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  • Writer's pictureDr. Keaton Massie

It's Coming...Parasite Armageddon

When horses have a substantial and overwhelming parasite load, we begin to observe clinical symptoms and health concerns like weight loss, colic, and diarrhea. In some severe cases, such as acute larval cyathostominosis, there may be a high mortality rate.

By Dr. Keaton Massie, DVM Massie Veterinary Services

Horses are susceptible to a variety of parasites, particularly: Small strongyles, large strongyles (Bloodworms), ascarids (roundworms), tapeworms, and bots.

One growing concern in the equine world is the increasing resistance of parasites to regular deworming medications. In the past, dewormers were the first line of defense in parasite management, but with the high number of drug-resistant parasites, we’ve had to rethink our usual treatment methods. I’ll elaborate on these methods as we go further.

Can’t we all get along, and what’s the goal?

I have found that horse owners often misunderstand the term “parasite control.” The real aim of any parasite control program is to manage and reduce the parasite burden, not eliminate parasites completely. It’s not feasible for horses to be free of internal parasites. In fact, trying to eradicate parasites completely has led to drug-resistant parasites, which is the biggest reason many dewormers don’t work effectively anymore. To understand how to control parasites, let’s take a look at how horses get infested.

For all parasites except bots; horses ingest parasitic larvae from pasture. Larvae become worms in the stomach of horses. The worms lay eggs, which are released in horse manure. The eggs hatch, find their way into the environment and pasture, and are again ingested by horses. The cycle continues. The reason bots are the exception is they are transmitted from the pasture but from flies that have a similar appearance to bees.

Since parasites have parts of their life cycle outside the horse's body and parts inside the horse's body, we can take advantage of the life cycle outside the horse's body to limit our horses parasite problems..

Therefore, the primary goals of any effective parasite control program can be summarized as follows:

  1. Minimizing the likelihood of parasitic diseases occurring.

  2. Managing the shedding of parasite eggs into the environment.

  3. Ensuring the efficacy of anthelmintic drugs and preventing further evolution of drug resistance in parasites.

Why fecal test when I can deworm?

Traditionally, horse owners have relied on a rotational deworming schedule to control internal parasites in their horses. This involves giving dewormers to horses every few months, regardless of whether or not they have a parasite infestation. The idea behind rotational deworming is to target different types of parasites with different dewormers throughout the year. However, research has shown that this approach is ineffective and can even harm horses.

The consistent use of dewormers led to the development of drug-resistant parasites, making dewormers much less effective in controlling an infestation. To slow this resistance, horse owners must remember that horses live naturally with parasites. Having a population of parasites continuously in your horse is called “refugia.” If non-resistant parasites can co-exist and compete with resistant parasites, they can dilute the population of resistant parasites.

Who’s responsible for these worms!

It might surprise you to know that the bulk of parasitic eggs shed on a farm come from only a few of the horse population There is a saying that 20% of your horses are responsible for 80% of the parasite burden. There is therefore no need to deworm every horse and needlessly build resistance. The best parasite control programs are tailored to individual horses. Horse owners must know the parasite egg-shedding rate of individual horses. This way, only horses with a higher worm burden that shed more into the environment are targeted with dewormers. This is why fecal testing is critical because it tells you who to deworm.

Fecal egg testing (FEC) involves collecting a small sample of the horse's manure and sending it to a laboratory for analysis to identify the type and quantity of parasites present. Typically, FECs identify mainly small strongyle and ascarid eggs. Horses can be divided into three groups based on their fecal egg count results:

  • Low shedders - < 50 EPG (Eggs per Gram of Manure)

  • Moderate shedders - 50-200 EPG

  • High shedders- > 200 EPG

Only those in the high-shedding group are dewormed frequently while the others are dewormed far less often.

To evaluate the efficacy of a parasite control program, I perform a fecal examination both before and four weeks after administering a specific dewormer. The extent of reduction in the egg population determines the dewormer's effectiveness. If the parasite egg counts remain high after using a particular dewormer, it indicates resistance and calls for a modification in the control program.

Is fecal testing a one-time fix for parasite control? Well, no. But it has greatly helped to control parasite infestations and reduce the risk of drug resistance.

So what can I do?

In general, we recommend twice yearly fecal testing prior to deworming. If this program does not work for your animals then we recommend only twice a year deworming. Once in the spring with a Fenbendazole-based dewormer and in the fall with an ivermectin-based dewormer.

*photo credit: Mother Earth News

Other Parasite Control Measures

Besides fecal testing and monitoring high egg shedders, here are other non-drug measures I recommend in every parasite control program:

  • Encourage frequent trimming of pastures and prevent overgrazing.

  • Cross-graze pastures with other species, such as cattle, sheep, or goats, as they serve as biological vacuums for equine parasites.

  • Compost manure properly to kill strongyle larvae and ascarid eggs.

  • During hot and dry weather, drag, or rake pastures to disperse manure piles and expose larvae to sunlight.

  • Feed hay and grain in raised containers instead of directly on the ground.

  • Remove or drag manure from paddocks and pastures every 24-72 hours before the strongyle eggs hatch and develop into infective larvae (5-7 days under optimal conditions).

  • Regularly clean water sources to prevent fecal contamination.

  • Quarantine new arrivals and conduct fecal examinations, administering a larvicidal treatment before turning them out on pastures.

Finally, it is no longer acceptable to randomly treat horses or keep horses on a deworming rotation calendar. Fecal testing should guide any deworming program. Remember, the goal is not to kill all parasites, but to keep parasite loads to a level compatible with health. Based on this new paradigm, I recommend a fecal exam on every horse at least once annually but ideally bi-annually. This is the only way to determine the effectiveness of a parasite control program and to detect the development of resistant parasites.

If you have further questions about internal parasite control in horses, reach out to Dr. Massie below.

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