Recommendations for parasite control in adult horses have changed in recent years. Traditional worming practices, where all horses are wormed every 6-8 weeks, have been shown to be less effective at parasite control and instead contribute to the development of parasite resistance. The goal of any worming program should be to maintain the health of the entire herd of horses rather than focussing on trying to rid individual horses of parasites completely.

A strategic worming program aims to:

• Minimise the risk of horses developing disease from parasitic burden
• Control contamination of the environment with worm eggs
• Prevent the development of parasite resistance to wormers

This can be achieved through

• Strategically worming horses at times when parasites are their most active and likely to cause disease
• Reducing usage of wormers by targeting horses that need treatment
• Using faecal egg counts (FECs) for monitoring
• Avoiding only using single active wormers in a worming program
• Quarantining and treating any new horses onto a property

In many cases, strategic worming means using only one or two worming treatments per year (typically autumn and spring) to control the major disease-causing parasites in adult horses. Beyond this, worming should be selectively applied to those horses that shed high numbers of eggs or are at greater risk of developing disease.  Where there are higher stocking densities (i.e. agistment properties) or frequent movement of horses on and off properties, worming may be required more often. Ideally, using FECs to selectively worm horses and testing the efficacy of drenches regularly with faecal egg count reduction testing, is the best way to ensure that parasites are managed effectively in your horses. 

It is important to note that these recommendations are for horses 2 years and older. Horses younger than 2 years of age, including foals, have different parasitic profiles and susceptibilities that require a different worming strategy to adults. It is always recommended that you consult your veterinarian to discuss tailoring a parasite management plan specific to your horse and your region.

Shedding, Faecal Egg Counts and Resistance Testing

Up to 80% of worm eggs shed onto pasture, on average, are shed by only 20% of horses. Horses are generally classified as low, moderate, or high egg shedders. Horses will normally have consistent egg shed rates over their life, if their health and stress levels stay the same. You typically can’t distinguish a high shedder from a low shedder just by looking at them. Performing a regular faecal egg count (FEC) on your horse helps to determine their egg shedding potential.

FECs are a relatively cheap and simple test to perform, as all that is needed is a fresh faecal sample from your horse. Horse owners can learn to do these tests themselves if they have the right equipment (microscope, faecal flotation solution, glass slides) but they are usually performed by veterinarians. FECs are performed by measuring the number of worm eggs present in a faecal sample. The results of an FEC are reported as eggs per gram (EPG).

Horses that have FECs <200 EPG are low shedders whereas horses with FECs >500 EPG are high shedders. It is important to note, just because a horse is a high shedder doesn’t mean they have a higher number of parasites. It does mean however that they are more likely to shed more eggs onto pasture and should be the target for selective worming.

*Speak to your veterinarian about which wormer would be effective for your horse/herd and your region.

It is important to ensure faecal samples are collected at the appropriate time. If samples are collected soon after worming, they are likely to be more reflective of the efficacy of the wormer rather than the horse’s actual egg shedding potential. Appropriate timing for collecting samples varies on the class of wormer used:

• PYRANTEL: 5-6 weeks
• FENBENDAZOLE: 6 weeks
• IVERMECTIN / ABAMECTIN: 9-13 weeks
• MOXIDECTIN: 16-22 weeks

FECs are useful for detecting strongyle (large and small) and ascarid eggs. Other parasites such as tapeworm, pinworms, bots or encysted cyathostomins (encysted small strongyles) are not detected using this method. Even if your horse has a consistently low FEC result, they may still benefit from strategic worming once or twice a year that targets these other parasites, in particular tapeworms and encysted cyathostomins. 

The Faecal Egg Count Reduction Test (FECRT) can be used to determine how effective your worming treatment is. To perform a FECRT, a FEC is performed prior to worming and again 14 days after worming. If the reduction in egg count is less than accepted (depending on the class of wormer used 85-95% reduction is required), then treatment failure is suspected. This should be investigated further with your veterinarian to determine if resistance is present on your property or if treatment is appropriate. Experts currently recommend performing a FECRT on your property annually to monitor for parasite resistance.   

When used as part of a strategic worming program, FECs play an important role. They can help decrease the need for worming treatments on your property and also assist in decreasing the rapidly growing resistance to wormers across the globe.

Worm Resistance

The World Association for the Advancement of Veterinary Parasitology (WAVAP) considers resistance to occur when less than 95% of the worms present in an animal are killed after treatment with a wormer. Due to the genetic diversity and large number of worms present in a population, resistance to wormers can arise naturally.

