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The Enemy Within - Internal Parasites
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For example, some types of intestinal parasites induce the host to release anti-bodies. Through a series of actions, these anti-bodies are prepared to protect the horse from a new invasion of parasites, but at the same time, will not harm the established worm colony. Other parasites are temporary passengers that invade the body and then pass through to begin the cycle again. Many of these invaders sequentially attack one or more alternate hosts. In doing so, they may prevent a more damaging predator from infecting your horse. But, even with a symbiotic relation, things can go wrong and a parasite overload can cause internal damage. Therefore, it is necessary to develop a control program that includes a deworming schedule.
Horseowners have the convenience of selecting when and how they protect against internal parasites. A variety of worms invade the stomach and intestines of your horses during all seasons. While they may all be present at one time, they are in various stages of their life cycles. This influences the types of products you select and when you deworm your horse. This week's tip covers some of the basics to help you select appropriate products and when to use them in your deworming program.
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Know Your Enemy
Gaining a better understanding of the parasites that plague your horse will help you develop an optimal parasite control program. The following table provides a summary of equine internal parasites and their characteristics:
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Methods of Deworming
Today's market affords several modes of parasite control. There are pastes, gels, pellets, and liquids that you can administer by squirting into the mouth or adding to feed. Additionally, some owners still prefer to have their veterinary administer products via stomach tube (referred to as "tube worming"). Pellets are often administered in a daily dose, while pastes, gels and liquids are provided at standard intervals throughout the year.
 Depending on the environment that you live in, you may need to deworm your horse monthly or bi-monthly. Generally, equine parasites hatch every 4-8 weeks; therefore, monthly intervals are recommended in areas where there is a suspected heavy soil infestation. In colder climates where freezes break the cycles, often a schedule of 6-8 weeks is ideal. Some owners prefer to control parasites through fastidious pasture maintenance and administer products according to fecal counts. However, fecal counts are not always a reliable way to assess parasite infestation. Some stages of parasites are not found in feces. Another form of control uses a seasonal schedule addressing the parasites that are most prevalent during those times and adding a couple of doses of alternate class dewormers to address pin, thread and strongyle worms in the interim. Daily dewormers help prevent parasite overloads by limiting egg producing adult worms. The other methods generally kill off active worm stages, thus breaking the cycle until the developing stages hatch. Either way, it is well documented that parasites can develop resistance to particular dewormers and therefore, it is recommended to rotate classes. There are four major classes that afford easy rotation throughout the year (see comparison chart below for details):
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 BenzimidazoleMacrocyclic Lactones |
TetrahydropyrimidinesPraziquantel |
Developing a Beneficial Schedule
There are a few easy assessments that will help you develop a deworming schedule that best suits your horse's needs and situation.
Age
Older and younger horses are those most often susceptible to parasite overload. Strongyle and ascrid overloads are most commonly found in younger or older horses.
Weight
Your horse's weight will determine the dose that you administer. Underweight horses may have heavy parasite overloads.
Condition
Some products may not be considered safe to administer to pregnant or nursing mares. Horses in poor condition generally have parasite overloads. Some seemingly fit horses may be in high stress work situations that open them up to the possibility of parasite overload.
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Environment
Worms require a warm, wet environment to flourish. Damp climates, stalls, or seasonal weather promote infestation. Cycles are generally broken during dry or cold weather. Pasture management helps prevent soil infestation. Too many horses in a small area and overgrazing promote infestation.
Types of Parasites
The environment in your area generally dictates which parasites may be of concern. Tapeworms are more common in the damper East Coast and Midwest. Colder climates allow for a break in the cycle of some parasites.
Seasonal Infestations
Some parasites are more common during certain times of the year and therefore should be addressed then. Bots, for example, are generally active in the stomach in late fall - recommendation is to use a boticide after the first heavy frost.
Which Brands for Which Parasites?
