articles

The Worm Turns

By  | 

Equine deworming is going through a transition in the conventional veterinary world. For three decades, it has been done using a six to eight weekly rotation schedule of anthelmintics. But parasite resistance has been identified with many of the drugs in current use, and there are no new classes of drug on the horizon.

For many years, Craig Reinemeyer, DVM, PhD, has been studying equine parasites, parasite resistance to anthelmintic drugs, and better ways to control parasites. His work is finally becoming mainstream, but it will still be many years before the horse world changes its longstanding habits permanently. However, the adaptation of Dr. Reinemeyer’s earlier work is refreshing. Fecal egg counts are performed on a regular basis to determine which horses on a farm shed eggs and which do not, or have very low counts. Deworming is only performed on horses that show a need for it.

Deworming drugs

Available drugs fall into three classes, and resistance has been shown in all:

1. Macrocyclic lactones — ivermectin and moxidectin
2. Benzemidazoles — fenbendazole
3. Pyrimidines — pyrantel

If drugs are used, select those that do not have resistance on that particular farm. Extensive drug use on a farm may result in contamination of the soil with drug residues, as well as parasite resistance on that farm. Conflicting results are found in research studies concerning microbial and dung beetle action on manure when drugs are present. There is evidence that the drugs affect manure breakdown by killing the bugs that degrade the manure. Anyone with an organic farm may be especially concerned about this residue in the soil.

Parasites are universal

Many horse owners as well as veterinarians consider that the only acceptable level of parasites is zero. However, parasites evolved in nature to co-exist with a host. If the host is dead, the parasite is without a home, so the most successful parasites have a non-destructive relationship with their hosts. Wild horses (and other animals) around the world have never received chemical deworming, yet they survive, reproduce and raise young. How wild horses naturally control parasites can help determine better management practices for domestic horses.

The parasites people are concerned about are the small strongyles, since the large strongyles that were common before modern anthelmintics came into use have been basically eliminated. Large strongyles did cause serious and devastating colics. Small strongyles, however, are not much of a clinical problem, unless the horse is in extremely poor condition and has a very heavy parasite load.

Why they become an issue

Parasites become a problem for domesticated horses for several reasons besides drug resistance.

• One is that confinement produces a low level of stress in the equine that can affect the immune system, allowing parasites to overpopulate in the animal. Even wild animals whose natural range is decreasing experience this. Horses are social animals that live in a herd, moving and eating 20 hours a day compared to domestic horses that may live in single stalls or paddocks, not moving much and having food rationed to certain times of the day.

• Wild horses range over a large region and therefore do not graze around the “roughs” or areas where manure has been left and parasite eggs hatch into larvae. Horses in confinement are often forced to graze close to or in an overgrazed pasture, directly on those long clumps of grass harboring larvae.

• Wild horses have a healthy gut ecosystem of beneficial bacteria and other microorganisms where parasites can live in balance. Modern horses have often been treated with antimicrobials and other drugs that have a negative effect on the gut ecosystem as well as the immune system, leaving the animals more susceptible to parasite overload.

An integrative perspective

Deworming with an integrative approach needs to include management changes on the farm, fecal egg count checks, and the use of natural products to decrease the parasite load in the pasture. The goal of modern deworming is to control the parasite load in the pasture through management and the identification and treatment of individual horses that shed high levels of eggs into the manure. Treatment of individuals with high egg counts can then be done using natural deworming agents. It has been my experience that horses in high stress competitive environments that shed eggs are harder to control with natural treatments, due to the ongoing levels of stress. So drug treatments may need to be done periodically.

Management changes in confinement situations include the removal of manure on a regular basis. Composting is the best way to remove the eggs from the environment. Spreading manure in pastures just carries the eggs around the farm and increase exposure to all horses. Clients can break up manure by harrowing during hot, dry weather (over 90ºF), or during pasture rotation when the horse is off the pasture. This will allow the eggs to hatch and the larvae to dry up before they’re ingested by the horse. Freezing does not kill the eggs, so tell clients not to break up manure during the winter and definitely not in the spring and fall.

The next step is to check fecals on all horses. Identify the high and low shedders. To establish a baseline for a horse or group, it is necessary to check fecals on a regular basis through the year — at least quarterly. Horses that consistently have a similar egg count through the year can be placed on a list of shedders, non-shedders and those in between.

Horses that consistently show a low or negative egg count can be listed as non-shedders and do not need to be dewormed with any product. Some horses never need to be dewormed! Horses with a moderate egg count should be dewormed until the count drops to the low range and stays there for several fecals. These horses can contaminate the pasture.

After using a deworming product, check the fecals again in ten to 14 days to be certain the protocol worked. The egg count should be at least 90% decreased. Horses that are shedders should be rechecked in six to eight weeks and dewormed again if needed. When parasite loads are high, it may be necessary to use chemical dewormers until the situation is under control. High shedders may benefit from a five-day dose protocol of Panacur®, which clinically seems to be the safest larvicidal deworming protocol in this author’s practice.

