When it comes to deciding when to worm your horse, everything can become quite muddled up in your mind, including the seasons, the parasites and the chemicals to use. To maintain effective control and prevent resistance in your herd, it is important to be proactive and target your worming, rather than relying on interval dosing or blanket treating.

Every horse is different, so managing worm burdens requires an individual approach. While some horses will be high egg shedders that need worming frequently, others will be low shedders that can get away with being wormed less frequently. It is important to determine which horses, if any, need worming, before you decide to treat.

Faecal egg counts (FEC’s) can help identify the high egg shedders from the low ones and determine whether the wormer chemicals you are using on your yard are still being effective. FEC’s are, however, limited to only detecting adult roundworm and not tapeworm or encysted larvae.

For this reason, worming at certain times of the year is unavoidable, such as in winter when encysted small redworm larvae are lying dormant in the gut lining, ready to emerge come spring.

Mature small redworm are very harmful and, if untreated in their larval stage, the adult burden to your horse in March/April can be life-threatening. Roundworm generally becomes less of a problem after age 6, when most horses should hopefully have developed a good immunity to adult worms.

Tapeworm used to be identified by a blood sample, although this took time and often proved expensive. Now, the EquiSal Tapeworm Test requires only a simple saliva swab to determine whether there is an existing problem to treat. You could carry out a saliva test twice a year and only treat for tapeworm if the results suggest a worm burden. Pinworm can also be detected by a simple sellotape test. For more information, E-SQPs at VioVet will be happy to advise you.


Key parasite to target

Wormer chemical

Spring (March/April)


Praziquantel OR


Summer (May-Aug)

Roundworm (only if FEC indicates a high burden)

Fenbendazole OR


Autumn (Sept/Oct)


Praziquantel OR


Winter (Nov-Feb)

Encysted Small Redworm

Moxidectin OR



(If treating only for Bots, use Ivermectin. If treating for Encysted Small Redworm and Bots, Moxidectin covers both.)


It is also crucial to mention that you should always worm in accordance with your horse's weight, as under-dosing can lead to resistance in surviving worms, and over-dosing provides no added benefit. If you aren't sure how to weigh your horse, ask your vet or E-SQP for advice or read our guide with handy tool here.

To try and make your lives easier, VioVet has produced this simple table to guide you through a typical year of worming. If you have any questions regarding which wormers contain which chemicals, either check out the ingredients' lists on the wormer product pages or contact us for more info: (01582) 842096 [email protected] Alternatively, 'A Quick Guide to Horse Worming' in our Knowledge Base offers more comprehensive info.

Written by: Hannah