Knowing which brands and classes of dewormers address which parasites will help you develop your deworming schedule by addressing particular species at particular times of the year. For example, bots are prevalent in late fall and a final deworming in spring will help break the cycle by preventing larva from passing through the horse into the pasture. Also, the stress of winter may compromise your horse's immune system and promote the overload of several species. A broad spectrum product (Macrocyclic Lactones) will provide a "fresh start" for summer.
The comparison chart below gives a breakdown of class, species addressed, and recommended seasonal applications. Brand names are listed to help prevent confusion.
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Comparison Chart of Dewormer Classes
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Class
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Controls
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Brand Names
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Season
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Benzimidazoles
Fenbendazole
Oxibendazole
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ascarids, large and small strongyles, pinworms
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Panacur, Safeguard
Anthelcide E.Q
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All
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Macrocyclic Lactones
Ivermectin
Moxidectin
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ascarids, large and small strongyles, pinworms, hairworms, stomach worms, bots, lungworms, threadworms
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Zimecterin, IverCare, Rotation 1
Quest
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Spring
Fall
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Tetrahydropyrimidines
Pyrantel pamoate
Pyrantel tartrate
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ascarids, large and small strongyles, pinworms
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Strongid, LiqiCare P, Rotation 2
Continuex (daily pellets). Strongid C, Strongid C 2x
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All
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Praziquantel
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tapeworm
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ComboCare (with moxidectrin)
EquiMax (with ivermectin)
Zimectrin Gold (with ivermectin)
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Spring
Fall
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Piperazines
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ascarids
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Administered by vet as tube worming
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All
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Dichlorvos, Trichlorfon and Carbon disulphide
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bots
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Administered by vet as tube worming
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Spring
Fall
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Comprehensive Prevention
Chemicals are just one part of a total parasite control program. Because parasites are primarily transferred through manure or grazing, good management is also key to a successful plan. The following intervention steps should be included in a well executed control program:
Remove manure from paddocks and stalls on a regular basis Rotate or rest pastures or alternate grazing with other livestock to interrupt the life cycles of equineparasites
Mow and harrow pastures regularly to break up manure piles and expose parasite eggs and larvae to the elements
Limit the number of horses per acre to prevent overgrazing and reduce contamination Avoid feeding on the ground Remove bot eggs to prevent ingestion Rotate deworming products to prevent chemical resistance Consult your veterinary for scheduling advice that addresses your area Some Concerns and Misconceptions
There have been many testimonials concerning possible negative effects of some of the newer dewormers. The best way to determine if a particular product might be of concern is to determine if the product fits your needs and to follow manufacturer's instructions. Not only are the types of parasites and time of year significant, but weight, age and condition are important factors when selecting a dewormer.
Some parasite control products have been tested and are recommended as safe for pregnant mares and foals. Others have specific instructions as to age and condition. All dewormers provide specific instruction about matching weight with dose. This is important to follow because some products are safe at 25 times the recommended dose and others can be potentially toxic or possibly lethal at only 5 times.
When rehabilitating malnourished or neglected horses that may have parasite overloads, sensitive digestive systems, or compromised immune systems, it is best not to try and clear the system all at once. Starting with a limited spectrum product and then moving into a broad spectrum dewormer is the best option for a safe program.
Older or ill horses may become heavily infested as their systems become less able to cope with normal parasite loads. Large die-offs of internal parasites can result in impaction colic, internal bleeding, or sloughing of intestinal wall. These problems can be potentially dangerous to your horse.
Contrary to generalizations about an infested horse's condition, a horse doesn't have to look unfit to be infested with worms. They may appear healthy and sleek outside and have a damaging level of infestation inside. If you are unfamiliar with the horse's previous history of deworming, then it would be best to consult your vet before beginning your program.
The object of beneficial parasite control is not to kill every worm that may live in your horse. The goal should be to help the horse in preventing overloads through management, rather than assault. Some parasites are a necessary addition to your horse's defense system.
As with many things, more is not better. Overuse of chemicals upsets the balance of bacteria that is used in digestion. This can result in colic, weight loss or other digestive disorders. Limit deworming to need and provide a course of probiotics (beneficial bacteria) after application.
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© Sue Baker, Russo's Tack Shack 2004
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