Russell Veterinary Hospital, P.C.​Lakeside Veterinary Medicine

"All Creatures Great and Small..."



At most boarding facilities, horses are at higher risk for parasite infection because all the horses are often on different deworming schedules and there are lots of different horses coming and going all the time. For that reason, I would recommend continuing to deworm your horse every 2 to 3 months. Ivermectin alone is fine for most dewormings, but you will want to use a product with praziquantel in it to treat tapeworms twice yearly. Also, it is important to use a boticide (ivermectin or moxidectin) AFTER the first good freeze to kill off any bots your horse may have picked up in the fall.

Daily dewormer does keep the parasite burden down at a lower level all the time (it does not actually PREVENT infection though, it just treats it at an early stage). Daily dewormers are very safe, and are no more “poisonous” than any other medication we give our horses (even salt is poisonous in excessive amounts). Daily dewormers do not kill every type of parasite, however (namely bots and tapeworms), so it is important to treat your horses with a product containing an avermectin (ivermectin or moxidectin) and praziquantel at least twice yearly. One downside to daily dewormers, is that occasionally horses that have been on it their entire lives do not develop some of the natural immunity against parasites that a normal horse would have. If you then stop the daily dewormer, some of these horses are more susceptible to parasite infestations for several months/years until their natural immunity develops. This is not a big issue as long as owners are aware of it and are more thorough in their deworming program during that time. For this reason, and because some parasites (like strongyles) can encyst inside the wall of the intestine where they are not affected by daily dewormers, I recommend using daily dewormers year-round if you are going to use them. Although rare, it is possible for resistance to develop against daily dewormers so periodic fecal exams should also be done. On the plus side, Pfizer, the company that makes the Strongid C2X you are using , has a program that helps pay for colic surgery (of any cause, not just parasites), if your horse colics while on their product. To be eligible, you must purchase your Strongid through your veterinarian (so they can verify that you are using the correct amounts), and your horse must have regular checkups (twice yearly) from your vet. This is a nice added “insurance,” especially if you are planning to feed the product anyway. Contact your veterinarian for more details and to see if they participate.


If your horse is alone is his pasture and his fecals have been negative he is at very low risk for most of the common intestinal parasites. Be sure to monitor his fecals closely over the next few months as the weather warms back up though, as many parasites are dormant in the winter months. If they continue to be negative my biggest concern would be bots and tapeworms, which do not need direct contact with other horses to be spread, and do not show up on fecal exams.

I would still recommend deworming your horse twice yearly to prevent bot and tapeworm infestation. Most dewormers are very safe, but it is always possible for reactions to occur in individual horses. It is impossible to tell what ingredient your horse might have reacted to (often horses are not reacting to the main drug, but rather the other components that the drug is administered in). To be safe, I would use a different brand and drug to deworm your horse. If he reacted to ivermectin, I would use moxidectin instead to treat the bots. If there was praziquantel in the dewormer he reacted to then I would use a double dose of pyrantel pamoate in addition to the boticide (ivermectin or moxidectin) to treat tapeworms. If you find you do need to deworm more often you can also experiment with some of the newer forms of dewormers (Ivermectin now comes in a pellet, for example), to see if that prevents a reaction. Anytime you are trying a new type of paste dewormer you might want to “spot” check a small area of your horse’s mouth with a dab of wormer first, then wait a few days for signs of a reaction.


If the fecal samples show a high worm burden, or if you can’t get the samples and want to play it safe, I would start with a half dose of a weaker dewormer like pyrantel pamoate (Strongid, etc.) or Fenbendazole/oxibendazole (Panacur, Anthelcide, etc). If that goes well, follow-up in two weeks with a full dose of the same medication. Two weeks later, use a full dose of ivermectin. To be safe, I would not use the combination Ivermectin/praziquantel (Zimectrin Gold, Equimax, etc.) until they have had a normal ivermectin dose first. Since the praziquantel will kill off tapeworms, it would be better to be sure the horse is clear of other worms first so you have less species of worms dying off at once. Four weeks after the ivermectin dose, you could safely administer the combination product. I would continue deworming every four weeks for the next 3 to 6 months depending on your weather. Very hot, dry weather or a hard freeze will help kill many of the parasites in the new pastures that may be reinfecting the horses. Warm, humid weather, on the other hand, will allow the parasites to last much longer outside the horse, so you will want to continue with the more frequent deworming.

I would avoid moxidectin (Quest, Combo Care) or the high dose fenbendazole (Panacur PowerPac, Safe-Guard Power Dose) for the first year or so unless specifically directed to use it by your veterinarian. Although it is tempting to try to wipe out any encysted strongyles, you can’t be sure of how severely infected your horses were (even a fecal now would not tell you if they were more severely infected previously). Horses with a high number of encysted larvae are at increased risk for complications if the larvae are all killed off at once. In about a year (around next spring), the number of encysted larvae should have decreased significantly and you can more safely use these products (and I would certainly recommend them at that time).

