Does My Horse Need Worming?

The short answer is yes. A thought through horse worming programme, specific to your circumstances, is essential – especially in spring and late autumn each and every year. 

Despite the promotion of Faecal Egg Counts (FEC) to monitor the worm burden, the general advice is that a low result, less than 200 epg, needs no action.

 This idea is flawed!

The flaw being FECS do not show encysted redworm larvae, which emerge to occupy the void when the mature redworms are cleared by wormers. Your horse can have a low result less than 200epg, but within three or four weeks be riddled with new mature redworms. FECS alone will not control the redworm population. 

You will need a sensible deworming schedule and proper pasture management, to reduce your horse’s worm population to less than 200epg, all year round.

When Should I Start?

Traditional de-worming of farm animals followed the moon cycles. The worming being started 2 days before a full moon (the moon waxing) and finishing 2 days after a full moon (the moon waning), 5 days in total. 

This practice is still followed in many parts of rural Europe. We have had consistently good results using the moon cycles every 3rd to 4th month. Adult red worms for unexplained reasons are attracted by the light and pull of gravity. Red worms become active over the full moon phase. Could this possibly be explained by some programming in their genes? What rural peasants have always known, science has yet to discover!

Whatever time of year or season you decide to de-worm your horse, the starting point is a Faecal Egg Count (FEC) test. The results will show the level of worm burden prior to using a wormer. 

A low result less than 200 eggs per gram (epg) but above zero will signify that deworming is necessary, but not urgent. Over 250 epg but less than 750 epg indicates deworming is needed as soon as possible – the worms are active. Over 750 epg indicates grazing infected with worm larvae and depending on season encysted larvae emerging. High worm burden points to poor pasture hygiene.

It cannot be stressed enough that poo picking, composting and seasonal deworming are essential to control your horse’s worm burden, reducing it to less than 200 epg, all year round.

How Do I Check Faecal Egg Count?

Faecal worm counts can prove a most useful aid in worm control. Regular worm counts taken 14 days after worming show the efficacy of the wormer used. Worm counts done before worming can show the level of worm infestation on your pasture. If your worming schedule and control measures are working, the results should be low to medium less than 250 epg, high counts over 500 epg are cause for concern

The first step in establishing your horse worming programme is to understand the faecal egg count.

Westlabs kits are an effective way of understanding your horses worm burden


Horse Worming Programme


Ideally, your deworming schedule should start in spring. The lengthening daylight hours coupled with warmth and moisture activates worm activity. Eggs that have over wintered on grazing, start to hatch once the temperatures reach double figures, over 10 degrees centigrade. 

The old rule was 3 weeks after horses go out to grass start deworming. No matter what your horse’s FEC result, it is essential to deworm in spring, followed by a FEC test. This will remove young new season larvae picked up from grazing and encysted larvae emerging from the gut wall. Think of this as an internal parasite Spring clean.

A FEC taken 14 days, after the last dose of wormer is essential to establish the worm burden. Results may not be as expected, a higher result than the FEC test taken prior to worming, confirms the presence of encysted larvae emerging into the gut. Encysted red worm larvae often fill the void when the small intestine is cleared of larvae and adult worms. Much like squatters they note a vacant home and waste no time in occupying. Immediately, re-dose to clear the unwanted (squatters) encysted larvae maturing into adult worms. Followed by a FEC.


In June, dose again to check the redworm population building up, followed by a FEC. A low egg count less than 200 epg means deworming can be put on hold over the hot summer months of July and August. The September rains, misty mornings with warmth and moisture again encourage worm activity, eggs hatch and larvae survive on grazing.

The Autumn dosing should be followed by a FEC. Results over 750 epg again point to emerging encysted larvae maturing into egg laying adults. Redworm larvae can encyst, hibernate for 2 or 3 years then emerge when conditions suit (Spring & Autumn). The squatters move in, once the wormer clears the resident mature worms. Dose again, to clear the newly arrived squatter redworms. The emergence of large numbers of encysted redworm larvae shows neglect of a proper approach to deworming in previous years.

Autumn / Winter

Mid to Late November, before temperatures dip under 10 degrees centigrade, do your last worming followed by a FEC test. This will clear any redworm larvae that would otherwise start encysting, to hibernate through winter.

The two most important deworming dates are Spring & late November, even though your horse’s FEC results are consistently low, don’t miss out on these crucial dates.

Regular de-worming is necessary to keep the worm population of your pasture under control. A regular schedule starts with the first flush of Spring grass within 3 weeks of horses being put out to grass. Followed by regular applications every 8 weeks throughout the summer and autumn months. This regime can be relaxed over the winter months November to late February if the horse is stabled, in a clean environment. 

The re-infestation rate from pasture is virtually nil once the temperatures drop below 10 degrees Celsuis, early December through to early March. If your horse is left out all year round then the last deworming will be in late November for encysted red worm, then left to over winter until early Spring when temperatures start to rise. Contrary to belief, frost and low temperatures do not kill worm larvae. They hibernate until Spring then moult into infective larvae.

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