Resistance occurs when wormers are administered to a horse that already has resistant worms present in their body. These resistant worms survive treatment and continue to breed, passing their resistant genes on to the next generation. With frequent worming treatments, the susceptible worms are killed off and only resistant worms remain. Over time resistance increases, decreasing the proportion of the population that is susceptible to deworming.

Contributing Factors to Resistance

1. Under-dosing – Under-dosing can occur through not accurately weighing your horse, incorrectly administering the wormer or the horse simply spitting the treatment out. Under-dosing exposes worms to sub-optimal, non-lethal doses of a wormer. Instead of killing the worms, they can instead build up a tolerance to the wormer making them more likely to survive after subsequent treatments. They then pass this tolerance on to the next generation.
2. Frequency of deworming - No worming treatment is 100% effective. Worms left behind after treatment have generally developed some tolerance to treatment or are naturally resistant. The number of resistant worms in a population increases with frequent worming, as susceptible worms are killed off more often.     
3. Stocking density - The more horses you have in a paddock, the more eggs that are shed and likely to be ingested. Paddocks that are overstocked are more likely to have less grass, forcing horses to graze closer to manure and worm eggs/larvae, thereby increasing worm burden in the horse. With more worms the likelihood of naturally resistant worms being present in the population increases as does parasitic disease. This in turn increases the need to worm more frequently and therefore increases the risk of resistance developing.
4. Pasture management – Paddocks that are not routinely rested, harrowed or cleaned are more likely to increase the worm burden in horses. Overgrazing leads to expansive areas that are grass free which decreases the population of worms present on pasture. This may sound great but this actually means a higher proportion of the worm population exists inside the horse and are exposed to worming treatments.   
5. Refugia – are the proportion of the worm population that have not been exposed to wormers. Refugia exist in untreated animals and eggs/larvae on pasture. If a high proportion of refugia exists, they dilute the resistant population. If there is very little refugia present (i.e. all horses are wormed frequently), resistance can increase rapidly in a population.

If resistance develops, worms will generally be resistant to one class of wormer and don’t usually develop resistance to multiple classes. For example if worms develop resistance to the benzimidazole class of wormers (e.g. fenbendazole, oxfendazole) they won’t be resistant to the macrocyclic lactone class of wormers (e.g. abermectin, ivermectin) and vice versa.

If resistance continues to increase at the current rate, horse owners will face a similar problem to sheep and goat owners. That is, there being high resistance to all classes of wormers with almost no treatment effective against parasitic infection. By understanding the factors that contribute to resistance and using strategic worming practices as part of parasite management of your horse, you can help slow the rate of resistance and prolong the life of wormers for the future.

Integrated Parasite Management

Traditionally control of parasites in horses has been heavily reliant on chemical methods only (i.e, wormers and drenches). Because of this, the development of resistant parasites is on the rise. Integrated parasite management (IPM) refers to a system where multiple approaches for parasite control are used instead of relying solely on chemical control. Whilst chemical control still forms part of an IPM program, reliance is significantly reduced by using chemicals strategically and utilising other practices to reduce worm numbers and exposure to them. IPM can be used on any sized property for any number of horses and includes the following principles.

1. Strategic use of wormers

Wormers should only be used when needed and when worm burdens in horses are likely to be at their highest (i.e., spring and autumn).  Horses that are more susceptible to disease or that have high faecal egg counts may need to be treated more frequently. Horses should always be treated with the correct dose and an appropriate product for the worm/parasite being treated. Efficacy of the wormers being used on horses should be regularly assessed by conducting faecal egg counts and resistance testing.

2. Husbandry Practices

Good husbandry practices can reduce the exposure horses have to worms and disease. Good hygiene is the most important aspect of reducing exposure. Cleaning yards, stables and small paddocks daily, decreases close contact with potentially contaminated manure. Ensuring feeders and water sources are kept manure free and where possible kept off the ground, reduces the risk of contamination. Feeding hay on the ground, particularly in yards and in grass free paddocks should be kept to a minimum, using hay nets instead where possible. Stocking densities should be kept to a minimum to prevent overgrazing and increased contamination of pastures. New horses should be quarantined and treated for parasites before being introduced to the rest of the property to prevent introduction of new or resistant parasites.

3. Pasture Management

Managing pasture so that they are minimally contaminated with parasites significantly reduces the need for treatment. Several strategies can be employed to keep parasite numbers to a minimum depending on the size of the property and number of horses kept.