Rotating dewormers is a hot topic these days. Resistance to Avermectins (ivermectin and moxidectin) is extremely uncommon so far (though a few cases of resistant pinworms have been reported in adult horses and resistant roundworms have been seen in young horses on some large breeding farms), and it is the only class of dewormer that is effective against bots, so you could just use ivermectin (and the occasional ivermectin/praziquantel combination for tapeworms) and be pretty safe. Periodic fecal checks (again, a few representative samples would be fine) would help confirm that resistance has not developed. Some people advocate rotating dewormers so that resistant parasites can not develop. The dilemma is if dewormers are rotated too often, you can actually develop “super resistant” parasites that are resistant to more than one class of dewormer. There is no good answer to the rotation debate yet. Regardless of what you do, the most important way to avoid the development of resistance is to use the full dose of dewormer recommended. Using half-doses should only be done in extreme situations (as described above) and should not be repeated. It is also important to not underestimate the weight of your horses. A weight tape is a cheap purchase to help with a better estimation, and when in doubt, guess high. Also, try to ensure your horse receives the full dose and doesn’t spit half of it out on the ground. All the dewormers on the market have a wide margin of safety for overdosing in adult horses (moxidectin has a narrower range, so should be used cautiously in young horses, small ponies and minis).


If your horse is the only horse that ever uses that paddock and his stall, he is at very low risk for many of the common intestinal parasites. Most parasites require horse to horse transmission (generally via feces). Some types of parasites, however, are spread by other means. Certain types of tapeworms, for example, are actually spread by a grass mite that can survive the hay making process, and stomach bots are spread by flies that lay eggs on the horse’s hair coat. These types of worms do not generally show up on regular fecal exams because they do not shed their eggs in the horse’s feces. For that reason, every horse should still be dewormed at least twice yearly with a product that treats both bots and tapeworms. Avermectins (ivermectin and moxidectin) are the only approved over-the-counter dewormers for bot control and praziquantel is the most effective drug for treating tapeworms (a double dose of pyrantel pamoate was the preferred method in the past, but praziquantel has been shown to be more effective and is the only drug licensed to treat tapeworms in horses).

There are several commercial products that contain both of these dewormers, so a single treatment can control both types of worms. Periodic fecal counts for the other types of intestinal worms (ascarids, strongyles, etc.) and appropriate treatment only when needed will help ensure your horse is well protected without exposing him to unneeded dewormings and will minimize the risk of developing parasites that are resistant to the common dewormers.

Small strongyles, a specific type of intestinal parasite, go through several stages as they develop from eggs to larvae to adult worms. Two of these stages (called the L3 and L4 stages) “encyst” within the lining of the intestines. These encysted larvae are protected from most deworming products because they are not actually within the intestines and they are not particularly active so they are not affected by many of the drugs we traditionally use. Over time, these encysted larvae can “hatch” and develop into adult worms, which will then begin to shed eggs in the feces. The length of time it takes for a larva to hatch varies considerably, sometimes taking as long as 2 to 3 years. Since strongyles can live a “protected” life for so long in the intestine, it only takes occasional exposure (every 2 to 3 years) to keep a horse infected. While the larvae are encysted, they do not produce any eggs, so they will not show up in a fecal. There are currently only two drugs on the market that have been shown to be effective against encysted larvae. Fenbendazole, given at a double-dose for 5 days or moxidectin, given in a single regular dose, treat for late L3 and L4 stages.

Your horse is probably at low risk for small strongyle infection from other horses, but since strongyles can “hide out” for so long, it would be possible for him to re-infect himself occasionally. Using a combination moxidectin/praziquantel product twice yearly would treat for encysted small strongyles, bots and tapeworms all at the same time. Periodic fecal exams in between deworming would ensure that your horse has not picked up any parasites in the meantime.


If the worm was fairly large and seen in fresh manure it is most likely an ascarid (also called a roundworm). Ascarids are very common in young horses, even on well managed farms. As horses mature, most seem to become more resistant to them, so it is unusual to see ascarids in adult horses unless they are exposed to them in very high numbers. Strongyles, bots and tapeworms can also be seen in the feces on occasion, but they are usually more difficult to notice unless high numbers are seen.

Here are a few sites that have pictures of the more common intestinal parasites in horses:

The best way to determine the type (or types) of worms your horses are infected with is to take a fresh fecal sample to your vet. The majority of worms that can be present in horses are only passed as eggs in the feces and cannot be seen by the naked eye. A fecal sample will also confirm that your new ponies are not shedding any eggs that will infect the rest of your horses. Keep in mind though, parasite eggs can be shed for several days AFTER you have dewormed your horse, and ascarid eggs in particular can live a very long time in the soil (up to several years). The area your ponies have been living in right now should be considered contaminated, and any horses that are housed on it will need to have fecal samples checked or receive regular dewormings.