Everything you need to know about Deworming Horses

Everyone knows how to deworm horses. You buy an inexpensive dewormer from the feed store and give it to your horse. You need to rotate the dewormer every two months to prevent resistance in the parasites. The old method of tube worming isn't used anymore since the newer dewormer compounds have become available.

Actually, recent research on parasite resistance has blown this whole concept of rotational dewormers out of the water. It turns out that (1) eliminating parasites completely from horses is neither beneficial nor desirable, and (2) our diligent rotational deworming program has not prevented resistance in equine parasites, but rather is creating super-parasites.

Historically, animals and their parasites have evolved together, reaching an “agreement” where the animal's immune system keeps the parasites under control, and the parasite agrees not to kill the host upon which it depends. By eliminating all of the parasites, the animals' immune system can go into overdrive, ultimately predisposing the horse to develop allergies, and possibly autoimmune diseases. So, in reality, a low level of parasitism functions as an “immune optimizer” for the animal. It turns out that a fecal egg count of 0-500 reflects a low level of parasitism, which is optimal for the horse, but unlikely to cause parasite related disease. Over 500 epg indicates a possibly disease causing infestation.

The more disturbing concern is the possible production of super-parasites. Resistance to every major class of dewormer has been identified.  In the case of the most common equine parasite, strongyles, eggs are passed in the feces, they are not infective to the horse. They must hatch and develop into stage 3 larvae (L3), a process which is weather dependent. They do not develop at all below 46F or over 100F, making April – November the prime time for parasite transmission in most of the United States, including Kentucky. It turns out that a solid deworming after the first hard frost in the late fall with moxidectin will take care of the parasites for the rest of the winter. Once the weather starts to break in the spring, a fecal on all the horses at a farm is indicated to determine which horses are shedding. Since 20 % of the horses shed 80% of the parasite eggs, the key is to identify those horses and treat them regularly to keep the overall worm burden on the pastures down. The 80% of horses which are low shedders should be allowed to maintain this lower worm burden, and shed eggs that are not subjected to the selection pressure of rotational dewormers. This way, only a small number of the eggs on the pasture belong to the potential “super-parasites.”

After the first spring fecals and treatment of all horses with fecal egg counts over 500 epg, the fecals should be repeated on those treated horses in 4 weeks to determine if the worms were sensitive to dewormer used. If the counts are still high, that indicates that the worms on that farm are no longer sensitive to that dewormer. This resistance is now permanent for that farm, and it is of no value for use on that farm. Worms on specific locations have been tested as long as 20 years after resistance develops, and they are still resistant on that farm. The ineffective dewormer should be permanently left out of the dewormer rotation for that farm

The pre-patent period (time from consuming the infective L3 larvae to adulthood and production of eggs in the feces) of strongyles is about 60 days, so fecals should be repeated on all the horses in 2 months. Again, any horses with 500 or lower epg counts are considered to have low level infestations and do not need to be treated. Horses with over 500 epg have high infestations and require treatment. As a rule, those horses with high egg counts will be the same horses over and over, because they lack the appropriate immunty to control the infestation on their own. Therefore, this subset of “high shedders” require a rotational deworming program. The typical rotational program for this group of horses will sound pretty familiar: Oxibendazole (Anthelcide ®), Ivermectin, and Pyrantel (Strongid ®). The reason that all the horses should not be on the rotational program is that we want to encourage the subset of parasites that can be easily controlled by the horse's own immune system, and not indiscriminately kill off that group. This is the best way to prevent the development of a super-parasite, and also allow the horses to maintain a healthy, small population of worms to properly stimulate immunity.

There are other parasites besides those that produce eggs that can be measured in the feces. Because of these groups of parasites, all the horses in a group should be dewormed twice a year with either ivermectin or moxidectin. Firstly, the large strongyles have a pre-patent period of almost 6 -9 months and the larvae can migrate through the liver, mesentary and other abdominal organs. Therefore, horses should be wormed before the eggs can be detected in the feces.

