Maintaining Happy, Healthy Horses Through Internal Parasite Control

Article by: Lily Stidham, UF/IFAS Extension Marion Co.

Whenever one has horses, the issue of internal parasites is one that cannot be avoided. Whether your horses are young or old, they are exposed to various worms. Small strongyles are the main parasites present in Florida horses, but others can be present as well depending on the horse’s age including ascarids, pinworms, tapeworms, bot flies, and others. It is very difficult to entirely eradicate them. However, horses can live happy, healthy lives with very low levels of worms in their systems, so it is more important to pursue proper management of parasites to keep these levels low in order to best promote the health of our horses. This can be done by understanding the issue of preventing parasite resistance, how to responsibly worm our horses, and what other management tips can be used. Before pursuing wormers and other management tools, it is important to understand the issue of resistance.

Dewormers do not usually get rid of 100% of internal parasites. This is due to some parasites being resistant to whatever dewormer was used. Dr. Martin Nielson, a professor at University of Kentucky stated, “Drug resistance is a natural biological consequence of incidentally selecting for a few individuals in these populations that happen to have the genes that allow them to survive” (Lesté-Lasserre, n.d.). Once parasites are resistant, the surviving worms pass down this trait to their offspring, leading to resistance occurring for generations. Resistance can get worse when dewormers are overly used in our horses. It is important to minimize resistance from developing, so a proper deworming program is very important based on tailoring your program to your own herd.

How would you go about developing a deworming program tailored to your herd? It starts with a fecal egg count (FEC). FEC’s should be collected for about 10% of the herd, or at least six horses after four months without being dewormed. A fecal egg count takes a sample of manure and looks at how many parasite eggs are present, which determines what kind of shedder a particular horse is. Some are higher shedders, and some are lower. In fact, about 80% of the parasites shed come from about 20% of the horses. The ones who shed more are the ones who should be treated more often. Upon receiving the results of the FEC, you can determine what kind of shedder your horse(s) is(are) and treat accordingly. Use products based on the parasites you are wanting to get rid of. About two weeks after treatment, you can have another FEC performed to determine the fecal egg count reduction rate (FECR), which should be at least 90%, otherwise there is likely some sort of resistance with a percentage lower than 80%. An FECR can be performed annually or at least every three years to determine anthelmintic efficacy. New horses should also be dewormed and held in a stall for at least 96 hours after initial arrival to minimize exposure for horses already there. When developing a deworming program, it is important to use anthelmintic products as little as possible but as much as needed while also pursuing other management practices to maintain parasitic control.

Figure 1

Drug Class Generic Name Brand Name Resistance Ascarids

(Mostly affect young horses)

Strongyles Pinworms Bots Tapeworms Lung and Stomach worms Threadworms
Benzimidazoles Fenbendazole



and Safe-Gard by Intervet

Small strongyles




Oxibendazole Antelcide E.Q by Pfizer




Tetrahydrophyrimidines Pyrantel pamoate Strongid

and Pyrantel Pamoate Paste

Small strongyles, ascarids



2x dose

Pyrantel tartrate Strongid C Small strongyles, ascarids


Macrocyclyc lactones Ivermectin Eqvalan,


Zimecterin Gold,









Moxidectin Quest, Quest Plus





Isoquinoline-pyrozines Praziquantel Equimax, Quest Plus,

Zimecterin Gold



Heterocyclic compounds Piperazine Small strongyles




Besides using anthelmintics, other management practices can be employed to prevent internal parasites as well. No matter what is done, the “key” to successful parasite control is interrupting the parasite’s life cycle and preventing environmental contamination, such as via manure (Heise & Reinemeyer, 2011). When spreading manure, it should ideally be composted and spread on pastures if animals are currently on it, or it can be spread raw if the pastures are not grazed. Dragging pastures should only be done during hot, dry conditions to expose the parasites to the heat, which kills them. Dragging during cooler temperatures would not cause the parasites to be killed since they can survive in those lower temperatures. Other recommendations include to not overgraze a pasture, avoid feeding horses on the ground, move feeding areas such as feed bunks and hay troughs periodically to minimize accumulation of manure, and keep water troughs and containers free of fecal material. Horses can be grazed with ruminants as well since the parasites of one do not affect the other, but there is not much evidence of specific benefits from this practice. These recommendations all strive to minimize exposure to and accumulation of parasite eggs in your pastures, which can help to mitigate the parasite load in your horses in addition to the use of dewormers.

While parasites are an ongoing issue in our horses, they can be managed through dewormers and other practices. Through fecal egg counts, which horses shed more parasites can be determined and a deworming plan can be adopted based on your herd that does not overly use dewormers and cause unnecessary parasite resistance. Every parasite control program will vary and be individualized to your facility based on the climate, season, age of your horses, the size and composition of your herd, the stocking density of your pastures, genetics, and prior anthelmintic use. As you develop or continue your own program, you should also involve your veterinarian. We all want our horses to be able to be happy and healthy, and through smart decisions in using anthelmintics and management practices at your own facility, your horses can continue to do their best by maintaining low internal parasite levels throughout their lives.


Heise, DVM, S. R. & Reinemeyer, DVM, C. R. (2011) Control of Internal Parasites of the Horse. In Horse Industry Handbook (pp. 430-1 – 430-10). American Youth Horse Council, Inc.

Lesté-Lasserre, MA, C. (n.d.) Equine Parasite Control: Deworming and Beyond. The Horse.


Posted: June 29, 2021

Category: Agriculture, Farm Management, Livestock, Pests & Disease, UF/IFAS Extension
Tags: Horses

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