There is increasing confusion over the parasites known as small strongyles. In the early years of parasite control, small strongyles were thought to be of little consequence to the health and well-being of the horse. After effectively controlling large strongyles and other common internal parasites in horses, researchers discovered that small strongyles cause much more damage than previously imagined; damage that can lead not only to poor performance, but to death in extreme cases.

We went to career-long parasite re-searcher and American Association of Equine Practioners member of 28 years John Paul, DVM, MS, to ask some basic questions about small strongyles. What we found out will help you understand this complicated world of worms a little better. You should discuss deworming options with your veterinarian; different species of parasites cause problems in different areas of the country, and the world. A practitioner who is taking care of a number of horses in your specific locale is best able to see what is happening, and adapt what he sees to a program for your individual horse.

Q: What are small strongyles?

A: They are a group of parasitic worms living in the cecum and large colon of the horse. There are about 40 species of small strongyles throughout the United States and the world. They belong to a family known as cyatho-stomes, all of which have a similar life cycle.

Small strongyles are the most numerous parasite of horses. If you were to open a normal horse, you would find many more small strongyles than any other parasite--tens of thousands to hundreds of thousands in some cases.

Most horse owners will never see an adult small strongyle in the feces of a horse. They are hair-like in diameter and only about a half-inch long. (The larvae that hatch in the fields are, of course, much smaller.) Don't expect to see dead worms in the manure after you deworm your horses for small strongyles.

Q: What is the typical life cycle of a small strongyle?

A: Because of the multiple species, there are some variations in the length of the life cycle. Six to 12 weeks is the normal range from the time an adult lays eggs until the next generation is able to lay eggs.

The adult stays in the horse's intestine. There can be 2,000 strongyle eggs per gram of feces in a horse with a severe infestation. There are 454 grams in a pound, and a horse can produce 30 pounds of manure per day--that's a lot of contaminated pasture!

It should be noted that a fecal egg count doesn't tell us the adult population, however. Not all adults lay eggs every day. So, we can't say if the horse has 2,000 eggs per gram of feces that it has "this number" of adults.

Once the eggs are on the ground they hatch and develop into larval stages depending on climatic conditions. If it is warm and moist, they develop rapidly. If it is cold and dry, they lay around and don't do anything (a lot like a seed for a plant that doesn't sprout unless conditions are right).

Eggs may not hatch and larvae might not survive when climatic conditions are very hot and dry or severely cold.

As small strongyles are growing up, there are first, second, third, and fourth stage larvae. If the larvae reach the third stage and are ingested by a horse, then the larvae can cause infection and proceed with the life cycle. If the horse picks up a larvae in the first or second stage, then nothing will happen--it won't grow and mature.

Once the third stage larvae are ingested, they pass through the stomach and go to the lower intestines (primarily the cecum and ventral part of the large colon). At that point, something tells them to burrow in to the wall of the intestine.

Not too long ago, we didn't think it was too harmful because the larvae were just in the intestinal wall; now we know they can cause damage to the horse.

Part of the life cycle is for the larvae to become encysted in the intestinal wall. That is when a fibrous capsule forms around them (like a cocoon). Under normal conditions, they develop through the early third stage, to a later third stage, and then to a fourth stage while encysted in the gut wall. Then they emerge, come back into the lumen (intestinal space), and become egg-laying adults. The amount of time these stages take is varied.

Owners need to understand that when horses are grazing every day they are continually picking up larvae at the same time other small strongyles are developing in the intestinal wall and coming out a few at a time. This is like dripping water into a bucket with a hole in it--water runs out the hole a little at a time so the bucket is never full.

The encysted stage is a normal part of the life cycle, and under normal circumstances it does not cause a lot of problems to the horse. The exception is if the gut wall becomes full of these encysted small strongyle larvae. Then it will interfere with absorption of nutrients. That is a function of severe parasitism.

Another aspect of the life cycle is the ability of the early third stage larvae to go into an "arrested" stage of development called hypobiosis. In this situation, the larvae decide not to develop. They can stay in that arrested stage for weeks, months, or even years.

Then, when the larvae get some kind of signal--probably from the environment--they all start developing again at once, and they all emerge at once. This causes severe problems for the horse, and even death.

Remember the water bucket? It's like that bucket had the hole plugged, water kept dripping in to fill it up, and suddenly the bucket was tipped over and all the water came out in one big splash!

This is the most severe situation this parasite can cause and is characterized by profuse diarrhea, rapid weight loss (several pounds per day), and can lead to death due to dehydration and weight loss.

The good news is that this scenario doesn't happen very often. The bad news is that no one knows when it will happen.

