When To Deworm

Chemical Resistance & Fecal Counts

For a horse’s current parasite baseline, start with a fecal test done by your veterinarian.

Horses are susceptible to worm and parasite contraction year round. It is important to watch for any symptoms that would indicate infestation in order to catch the worms at an early life stage. It is also imperative to get a prevention plan prepared to avoid contraction. Many questions arise when dealing with equine worms and parasites. What type of de-wormer should I use? What type of worms does my horse have? How do I know when my horse has worms? Let’s get some answers!

How can my horse contract worms/parasites? The lifecycle of most internal parasites begins when horses ingest worm eggs, simply by grazing in an area that contains eggs. Horses often graze or consume hay or other feed from the ground while confined in small pens or small acreages. Since removing manure from outdoor areas is not a typical chore, this is usually an opportunity for maximum exposure to parasite eggs. Horses can also contract worms from contaminated hay or feed. This is one reason why pasture rotation is an effective preventative for parasite control.

How do I know if my horse has worms/parasites?

• Shaggy/dull coat
• Eye/nasal drainage
• Frequent loose stool
• Bloating
• Tail/mane rubbing
• Pale gums
• Summer sores
• Trouble keeping weight on/emaciation/loss of appetite
• Lethargy/depression
• Compromised immune system
• Colic/gut torsion

High Shedders vs. Low Shedders
Healthy horses usually have a low amount of worm eggs in their digestive tracts. “High shedders” are more susceptible to carrying a high amount of eggs and in turn shed more eggs through their manure. “Low shedders” carry low amounts of eggs and rarely shed them out through their manure. Interestingly, in most cases, “high shedders” are resistant to de-wormer chemicals.

Chemical Resistance
Over the past couple of decades, overuse of chemical de-wormers has become a prevalent issue for treating worm/parasite infestations. Ivermectin is among the many chemicals that have been overused because of traditional ways. In a traditional barn setting, horses are usually on a scheduled de-wormer rotation, sometimes every 3 months, or in some cases every 6-8 weeks. The thought process behind this is that during a change in season, different parasites are available for horses to ingest. The horses then need to have preventative chemical treatment so that the entire barn is safe from infestation. Horses brought into a new barn are also dewormed even if they show no parasital symptoms.

Deworming a horse that does not have a high infestation of parasites or any parasites at all can be detrimental to their systems, sometimes causing ulcers and colic. The same thinking can be applied to taking a Tylenol when you don’t have a headache. Why take medicine if you show no symptoms? If the horse’s system is being exposed to these chemicals for no reason other than to ease the mind of the owner, this can cause long-term effects leading up to chemical resistance. Every day, horse owners are overusing and misusing chemical de-wormers and in turn, equine parasites are building up a strong resistance, making it more difficult to treat for worms and parasites. There are alternatives to scheduled chemical deworming that one might consider for the horse’s sake.

Fecal Egg Counts
Fecal egg counts (FEC) are a technical measure for finding out exactly what types of worms and parasites are present in your horse’s digestive system. A fecal egg count is a simple process that can be conducted on your horse by your veterinarian, and can even be a process that you can learn to conduct yourself. FEC measures the eggs per gram in the horse’s manure. According to the McMaster method, a low count is 0-200 eggs per gram, a moderate count is 200-500 eggs per gram and a high contaminator is 500+ eggs per gram.

If there is any chemical resistance present, the amount of reduction will be less than 90%. Fecal egg counts should be conducted every 3 months, or when necessary. Deworming a horse does not guarantee removal of any or all parasites. Always consult with your veterinarian before deworming.

Preventatives for Infestation
If available, pasture rotation will help interrupt the life cycles of the parasites. If the parasites cannot find an appropriate host (your horse), they will not survive past the larval life stage. Spreading manure to ensure that it dries out over a large area of land is one way to help break the parasite life cycle. Additionally, piling manure in one spot, similar to composting, can also help eliminate infestation as the manure pile will heat up as it decomposes, in turn killing parasite eggs before they hatch.

A natural parasite control is food-grade diatomaceous earth that can be found at most feed stores. Diatomaceous earth will make its way through the digestive tract, shredding anything with an exoskeleton and is usually given as 10% of the feed ration. In response to the increasing chemical resistance concerns, there are a growing number of natural (non-chemical) “anthelmintic” (de-wormer) products available on the market. “Vermicides” kill intestinal parasites/worms and “verifuges” expel dead parasites/worms from the bowels. “Taenicides” are specific to killing tapeworms and “taeniafuges” in turn, expel dead tapeworms from the bowels.

Final Thoughts
Healthy horses can go months, sometimes years without needing to be dewormed, even in the presence of a “high shedder”. If their immune systems are in full force, they will naturally fight off worms and parasites. If you are weary about applying a chemical de-wormer every year, try one of the many natural parasite controls. You can also get fecal egg counts done at your veterinarian’s office as often as you would like to insure a low egg count in your horse’s digestive tract. If worm egg counts are low, daily de-wormers can be used safely and effectively. Remember to be on the lookout for infestation symptoms. Sometimes de-wormers do not rid the digestive tract of those pesky parasites and the horse will still show symptoms. Managing your pasture to decrease production and contraction of parasites is a key factor to eliminating the need to deworm. Working together with your veterinarian, and doing routine fecal egg counts will help insure a healthy future for your horse, and the end of the line for excessive parasites and chemical de-wormer resistance.