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All horses have parasites and regular worm drenching is a vital component of any parasite control programme. Lots of different parasites infect horses and ponies and these have a wide range of life cycles and behaviours that can produce many different signs in the horse. Some can produce very severe disease and even death. As a result, drugs have been developed that have different actions and effects to target the various types of parasitic worms.
Amongst the group of worms that live in the horses gut, there are some that are far more dangerous and sometimes much more difficult to eliminate with routine drenches. Resistance to certain drug types and specialised life-cycles have made some of these worms very difficult to kill.
Resistance to drench drugs is a very real issue with some worms building up resistance to the various drugs and action families over time. Resistance tends to result from drenching too frequently, under-dosing and over-using certain drench families. Discuss with your vet what the most appropriate drenching frequency should be on your property and what drugs to use, and when to use them to avoid contributing to the development of resistance.