Glossary: Common Terms in Equine Parasite Control

How to Use This Glossary

This glossary defines common terms you may encounter when reading product labels, veterinary reports, or educational materials about equine parasite control. Understanding these terms helps you communicate more effectively with your veterinarian and make better-informed decisions about your horse's care.

Terms are organized alphabetically for easy reference.

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A

Active Ingredient — The specific chemical compound in a deworming product that is responsible for killing or controlling parasites. For example, ivermectin is the active ingredient in the products reviewed on our products page.

Anthelmintic — A class of drugs used to treat infections caused by parasitic worms (helminths). Equine dewormers are anthelmintics. Common classes include macrocyclic lactones, benzimidazoles, and pyrantel.

Ascarids (Roundworms) — A type of intestinal parasite primarily affecting young horses. Heavy ascarid infections can cause poor growth, a pot-bellied appearance, and in severe cases, intestinal obstruction.

B

Benzimidazole — A class of anthelmintic drugs that includes fenbendazole and oxibendazole. These are among the oldest equine dewormers and have experienced significant resistance in some parasite populations.

Bots (Bot Flies) — The larvae of bot flies (genus Gasterophilus) that attach to the horse's stomach lining. Bot eggs are visible as small yellow specks on the horse's hair, particularly on the legs and chin.

Broad-Spectrum — Describes a dewormer that is effective against a wide range of parasite species, as opposed to one that targets only specific parasites.

C

Cyathostomins — See "Small Strongyles." The scientific name for small strongyles, which are currently considered the most significant equine parasites.

D

Dewormer — A colloquial term for an anthelmintic product used to treat or prevent parasitic worm infections in horses. Available in various forms including paste, gel, pellets, and liquid.

E

Egg Reappearance Period (ERP) — The time after deworming treatment before parasite eggs reappear in a horse's feces. A shortened ERP may indicate developing resistance to the deworming product used.

Encysted Larvae — Parasite larvae (particularly small strongyles) that burrow into the wall of the horse's intestine and enter a dormant stage. Encysted larvae can be difficult to treat and may emerge from the intestinal wall in large numbers (larval cyathostominosis), causing serious illness.

F

Fecal Egg Count (FEC) — A diagnostic test that measures the number of parasite eggs in a manure sample. Results are expressed as eggs per gram (EPG) and help determine a horse's parasite burden and the need for treatment.

Fecal Egg Count Reduction Test (FECRT) — A comparison of FEC results before and after deworming treatment. Used to evaluate whether a deworming product achieved adequate parasite reduction, typically performed 10–14 days post-treatment.

Fenbendazole — A benzimidazole-class anthelmintic used in equine dewormers. Sometimes used at elevated doses for extended periods to address encysted small strongyles, but only under veterinary direction.

H

Helminth — A general term for parasitic worms, including nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes).

High Shedder — A horse that consistently shows high parasite egg counts on fecal testing. Research indicates that a small percentage of horses (approximately 20%) shed the majority of parasite eggs in a herd.

I

Ivermectin — A macrocyclic lactone anthelmintic widely used in equine medicine since the early 1980s. Effective against a broad spectrum of internal parasites. Available in oral paste form, often with apple flavoring.

L

Large Strongyles — A group of equine parasites (including Strongylus vulgaris) that were historically the most dangerous equine parasites. Effective deworming programs have significantly reduced their prevalence, but they remain a concern in some populations.

Low Shedder — A horse that consistently shows low parasite egg counts on fecal testing. Low shedders may need less frequent deworming than high shedders.

M

Macrocyclic Lactone — A class of anthelmintic drugs that includes ivermectin and moxidectin. These are among the most widely used equine dewormers and have generally maintained good efficacy.

McMaster Method — A standardized laboratory technique used for fecal egg counting. It is one of the most common methods used to perform FEC testing in veterinary practice.

Moxidectin — A macrocyclic lactone anthelmintic considered more potent and longer-acting than ivermectin. Used in equine medicine for broad-spectrum parasite control.

N

Nematode — A type of parasitic roundworm. Most equine internal parasites are nematodes, including strongyles, ascarids, and pinworms.

O

Oral Paste — The most common administration form for equine dewormers. Product is dispensed from a calibrated syringe-style tube directly into the horse's mouth.

P

Pinworms (Oxyuris equi) — Equine parasites that cause itching around the tail and perianal area. While generally not a serious health threat, they cause discomfort and can lead to hair loss and skin irritation from rubbing.

Praziquantel — An anthelmintic specifically effective against tapeworms. Commonly combined with ivermectin or moxidectin in combination dewormer products.

Pyrantel — An anthelmintic available in both paste (single-dose) and daily feed-through pellet formulations. At elevated doses, pyrantel pamoate has activity against tapeworms.

R

Refugia — The proportion of a parasite population that is not exposed to deworming drugs. Maintaining refugia (i.e., not treating every horse on a fixed schedule) helps slow the development of resistance by preserving susceptible parasites in the population.

Resistance — See "Anthelmintic Resistance." The ability of parasites to survive exposure to deworming drugs that previously controlled them.

S

Small Strongyles (Cyathostomins) — Currently the most prevalent and clinically significant equine parasites. Small strongyles can encyst in the intestinal wall, making them difficult to control. They represent the primary target of most modern equine deworming programs.

T

Tapeworms (Anoplocephala) — Flatworm parasites found in the horse's intestinal tract. Associated with certain types of colic. Controlled primarily by praziquantel.

Targeted Selective Treatment (TST) — A modern approach to deworming that uses fecal egg count results to determine which individual horses need treatment, rather than treating all horses on a fixed schedule.

W

Weight Tape — A specialized measuring tape used to estimate a horse's body weight based on heart girth measurement. Accurate weight estimation is important for proper dewormer administration, as most paste products are calibrated by body weight.

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This glossary is for educational purposes only. It does not constitute veterinary advice. For questions about any of these terms as they relate to your horse's specific needs, consult a licensed veterinarian.