It shouldn't be overlooked, how-ever, that the mild situation occurs more commonly. I'll suspect a performance horse with a dull or shaggy hair coat (of having small strongyles). He doesn't have that nice bloom, and his performance isn't up to par--he is beaten in races, or isn't doing well in events. He's got "ADR," ain't doin' right. That probably is due to an accumulation of cysts in the gut wall interfering with absorption, but we don't know that for sure. In some horses, a few encysted larvae deciding to come out at once could be enough to upset the balance of the gut in that horse.

A lot of things can cause "ADR," but we know that one of those things is parasites.

Q: What is the difference between the early and late third stage larvae?

A: The early third stage larvae is the only stage that has the ability to go into arrested development.This is the time when it is important to select optimum parasite control procedures.

Q: How can we detect small strongyles?

A: We can determine if a horse has adult cyathostomes because they lay eggs, and a veterinarian can run a fecal exam. We don't know about the larvae because they are immature and are not laying eggs. And we don't know if a horse has encysted larvae or not.

If you have adult small strongyles, you know they've gone through the life cycle and that horses are picking up the larvae in the field. But, you don't know if any of them are in an arrested stage.

Q: Why do some larvae develop normally and others go into the arrested stage?

A: We think it is because of climatic conditions outside the horse. Think of it this way: If a horse in the northern United States has an adult small strongyle that lays eggs in winter, then the eggs might not survive until conditions are right for the life cycle to continue. If it were late fall, a heavy frost could kill the eggs and larvae, and the life cycle couldn't continue.

So, in late fall something triggers larvae to encyst and wait until spring to develop, when the life cycle can continue outside the horse. No one has collected data to know exactly how climate affects small strongyle development.

At the University of Kentucky, larval cyathostomes were diagnosed as the cause of death at the diagnostic lab 45 times in six years. That's more than seven cases a year. The horses ranged in age from six weeks to 33 years, but they primarily were adults.

Q: How can we get rid of encysted small strongyles?

A: Become familiar with the life cycle, and become familiar with the active ingredients in your dewormer. Then you can determine which products affect the early third stage larvae. That is the only stage in which they can enter the arrested development. Currently, fenbendazole is the only anthelmintic (dewormer) that is recognized by the FDA to be effective against early third stage larvae, but that drug must be given at a larvicidal dose. That is 10 mg/kg of body weight of fenbendazole daily for five days. (The usual de-worming dose is 5 mg/kg once.) Fenbendazole also kills late third stage larvae and fourth stage larvae.

Moxidectin has label claims against late third stage and fourth stage larvae. Ivermectin is effective against fourth stage larvae, but not early or late third stage larvae. All of these data are cited in scientific literature.

Pyrantel tartrate is a daily anthelmintic that can be top-dressed on the feed. If a horse consumes a daily ration, it will kill the infective third stage larvae before it goes into the gut wall. The important note here is that you have to assume the horse is picking up larvae every day in the pasture. Thus, if he misses a day of his dewormer, then the larvae picked up that day can get into the gut wall.

You can give fenbendazole on top of the daily pyrantel tartrate or any other dewormer. Your veterinarian can help you decide when would be the best time to treat your horse. In most northern climes (considered by one parasite researcher to be from the Ohio River Valley to the north), that time would be November or December. As you go south, the time the encysted larvae would be expected to have an abrupt development would shift earlier in the year.

In the dry Mid-South and Southwest, summer is the worst time for survival on pasture, so you get a different development. These are assumptions and are not based on research in the horse. There is a worm in cattle that has a life cycle very much like the small strongyles in horses, and much research has been done on that parasite. Based on that research, we assume this is what to expect in horses.

Q: Does fenbendazole trigger the encysted small strongyles to come out and kill them, or does it kill them in the gut wall? Can that "kill-off" cause any problems in a heavily infested horse?

A: Fenbendazole, at the larvicidal dose, kills the early third, late third, and fourth stage larvae in the gut wall. No problems have ever been reported due to a massive killing of larvae.

Q: Are there any other specific scenarios we need to be concerned about with small strongyles?

A: Some parasitism in animals is probably good because there is some immune response in the horse to parasites. There is the possibility that some young horses which have been on a daily dewormer consistently are "too clean" of parasites. (Daily dewormers are shown to kill 80% or more of the parasite population in the horse.) That young horse might be sold at auction or privately and pick up small strongyles at the sale grounds or in its new environment. If the new owner does not use a daily dewormer--or if the animal misses a few days of dewormer--then the young horse could become overwhelmed with small strongyles because his immune system won't be offering much protection.

The take-home message here is not to count on any one product all of the time. Some products are not effective on tapeworms, others aren't effective on bots, and still others don't control all larvae stages of small strongyles. This is why you need a veterinarian's input to develop a deworming program that actually protects your horse!

By Kimberly S. Graetz With John Paul, DVM, MS

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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