Over the past two decades our knowledge and understanding of worm burdens in the horse has changed significantly. Traditional thinking was to attempt to completely rid horses of worms – this involved drenching all horses in a herd at regular set intervals (often every 8 weeks), with seasonal rotation of the drenching drug class. These traditional practices were extremely effective in controlling the most common disease-causing worm of the time, Strongylus vulgaris, a large red worm that caused verminous arteritis.

However, these traditional practices have also directly lead to:

1. the development of drench resistance in worms, and

2. a change in the disease-causing worms we see in horses today.

Firstly, the traditional interval, blanket treatment of all horses creates a strong selection pressure for drench resistance in worms – and once resistance is present within a worm population, they do not appear to go back to susceptibility.

It is important to note however, that the occurrence of drench resistance is very variable between properties, even those within the same region, and therefore resistance cannot be concluded on any given population of horses without specific, appropriate testing.

Secondly, the traditional drenching methods also led to the worm population steadily evolving so that today we see the main disease-causing worms in adult horses being the small red worms (cyathostomins) and tapeworms, with Strongylus vulgaris not nearly as common, and in foals ascarid worms are the main issue.

Aside from these changes associated with the traditional drenching practices, there have been other developments in our knowledge:

individual horses vary in their immune system capabilities to control worms – up to 80% of worm eggs are shed onto pastures by only 15-30% of horses in that mob – the shedding potential for a horse is very stable over time (i.e. a healthy pastured horse with a low egg shedding potential will tend to always have a low FEC, and vice versa for a high shedder),

the development of new drenches is MINIMAL to NON-EXISTENT – with no new class of drench reaching the equine market for over 25 years,

worm free horses are not possible NOR are they desirable – the goal should instead be to minimise worm egg contamination of pastures,

This brings us to strategic deworming – a concept that attempts to find a balance between:

1. minimising the risk of worm diseases,

2. controlling worm egg contamination of the environment,

3. but also preventing the development of drench resistance.

It is important to note that the following recommendations only apply to horses 3 years of age or greater.

Because horses vary greatly in immune system capabilities, we should target worms on an individual level. This can be achieved by performing faecal egg counts (FEC) at regular intervals and identifying and treating the “moderate to high shedders”.

FEC’s are performed at intervals according to the egg reappearance periods (ERP’s) of the drench used, with the guideline being the ERP + 2 weeks. These are the current accepted egg reappearance periods for the common drench classes:

PYRANTEL:5-6 weeks

FENBENDAZOLE:6 weeks

IVERMECTIN / ABAMECTIN:9-13 weeks

MOXIDECTIN: 16-22 weeks

However, as discussed previously drench resistance is increasing to the common worms, and this level of resistance varies from one farm to the next. Therefore, in choosing a drench for your horses, don’t simply buy the cheapest or the one recommended to you by a neighbour or the feedstore, you need to consult your veterinarian to perform faecal egg count reduction testing (FECRT). This should be undertaken every 2-3 years on each farm and provides specific drench resistance information for your property.

Deworming programs for adult horses should be designed with the following principles in mind:

Evaluate the effectiveness of drenches every 2-3 years using the FECRT. The old concept of “rotating” drenches is no longer recommended.

Focus drenching during seasons of peak transmission (usually Spring and Autumn).

One to two drenches per annum to all horses will be sufficient to control large stongyles, bots, and habronema.

Properly timed single annual tapeworm treatment is beneficial for most horses - this treatment should be given in late Autumn or Winter.

Consider including a treatment effective against encysted cyathostomins at a time when the mucosal burden is at its peak, typically Spring.

All further treatments should be targeting horses with a high egg shedding potential as identified through FEC’s.Horses less than 3 years of age require special attention as they are more susceptible to worm infestation, and are more at risk for developing disease. During the first year of life foals should receive a minimum of four anthelmintic treatments.

1.2 months of age - Either a benzimidazole drug or a combination product containing a non-macrocyclic lactone drug is recommended to ensure efficacy against ascarids.

2.Just prior to weaning. An extra treatment can be justified before weaning if the period between the two treatments exceeds 3 months. At weaning FEC are recommended to determine whether worm burdens are primarily strongyles or ascarids, to facilitate the right choice of drug class.

3.9 months of age - treatment should primarily be targeting strongyles. Tapeworm treatment should be included.

4.12 months of age - treatment should primarily be targeting strongyles. Tapeworm treatment should be included in the 9-month treatment.

Recently weaned foals should be turned out onto the “cleanest” pastures with the lowest parasite burdens.

Yearlings and two-year olds should continue to be treated as “high” shedders, and receive about three yearly treatments with efficacious drugs.

The take home message is that our knowledge of effective drenching of horses has evolved. It is recommended that you consult your veterinarian to discuss the above information, and tailor an annual drenching protocol specific to your horses.