The second group of parasites that are not typically found in a fecal egg count is the tapeworms. This group of worms has a stage of its life cycle in mites that live on the pasture, and horses actually pick up the infection by eating the mites on the grass. The larvae develop into adults which attach themselves to the mucosa in the region between the ileum and cecum and can cause colic. Tapeworm eggs are not usually found in the feces because they do not float in the typical fecal lab test. In order to keep tapeworms under control, we deworm at least once a year with either praziquantel, which is found in Quest Plus ® (moxidectin with praziquantel), or Equimax ® or Zimectrin Gold ® (ivermectin with praziquantel).

Next are the bots. Bot flies lay eggs on the horses hair, commonly on the neck and inner portions of the front limbs. As the horses itch themselves with their teeth, they pick off the eggs, which hatch in the mouth and travel to the stomach. The bot larvae attach to the stomach lining, causing stomach irritation, including ulcers. Typically we control bots by keeping the bot eggs scraped off the hair to prevent ingestion and also by deworming with either ivermectin or moxidection after the first frost in the fall to eliminate the bots.

Other parasites include habronema, which contributes to summer sores (non-healing wounds on the legs), conjunctivitis and other eye lesions, and onchocerca, which causes skin and eye lesions. Since these parasites don't produce eggs in the feces, twice yearly deworming with either ivermectin or moxidection controls these parasites.

Summary of Deworming Recommendations for Adult Horses:

1. Deworm with moxidection with praziquantel (Quest Plus ®) in the fall after the first frost to eliminate encysted strongyles, bots, onchocerca, habronema and tapeworms. This should control parasites for the rest of the winter.

2. Check fecals on all horses in the group about 4 weeks after the first warm days (7 – 10 days of consistently warm days in a row). Deworm all horses with egg counts over 500 with 5 day double dose fenbendazole (Panacur ®) to eliminate migrating larvae and encysted larvae.
3. Recheck all treated horses again in 10 – 14 days to determine if the fenbendazole was effective. If yes, then the Strongyles on the farm are not resistant to fenbendazole, so it is a good product to keep in the rotation.
4. Check fecals on all horses in the group 8 weeks after the first. This allows a full cycle of infective L3 larvae picked up after the first taste of Spring maturing into adult worms which shed eggs in the feces. Any horses whose immune systems are unable to control their parasites (high shedders) will have egg counts over 500 epg, and should be dewormed with the next compound in the rotation, pyrantel pamoate (Strongid®) . Again, fecals should be checked on those that were dewormed at 10-14 days to determine if the pyrantel was effective. Again, if the parasites are not resistant, this product can be kept in the rotation. Low shedders will have low egg counts (<500 epg) at both times.
5. Now you know which horses are “high shedders” and which are “low shedders.” After the next 8 weeks, all horses should be dewormed with Ivermectin with praziquantel (Ivermectin Gold ®, Equimax ®).
6. At the next 8 week point, deworm the “high shedders” with the next product in the rotation, oxibendazole (anthelcide). Again, recheck the fecals at 10-14 days to determine if this dewormer is effective on your farm.
7. At the next 8 week point, repeat the process, this time using pyrantel again, only if it effective in your location.
8. Next, deworm with Quest Plus after the first frost.

After following this program for a year, you should have a good idea which horses are the high shedders and which are the low shedders. If you have a stable population, then, your program will be similar year after year. All horses coming into the group should be dewormed with Quest Plus before joining the herd.

Foals add a different dimension to the program. Most adults develop immunity to worms, but foals have never been exposed, so deworming is of critical importance. The first worms that foals pick up are Strongyloides (threadworms), which they actually ingest in the milk of the mare. There is some debate about whether Strongyloides causes any disease in foals. However, current recommendations are to dewormed the mares with ivermectin within 24 – 48 hours of foaling to eliminate this parasite.