• Stocking density – is by far the most common problem for horse establishments. Overstocking and overgrazing considerably increases the number of parasites present and increases the likelihood of developing resistant parasites. Reducing the number of horses per paddock or having horses with low egg shedding potential can decrease the reliance on chemical treatment.
• Rotational grazing – paddocks are grazed for a few days only before horses are moved to a new paddock. This allows paddocks not being used, to be spelled for longer periods. This strategy can also include using a paddock for forage or crops and allowing horses to graze after they have been harvested.
• Cross grazing – as parasites show little cross infectivity between species, using other animals such as cattle or sheep to graze paddocks before or after horses can help reduce the number of infective parasites present on pasture.
• Alternate grazing – grazing more susceptible horses (i.e., weanlings, yearlings) on clean paddocks first, followed by adult horses ensures that the animals more likely to develop disease are subjected to the lowest possible number of parasites.
• Resting pasture – this is only possible on larger properties where no grazing occurs in a paddock for 3-6 months (depending on the time of year) but significantly reduces parasite numbers.

4. Nutritional Management

Improved nutrition can increase host resilience making them less susceptible to disease as well as infection. Foals, their mothers, young and geriatric horses are at higher risk of infection due to immature or compromised immune systems. Nutrition in these horses is particularly important in helping them to build their immunity and help reduce the consequences of disease caused by worms. Horses that are well nourished are more likely to cope better with a worm burden and maintain control over infection than malnourished horses.

5. Biological Control

Using worm’s natural enemies, population levels can be kept lower than if they weren’t present. Biological control generally targets the stages of the worm lifecycle that live on pastures. Natural enemies such as dung beetles, earthworms, birds and microorganisms (protozoa, bacteria, fungi etc) can be used to decrease the number of worm larvae and eggs present on pasture. Using biological control is sustainable, has very little or no side effects and generally continues to regenerate. It cannot control infection already present in a horse and can take some time to become established however, can be very effective at helping to keep worm populations a lot lower in paddocks and yards.

6. Selective Breeding

Although selective breeding has been used in the horse population for attributes such as speed, strength, agility and reproductive performance, very little has been focussed on resilience to parasitic infection. Selective breeding for parasitic resilience has been successfully achieved in sheep, cattle and pigs and has been shown to be a heritable trait in horses. Animals that are more resilient to parasitic infection can withstand a greater parasitic burden without showing signs of disease but also tend to clear infection on their own without requiring any treatment. Breeding horses that have low egg shedding potential should be a consideration particularly in areas where resistant parasites are an increasing problem.

The principles of IPM can be integrated into any horse establishments management practices. Whether you have one or a hundred horses on your property IPM is a simple, cost effective and more sustainable way to control worms and other parasites whilst also helping to combat ever increasing resistance to worming treatments.    

New Horses

Having a new horse arrive on your property can be exciting but they can also bring with them unwanted guests. A new horse can bring with it any number of infections, including worms. New horses have the potential to introduce new strains of worms onto your property which could also be resistant to several worming treatments. This could potentially expose horses already present on the property to different worms that may be a lot harder to treat. This can be avoided with a few simple steps.

1. Isolate and quarantine – any new horse should be unloaded and kept in an area away from other horses. Separate equipment should be used for any new horse including feeders, hay nets, water troughs or grooming equipment and these should be able to be adequately cleaned after use. Where possible only one person should have contact with a new horse to reduce the possibility of spreading infection. Manure from new horses should not be used as fertiliser and should be disposed of. 
2. Test and Worm – even if the new horse comes with a complete worming history, you can’t be sure that a wormer has been applied properly or at the right dose. It is better to use caution and conduct a faecal egg count (FEC) and then use an appropriate wormer at the correct dose that treats strongyles (large, small and encysted), tapeworm, bots and pinworms. 
3. Hold and then move – after worming treatment is applied new horses should be kept in the same yard for a minimum of 2 days to allow them to pass any untreated worm eggs in manure. New horses should then be moved into a small paddock that has been recently grazed by other horses on the property. This allows them to be exposed to any worms that are already present on the property. 
4. Retest and integrate – 14 days after worming treatment another FEC should be performed to ensure that deworming treatment was successful. If the FEC is high another treatment should be administered that contains different active ingredients to the first treatment and the horse held in the small paddock for another 14 days and again retested. If the FEC is low the new horse can be integrated into the rest of the herd. 

Due to the stress of a new environment, new horses may shed more worm eggs than they normally would.  Conducting regular FECs on new horses (every 8-12 weeks) can indicate how they are coping with their new environment and provide information on their ongoing egg shedding potential.

Don’t import someone else’s issues onto your property when introducing a new horse. Taking a few simple steps at the start can protect your property, your horses and make sure new horses are a pleasure, not a burden.