Your horse is stiff, he sweats a lot, or sometimes he refuses to walk after exercice? It could be tying-up.
Dr. Julie DAUVILLIER explains a little more about this syndrome.
Historically, the most effective way to control parasites was believed to be by deworming every 8 weeks with a rotation of the class of active ingredient used. This approach has resulted in the development of resistant strains of parasites. Individual worms develop resistance to a particular drug due to random genetic mutation. Indiscriminate deworming kills off the competition and gives these worms an advantage for survival. The more frequently the horse is dewormed with the resistant chemical, the quicker the population of resistant worms increases.