a horse should be wormed evey 6-8 weeks considering its healths issues if a horse has been in the open eatin grass an a field he needs to be wormed or if he has been in stables or close to other horses for a good time it needs to be wormed...
the horse should be de-wormed about twice every year, especially during winter.
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From Horseman's U.com: Horses need routine worming, especially when living in close contact with each other in stables and facilities. The average is every 2 months and no sooner (they build resistances to the wormers). Problem is, only about 20% of stabled horses suffer from worm infestations; the other 80% are not as affected by them. To find out which category your horse is in, have a stool sample tested.
Worms rotate through your horse at seasonal intervals. Wormers should be rotated with the seasonal parasite. For really good information on wormers and their use, visit Horseman's U.com
they should be wormed at least every two months to prevent outbreaks of worms and to stop the worms breeding cycle