Worms vs. Drugs: The Fundamentals

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Worms vs. Drugs: The Fundamentals

It is important that you know the basic characteristics of common equine parasites, as well as the main mechanisms of action and affects of the common deworming compounds.

Photo: Kristen M. Janicki, MS, PAS

All those tubes of anthelmintics (deworming drugs) look pretty similar lined up on a store shelf, but the drug classes within them vary significantly. How do you know which type of dewormer to choose and whether the one you’re administering to your horse is doing its job? We’ve learned that each horse and farm requires an individualized parasite control program, but confusion about words like benzimidazole and cyathostomin likely still exist. 

It is important that you know what these terms mean, the basic characteristics of common equine parasites, as well as the main mechanisms of action and effects of the common deworming compounds. So we’re here to help.

The "Classic" Parasite Life Cycle

While there are great differences among the important internal parasites, they share a fundamentally similar life cycle: They all spend part of their lives in the horse and part of their lives in the environment. 

A quick review: Adult worms live and lay their eggs in the horse’s intestine. These eggs pass from the horse into the environment with the manure. On pasture, the eggs hatch over time, releasing larvae that develop to a point at which they are “infective” and a horse ingests them, bringing us back to how the worms gained access to the intestines in the first place. The infective larvae mature within the horse into adult worms, and the cycle repeats. 

“In the environment, parasite eggs flourish and develop into infective larvae when temperatures are between 45° and 85° Fahrenheit,” explains Martin Nielsen, DVM, PhD, Dipl. EVPC, ACVM, co-author of The Handbook of Equine Parasite Control, chair of the AAEP Parasite Control Subcommittee, and an equine parasitologist, veterinarian, and assistant professor at the University of Kentucky’s Gluck Equine Research Center, in Lexington. “When these conditions exist during the year depends on location, but one thing we know is that eggs and larvae actually survive much better in cold than hot conditions. There is no such thing as a killing frost for these parasites.”

Know the Worms

Large strongyles Historically, veterinarians considered the large strongyle Strongylus vulgaris (bloodworm) to be the most important equine parasite. Bloodworms were common, and they caused a life-threatening problem in horses called thromboembolic colic, in which worms enter, damage, and block the arteries that supply blood to segments of intestine. Over the past 40 years, much of our parasite control efforts went into eliminating these worms, and to a great extent we were successful. Thromboembolic colic, and bloodworms, are now rare in managed horse populations. But those same efforts have caused severe drug resistance in other worm populations. 

Small strongyles (cyathostomins) These parasites are common in almost all horses. They are small, threadlike worms that can sometimes be seen in manure after deworming. Small and large strongyles share the classic life cycle described, but small strongyles show one important difference: Their larvae burrow into the intestinal wall and stay there for extended periods in a dormant state until the environment is optimal for reproduction, at which time they emerge. This makes the species more adaptable. It also protects them from most of our deworming compounds, which cannot penetrate the intestinal wall well enough to kill the encysted larvae there. Small strongyles are now resistant to many of our common dewormers. The only good news is that these parasites do not cause severe disease unless they are present in extremely large numbers.

Ascarids One of the most important parasites in young, growing horses is Parascaris equorum, a large, pale roundworm resembling a bean sprout. This parasite is primarily of clinical concern in horses younger than 6 months old; adult horses can ingest ascarid eggs, but their acquired immunity prevents infection. 

Ascarid eggs are extremely hardy in the environment, living for years on pasture. When temperatures warm above 45°F, the larvae develop to an infective stage within the eggs that the horse ingests. The eggs are coated with a sticky substance that enables them to adhere to all types of surfaces, including fences, walls, and the mare’s udder. Mouthy, curious foals can pick them up from these surfaces or as they nurse. Once ingested, the eggs hatch in the small intestine, and the larvae penetrate the intestinal wall and enter the bloodstream. They enter the liver, travel to the lungs, and then back to the intestine, where they grow to be adults and lay eggs. The whole cycle takes about 2 ½ months.

What About Bots?

