The Year In Horse Worming: A Seasonal Schedule

By Wendy Talbot on 09 March 2017

To help keep your horse free from illness, you need a proper plan of action for controlling equine worms. Parasite life cycles are linked to the seasons, so your worming schedule should take into account the time of year.

In addition, you need to consider the problem of resistance. The growing threat of worms becoming resistant to the chemical ingredients in wormers means you should always look at the bigger picture – it’s no longer acceptable to simply dose all horses routinely throughout the year.  Dosing correctly, as little as possible but as much as necessary, is the key and this can be achieved by assessing every horse individually, always practising good pasture management and using faecal worm egg counts (FWECs) appropriately.


This is when encysted small redworms (ESRW) mature and emerge from the large intestine, damaging the gut wall and potentially causing health problems for your horse, including weight loss, diarrhoea colic and sometimes death. If you have treated effectively for ESRW over the winter your horse shouldn’t have a problem; however, if there are any unexplained health problems it is important to contact your vet immediately.

Spring is the perfect time to start faecal worm egg counts (FWECs) to identify horses needing treatment. FWECs are never 100% accurate. This is because not all parasites eggs are easily detectable and some parasite stages don’t lay eggs. To confuse matters further the number of eggs produced by parasites will vary from horses to horse even if they have the same level of equine worm burden. That said FWECs can indicate horses with particularly high equine worm burdens and can also identify the horses passing (shedding) the most eggs and contaminating the pasture – which is very important to know.

Treat your horse in spring for:

  • Tapeworms –treating at the beginning and end of the grazing season at roughly six months apart is usually recommended to keep this parasite under control. Alternatively, you may test for antibodies to tapeworm but expert advice is needed to correctly interpret the result
  • Encysted small redworm– treating any horse that has not previously been given a wormer effective against ESRW in the preceding late autumn/winter period is advised.
  • Redworms – treatment is usually recommended if FWECs > 200epg


Ideal environmental conditions for the development of eggs and equine worm larvae on the pasture in the spring lead to high pasture contamination and increase the potential for your horse to acquire new parasite burdens.

In summer you can easily reduce pasture contamination by poo-picking at least twice a week.

Ideally, you should also conduct an FWEC every two to three months to identify horses that need dosing for redworms, which are the main summer culprits, and those that don’t.

Treat your horse in summer for:

  • Redworms – treatment is usually recommended if FWECs > 200epg
  • Roundworms – foals should receive at least two doses effective against roundworms in the first six months of life. Speak to your vet or SQP for specific advice regarding timing and the most appropriate wormer for your foal.


In autumn, most parasites are entering a less active phase and pasture contamination is reduced.  Ideally, you should continue with FWECs until the end of the grazing season (roughly when the grass stops growing) and if your horse has a strongyle count of >200epg he should be treated.

Depending on their age, many young foals may need their second roundworm dose in autumn too.

In late autumn if your horse has picked up any new small redworm larvae from the pasture these may encyst within the gut wall and they must be treated. In addition, late autumn/early winter is the time to tackle tapeworm and bots:

  • Encysted small redworm – dose all horses (> six months of age) in November/December regardless of the FWEC, using a specifically indicated wormer
  • Tapeworms – dose now, if your previous dose was in the spring
  • Bots – the bot fly lay eggs in late autumn and bot larvae develop in the horse’s stomach so they should be treated now


This is a deceptively quiet time of year for equine worms. Roundworm eggs can lie dormant in the soil for long periods, even in freezing weather. Small redworms (cyathostomes) will ‘hibernate’ in their encysted state in your horse’s gut wall and won’t lay any eggs so a faecal worm egg count will not reveal them.

Treat your horse in winter for:

In addition to these seasonal threats ensure at least one of the doses given throughout the year is effective against large redworm.  Pinwormliver fluke and lungworm may be a concern for some horses – it’s best to talk to your vet to check if your horse may be at risk of these.

For more information about worming and timing it right click here.



Wendy graduated from Bristol University in 1999. She then went on to complete a residency at Liverpool University and holds a European Diploma in Equine Internal Medicine. After working in practice for 13 years, she joined Zoetis in 2012 as the National Equine Veterinary Manager.

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