CH Top Links

Understanding Deworming with Katy Raynor, DVM

Rotating and Resistance

Recommendations for deworming horses have changed over time due to the development of parasitic resistance. Rotating dewormers still is required at many boarding facilities, although this approach is just one of the reasons why resistance has developed in some horses to the common dewormers.

Fecal Egg Counts (FECs)

Rotational deworming programs have started to decrease in popularity, and the use of fecal egg counts (FEC) has become the new trend. FECs are important to have analyzed; however a low shedder does not indicate your horse is parasite-free, since FECs do not reliably get an appropriate number of eggs shed by small strongyles. This is because small strongyle eggs are only seen on a FEC if there are mature female worms shedding. The encysted form may still be present without showing up in the FEC, since they do not actively shed eggs. Another parasite commonly missed with FECs are tapeworms, since the egg of a tapeworm is too heavy to be seen on the fecal float method. Small strongyles and tapeworms are a common cause of colic, which means that monitoring the presence of these two common parasites is paramount in the fight against parasites.

Since the FEC test does not account for two of the common equine parasites, it is important to deworm your horse at least two times a year with a product that works on encysted small strongyles and tapeworms regardless of a possible low shedding result on FECs. Moxidectin and a higher dose of fenbendazole Panacur PowerPac are anthelmintics effective against encysted small strongyles. Moxidectin must be used with great caution due to the potential of overdosing. Moxidectin when used properly is a safe deworming tool. Overdoses happen when the horse’s weight is not measured properly (using a weight tape), or when used in young or debilitated horses. For this reason, be sure to weigh your horse as accurately as possible, and do not deworm young or elderly horses with this product. If your horse has a recent history of gastrointestinal upset and needs to be dewormed for suspected encysted small strongyles, use the high dose fenbendazole products instead of Quest. Praziquantel has been shown to be an effective drug against tapeworms. As a hint, any dewormer with “Plus” added onto the name typically carries praziquantel. Products containing Ivermectin are still a great tool for the higher shedding horses with moderate to high FECs.

FECs are still very important to perform at least twice a year, because each horse has a different level of immunity to the common parasites. FEC results will be shared with you by your veterinarian, but what you do with those results is most important. There are three levels of FECs: low, moderate, and heavy. Low means there are less then 200 eggs per gram (epg), moderate is a range between 200-500 epg, and heavy is over 500 epg. If your horse is low, still deworm with moxidectin and a praziquantel product once a year. If your horse is moderate add a third dewormer that year, and if your horse is a heavy shedder a fourth dewormer that year is necessary. With the moderate and heavy shedders, a follow-up FEC should be performed 4-6 weeks after deworming to see if there is a resistance pattern present.


Deworming Foals

While intermittent deworming is adequate for adult horses, the immune system of foals is not quite ready to handle intestinal worms on its own. A deworming program starting at 30 to 60 days of age and continuing every 30-60 days until the foal is at least 12-18 months should be maintained. There are many effective deworming compounds (anthelmintics), that are safe for use in foals. Examples include ivermectin, pyrantel, and fenbendazole. It is important to read the label to determine the active ingredient of the product and understand if it is safe to use in foals. The body weight of the foal should be determined as accurately as possible and the appropriate amount of de-worming medication administered for that body weight. Rotation between classes of deworming medications is often recommended, but is somewhat controversial. Results of recent studies have shown that resistance to certain types of dewormers has become increasingly more common. Fecal egg count reduction tests before and after deworming are important to determine resistance in individuals. Remember that moxidectin should never be used in foals less than 6 months of age.


Deworming strategies do not stop at the hard-to-pronounce drug names in the bright colored boxes. It is multi-faceted and includes proper pasture management, geographic location and climate, housing conditions, proper storage of the dewormers, proper dosing, choosing which horses to deworm more often depending on individual immunity, and consulting with your veterinarian. In other words, there is not a single deworming strategy recommended for all farms, so work with your veterinarian to come up with the best strategy for you and your farm.

Katy Raynor

Written by Katy Raynor, DVM, New England Equine Medical and Surgical Center. Katy can be reached at 603-749-9112 or



, ,

No comments yet.

Leave a Reply

Powered by WordPress. Designed by WooThemes