Shaping the future of animal health

Horses requiring special attention

Some horses are at greater risk of worm-related disease than others, and therefore, some special considerations need to be taken into account when preparing a worming program for them. Young horses, pregnant mares, thin or sick horses and any horse with an unknown worming history or who has gone an extended period without being wormed need to be given special consideration when worming them.

Pregnant mares

worming foals Pregnant and breeding mares require special management with worming for a couple of reasons. First it is very important to keep a pregnant mare on a regular worming program to protect her and her foal from damage by worms during a stressful time for her body. Pregnant mares should be wormed every six to eight weeks during their pregnancy, timed so that they are wormed two weeks before they are due to foal. The second area of concern with pregnant mares is that some worming compounds are not safe for use in pregnancy and may cause complications or abortions. Always check that the wormer you intend to use is safe for use in pregnant mares.

Foals and young horses

The immature immune systems of foals and young horses mean they are far more susceptible to worms than older animals. Some worm species are very common in young horses but rarely seen in older animals as horses seem to develop a natural immunity to them as they age. Young animals are also more susceptible to the damage caused by worms due to their smaller size. Immature lungs and digestive systems are easily damaged by migrating worms and a worm burden that may not cause a problem in an adult horse could easily block the gut of a foal. It is vitally important that young horses are regularly wormed with first drench at twelve weeks of age and subsequently every eight weeks so that large burdens are not allowed to develop.

Ascarids and young horses

Ascarids are worms that primarily only affect horses under two years of age. Once horses mature, they develop a natural immunity to ascarids and so these worms are rarely seen in older horses. As ascarids are such a large worm that can quickly develop into life threatening numbers, they are the most significant parasite in horses under two years of age. Unfortunately it appears that ascarids are the worm species developing resistance to the mectins. To protect young horses from potential mectin resistance, it is recommended that a combination product registered to treat mectin resistant ascarids, such as Equimax Elevation or Strategy-T, be used. Young horses should be wormed with Strategy-T in spring and summer and Equimax Elevation in autumn and winter from twelve weeks of age until they are two years old. At this stage they can be put on the Equimax/Strategy-T strategic worming program.

Thin or sick animals

Special care should be taken when worming thin or sick animals as they will be much more susceptible to ill effects after worming and this may aggravate their condition. Veterinarian advice should first be sought, particularly if there is a suspicion that their condition may be caused by worms as some wormers/worming compounds are not suitable to be used in thin or sick horses. It is best to check the safety directions or with the manufacturer before using a worming product in this type of horse.

Horses with unknown worming histories or who have not been wormed for extended periods.

Occasionally owners may acquire horses with unknown worming histories or a horse that has been left an extended period without being wormed.

These horses need special treatment as they may be carrying large worm burdens and the sudden death of large numbers of worms can lead to life threatening colics in some horses. The best way to handle this type of horse is to worm it initially with a straight BZ such as Fensolve®* which will kill some of the worms more slowly. Then to ensure a full clean out two weeks later, a dose of a mectin wormer such as Equimax should be given.


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