Whether or not getting a "huge infestation" of parasites is a concern depends very much on your horse's own resistance to the parasites, how your horse lives (situation) and the contamination of your pasture/paddock if your horse grazes. In an adult horse, northern US middle of winter, probably stalled and living on hay and concentrates, the risk of heavy parasite infection due to the removal of daily dewormer isn't too likely.
There is no need to give ivermectin the same day you discontinue a daily dewormer. When you should deworm again is going to depend on the risk factors for reinfection. It would be a really be best to have a fecal egg count run in a month or so and see what kind of reinfection rate you are looking at. At the very soonest you should need to start deworming again in a month--before that you aren't going to have many if any adult parasites. Most likely (if your situation is what I described above) you won't need to deworm again until your temps start warming up again and then you would want to do it to help minimize the shedding of any tapeworm ova onto the pastures when grass is starting to grow.
Licensed Veterinary Technician
Last edited by Ryle; 01-06-2009 at 10:20 PM.