Worm Egg Counts Explained

faecal sample

The strategy of simply worming your horse when faecal worm egg counts reach a certain level may be used with some horses to reduce the use of wormers. However, faecal worm egg count tests do have their limitations. As the name of these tests clearly states, they can only detect the presence of eggs in the horse’s droppings, and are limited to detecting a few roundworm species: small redworm, large redworm and large roundworm.

So even if you get a low egg count reading, your horse may be harbouring a high and potentially fatal number of larvae, which don’t produce eggs. With this approach, you’ll at the very least need to worm your horse in the late autumn/winter against encysted small redworm larvae, and in the spring and autumn against tapeworm.

So the answer isn’t a straightforward decision between worming and faecal worm egg count tests. To ensure the health of your horse, it’s advisable to use the faecal worm egg count tests as an integral part of your worm control programme.

What happens to your sample at the lab?

Once prepared, your horse's sample will be looked at under a microscope

Once prepared, your horse's sample will be looked at under a microscope

When using the Faecal Worm Egg Counts (FWEC) approach requires you, the horse owner, to supply a poo sample from your horse. When your horse's sample arrives at the lab, this is what happens:

STEP 1: First 3g of faecal sample is weighed out and then diluted, with 42ml of saturated salt solution.

STEP 2: Using a pestle and mortar (that’s right people a pestle and mortar!) the sample is ground up with the solution. Emma tells me the solution acts a ‘floatation medium’ - translation: something for the eggs to float to the top of!

STEP 3: Next the solution is sieved using…a tea strainer. This removes all the debris leaving just the liquid and eggs (if there are any).

STEP 4: With a pipette the solution is put into a McMasters Chamber, a small device broken up into counting chambers. These chambers are used as a guide to work out how out the egg count.

STEP 2: Finally the solution can be looked at under a microscope. If eggs can be seen in the chambers they’re counted then multiplied by 50 to get the total egg count. For example if there are eight eggs in the two chambers that gives you a worm count of 400eggs per gram which indicates that the horse needs worming.

When to worm your horse

It's important to know when you should worm your horse

It's important to know when you should worm your horse

All year round

ROUNDWORM Routine roundworm control, once the preserve of the grazingseason, should now be
undertaken all year round, even if your horse is only at grass for short periods of time during the winter months. This is due to climate changes, with milder and wetter winters meaning horses are at risk of accidentally eating infected larvae late into the year. The most common roundworm, andmost common and harmful parasite to infect horses today, is the small redworm.

Treatment Dosing intervals for roundworm are based on the active ingredient of wormer: moxidectin every 13 weeks, ivermectin every eight to 10 weeks, pyrantel every four to eight weeks, fenbendazole every six to eight weeks, or mebendazole every six weeks. When planning your routine roundworm worm control, it’s important to not use wormers against which there is known resistance. With resistance inhorses to benzimidazole-,  pyrantel- and ivermectin-based wormers, you may want to seek guidance from your vet prior to their use.

Suggested treatment time - all year round. Depending on the active ingredient used for routine roundworm control, additional treatments may be needed at certain times of the year if not covered as part of routine treatment.


TAPEWORM With treatment for tapeworm recommended every six months, treatment should be repeated in the spring.

Treatment Single dose of praziquantel-based wormer or double dose of pyrantel-based wormer.

Suggested treatment time March, April or May.


TAPEWORM Exposure to tapeworm is greater during periods of prolonged grazing, so treatment should be undertaken in the autumn following summer turnout. Treatment for tapeworm is recommended every six months.

Treatment Single dose of praziquantelbased wormer or double dose of pyrantelbased wormer.

Suggested treatment time September, October or November. 

ENCYSTED SMALL REDWORM Treat your horse against encysted small redworm larvae to reduce the burden of these lifethreatening encysted larvae that have accumulated in his gut wall as he grazes.

Treatment A single dose of a moxidectin based wormer or five-day course of fenbendazolebased wormer.

Suggested treatment time November.


BOTS Treat your horse after the first frost when the adultflies die off, and prior to the larvae maturing and emerging from your horse in the spring.

Treatment A single dose of a moxidectin or ivermectin-based wormer.

Suggested treatment time November, December or January.

ENCYSTED SMALL REDWORM Treat your horse in late winter against encysted small redworm, which are hidden in the horse’s gut wall. These will typically emerge from your horse’s gut wall in late winter or early spring. This mass emergence of encysted small redworm, known as larval cyathostominosis, is potentially fatal.

Treatment A single dose of moxidectinbased wormer or five-day course of fenbendazole based wormer.

Suggested treatment time February.

Worming: Top tips for fighting resistance

Ensuring that the worms in our horse are effectively controlled means that the choice of wormer for your horse is important. Here’s how to resist the resistance!

1. If you’re rotating wormers each grazing season, ensure you change the active ingredient – don’t just switch brand names.

2 Don’t rotate between wormers that belong to the same chemical family. There’s no point rotating between ivermectin and moxidectin-based wormers as both belong to the same chemical family. Worms develop resistence to moxidectin more slowly than ivermectin, so moxidectin-based wormers should be your first choice when choosing a wormer from this family.

3 Check with your vet that there’s no confirmed resistance in your area to the active ingredient you plan to use.

Determine the right dosage by weighing your horse using a weightape (pictured) or weighbridge

4 Give your horse the correct dose according to his bodyweight. Weight can be assessed by means of a weightape or weighbridge.

5 Reduce usage of wormers by worming less frequently. This can be achieved in two ways: firstly by using wormers with longer dosing intervals; moxidectinbased wormers have a 13 week dosing interval. Secondly, by using faecal worm egg counts and only treating horses when the worm egg count in the horses’ droppings reaches a certain level, normally above 200 eggs per gram.

Regular poo picking is vital in the war against resistance

Regular poo picking is vital in the war against resistance

6 Poo pick. By removing horse droppings from your pasture you’re removing most of the worms that have managed to survive your horses’ worming treatment. These worms are the ones that are – or have the potential to become – resistant to the active ingredient that was used at the time of treatment. By removing these worms, they are unable to reinfect your horse, and so are not able to complete their life cycle, nor produce resistant offspring.