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Given that the majority of horses all over the world have been chemically carpet bombed with dewormers for the last 60 years, it’s not surprising that many of these dewormers are no longer effective in killing the worms inside your horse.
Through a process of natural selection worms have evolved to become resistant to one or more classes of dewormers. What does this mean ? Well, the more you keep deworming your horse, the quicker resistant worms will develop. It’s important to remember that horses have evolved for centuries alongside their worms. A recent study in NZ even found no harmful health effects for reducing deworming.
One of the best lines I’ve heard was from leading equine parasitologist, Dr Craig Reinemeyer at a recent Parasite Roundtable Discussion : “Remember Seabiscuit ? Well Seabiscuit more than likely raced with a belly full of worms and he seemed to do ok!” Seabiscuit lived from 1933-1947 – long before people were firing dewormers down the throats of horses!
There are three classes of dewormers currently available to treat strongyles and ascarids in horses : the benzimidazole (BZ’s), pyrimidines (PY’s) and the macrocyclic lactones (ML’s). Ascarids (the worms commonly found in foals) are widely resistant to ML’s. This has been clearly documented on many stud properties in Australia. Studies have also shown that strongyles are widely resistant to both BZ’s and PY’s. More importantly, a 2020 study also found that there are reports of strongyle worms being resistant to ML’s (ivermectin and moxidectin). ML’s are the newest class of drugs we have and strongyles can pose a significant health threat to to your horse. There are NO new deworming compounds available for horses.
Remember, if we keep going down this path of deworming horses at regular intervals or at the “change of season” or whatever date your agistment owner pulls out of the clear blue sky then you are only speeding up the development of resistance in your horse. Whilst many agistment centers and horse owners are already adopting a diagnostic-led approach to deworming, the vast majority have yet to educate themselves on the catastrophic issues we are facing. It’s not a matter of if, but when we find ourselves with more and more horses suffering parasite related colic…. and we are not going to have any new dewormers to treat them with.
So what can you do about it ?
Fecal egg counts should be the cornerstone of your deworming program. They serve three purposes :
To perform the FECRT a fecal sample is collected prior to deworming. The dewormer in question is administered and a fecal sample is collected 14 days following treatment. Using a simple equation we can then determine if the dewormer has been effective. As a generalisation, we want a reduction in more than 95% of the egg count after deworming compared to before deworming.
Another tool our clients are utilising to determine if resistance is developing on their property is to measure the egg reappearance period (ERP). This basically measures how long the eggs takes to reappear after your horse has been dewormed. A shortened ERP is an early predictor of resistance on your property. For example, the ERP for moxidectin (Equest) used to be 16weeks…. but many are seeing eggs reappear in as little as 4-6 weeks.
2. Classify horses into low, moderate and high shedders for a targeted parasite control approach
Remember, 80% of horses shed 20% of eggs onto pasture. These “high” shedding horses may need deworming more frequently (2-3 x per year) whereas the low shedding horses (the majority of horses!) only need deworming once a year. Fecal egg counts offer important information when we conduct a “risk analysis” of your farm. They shouldn’t be used in isolation and they are definitely NOT an indication of your horse’s worm burden. Instead, they tell us which horse is responsible for contaminating your pasture with worm larvae that could be ingested by other horses.
Previously we have used a cut-off of 200 eggs per gram as an indication that your horse needs deworming. However, a recent consensus document written by experts in the field suggests that an egg count of over 500 epg is a more suitable cutoff. The overall risk of your horse developing parasite related disease is not just based on fecal egg counts but rather their entire management. How often is manure picked up from paddocks ? Are paddocks irrigated ? Do they co-graze with young animals ? Does your property have a good quarantine protocol for new arrivals ?
For example, a 12yo Thoroughbred gelding horse returns an egg count of 350. They live with one other mature horse on a property with good pasture management and manure is removed from paddocks 2-3x per week. This horse probably only needs deworming once a year.
3. Monitor the presence of strongyles or ascarids in foals
Foals and youngstock should be treated very differently to adult horses. Whilst their immature immune system makes them more susceptible to parasite-related disease, they are also the ones who stand to pay the biggest price if resistance develops.
Whilst the intricate details about parasite management in foals will be discussed in a separate article, here are a few points to consider. Foals should receive their first dewormer no earlier than 2-3mths of age – treating before this time is not recommended as there will be very few adult worms present before 2mths. The dewormer should be from the BZ class (eg. Ammo Green). As previously mentioned ascarids which commonly affect foals are resistant to ML’s (eg. ivermectin/moxidectin). From 4-6mths fecal egg counts should be performed to determine whether you need to target your worming towards ascarids or strongyles. Young animals should be placed on the cleanest pasture. Foals and youngstock will secrete the highest amount of eggs onto pasture and the paddocks should be suitably rested prior to turning out other horses.
Just like fecal egg counts, good pasture management is one of the cornerstones to an effective parasite management strategy. Regularly picking up manure reduces your horse’s exposure to parasites and hence the dependence on dewormers can be minimised.
We do not recommend “dose and move” strategies as this just contaminates “clean” pasture with resistant worms that have survived the worming treatment. Harrowing of paddocks is common, but this should only be done in hot, dry weather to prevent spreading of infective parasite eggs.
Your horse lives with parasites and has done for centuries. It also lives with a whole array of bacteria. Does it make sense to start giving your horse antibiotics at regular intervals ? Of course not, so why does it make sense to continue to deworm your horse with chemicals it does not need.
We would be more than happy to discuss how you can get started on a diagnostic-driven parasite control program that is tailored for your horse. Start with fecal egg counts and determine if you have any resistance on your property. Implementing a diagnostic-driven approach to deworming will pay dividends in the long run.
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5. Nielsen, M.K., Mittel, L., Grice, A., Erskine, M., Graves, E., Vaala, W. et al. AAEP Parasite Control Guidelines. https://aaep. org/sites/default/files/Guidelines/AAEPParasiteControlGuidelines_ 0.pdf (2019)
6. Nielsen, M. K. “Parasite faecal egg counts in equine veterinary practice.” Equine Veterinary Education (2021).
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