Is the Panacur (5 day) Powerpac safe to give to a 700 lb EXTREMELY WORMY 3 year old colt? Should he be given the full tube or just the amount of tube for his weight? KM from FL

Dear KM,

This product may not be your best choice when it comes to deworming a horse that you know or suspect has a high parasite load. That’s because it’s designed to be given for five days in a row at a double dose to control the hard-to-get encysted cyathostomes, the juvenile small strongyles that hibernate within the intestinal wall. Since the Panacur Powerpac is a potent means of parasite control, it may be a bit too much for a horse with a high worm burden. Killing too many worms at once can lead to a number of serious problems, such as an impaction colic or an allergic reaction caused by the sudden release of antigens by dying parasites.

A better choice might be to give a product with the same active ingredient, fenbendazole, but at the regular adult dose of 5mg/kg, and to give it one time only rather than once a day for five days in a row. This way you’ll remove the large strongyles and adult small strongyles that he’s probably full of as well as any roundworms and pinworms hanging around. You would expect a three-year-old to have developed immunity to roundworms (ascarids) but if he’s starving or unhealthy he may still have these parasites lingering in his system. Follow up about a month later with one regular dose of ivermectin, and after that you should be able to put him on the same parasite control program the rest of your horses are on. And of course, keep your local veterinarian in the loop any time you’re making health decisions about a horse, especially one in questionable health.

In regard to your second question, it’s always best to accurately determine an individual horse’s weight (weight tape, simple calculation, scale), then dose as closely as you can to this. When you underdose a product—whether it’s a dewormer, antibiotic or other product that targets an organism—you could be creating resistance by not giving enough active ingredient to actually kill the “bug.” On the other hand when you overdose a product, you could be putting your horse’s health at risk depending on the margin of safety of that particular agent. You might find SmartPak’s Equine Weight Calculator to be a helpful tool.