Ask the Vet: Internal Parasites

Courtesy of AAEP

Question: My horses are turned out primarily on a dry lot with access to a small area of grass for approximately 2 hours/day. I have had yearly fecals done and they are always negative. Is it necessary to deworm my horses at all under the current conditions?

Answer: You are in a perfect situation to use dewormers very sparingly! My suggestion is to deworm them at least once a year with a broad-spectrum dewormer such as ivermectin gold or quest plus.

The reason for this is that fecal egg counts are dependent on adult parasites not only being present but also shedding eggs. The fact that yours are negative suggests that your horses have a very low burden and that is great. They could have minor infections with pinworms or tapeworms that do not show up on the fecal counts and that is the reason I would suggest a once-a-year treatment. Thanks for a great question and good luck to you with your horses.


Question: There is a new idea circulating in the horse world pertaining to performing fecal egg counts and deworming based on the results of this test, and getting away from deworming every two months. Rather, deworming based on the results of the fecal egg count. What are your thoughts on this new idea?

Answer: This is the right way to go in my opinion. Only 20% of horses are shedding eggs and contaminating the environment and those are the ones that we need to be treating. The rest of the horses, even if they have a few parasites are not at risk for any clinical signs, and provide a dilution factor if any resistant parasites exist on your farm.

I believe it is still necessary to treat all horses at least once a year to control for tapeworms (if you have them in your area of the country) and for the few remaining large strongyles that may be on your farm. I suggest treatment at the beginning of the most favorable time for larval transmission of parasites, spring in the north and fall in the south. Fecal egg counts should then be conducted on all of the horses on the farm following the anticipated egg reappearance time (4 weeks for Panacur, Anthelcide; 6 weeks for ivermectin; 10 weeks for moxidectin) to identify the high egg shedders. The high shedders (those with egg counts over 300 eggs per gram) should be retreated and then follow-up on them 10-14 days after treatment to see if there has been at least a 90% reduction in their egg counts. This will also provide information about the efficacy of the dewormer that has been used. Good luck with this new technique. I believe you will be happy with the results and your deworming programs will actually become more effective.


Question: Does it hurt or help to use the entire tube of paste dewormer if within a couple hundred pounds of weight of the horse?

Answer: All of the dewormers that are in use today have a wide margin of safety so my initial response is No, it does not hurt to overdose with the paste dewormers. The exception would be if you have Miniature Horses or ponies, some of which can be very sensitive to products containing moxidectin as the active ingredient (Quest or Quest plus).