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Deworming an Extremely Wormy Horse

Is the Panacur (5 day) Power Pack 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, Florida

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.



Lydia F. Gray, DVM, MA SmartPak Staff Veterinarian and Medical Director Dr. Lydia Gray has earned a Bachelor of Science in agriculture, a Doctor of Veterinary Medicine (DVM), and a Master of Arts focusing on interpersonal and organizational communication. After “retiring” from private practice, she put her experience and education to work as the American Association of Equine Practitioner’s first-ever Director of Owner Education. Dr. Gray continues to provide health and nutrition information to horse owners through her position at SmartPak, through publication in more than a dozen general and trade publications, and through presentations around the country. She is the very proud owner of a Trakehner named Newman that she actively competes with in dressage and combined driving. In addition to memberships in the USDF and USEF, Dr. Gray is also a member of the Illinois Dressage and Combined Training Association (IDCTA). She is a USDF “L” Program Graduate and is currently working on her Bronze Medal. Find Dr. Gray on Google+

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2 comments on “Deworming an Extremely Wormy Horse
  1. s knettle says:

    I have been reading a few articles on the problem of over deworming. Two articles I read said that 50% of the herds tested have already built up resistance to 2 types of dewormer.The thought was that alternating wormers would keep this problem from happening. From what I understand the harder and faster we slam parasites,the harder and faster they scamble to build up resistance. Is someone working of the next line of dewormers?

  2. lynne says:

    I have a hard time trying to figure out which one to use and how much to use and what time to

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