Internal Parasite Control for Horses

Internal parasites (worms) have a major role in the health, or lack of it, of your horse. Internal parasites can cause a dull haircoat and unthriftiness all the way to colic and death. There are over 150 species of internal parasites affecting horses, including large and small strongyles, roundworms (ascarids), tapeworms, lungworms, pinworms, bots and threadworms. The most important health risks are from the strongyles, roundworms and tapeworms. The lifecycle of these worms involves eggs, larvae (baby worms) and adults (mature worms). The eggs (or larvae) are deposited on the ground in the manure from an infected horse. They are then swallowed by other horses while grazing and mature to adult worms in the intestine. Some species migrate out of the intestine as larvae and go to other tissues/organs and then return to the intestine as adults to lay eggs and complete the life cycle. Each of these parasites has its own course of disease in horses but all have a negative effect on the health of your horse.

Signs of parasitism in horses include dull, coarse hair coat, lethargy and depression, decreased stamina, unthriftiness or loss of condition, slowed growth and pot belly in young horses, colic and diarrhea. Although these signs are common, many horses affected by parasites can appear completely normal. The best way to monitor for parasitism in horses is to determine the fecal egg count. This will allow your veterinarian to determine which parasites are present and at what levels. It is important to remember that a negative fecal egg count does not mean the horse has no parasites as shedding can be intermittent. This information is used to determine an appropriate parasite control program for your farm/stable.

There are a number of different classes of deworming agents (anthelmintics) for use in horses: avermectins/milbemycins, benzimidazoles and pyrimidines. Most are broad-spectrum and work on several types of internal parasites. A rotation of these classes should be the basis for your deworming program, while specific spectrum agents can be used when a specific parasite is involved. Although none of these agents will kill all internal parasites, by keeping their population as low as possible, you significantly improve the health of your horses.

There are two basic types of deworming programs, continuous or strategic. Continuous deworming uses a small amount of deworming agent given every day. This is especially effective for grazing horses as it stops new infection by larvae during grazing. Strategic deworming uses a larger dose of dewormer at specific times per year or when fecal egg counts go up. Combination programs can also be used. There is no ideal deworming program for all horses. The best program for your horse will consider the type, number and ages of horses on your farm, pasture management and geographic location, along with fecal egg counts to determine the protocol.

Dewormers can be given as feed additives, oral paste in a syringe or by nasogastic tube. All of these methods are effective as long as the complete proper dose is given at the right time. It is best to determine each horse’s weight (by scale or weight tape) at the time of deworming.

Remember that chemical control of parasites using dewormers is only one part of a complete parasite control program. A pasture management program is also critical and should involve:
• Keeping the number of horses per acre at a minimum to reduce overgrazing and minimize exposure to eggs and larvae
• Picking and removing manure frequently
• Composting manure away from pasture
• Mowing and harrowing pastures frequently to break up manure piles and expose egg/larvae to sun
• Rotating pastures with cattle or sheep to interrupt the equine parasite lifecycles
• Keeping foals and weanlings away from yearlings and two year olds to reduce exposure to roundworms and other parasites
• Removing bot eggs from horse’s coats

Consult your veterinarian for determination of fecal egg counts and to determine which deworming agents should be used in your rotation and how frequently. Remember to complete your parasite control program with appropriate pasture management.
-based on Internal Parasites: Strategies for Effective Parasite Control, AAEP and Bayer Animal Health

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