DEWORMING of horses is an integral part of basic equine husbandry, but the advice for deworming horses has changed dramatically in the last number of years.

The old practice of deworming all horses at regular intervals, the frequency being determined by the active ingredient in the wormer product used, is now defunct! We have learned that this protocol, faithfully followed for years and performed with the best of intentions, unnecessarily ‘over-wormed’ the majority of horses.

Over-worming does not provide any health benefits to horses, costs owners a small fortune and actually does more harm than good in that it promotes resistance in parasites against wormers.

The aim of deworming is not to remove all worms from every horse (which would be impossible), but instead to keep the levels below that which cause ill-health.

The good news is that, if you haven’t already updated your deworming regime, investing in doing so now can save you a lot of money down the line. The bad news is that it is going to require significantly more effort!

HOW OFTEN SHOULD I WORM MY HORSE?

The simple answer to this is ‘At least once a year’, unfortunately, that’s where the simplicity stops!

The number of additional treatments a horse needs, will depend on the results of laboratory tests called ‘faecal egg counts’ (FECs). Several of these tests will need to be performed to determine the classification of each horse as a ‘low shedder’, ‘moderate shedder’ or a ‘high shedder’.

These classifications are based on the number of worm eggs shed in the faeces of each horse and are described in terms of eggs per gram of faeces, or epg for short. Table 1 shows how horses may be classified.

The American Association of Equine Practitioners (AAEP) estimates that low shedders account for the majority (50-75%) of the horse population.

Low shedders only require one or two worming treatments per year. A single treatment should be administered in autumn, and if opting for a second treatment, this should be given in spring.

Both treatments should target ‘strongyle’ worms and treatment in autumn should also target tapeworms.

In addition to classifying the horse, FECs also allow you to determine when a horse has a clinical burden of worms. If the result is >200epg, that horse needs to be wormed!

Moderate and high shedders account for approximately 10-20% and 15-30% of the horse population respectively.

Yearlings and two-year olds are always treated as high shedders. These horses will require additional treatments based on these FEC results, typically three or four treatments per year.

WHERE DO I START?

Contact your local equine veterinary surgeon or veterinary nurse. Guided by them, you will need to collect faecal samples from all of your horses for individual testing. Your veterinary surgeon or nurse can organize the testing for you, interpret the results and advise you on which horses to worm, how often and with what wormer.

They will also advise you on testing for anthelmintic resistance.

You will need to keep records of all FECs and treatments administered.

WHEN SHOULD I TEST FAECAL SAMPLES?

If you want to classify the horse in terms of egg shedding, and determine whether worming is needed, there is absolutely no point in testing the faeces of horses while the effects of the last wormer administered are still present.

A little bit of detective work is needed to determine how long this is.

Look up your records and see what product was used last and when. Read the fine print on the packaging or check it out online to see what the active ingredient was.

HOW DO I COLLECT FAECAL SAMPLES?

You will need to collect as fresh a sample as possible from each horse. Ideally the sample will be so fresh that it will steam up the container! However, if this is impractical, collect samples within 12 hours of being passed.

Use a new pair of clean, disposable gloves for picking up each different faecal sample, and a separate, air-tight, leak-proof container for each. Proper, screw-cap faecal pots may be available from your veterinary practice but in general, appropriately sized ziplock bags are perfectly adequate.

Label each bag with the name/identity of each horse before you start collecting samples. Use a permanent marker and ensure the writing is legible!

After placing each sample into the correct bag, squeeze out as much air as possible from the bag before sealing. The samples should be refrigerated between collection and testing.

Be warned that if a domestic fridge is the only option, this may not go down too well with some sharers of said fridge! Put all of the ziplock bags into another air-tight, leak-proof, non-transparent container to prevent smells, spills and potential divorce!

WHAT PARASITES DO WE TREAT FOR?

1. Small strongyles:

Small strongyles are described by the American Association of Equine Practitioners (AAEP) as the “primary equine parasite pathogen”.

These parasites are the focus of current worming regimes. Small strongyles are a collection of different species of related worms that are also referred to as ‘cyathostomins’ or ‘small redworms’.

When we perform faecal egg counts, the strongyle eggs that are seen are >99% likely to be from small strongyle worms.

They are relatively mild parasites which only cause disease when present in high numbers; hence why additional worming is only advised if horses have FEC results of >200epg.

Small strongyles can cause a potentially fatal condition in horses called ‘Acute Larval Cyathostomosis’. These rare cases are seen in springtime when weather conditions improve causing huge numbers of dormant small strongyle larvae to simultaneously emerge from the large intestine.

This causes an intense immune reaction and severe inflammation of the intestine, resulting in colic.

Small strongyles are incredibly clever parasites that appear to be able to tell the weather outside, from the inside of the horse! If the weather is bad (typically in winter) their larvae will ‘hibernate’ in the lining of the large intestine.

There are only two worming protocols that can kill the dormant small strongyle larvae in horses: either a single dose of moxidectin (e.g. Equest) or a five-day course of fenbendazole (e.g. Panacur).

