Why You Need a Seasonal Worming Schedule for Horses

To help keep your horse free from illness, you need a proper plan of action for controlling 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. This can be achieved by assessing every horse individually, always practicing good pasture management and using faecal worm egg counts (FWECs) appropriately.

Spring

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 tested or 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 horse to horse even if they have the same level of worm burden. That said, FWECs can indicate horses with particularly high 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 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. Alternatively, a specific antibody test can be carried out which will show if encysted small redworm are present.
• Redworms – treatment is usually recommended if FWECs > 200epg.
• Bots – the bot fly lays eggs in late autumn and bot larvae develop in the horse’s stomach so they should be treated now.

Summer

Ideal environmental conditions for the development of eggs and larvae on the pasture in the spring may 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.¹

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 RAMA for specific advice regarding timing and the most appropriate wormer for your foal.

Autumn

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 tested or treated for. In addition, late autumn/early winter is the time to tackle tapeworm and bots.

Treat your horse in autumn for:

• Encysted small redworm – test or dose all horses (> six months of age) in November/December regardless of the FWEC, using a specifically indicated wormer.
• Tapeworms – test or 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.

For optimum control of bots, treatment should be administered in the autumn, after the end of the fly season and before spring as the larvae may start to pupate and therefore are less sensitive to treatment.

You can find the correct worm treatment on our website here

Winter

This is a deceptively quiet time of year for horse parasites. 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:

• Encysted small redworm – test or treat now if not previously treated in the late autumn.

In addition to these seasonal threats ensure at least one of the doses given throughout the year is effective against large redworm.  Pinworm, liver 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.

Written by Dr Wendy Talbot. 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.

Related link: www.horsedialog.co.uk