Deworming Principles (Book Excerpt)

Editor's Note: This is from Understanding Equine Preventive Medicine by author and veterinarian Bradford G. Bentz, VMD. The book is available from

Parasite control programs should be designed to reduce infections and diseases caused by parasites and to minimize transmission of parasites among horses. Infestation with gastrointestinal parasites reduces the benefits of feed and nutrients and increases incidence of colic and loose manure or diarrhea. Larval migration of gastrointestinal parasites can be associated with other clinical diseases such as parasitic pneumonia and neurologic disorders. Other parasites, such as some external parasites, may cause hypersensitivity reactions in the skin and development of wounds in certain areas.

A thorough parasite control program involves a complete understanding of helpful management practices and knowing how different types of dewormers work. Because management practices, economics, location, and disease outbreaks differ, no one program for parasite control applies universally.

A veterinarian should help customize an individual program for each farm or situation. Today's programs might include interval deworming (fast, slow, and no rotation), daily deworming, strategic deworming, and targeted deworming. All have advantages and disadvantages. The success, selection, and need to modify a deworming program are generally dictated by the degree of control of the cyathostome parasites (small strongyles) and their encysted stages within the individual horse and/or the population of horses.

A program that includes "tube deworming" is no longer necessary and probably offers no real advantage other than assuring the complete delivery of the dewormer to the stomach. The risk of overdosing the horse by overestimating weight is reduced by the safety margins of today's dewormers. Furthermore, an experienced horseman is able to administer paste dewormers effectively without significant loss of the medication.  Horses that are to receive a paste of any kind should have all hay and feed removed from the mouth to prevent loss of the medication with the dropping of the feedstuff.

Fast Rotation Interval Deworming

Fast rotation interval deworming alternates different classes of dewormers during the year at predetermined periods (usually four to six weeks).  It increases intervals between treatments with the same class of drug and thus reduces development of resistance.  This allows elimination of parasites not uniformly killed by all drugs (Gasterophilus spp., Anoplocephala perfoliata/A. magna).  A disadvantage is that the egg reappearance period for different drugs is variable (ivermectin, eight to 10 weeks; other drugs, four to six weeks). 

Therefore, using standard intervals between treatments with different drug classes may not minimize pasture contamination. Despite this, some exposure to parasites in this way may actually help by allowing some protective immunity to develop. A fast rotation interval deworming program will likely cost more than most deworming programs.

As with all deworming programs, well-timed administration of ivermectin or moxidectin to eliminate stomach bots and administration of 2-3x pyrantel to eliminate tapeworms is necessary.  Treat with ivermectin in late spring (May) and fall (November) for elimination of bots. Yearly deworming with a double dose of fenbendazole for five consecutive days to control encysted small strongyles may be appropriate despite the apparent broad spectrum of control offered by the fast rotation interval deworming program.

Annual (slow) Rotation

Slow or annual rotation uses the same anthelmintic or class of anthelmintic at appropriate intervals throughout the year. The drug classes are alternated yearly rather than with every dosing.  This approach does not account for the varying effectiveness of dewormers against a broad range of parasites but particularly focuses on treatment and control of cyathostomes.  Therefore, it is important to administer other dewormers such as ivermectin and pyrantel to eliminate parasites not killed by the drug being used that year.  Timely administrations of ivermectin and 2-3x pyrantel will be necessary to bolster the program to address stomach bots and tapeworms.  Larvicidal doses (double doses) of fenbendazole for five consecutive days also may be appropriate.

No Rotation

Programs that use no rotation involve the continued use of one effective drug until it no longer reduces small strongyle numbers as indicated by fecal egg counts.  The intervals of administration depend on the dewormer used, but this program is limited to the use of ivermectin or the daily administration of pyrantel tartrate (Strongid™C).

One benefit is that such a program is easily implemented and provides maximal reduction of parasite burdens.  Rapid resistance has not been seen to develop with such programs. Horses raised on no-rotation programs will acquire resistance more slowly.  The administration of 2-3x pyrantel to control tapeworms would be necessary with either the use of ivermectin or pyrantel as the anthelmintic.  This is necessary even with the use of pyrantel tartrate, as the daily dosage is well below the required 2-3x dosage.  If pyrantel tartrate is used, ivermectin would be necessary at the appropriate times (May and November) to control stomach bots.  It has been suggested that daily administration of pyrantel tartrate may aid in preventing infection with Sarcosyctis neurona, the causative organism of equine protozoal myeloencephalitis (EPM).

Targeted Treatment
Only those horses with significant fecal egg counts should be treated to control cyathostomes.  The benefit is that this deworming program minimizes use of anthelmintics and encourages the naturally acquired resistance that is developed against the parasites by the immune system.

Management of this program requires regular quantitative fecal egg counts and treatment when cyathostome counts are higher than 100 to 200 eggs per gram of feces (EPG) in order to be effective in minimizing pasture contamination.  Stomach bots and tapeworms are not killed by typical dewormers and regular doses normally employed. 

This program is not useful in young animals because of the absence of acquired immunity. 

Strategic Treatment

Strategic treatment deworming is based on the use of effective anthelmintics to eliminate small strongyles in the intestine before the season of the year that is optimal for parasite development in pastures.  This approach reduces pasture contamination and thus reinfection of resident horses.  It minimizes use of anthelmintics but requires an adequate understanding of the local epidemiology of cyathostome infections.  This program may not be as effective in young animals. 

In northern temperate regions, cyathostome infections have been controlled by treatment administration in the spring, early summer, and fall.  In southeastern regions, cyathostome infections may be controlled by beginning these treatments in the early fall.  This program can be modified by treating horses with a larvicidal dose of anthelmintic before exposure to pasture during the period of optimal parasite transmission.

About the Author

Bradford G. Bentz, VMD, MS, Dipl. ACVIM, ABVP (equine)

Brad Bentz, VMD, Dipl. ACVIM, ABVP, ACVECC, owns Bluegrass Equine Performance and Internal Medicine in Lexington, Ky., where he specializes in advanced internal medicine and critical care focused on helping equine patients recuperate at home. He’s authored numerous books, articles, and papers about horse health and currently serves as commission veterinarian for the Kentucky State Racing Commission.

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