If the weather is good, most small strongyle larvae won’t bother hibernating and so other wormers can be used which will kill the adults and non-dormant larvae.

2. Large strongyles:

Our old system of worming routinely every so many weeks was focused on killing ‘large redworms’. Large strongyles are far more pathogenic than small strongyles; in the past they caused many fatalities due to embolism, aneurysm and colic.

Thankfully their numbers have drastically reduced and we are more concerned with small strongyle worms today.

Having said that, we don’t want these vicious parasites to make a comeback; hence why it is advised to administer at least one preventative treatment annually to all horses.

3. Tapeworms:

It is extremely important to routinely treat horses for tapeworm as they can cause impaction colic and spasmodic colic. They are transmitted through mites found in the soil, so horses grazing on pasture are inevitably at risk.

Tapeworm infections are difficult to diagnose as their eggs are not usually detected on FEC. Therefore diagnosis by a vet will potentially only happen after the damage has been done and the horse is already colicing. Only two active ingredients of wormers can treat tapeworms in horses: praziquantel at a single dose, or pyrantel embonate at a double dose.

A single, annual treatment against tapeworms is recommended – therefore the ‘once a year’ autumn worming treatment advised for all horses, should also be effective against tapeworm.

4. Pinworms:

Pinworms are located at the very end of the large intestine and within the rectum. Adult female pinworms crawl out of the anus to lay their eggs on the surrounding skin of infected horses. This causes, something I like to call ‘itchy ass syndrome’!

Severely affected horses will scratch their rumps and tails off just about anything they can, in turn contaminating these items in the environment with the pinworm eggs and spreading the infection. Horses may also have pinworms but show no clinical signs and pinworm eggs are generally not detected on FECs as the eggs are laid on the skin as opposed to being passed in the faeces.

In the grand scheme of things, pinworms aren’t the worst in the world and are easily treated by most wormers.

5. Bots:

Bots are not worms at all, they are the larvae of bot flies. The adult flies can stress horses during the warmer months when they fly around their legs laying their eggs.

Horses lick off the eggs causing them to hatch into larvae which then live in their stomachs. Bots are generally not associated with disease in horses; they are usually only detected by chance if a horse is gastroscoped, say if investigating stomach ulcers.

Bots are more disgusting than dangerous (see picture). However, treating horses with ivermectin or moxidectin based wormers has the happy side-effect of also killing any bot larvae present!

A good hard frost will kill the adult flies, so when administering the annual autumn treatment, wait until after the first decent frost so that no more bot fly eggs will be laid on the horse either.

6 and 7. Roundworms and Threadworms:

These worms principally affect foals and weanlings and so are of minimal concern when worming adult horses.

WHAT WORMER SHOULD I USE?

This is probably the most common question that clients ask veterinary professionals in relation to worming. It is not a question that can be answered easily!

There are a myriad of equine wormers on the market that typically list a dizzying array of parasites that they treat. Some key questions need to be addressed:

What does this wormer treat, and more importantly what does it NOT treat?

Table 2 provides a general summary of the activity of common active ingredients in horse wormers.

It is strongly advised to consult your veterinary surgeon or nurse for advice on selecting specific products. In all cases the data sheet should be thoroughly read before any product is administered and horses’ weights should be accurately obtained using a scales or weigh-band.

What parasites do I need to target?

If it is the annual autumn treatment, we want to ‘nook’ everything ideally! Large, small and dormant small strongyles, tapeworms, bots and pinworms.

Options for the autumn broad-spectrum treatment therefore include:

Additional treatments throughout the year will be warranted if subsequent FECs >200epg.

These will focus on targeting small strongyles. Vets may also administer or prescribe a specific product to treat diagnosed parasitic infections.

How long does it take after worming before strongyle eggs will reappear in the faeces?

Minimising egg shedding on the pastures will keep pastures ‘clean’ and help prevent reinfection of horses.

Table 3 shows the intervals between various worming treatments and the reappearance of eggs in the faeces.

Pasture hygiene can also be improved by removing faeces twice weekly, mixed grazing (cattle or sheep) or paddock rotation.

Do I know that the parasites on my grazing land are not resistant to this product?

Anthelmintic resistance is a growing problem in all herd animals. There is no point spending money on wormers that won’t work on resistant parasites!

The good news is that this can be easily investigated by performing what is called a ‘Faecal Egg Count Reduction Test’ (FECRT) on high shedding horses.

Two FECs are performed, the first before worming and the second test, two weeks after worming.

Your veterinary surgeon or nurse can interpret the results for you.

Doireann Dowling is a veterinary nurse and environmental scientist who is currently completing a Masters in Livestock Health and Production.

She worked in equine practice on the Curragh for five and a half years and has worked for Dundalk Institute of Technology for the last eight years where she lectures om the BSc. in Veterinary Nursing.

Dowling has a keen interest in parasitology and is happy to be contacted at doireann.dowling@dkit.ie if you have any queries.