Natural deworming compounds can be used in many cases. If the horses are under stress or are living on heavily contaminated properties, however, natural methods may not work, or may need to be supplemented with chemicals at times. Many companies sell products claiming 100% efficacy. This is impossible, knowing the complexities of the living horse and environmental issues. It is also impossible for a natural product to be given as a single dose and be strong enough to clear parasites in a similar way as a single drug dose.

Complementary therapies

• Probiotics and prebiotics help restore the natural balance of healthy bacteria in the gut. In creating a natural health program for a horse, a two to three month course of these should be considered. Horses that have been ill, have poor immune systems, or have been treated extensively with antibiotics may need to use these products routinely. There is no harm in long-term use of pre and probiotics as long as they contain natural ingredients and no preservatives.

• Natural deworming is usually done with herbs, homeopathics (see sidebar) or mechanically with diatomaceous earth (DE), a fine powder from the shells of diatoms or microscopic algae. The edges are very sharp and act mechanically to slice the outer skin of the parasite, adult, egg or larvae. DE can actually absorb moisture from the parasite into its fine structure. Once this occurs, the parasite dies. Because the damage is mechanical, there is no chance of resistance. There is some theoretical concern about intestinal irritation, but that has not been clinically apparent in many years of using products containing DE.

Diatomaceous earth is often mixed with vermifuge herbs. This can enhance the effectiveness of the product. DE can also be offered free choice to horses, and in many cases, but not all, horses will eat the DE, usually just before the full moon as the parasites are becoming more active. One such product mix is NOMS, by Advanced Biological Concepts.

• Many herbs have an anthelmintic or vermifuge action. Some are relatively harsh or slightly toxic and should be used for short periods or mixed with herbs that soothe the digestive tract, such as marshmallow, slippery elm or those that help prevent cramping such as chamomile or valerian. Some herbs are contraindicated in pregnancy. A well-trained, experienced equine herbalist, not a well-intentioned horse person who has looked up herbs that clear parasites, should formulate any herbal formula.

Some of the herbs that have vermifugal actions include garlic, peppermint, common thyme, cinnamon, Echinacea, hyssop, tansy, cayenne, fennel, wormwood, elecampane, cloves, southernwood, and mugwort. One successful certified organic herbal deworming product is Verm-x, from a British company by the same name.

Chinese medicine also has herbs with vermifuge action. The companies that sell those herbs generally sell directly to the profession and not over the counter. Jing Tang Herbal and Institute for Traditional Medicine are companies that have useful Chinese herbal products.

Homeopathic solutions

Classical, single remedy homeopathy

A number of homeopathic remedies have been used for specific types of parasites. These are placed in water buckets and used over a period of five days around the time of the full moon, or for a few weeks at a time.

a) For tapeworms: o Granatum 3X — any age o Cina 3X –young horses o Chenopodium 3X — mature horses
b) General worms – strongyles: o Santonium 3X o Cina 3X — best with young horses
c) Ascarids — usually just in young horses: o Abrotanum 3X

Combination homeopathy

Equiopathics “Wrm Clr®” (manufactured by Washington Homeopathic Products for Equiopathics, LLC) needs to be administered for three weeks during the initial course. Homeopathics do not kill parasites; they improve gut health so the parasites do not wish to live there.

Heads up

One word of caution about black walnut. It is a traditional herb used by many cultures for parasite control. It is also a well known toxin in horses, and causes laminitis when horses are bedded on shavings made from the wood. Horses have also foundered when exposed to black walnut in the pasture. Some people have used black walnut extracts and other products successfully, and companies do sell it for use in horses. However, it is my opinion, and that of many others’, that it is not worth the risk when there are plenty of alternative herbs to use.

Deworming with an integrative approach needs to include management changes on the farm, fecal egg count checks, and the use of natural products to decrease the parasite load in the pasture.


Joyce Harman, DVM, MRCVS, graduated in 1984 from Virginia Maryland Regional College of Veterinary Medicine. She is certified in veterinary acupuncture and chiropractic, and has completed advanced training in homeopathy and herbal medicine. Her practice in Virginia (harmanyequine.com) uses 100% holistic medicine to treat all types of horses. Her publications include The Horse’s Pain-Free Back and Saddle-Fit Book — the most complete source of information about English saddles – and The Western Horse’s Pain-Free Back and Saddle-Fit Book.

Dr. Joyce Harman graduated in 1984 from Virginia Maryland Regional College of Veterinary Medicine. She is certified in veterinary acupuncture and chiropractic and has completed advanced training in homeopathy and herbal medicine. Her practice in Virginia uses holistic medicine to treat horses. Her publications include The Horse’s Pain-Free Back and Saddle-Fit Book – the most complete source of information about English saddles.