What You Should Know About Horse Worming

Horse If you own a horse, you most likely, also own a few parasites. Parasites are an inherent part of our world and can exist in an array of environments. More specifically, these parasites living inside of horses can produce millions of microscopic eggs every day. Horses are able to cope well with a low burden of parasites, but if not monitored and treated or if exposed to multiple parasitic invasions, this burden could create a pathological and life-threatening equine disease hence why it is key for you to look at equine worming as a crucial responsibility and a strategically planned task.

Prior to the 1960s and the development of effective parasitic treatments, many horses died from worm burdens; however, the worming revolution of the '60s & '70s, made it commonplace to worm at regular intervals, first with pyrantel and fenbendazole based wormers and then later on with ivermectin, moxidectin and praziquantel. Unfortunately, all equine wormer brands that have been available in the UK have only ever consisted of these five active ingredients, which has created a new problem. As with diseases becoming resistant to heavily used antibiotics, equine worms have become resistant to deworming drugs, and there are no foreseeable alternatives in the near future. Due to this resistance evolution to repeated chemical exposure, we need to be using these chemicals wisely and strategically.

Worm Resistance and Targeting

It takes exposure to these worming drugs for parasites to gradually develop resistance to treatments, so it is important to limit usage to only what is necessary. For example, there is no treatment for small redworm that hasn't shown the development of drug resistance. Unfortunately, small redworm is the most dangerous and numerous of equine parasites, so actions to slow this resistance development down is vital.

Chemical worming resistance is determined firstly by shortened egg reappearance times on a worm egg count. Secondly, resistance can be evidenced when there is no or a low worm egg count reduction after a wormer treatment, showing that the drug dose has not killed the worms that are known to be present. In some areas, the product moxidectin, with an interval dosing of 13 weeks, has shown small redworm infections rising sooner than expected, which is indicative of drug resistance. Additionally, the small redworm resistance to the fenbendazole (Panacur) is believed to be as high as 60%-80%, in some locations.

Since these two chemicals are the only wormers that are licenced to treat the encysted stages of small redworm, this highlights the need to reserve moxidectin for the winter dose, wherever possible, unless a specific situation requires treatment for larval cyathostomins, at other times of the year. For the remainder of the year, we can rely on ivermectin or pyrantel (where no resistance is present) to treat adult stages of redworm, when test results rise.

Prescribed Worming Programme

To reduce worm resistance proliferation and optimise the use of worming chemicals, it is important to lab test, prior to any treatments. This way you will only be using the drug that is absolutely needed. It is important to test for the right parasite at the right time of year. It is recommended also to incorporate resistance testing to check if the treatment has been effective. This may sound complicated, but if the horse is generally healthy, then a very simple plan of testing and dosing can be followed.

The following programme is a good basis for a healthy adult horse:

You should check the worm egg count for redworm and ascarids in spring, summer and autumn and worm for the possibility of encysted redworm in the winter. Tapeworm testing should be conducted every six months. You should always be on the lookout for bots, pinworm, lungworm and liverfluke by ensuring that you know and are watching for signs and symptoms.

For a healthy horse, worming treatments are only required if the tests indicate infection above a certain level, but at the end of the year, in the winter months, you should treat for possible encysted redworm. However, foals, youngsters, neglected or older horses, will require much closer monitoring and attention.

When treatment is required, worm egg counts should be repeated 10-14 days after the dosage is administered, and an EquiSal saliva test should be carried out two months after treatment. If the count or saliva score hasn't reduced significantly, this could indicate drug resistance, providing the dose was correct for the horse's weight and the full amount was administered.

Encysted stages of redworm, which are counted in the dung sample, are not mature enough to lay eggs, so it is important to either carry out a blood test with your vet for encysted redworm or treat with an effective product during the winter months (December to February). You can then rely on your worm count test results over the next season, from which the laboratory will provide advice to aid your decision to worm or not. The idea of worming is not to eradicate all worms, as this would be near impossible, but to help ensure that the worm burden is at an acceptable level, that shouldn't cause any issues for the horse in question.

Worm counts are reported as the number of eggs per gram (epg). It is common for there to be no worm eggs seen which is represented as less than 50 epg (<50 epg).

• A count of less than 200 epg is a LOW count and shows that your worming measures are working. There is no need to worm at this level.
• A count between 200 epg and 1200 epg is a MEDIUM count, the horse will need worming.
• A count over 1200 epg is a HIGH count, the horse needs worming and the worming programme also needs attention.

Because of the way tapeworm eggs are excreted, worm counts are not a definitive test for this parasite, instead, use the EquiSal saliva test to determine infection levels. 

Additional Tips

Wherever possible we should look to break the lifecycle of worms mechanically rather than chemically. Good pasture management and animal husbandry techniques are key to helping to control your horse's worm burden:

• If possible, keep horses with the same grazing companions, not constantly changing groups. 
• Poo-pick as much as you can, at least twice a week to keep parasite levels down. 
• Keep stables, buckets and communal areas clean. Disinfect from time to time. 
• Rest and rotate grazing and don't overcrowd fields. 
• Cross graze pasture with other species, like sheep. 
• Keep new horses separate until tested and treated accordingly.
• Don't worm and move; after worming ensure horses stay on the same pasture for a few days to help slow down resistance. 
• When first starting to use a targeted worming programme all horses should be tested, at a point when worming is due or slightly overdue, so as to get a true result.

In closing, it is important to remember that you should develop a testing and worming regime to help slow the worming drug resistance in equine parasites and to support the efficiency and effectiveness of these drugs. When in doubt, contact worming specialists such as those at Westgate Labs for more information and guidance. 

Westgate Labs have a friendly, knowledgeable team of Registered Animal Medicine Advisors (formerly known as SQP's) and veterinary approved follow-up advice is included, free of charge, for worm counts and testing services.

Westgate Labs were integral in the creation of this content, in order better advise natural equine practitioners and horse owners for the betterment of equine health.