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Symptoms of Parasite Infestation

To determine whether your horse has parasite infestation, consider these general symptoms:

General lack of health

Dull hair coat that doesn’t shed

Weight loss

Poor hoof condition

Frequent colic

Cough or nasal discharge (may indicate roundworm migration)

If your horse exhibits one or more of these symptoms, you may want to examine your deworming program more closely. You’ll want to consider how you control parasites, at what intervals you deworm your horse, and the necessity of using a rotational deworming strategy.


Although there are some excellent and safe dewormers on the market, management is still the key to controlling parasites. Horses become infested with strongyles, ascarids and tapeworms by ingesting the eggs or larvae from contaminated pastures and paddocks. The best control is to remove manure promptly and compost it properly before spreading it back on pastures. Another option is to provide very large pastures or rotate pastures so that horses are not forced to graze areas contaminated with manure. This arrangement most closely matches the wild horses’ pattern of grazing in one area very heavily then moving on to another area. Despite your best management at home, if you travel to show grounds or rodeos and allow your horse to graze, he may pick up larvae left there by other horses.

Climate must also be considered when designing your deworming program. Parasites are most active in the warmer, humid times of year. This is when you must be most diligent about control. Pasture management combined with regular fecal and blood exams can keep the use of chemical dewormers to a minimum and sometimes eliminate them altogether. It is also important to consider the overall immune system of the horse. A very healthy horse will be resistant to mild parasite burdens. I have been able to go as long as two years without needing to use chemical dewormers on my pasture horses. I gave them a probiotic/enzyme combination each new and full moon to support normal gut function and detoxification. (It is an old animal husbandry premise that parasites are more active during the full moon. Whether this is true or not, using the new and full moons as worming dates is an easy way to mark the calendar and to remind yourself to stay on a schedule.) The following spring, however, I detected some strongyle eggs on the fecal exam, which I suspected was due to the combination of a mild winter and the introduction of a new horse who might have had a heavy load of encysted small strongyles. Some people report good results feeding diatomaceous earth on a regular basis. This did not work for me, but I could see how it might in a drier climate. N.O.M.S. is another product that, fed daily, may help prevent parasite infestation.

Deworming Intervals

The standard frequency for chemical deworming is every six to eight weeks. This is called “purge deworming” and, depending upon the product used, removes parasites that are in the gut and/or that are migrating through the tissues. Unfortunately there is little if any residual action. In other words, your horse can immediately start picking up new parasites if exposed. This is why products such as StrongidC, which is fed daily, may be your best choice in a highly contaminated environment. Another choice is the natural or herbal dewormers that may decrease the numbers of parasites surviving in the gut. By using these, you may be able to increase the time period between dewormings in cold or very dry conditions. If using a purge deworming plan for foals, start the program when the foals are eight to ten weeks of age. There is some concern that feeding StrongidC on a daily basis to foals may delay onset of acquired immunity to ascarids. Fecal flotation exams or blood evaluations done at two- to four-month intervals allow you to properly evaluate your deworming program, identify horses who are particularly susceptible, and focus your efforts on these horses.

Rotational Deworming

Rotational deworming is the practice of using a different class of dewormer each time you deworm your horse. The small strongyles are the most likely to develop drug resistance. The theory behind rotation of dewormers is that the less the strongyles are exposed to a chemical, the longer it will take them to become resistant to that chemical. In reality this has not proven to be the case. Resistance of small strongyles to normal doses of benzimidole dewormers, at their recommended dosage, is well documented, yet large doses of some drugs in this class are effective against encysted larvae. Your best plan is to work closely with your veterinarian to choose an effective product that has the least toxicity and to deworm at the longest interval possible to maintain minimum but not necessarily zero parasite burden.

About the Author

Madalyn Ward, DVM, owns Bear Creek Veterinary Clinic in Austin, Texas. She is certified in Veterinary Homeopathy and Equine Osteopathy.

Memberships include American Veterinary Medical Association, American Association of Equine Practitioners, American Holistic Veterinary Medical Association, Texas Veterinary Medical Association and the Academy of Veterinary Homeopathy.

She has authored several books and publishes at her blog.

Madalyn Ward DVM