The traditional deworming strategy of interval treatment has resulted in parasite resistance to most dewomers. | Photo: iStock

Q. When planning a deworming program, how do you prevent new parasite eggs from hatching? You deworm and kill the worms, but then more eggs hatch and reinfect. Is it beneficial to give, say, 5 grams of dewormer per day for four days or every other day to kill newly hatched worms? Or is it best to give it all at once? — Sherri Knippling, via email

A. Pastured horses are exposed to a nearly continuous cycle of reinfection with cyathostomins (small strongyles). Effective deworming provides a temporary respite from adult worm populations (which are relatively harmless), but the same pool of pasture larvae that infected your grazers yesterday is still out there in the grass waiting to do it again tomorrow. Deworming disrupts parasite egg production temporarily because the anthelmintic treatment kills the egg-laying adult female worms. No hen, no eggs.

Deworming might even halt egg production for up to several weeks after administration, especially when using ivermectin or moxidectin. However, any benefits from reducing the infective potential of the environment will not occur immediately but, rather, in weeks or months. Depending on the season and local geoclimatic conditions, the cyathostomin larvae currently threatening your horses could be as young as one week or as old as six months, and they can survive on pastures for a fairly long time. So, the overall risk of reinfection doesn’t stop immediately with deworming. However, because worm eggs turn into infective larvae, the risk of infection should be diminished, at least theoretically, thanks to deworming