The next group of parasites that are important to foals are the ascarids, or roundworms. This group is one of the biggest problems groups, because the eggs hatch in the intestines and then burrow through the tissue of the foal and migrate through the lungs. Many foals develop a cough and even pneumonia as a result of the parasites migrating through the lung tissue. For this reason, the first deworming of foals should be with a product that will eliminate most of these migrating larvae. Therefore, this early deworming at 3-4 weeks of age is with ivermectin. Those ascarids that escape the ivermectin deworming will go on to develop into adults, which can then cause generalized unthriftiness and colic. Therefore, at 8 weeks, foals should be dewormed with pyrantel.

The early deworming program in foals is designed to treat for parasites that are not mature enough to produce eggs, so fecal egg counts are not of value. However, once the foals are out to 16 weeks, they should start on the fecal egg count program. Young horses should be checked every two months until after the first frost of their yearling year, after which they join the adult program. Yearlings are still at risk for ascarids, and consequently vigilance should be maintained until they are two-year-olds.

Summary of Deworming Recommendations for Foals:

1. Deworm mares with ivermectin within 24 – 48 hours of foaling to control Strongyloides (threadworms) infections in the foals.
2. Deworm with ivermectin at 3-4 weeks of age to treat migrating larvae of ascarids (roundworms).
3. Deworm with pyrantel at 8 weeks of age.
4. Start checking fecal egg counts at 8 week intervals starting at 16 weeks of age, and use rotational dewormers of oxibendazole, and pyrantel, when counts are high. Respiratory signs without evidence of infectious disease may be associated with migrating ascarids, and may require an additional deworming with either ivermectin or double fenbendazole. Recheck fecal egg counts 10-14 days after deworming to determine if the worms are susceptible to the dewormer product you are using. If the product you are using is not effective, you need to eliminate that product from your rotation.
5. Ivermectin is indicated at the next 8 week point, similar to the adult program.
6. The terms “high shedders” and “low shedders” aren't useful in this age group, because they tend to all have high egg counts, probably because of limited immunity at this age.
7. Continue to deworm at 8 week intervals until after the first frost, when moxidectin with praziquantel (Quest Plus ®) is indicated. Always carefully estimate the foal's weight and dose exactly according to weight to avoid accidental overdose.
8. Young horses join the adult fecal egg count and deworming program 4 weeks after the first break in the weather in the spring, although most yearlings of this age have not developed sufficient immunity to control the parasites themselves, and will carry a high worm burden. This group needs to continue on the rotational deworming program, with spot checking egg counts at 10-14 days post dewormer to determine resistance among the parasites on the farm.
9. By the spring of their two-year-old year, the pattern of “high shedder” and “low shedder” emerges, and you will be able to determine which horses must remain on a high frequency rotation, and which require only ivermectin or moxidectin with praziquantel twice a year.

Management Program to keep Parasitism in Check

Management programs will also help limit parasitism in horses, and are probably a bigger part of parasite control in warm regions of the country, where you cannot count on a hard frost to “reset” the environmental parasite clock.

The most critical factor is pasture rotation. If pastures can be rested for 2 – 4 weeks at a time, the fields can be harrowed to break up the manure piles, and over a 2 – 4 week period (shorter in temperatures over 70F, longer below 70F) the strongyle eggs hatch, and the infective L3 larvae emerge. After a period of time, if they are not ingested by a susceptible host (horses), they die off, leaving the pasture strongyle free. If pastures cannot be completely rested, an alternative species, such as sheep, cattle or goats can be grazed instead, because the parasites do not cross host species lines. If horses are kept in a sufficiently small pasture, an alternative to pasture rest is manure removal. The manure can be picked up and disposed of similar to stall muck. Composting, where the internal temperature of the piles reaches and maintains 165F will also kill off the parasites. Unfortunately, very little kills off ascarid eggs, and these eggs remain viable on pasture for up to 20 years.

This is the current state of the recommendations of the parasite experts at this time in order to optimize equine health and immunity as well as prevent further resistance among the equine parasites. For specific recommendations for your area of the country, contact your veterinarian.