These parasites are not actually worms but the larval stage of a fly that looks a bit like a honeybee. Botflies buzz around horses' legs and lower bodies, depositing their sticky pale eggs on the hair. Horses lick areas where the eggs have adhered, ingesting them in the process.

Once consumed, the eggs quickly hatch within the mouth, and the larvae burrow in the tissues and stay there for several weeks until the horse swallows them and they attach to a very particular location within the stomach, where they spend the winter months. When conditions are right in the spring, the larvae release and are passed into the environment in the manure. They burrow into the soil and pupate, emerging later as adult flies to lay eggs and repeat the cycle. Our sources say the internal larval stages of bots probably cause no real problems for horses. The biggest problem associated with bots is likely the annoyance the adult flies cause.

The only dewormers that control bot larvae are moxidectin and ivermectin. But aggressive use of these drugs to eliminate bots might also increase resistance in other parasites.

Doug Thal, DVM, Dipl. ABVP

In large numbers, these parasites can cause ill thrift and poor growth. When you administer a dewormer to a heavily parasitized youngster, large numbers of these worms can die simultaneously in the intestine and drift downstream, forming a tangled mass that blocks the intestine and causes severe abdominal pain. This life-threatening episode is known as ascarid impaction. Managing and cleaning the environment where foals reside helps reduce the number of eggs they ingest, but it is almost impossible to prevent infection in young horses altogether. 

Pinworms Unlike the parasites described previously, pinworms do not cause serious disease. Pinworm adults live in the colon. Instead of laying eggs that are passed into the environment through manure, female pinworms actually leave the intestine and lay their eggs on the skin of and around the anus. The eggs irritate the skin there and cause itchiness. In rubbing his tail base and anus on stall walls, fences, trees, etc., the horse deposits the eggs back in the environment. The next horse to lick the object will ingest the eggs, and the cycle will repeat itself. 

Historically, pinworms were primarily a problem in younger horses, but today they are seen more in adult horses, too. “There is evidence that pinworms are developing drug resistance and new approaches to managing them are needed,” cautions Craig Reinemeyer, DVM, PhD, president of East Tennessee Clinical Research Inc. and co-author of The Handbook of Equine Parasite Control.  

Tapeworms These are very different from other equine internal parasites, and most of the common dewormers do not affect them. One important difference is that tapeworms have an indirect life cycle; they must spend time within a second “intermediate host,” a tiny oribatid mite, to complete their circuit. These mites live in large numbers on pasture and ingest tapeworm eggs that horses shed in manure. The eggs hatch into larvae within the mites, and the larvae grow. 

Grazing horses inadvertently eat the mites containing the infective larvae, and during equine digestion the larvae are released and travel to a specific location within the intestine—the point at which the large and small intestines meet (called the ileocecal valve). Here the larvae mature into adults over six to 10 weeks and begin shedding segments containing eggs that are passed in the manure.

It’s difficult for veterinarians to detect tapeworms. “They are usually not diagnosable using fecal floatation techniques, and the available blood tests have some problems, too,” says -Reinemeyer.

The mites live only on moist, growing pasture, so these areas are more prone to harbor tapeworms, and drylot situations and arid environments generally aren’t affected. Tapeworms are thought to contribute to a condition causing colic—irritation and blockage of the ileocecal valve area where they gather. But they can live in small numbers there and usually not cause a problem.

Equine Internal Parasite Guide

Parasite What Horses it Affects Prevalence Health Concerns it Causes Drug Resistance
Large strongyles, bloodworms (Strongylus vulgaris) All horses Rare in managed horse populations Severe colic, thromboembolism (blood clots in the arteries), and intestinal damage No reported resistance
Small strongyles (cyathostomins) All horses 6 months of age and older Widespread throughout the United States, but more prevalent in moist environments. Weight loss, lack of appetite, diarrhea, fever, lethargy, dull hair coat, poor performance, etc. Widely resistant to benzimidazoles, somewhat resistant to pyrantels, early evidence of macrocyclic lactone resistance
Tapeworms (Anoplocephala perfoliata) All horses 6 months of age and older Widespread throughout the United States, but more prevalent in moist environments. Ileocecal impactions and spasmodic colic No reported resistance
Roundworms, ascarids (Parascaris equorum) Primarily foals younger than 6 months Widespread on breeding farms Airway inflammation, small intestinal impaction, ill thrift, poor growth Widespread macrolide lactone resistance, early evidence of pyrantel resistance
Pinworms (Oxyuris equi) All horses Widespread but less common in managed adult horse populations Intense tail rubbing, skin irritation around the anus, pruritis Reported resistance to several classes, some macrocyclic lactone resistance
Threadworms (Strongyloides westeri) Primarily foals younger than 4 weeks Widespread throughout the United States, but more prevalent in moist environments. Possible unthriftiness, diarrhea No reported resistance
Bots (Gasterophilus spp.) All horses Widespread during summer Annoyance No reported resistance; however, most anthelmintics are ineffective against insects

