All horses have worms. Small numbers are totally harmless and help maintain a healthy intestine, but large worm burdens can cause serious disease, as well as contaminating the pasture that they are grazing, thereby increasing the risk to other horses grazing the same pasture. It is essential, therefore, that worms are kept to manageable numbers both in our horses and on the pastures that they graze.

For the last fifty years we have had drugs (called anthelmintics) that we can use to kill horse worms, but recently many of the worms have become increasingly resistant to these drugs. In addition, we now know that these drugs can have serious adverse effects on invertebrates and the environment. This has necessitated a change in the way that we manage horses to control worm burdens, and the way that we use the deworming drugs