Larvacidal De-Worming Of Horses

"Larvacidal de-worming" is a rather broad term used to describe the killing of worms or parasites in the larvae stage inside of our horses. However, the term is often intended to be much more specific than simply killing any species of larvae at any stage. "Larvacidal de-worming" is frequently used by horse owners to specifically mean the killing of small strongyle larvae during the stage of development when they are buried in the horses' intestinal wall. This is frequently referred to as the "encysted" stage since cysts form around the larvae.

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Life Cycle Of The Small Strongyle

The more researchers learn about encysted small strongyles, the more reasons horse owners have to hate them. Before we go any further, it's going to be helpful to understand the life cycle of the small strongyle:

Adult small strongyles lay eggs inside of the horse that are passed into the horse's feces and subsequently dropped into pastures, stalls, etc.
The eggs hatch and go through several stages of development. During the stage when they are known as "third-stage" larvae (L3) they are ready to be ingested by a horse. Once inside, they migrate into the horse's intestinal wall.
When the third-stage (L3) small strongyle larvae have entered the intestinal wall, cysts form around them. The encysted larvae have an early stage (EL3) and a late stage (LL3) of development. While in the early third-stage of development the larvae can remain encysted in the horse's intestine for up to 3 years before they develop into the later stage.
The larvae develop into a fourth stage (L4) of development and emerge from the cysts in the intestinal wall. They then develop into a fifth stage (L5), then adulthood where the lifecycle starts over again.

Damage and Death

Encysted small strongyles in the horse can cause significant inflammation, unexplained weight loss, diarrhea, poor hair coat, and in extreme cases death. Even so, it is when the larva enter the fourth stage and emerge from the intestinal wall that they are even more of a threat to the horse. The damage caused by thousands or millions of these larvae emerging at once can result in colic, diarrhea, swelling, unexplained lethargy, sudden weight loss, and even death. Since encysted small strongyles are not killed by typical de-worming procedures, and since they cannot be detected by fecal analyses, even conscientious horse owners on a regular de-worming program may be hit by surprise with encysted small strongyles.

Equine Illness After "Regular" De-Worming

Occasionally a horse will act a bit listless or as though it doesn't feel well after it has been de-wormed (it's important to note here that we're talking about a "regular" de-worming, not a larvacidal de-worming as described below). In the past, it was assumed that the cause of a horse acting as though it felt a bit off after being de-wormed was due to the results of a heavy parasite load being killed off.

However, newer research suggests there could be a much more sinister cause. Adult small strongyles in the intestine seem to have some way of communicating to their encysted brethren in the horse's intestinal wall. When the adult population is sufficient, the encysted larvae remain where they are. But when the adult population is lowered (for example, after a de-worming) the encysted, stage three larvae (L3) are somehow prompted to develop into stage four (L4) larvae and emerge from their encysted state. Many researchers now think that a horse who shows signs of listlessness or illness following de-worming may be experiencing large numbers of small strongyle larvae emerging out of their cysts.

Larvacidal De-Worming

To kill encysted small strongyles horse owners need to use a very specific de-worming procedure called larvacidal de-worming. This is commonly done by administering twice the usual dosage of fenbendazole for horses once a day for 5 consecutive days. Fenbendazole for horses is marketed under the names of Safe-Guard and Panacur. Currently, fenbendazole is the only FDA-approved product to treat all stages of encysted small strongyles in horses. Do NOT consider giving any horse, foal, or pregnant mare a larvacidal dose of de-wormer (febendazole or any other) without first consulting your veterinarian. To do so could result in the death or severe illness or injury of the horse, foal, pregnant mare, or unborn foal.

When Is Larvacidal De-Worming Usually Done?

When or if a horse should be given a larvacidal dose of de-wormer can vary according to the age of horse, pregnancy status, overall health, and what is known about the horse's medical history. For example, when some people purchase a new horse they may not know its vaccination or de-worming history, so they feel most comfortable giving it any vaccinations they feel it needs, as well as de-worming it, including a larvacidal de-worming. Some horse owners give their horses larvacidal doses of de-wormers once or twice a year as a part of their regular de-worming program. Consult your veterinarian to see what is best for your horse.

Related Terms And Information

What is an anthelmintic?
An anthelmintic is a parasite killing drug.
What's the difference between Safe-Guard and Panacur?
Safe-Guard and Panacur both contain febendazole. Safe-Guard is the over-the-counter name, and Panacur is distributed by veterinarians.
What is "S. vulgaris?"
S. vulgaris is the abbreviation for large strongyles, not small strongyles. It stands for "Strongylus vulgaris."
What is "larval cyathostomosis"?
"Cyathostomosis" is another name for small strongyles, so "larval cyathostomosis" is referring to small strongyles in the larval state. Usually, the term is used to refer to the encysted stage of small strongyles and the health problems they cause.
More Information
More information on encysted small strongyles and larvacidal de-worming of horses can be found at www.power-dose.com.