Faecal Worm Egg Counts

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In recent years, we have changed the way we worm (technically ‘de-worm’)
our horses. Instead of dosing at regular intervals throughout the year, we now
realise a targeted approach to individual horses is far safer and more efficient.
A ‘one size fits all’ approach is no longer viable, as every yard has different
risks depending on grazing strategies and numbers of horses coming and
There are 3 classes of worming drug on the market but sadly, due to their past
overuse, only 1 remains fully effective. The worms have simply become
‘immune’ to the drugs. By testing horses and treating only those that need
worming, and only when they need worming, we will keep the remaining drugs
effective for as long as possible. No new classes of wormer have been
developed so we must protect the capability of the ones we have.

The most useful tool we have to ascertain which horses need worming, is a
faecal worm egg count, or FWEC. Adult worms inside the horse’s gut lay eggs,
which are passed out in the droppings. We can count these eggs by looking at
the droppings under a microscope, and thus decide if a horse needs treatment
or not. This count can be carried out quickly and cheaply at any time, but we
use it for different purposes during the different seasons.

Spring and Summer -

Warm weather means worms are active; growing and reproducing both inside
your horse, and on the ground (hatching from eggs into larvae inside a dung
pile). Fresh droppings will contain any eggs from an internal worm population,
and a FWEC will tell us how big that population is. 4-5 nuggets of droppings is
enough for us to work with. A FWEC should be performed at the start of
Spring and then every 3 months throughout Summer into early Autumn.

Autumn and Winter -

Worms tend to hibernate and cease activity during cold weather. FWECs are
not routinely necessary during this time, but may be recommended if we have
Horses may be infested with different species of worm, broadly speaking
these are large roundworms, small red worms and tapeworms. FWECs have
their limitations. They do not accurately pick up tapeworms, and cannot detect
red worms if they have become encysted (trapped) in the wall of the horse’s
A saliva test for tapeworm is recommended at least once during the Summer,
but it must be carried out at least 3 months after any worming treatment.
A blood test for small red worms may be advised if your horse is at high risk, or
is showing clinical signs of disease.

In any given population of horses, there are large variations in their immunity to
worms. It is known that 20% of horses on a yard are responsible for 80% of
the worm eggs shed in droppings. These 20% may never show clinical signs of
a heavy worm burden, they just live with them. But they do infest the paddocks
with vast numbers of eggs, increasing the risk of transmitting worms to others.
FWECs enable us to identify these animals and treat them accordingly.

Faecal Egg Reduction Tests

A certain species of roundworm which affects foals and young stock, called
Parascaris, is resistant to many wormers. Small red worms, too, are difficult to
eliminate using many of the wormers on the market. We often need to find out
if a drug is effective against the worms affecting a certain yard.
We can use FWECs to do this:
At least 6 horses should be used in the test, and should not have received a
wormer in the last 8-12 weeks depending on which product was used.
FWECs are performed, and then the same worming drug is given to all the
horses in the trial. Second FWECs are carried out 14 days later. The number of
eggs in the pre-treatment and post-treatment samples are used to calculate
the percentage reduction in eggs for that drug.
Remember that all horses sharing a paddock share the same population of
It is important to acknowledge that under-dosing of worming treatments, or
failure to administer the full dose, may result in the incorrect assumption that a
wormer isn’t working