Fall Deworming

Fall is approaching rapidly and brings with it such predictable events as raking leaves, Thanksgiving dinner, and weekend football. Autumn also heralds changes in parasite activity that are equally predictable. However, just as the environmental effects of autumn differ throughout North America (scarlet maples in Vermont vs. green magnolias in Savannah), the seasonal changes in parasite activity also vary geographically. Therefore, autumn parasite control measures must be designed for local conditions.

Strongyles are the most important internal parasites of mature horses, and are affected significantly by these seasonal changes.

Northern Temperate Regions

For regions located above about 37ºN latitude (i.e., north of the Ohio River), autumn means high pasture contamination with infective larval strongyles. Larvae accumulate after mid-summer and survive through winter into spring. If northern horses are pastured during fall and winter, they can acquire sizable strongyle infections.

Fall larval buildup is best prevented by summer deworming. If that hasn't been done, then horses should be dewormed in the fall and moved to the cleanest pastures available (hay fields, cattle pastures, or horse pastures vacant since early summer).

If clean pastures are unavailable, stabling during the winter effectively removes horses from the chief source of strongyle infection. Strongyle eggs lead to future infections only when feces are deposited on pasture, so reinfection doesn't occur in stalls or when manure is composted. Once horses are confined for winter, they do not require additional deworming until just prior to spring pasture turnout.

If horses must remain on highly infective pastures through autumn and winter, consider a daily dewormer (i.e. pyrantel tartrate) to prevent strongyle accumulation.

Southern Temperate Regions

In U.S. regions below about 37ºN latitude, high summer temperatures suppress larval amplification, and the pasture reinfective potential is lower than at any other time in the annual cycle. Autumn's moderate temperatures signal the return of successful translation of strongyle eggs into infective larvae on pasture. A mature horse easily can pass one million eggs per day in his manure.

I consider autumn to be the beginning of the annual deworming cycle in southern temperate regions. Southern horses should be dewormed no later than early October, and subsequent treatments should be administered at intervals determined by Egg Reappearance Periods (ERP) (see table). The ERP is the interval between treatment and the return of significant numbers of worm eggs in the feces. Egg contamination can be minimized by deworming within the ERP of the last dewormer used. Thus, if moxidectin were used on Jan. 1, the next treatment should be administered in late March or early April, based on moxidectin's 12-week ERP. Similarly, the use of pyrantel pamoate in early January would require a follow-up treatment one month later. Anthelmintic resistance is a serious, common problem; your veterinarian can help determine which dewormers are still effective in your herd.

Bots (Gasterophilus spp.)

Adult bot flies begin laying eggs on equine hair in summer and continue to do so until the first hard frost in November or December. No drugs are effective against the egg stage, but ivermectin and moxidectin products kill bot larvae in the oral cavity and the gut.

Treatment timing is critical for bots. If horses are treated too early (before the first frost), surviving flies can lay more eggs on the horse, leading to reinfection and requiring additional treatment. Ideally, bot treatments should be scheduled after the first frost, and should be combined with mechanical removal of eggs from the legs, shoulders, and mane. All horses in the herd should be treated. Despite their relatively large size, larval bots don't cause much damage to the equine gut, but treatment can decrease their numbers in the subsequent summer.

Ivermectin and moxidectin are effective against bots plus a wide range of internal parasites; these products are optimal for incorporation into your fall deworming program.

Other Parasitic Considerations

Control of ascarids or roundworms (Parascaris equorum) isn't determined by seasons, but by the horse's age. These parasites are most common in young horses, and can be managed by deworming foals at two months of age and continuing at bimonthly intervals until 12-16 months of age.

Tapeworms are fairly common in horses, but no equine dewormers are labeled for efficacy against them. In addition, next to nothing is known about tapeworm biology; control recommendations based on seasonality are purely theoretical.


Parasitic activity during autumn is predictable, which means that it can be disrupted. Plan fall treatments for killing bots and strongyles early; your pastures will be cleaner and your horses healthier. Then you can relax while eating turkey and watching football.


4 weeks
4 weeks
4 weeks
Pyrantel pamoate
4 weeks
6-8 weeks
12 weeks
Pyrantel tartrate

About the Author

Craig R. Reinemeyer, DVM, PhD

Craig R. Reinemeyer, DVM, PhD, is president of East Tennessee Clinical Research, Inc., an independent business in Knoxville, Tenn., that conducts clinical pharmaceutical research for animal health companies.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from TheHorse.com. Learn More

Free Newsletters

Sign up for the latest in:

From our partners