in making adjustment in their programs to make them more effective in the prevention of parasitic disease. Giving de-worming medication does not give owners any evidence that their horses are being protected.

Are there other areas of horse care that may benefit from having stool sample examinations done?

When a horse enters training, is switched from one stable to another, or goes back to a farm for a lay up. When these changes occur the horse is frequently de-wormed with each change. This not only may put the horse at some risk it also irritates the owner who has to pay for each de-worming. If there were no parasite eggs in the horse’s stool it would not be necessary to de-worm it. Also when the horse is in full training there would be no need to lay the horse off for a day or two for de-worming. Often these breaks in a horse’s daily routine can cause them to develop problem with tying up, colic, or going off feed. Many of these problems might be avoided if their stool was checked and they were only de-wormed when needed.

Does the de-worming of horse give it any long-term protection against future worm infections?

Contrary to what many owners and trainers seem to believe there is no long terms protection. For example if a horse is feeding on a pasture heavily contaminated with infective strongle larva this horse will begin to be re-infected immediately and the previously given de-worming medication will have no effect on the rate of re-infection. Most de-worming medications only kill the adult worms in the digestive tract, however there are some of the newer drugs that appear to kill the encysted stages of small strongles that are about to emerge and begin laying eggs. The de-wormer does not affect many of the larva traveling throughout the horse’s body, other than in the lining of the intestine. By killing the larva that are about to emerge some of these newer de-worming medication prolong the time after de-worming that a horse will not be passing eggs. Therefore these medications do give the owner added protection. Any adults developing after the de-wormer has passed are free to live there until they are detected by their egg production and the horse is de-wormed again.