Deworming Your Horse
Horses should have a microscopic fecal flotation exam done at the beginning of spring (by the middle of April in this area) to best plan a parasite control strategy. The exam allows us to count the number of worm eggs per gram (EPG) of manure. Horses with a zero EPG should not be dewormed until either a positive sample is tested or in the fall, after pasture grazing to ensure removal of any tapeworms and any bot larvae. To run a fecal flotation exam, we need a single fairly fresh manure ball placed in a ziplock back and kept out of the sun or refrigerated. The sample should be run within 7 days. The cost for a single sample is $27 or $17 if we are doing more than one sample at a time.

In a herd situation, there may be individual horses who are less immune to small strongyles, the main intestinal parasite of horses, and these horses will pass large numbers of worm eggs, contaminating the pasture and allowing infestation of the rest of the herd. It is therefore important to identify these horses and deworm them on a strict schedule, based on the effective suppression time of the chosen dewormer, rather than based on the calendar. Because resistance by strongyles to a number of dewormers has been documented and is increasing in the United States, a fecal flotation exam should be done two weeks after deworming to quantify the reduction in EPG. Resistance to dewormers occurs when dewormers are used on a schedule, regardless of the duration of their effectiveness, in all horses in a herd. Only the most resistant worms will remain to repopulate the herd. Resistance to a daily product (pyrantel) has also been demonstrated and research has shown that the daily product is less effective than periodic deworming. We do not recommend them. In addition, pyrantel is not efficacious against stomach bots or encysted larvae, although a double dose of pyrantel will remove most tapeworms.

Dewormers are very heavily marketed, but many are no longer very effective in some areas of the country because of acquired resistance over the years. At this date, the best products to kill adult internal strongyloid parasites and reduce manure egg counts are Ivermectin, sold under the trade names of Zimectrin, Ivercide, Eqvalan, and others, and moxidectin, sold as Quest Gel.

Ivermectin has a residual effect suppressing egg production for 6-8 weeks, moxidectin for 12-16 weeks.

Two products are used to reduce encysted small strongyle (cyathostome) larvae in the wall of the large intestine. Panacur Power Pack, which requires a 5 day double dose of fenbendazole, is becoming less effective. Moxidectin (Quest) Gel is a single dose application that is probably 70-80% effective against most larvae and kills 100 % of adult parasites as well

Praziquantel which is now combined with ivermectin (Equimax, Zimecterin Gold) and also moxidectin(Quest Plus, Combicare) , will kill tape worms. Surveys have shown about 19% of horses in Colorado are infested with tapeworms during the summer and fall. Use these combination products as your last deworming at the beginning of winter.

Other de-wormers in the Benzimidazole class like Anthelcide (oxibendazole), Safeguard and Panacur (fenbendazole), and others, are now less effective against strongyles due to parasite resistance, and they have a very short window of suppression if they work at all—as short as 4 weeks. Pyrantal (Strongid and other trade names) also has a high rate of strongyloid parasite resistance possibly because of the use of daily wormers but is still efficacious against ascarids (roundworms). It is also only effective for 4 weeks.

Important: We do not currently recommend rotational worming unless we do parasite egg counts before and 14 days after worming and are sure of their effectiveness.

Occasionally we may recommend concurrent use of Anthelcide (oxibendazole) and Strongid (pyrantel) in a deworming rotation with ivermectin or moxidectin, and we use fenbendazole and pyrantel in weanlings if there have been ascarids contaminating the stalls, paddocks or pastures at the roundworm dose.

Panacur (Fenbendazole) is currently the most effective dewormer against ascarids.

Suggested adult worming schedules ( Herd < 10 )

Mid to late March Fecal flotation exam to determine EPG parasite load. DO NOT DEWORM IF NEGATIVE
If Positive:
April 1 Quest or Zimectrin, or other ivermectin or moxidectin product
April 15 Fecal flotation to determine any resistance
June 1 If Ivermectin product used (Zimecterin, etc) on April 1: Deworm with Quest, Zimectrin or other ivermectin or moxidectin product
July 1: If Moxidectin product used (Quest) on April 1: Deworm with Quest, Zimectrin or other ivermectin or moxidectin product
August-September fecal flotation exam. Do not deworm if Negative. Deworm with Quest if positive.
Nov-15 to Dec-15 (All) at end of the grazing season: Quest Plus, CombiCare, Equimax or Zimecterin Gold (to kill all adult parasites including tapeworms and stomach bots)
Horses with a significantly positive fecal exam (over 200 EPG) in March may have a more intense deworming schedule and should be dewormed with Quest (moxidectin) products every 12 weeks for 1 year.

Special Cases
Nursing foals, weanlings, old or debilitated horses should not be dewormed with Quest.

Important: Dewormers pass through the manure, and may be toxic to dogs for an extended length of time!
A broodmare should be de-wormed regularly during her pregnancy if she had a positive fecal sample, then one month before and possibly the day after foaling with an ivermectin product.

Foals should begin their de-worming schedules at 4 weeks of age, and every 4-6 weeks thereafter beginning with fenbendazole and additionally with ivermectin at 4 months of age. Do not use Quest in foals under 1 year of age. Then begin the regular adult schedule.

Important: Horses should be measured by a weight tape prior to administering the de-wormer to ensure the proper dose. Tapes are available at the feed store. See article on weight management and proper use of weight tapes.
Reducing pasture contamination with worm eggs and larval ingestion by horses
  • Do not feed in manure contaminated areas
  • Deworm all horses on pasture as directed. Deworm and isolate any newcomers for two weeks before turnout.
  • Remove manure contamination from areas where horses eat. Horses tend to select “islands” in a pasture where they will defecate and they do not eat in these areas given enough available food and room. Compost manure to kill worm eggs and larvae. Ascarid eggs may not be killed by composting
  • Do not spread composted manure in pastures if ascarid infestation exists on the ranch.
In herds with over 10 head of horses, we recommend checking fecal samples on at least 10, or 20% of a very large herd. Identifying the parasite egg “shedders” is important to the health of the entire herd.

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