Horse Dewormer (Wormer)
What Kind and How and When to Use Them

     Once the chemical class has been selected, the cheapest brand is often the best brand. An ivermectin is an ivermectin.

     In order for a dewormer to be effective it must be administered as the manufacturer suggests, but you do not have to be a veterinarian to follow directions. You should know the approximate weight of your horse before administering a dewormer. And you must know proper deworming procedure.

The Big Rotation Question

     Rotation refers to using dewormers from different drug classifications. A fast rotation plan uses a different class of dewormer each time. A slow rotation plan uses one dewormer for a year before rotating to another dewormer. No rotation uses one dewormer all of the time.

     The two main reasons given for rotating dewormers is cost and parasite "resistance."

COST: If you can effectively use a cheaper dewormer at certain times during the year, it makes economic sense to rotate to that less expensive dewormer when you can.

RESISTANCE: The other reason for rotation is based on research reports that parasites (specifically small strongyles) can develop strains resistant to dewormers and in fact resistance has been shown to occur with most of the benzimidazoles. Further research on resistance to dewormers has shown that resistance (of small strongyles to the use of benzimidazoles) is neither delayed nor enhanced by rotation of drug classes. Therefore, when considering the use of benzimidazoles in your deworming program, slow and fast rotation are both acceptable strategies in the control of parasites.

     Some horsemen believe resistance claims are merely anecdotal and may be better explained by "bad timing" in deworming schedules. Research is underway to see if dewormers given to a horse at a time in a parasite's life cycle when the parasite is not vulnerable might explain persistent problems with small strongyles.

     Ivermectin has not been shown to have resistance problems so would be the only dewormer appropriate for ano-rotation program. However, for economic reasons, use other dewormers for your horse's deworming regimen.

     If you use the daily dewormer Strongid C, realize that it does not kill bots. Therefore, you should also administer an ivermectin paste in the spring and fall to control bot infestations.

• Deworm six times per year (every eight weeks).

• Twice a year use an ivermectin product (Zimecterin, Rotectin 1, Equimectrin, or Eqvalan, whichever is cheapest):

1. During early spring (approximately April or May) just before bot larvae leave the stomach

2. In late fall after a killing frost and after all bot eggs have been removed from the horse's coat (approximately October or November)

• In the summer, treat your horse for tapeworms (Strongid P or Rotectin 2 or praziquantel)

• The other three times, alternate dewormers according to your veterinarian's recommendations. Consider using moxidectin once or twice a year for encysted larvae following ivermectin treatment

     Keep on the lookout for the presence of bot flies or eggs on your horse's legs or flanks. If you are in a heavy bot fly-way (for instance, your neighbors deworm their horses once a year whether they need it or not) you may have to step up your bot control program to three or four times per year—two spring and two fall treatments.

SAMPLE SCHEDULE
(shift one way or another according to your climate:
confer with your veterinarian)

January/February Variable (fenbendazole)

March/April Ivermectin

May/June Moxidectin

July/August Double-dose Strongid P or Praziquantel

September/October Ivermectin

November/December Moxidectin

     You may wish to do a post-treatment fecal check to see if the dewormers you are using are doing their job. The routine annual fecal exam should be performed two weeks after treatment to determine if there is a problem with a specific parasite not normally covered by these dewormers.

     As a final note, deworming your horse is usually routine and does not require veterinary help. However, if you have questions, are concerned about particular worms, or question whether a deworming drug, consult your veterinarian. These are, after all, medications used to eliminate potentially harmful parasites. And, of course, fecal checks are a good check and do require veterinary assistance.

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