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 sum