Know the Drugs

Property management techniques are the true cornerstone of effective parasite control and are more important than drug administration. There are many management points to consider, (See "Creating a Parasite Control Program"), but here we will focus on the common anthelmintic drugs used in horses:

Benzimidazoles A class of compounds called benzimidazoles have been a mainstay of equine parasite control for more than 40 years. These chemicals interfere with worms’ energy metabolism on a cellular level, causing a slower kill of the parasites than the so-called paralytic compounds we’ll describe next. Examples of benzimidazoles include fenbendazole (Panacur) and oxibendazole (Anthelcide EQ). These continue to have good efficacy against ascarids, but small strongyles are now mostly resistant to this class. For this reason, use these drugs primarily in foals.

Pyrimidines These drugs (pyrantel pamoate/tartrate, known by the trade name Strongid) act at the junction between parasites’ nerve cells and muscle cells, causing paralysis and rapid kill of worms. Pyrantel does not penetrate the intestinal wall and, so, will not kill encysted strongyles or any parasite stage outside the intestine. There is now significant resistance to pyrantel among strongyles. Pyrantel comes in several forms: a paste, a fluid, and at low levels in a pellet (continuous dewormers like Strongid-C). Double-dose pyrantel is labeled for tapeworm treatment, with no evidence of resistance. It is generally very safe for all ages and classes of horses. While some study results have implicated continuous dewormers in resistance development, Reinemeyer estimates they still might have a niche role “for selected horses on the farm, for a selected time period, but never for life.”

Macrocyclic lactones (ivermectin and moxidectin) Ivermectin has been around for about 30 years and has been our most relied-upon dewormer, but now there is evidence that certain parasites are developing resistance to it. 

Ivermectin and moxidectin also work by paralyzing worms. But unlike the other drug classes, macrocyclic lactones also kill external parasites such as lice, mites, and larval skin forms involved in summer sores. Moxidectin can penetrate the intestinal wall and kill encysted strongyles, and it probably is the most effective compound for this purpose. 

Diatomaceous earth and other ‘natural’ products have to date not been shown to effectively kill parasites.

Dr. Craig Reinemeyer

Praziquantel This drug kills only tapeworms and is currently marketed in combination with either ivermectin or moxidectin. Praziquantel is probably also being overused, especially in regions that have very few tapeworms. 

Other compounds “Diatomaceous earth and other ‘natural’ products have to date not been shown to effectively kill parasites,” Reinemeyer adds. If you rely on these products, you might be putting your horses at risk.

Take-Home Message

With these basics in mind, consider your own unique circumstance and ask yourself some questions: What is your current deworming program and how do you evaluate its efficacy? Have you changed your approach as the industry has become more aware of drug resistance? Have you done all you can do to reduce the number of worms your horses ingest? What worms are likely to be a problem for your horses, given your geographic region and management?

By understanding worms and their life cycles, along with the drugs used to combat them, you can take a smarter approach to parasite control, helping slow the onset of drug resistance while still protecting your horses’ health. Work with your veterinarian to develop a targeted parasite control plan tailor-made for your situation.

About the Author

Doug Thal, DVM, Dipl. ABVP

Doug Thal, DVM, Dipl. ABVP, is an equine veterinarian with 18 years experience in clinical practice. Thal Equine (www.ThalEquine.com) is his full-service equine hospital near Santa Fe, N.M.

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