Should I Deworm My Horse in the Winter?

Q. - Is it worthwhile to deworm my horses during the winter? I've heard that internal parasites aren't around this time of year.

A. - Yes, you are correct that we recommend deworming horses during the active grazing season rather than during the winter. The reason is that the small strongyles tend to arrest their development over the winter at the larval stage. Most of the available dewormers have no effect against these arrested larvae, and the few that do are only partially effective. 

The arrested development can lead to an accumulation of small strongyle larvae in the mucosal walls of the large intestine. If a large number of these larvae are present in a horse, deworming it at this time might actually cause more harm than good. Most dewormers mainly act on the adult parasites present in the lumen of the intestine, and removing these may activate the arrested larvae and cause them to mass emerge from their encysted stage. This can lead to serious diarrhea. 

There are situations where it can be necessary to deworm a horse during the winter months, such as if the horse hasn’t been dewormed adequately during the latter part of the grazing season. Other examples of winter deworming include foals that harbor ascarid parasites, which aren't seasonal to the same extent as the strongyles. In both cases you should consult with your veterinarian regarding your choice of dewormer.

We find that horses actively shed parasite eggs year-round, but there is a tendency toward lower counts in the off-season months. This makes sense from the parasite's standpoint, as winter conditions do not favor parasite transmission. 

So the bottom line is that we don't typically recommend treatments directed at strongyle parasites during the winter months in the colder climates. Remember, though, that in warmer climates (i.e., the southern United States) parasite transmission is more likely to occur during the winter than in the hot summers. Most importantly, make sure your dewormers are working by running fecal exams to determine parasite egg counts before and after treatments.

Electric Fencing

Q. - We just got a four year-old stallion at our barn, and I wanted to know if even with electric fencing will he be able to break out to get in with the mares that are in the paddock next to him? Electric wires are the only thing separating him from the geldings and mares. The mares are just coming into heat, and he has been smelling the air. Otherwise, he’s really quiet.

A. - Electric fence is not my favorite fencing option for many reasons. Even though it may be just fine for some stallions, I would not depend on an electric wire as the only barrier between a stallion and geldings and cycling mares. More effective fencing options include 6-foot-high sturdy post and board (with boards on both sides of the posts), especially if lined with stallion-safe diamond mesh woven wire. The best option is a double line of fencing with an alley between.