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Equine internal parasites

Internal parasites of horses live in one of two places: in the horse's environment as larvae (typically pastures) or inside the horse. Once inside the horse, the larvae mature and may migrate to various areas of the horse,  such as the stomach, intestine, lungs or blood vessels. Each parasite has a specific life cycle, but in general they follow the pattern below:

Ingest

Equine parasite lifecycle stage 1

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Horses ingest the egg or larvae while grazing.

Mature / Migrate

Equine parasite lifecycle stage 2

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Inside the horse the larvae develop and mature and may migrate to various sites, such as the stomach, intestine, lungs and blood vessels and may cause damage there.

Reproduce

Equine parasite lifecycle stage 3

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The adult worms lay eggs that are passed in the manure (or develop into larvae that are passed) to start the cycle again.

Manure

Equine parasite lifecycle stage 4

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Eggs or larvae are passed in the manure and contaminate the pasture.

There are a lot of different internal parasites, but some are especially important because they could be a significant health threat to horses. The nature and extent of damage varies with the type of parasites. They can cause anemia, weight loss, colic, diarrhea, reduced reproductive and athletic performance, poor growth, and immune system suppression.

 

At this time, the most common and important types of internal parasites in horses:

 

  • Nematodes, primarily small strongyles, cyathostomins (due to traditional parasite control programs involving rotational treatment, large strongyles (S. vulgaris) are now rare in managed horse populations)
  • Tapeworms (Anoplocephala perfoliata)
  • Roundworms (Parascaris spp.) in foals and weanlings

Prevention

Environmental measures are aimed to break the life cycle of internal parasites. Following good management practices to control the frequency and spread of internal parasites can be as important as deworming.

 

  • Remove manure daily from stalls and run-ins and weekly (or more frequently) from paddocks and pastures

  • Be sure pastures and paddocks are well-drained and not overpopulated

  • Compost manure rather than spreading it on fields where horses graze

  • Use a feeder for hay and grain and avoid feeding on the ground

  • Implement fly control programs

  • Keep water troughs and feed bins clean

  • Routinely examine horses for telltale signs of infestation

  • Set up a biosecurity program for all new horses that includes a fecal egg count and larvicidal deworming prior to turn-out with resident horses 

  • Establish a parasite prevention and monitoring program with your veterinarian. This may include regular manure checks and deworming programs tailored to the needs of your horses

Treatment

The aim of today’s equine parasite-control program isn’t to eliminate all parasites. What is needed are properly timed treatments with effective anthelmintics to control parasite egg shedding, to help keep horses healthy and to avoid development of resistance as much as possible.1


To achieve these goals, it is important to know the magnitude of egg shedding of individual horses and of detecting resistance of farms by performing periodic fecal egg count (FEC) surveillance and fecal egg count reduction tests (FECRT).  

 

Parasite-control programs are best viewed as a yearly cycle starting at the time of year when parasite transmission to horses changes from negligible to probable.1 There is no such thing as a “one size fits all” horse deworming schedule. Each farm, with veterinary guidance, should develop its own program tailored to their specific needs. All adult horses should benefit from one or two treatments per year. Low-egg shedding horses with naturally strong immunity may not need additional treatments. Moderate and high egg shedders may need additional strategic treatments. Any additional treatments may be given on an “as needed” basis.  Young horses (typically less than 3 years of age) require special attention due to their higher susceptibility to parasite infection and to being more at risk for developing clinical illness.

 

Treatments should be timed to control the level of egg shedding into the environment. This relies on the use of deworming medications that are effective for their intended use.1 The 3 common classes of horse dewormers are macrocyclic lactones (ivermectin, moxidectin), benzimidazoles (fenbendazole, oxibendazole), and tetrahydropyrimidines (pyrantel pamoate). Small strongyles and roundworms are the greatest resistance threats. Small strongyles have developed resistance to two of the three major dewormer classes (benzimidazoles and tetrahydropyrimidines).2 It was reported that Ivermectin, the active ingredient in ZIMECTERIN® GOLD (ivermectin 1.55%/ praziquantel 7.75%) Paste and ZIMECTERIN® (ivermectin paste) 1.87% have been shown to be effective against benzimidazole-resistant small strongyles.4 Ivermectin and excreted ivermectin residues may adversely affect aquatic organisms. Do not contaminate ground or surface water. 

 

ZIMECTERIN GOLD controls more species and stages of parasites than any other product (47 species and stages of equine parasites, including tapeworms).4 ZIMECTERIN GOLD may be used in horses two months of age or older. 

Zimecterin Product Shot with Syringe

ZIMECTERIN® (ivermectin paste) 1.87%

Kills important internal parasites with a single dose. Not for use in humans. This product should not be used in other animal species as severe adverse reactions, including fatalities in dogs, may result.

See product detail

ZIMECTERIN® (ivermectin paste) 1.87%

Kills important internal parasites with a single dose. Not for use in humans. This product should not be used in other animal species as severe adverse reactions, including fatalities in dogs, may result.

See product detail
Zimecterin Product Shot with Syringe
Zimecterin Gold Product Shot with Syringe

ZIMECTERIN® GOLD (ivermectin 1.55%/praziquantel 7.75%) Paste

Controls more species and stages of parasites than any other horse dewormer. ZIMECTERIN GOLD has not been tested in breeding, pregnant or lactating mares, or breeding stallions.

See product detail

ZIMECTERIN® GOLD (ivermectin 1.55%/praziquantel 7.75%) Paste

Controls more species and stages of parasites than any other horse dewormer. ZIMECTERIN GOLD has not been tested in breeding, pregnant or lactating mares, or breeding stallions.

See product detail
Zimecterin Gold Product Shot with Syringe

ZIMECTERIN IMPORTANT SAFETY INFORMATION: Not for use in humans. Keep this and all drugs out of reach of children. Do not use in other animal species as severe adverse reactions, including fatalities in dogs, may result. Ivermectin and excreted ivermectin residues may adversely affect aquatic organisms. Do not contaminate ground or surface water. Dispose of the syringe in an approved landfill or by incineration.

 

ZIMECTERIN GOLD IMPORTANT SAFETY INFORMATION: ZIMECTERIN GOLD has not been tested in foals younger than two months of age, mares at or near the time of breeding, pregnant or lactating mares, and breeding stallions. There have been rare reports of swelling and irritation of the mouth, lips, and tongue following administration of ZIMECTERIN GOLD. This product should not be used in other animal species as severe adverse reactions, including fatalities in dogs, may result. Not for use in humans.

1. AAEP Parasite Control Guidelines Available at: https://aaep.org/sites/default/files/Documents/InternalParasiteGuidelinesFinal5.23.19_0.pdf. Accessed Nov. 16, 2020.

2. Kaplan RM, et al. Prevalence of anthelmintic -resistant cyathostomes on horse farms. J AM Vet Med Assoc. 2004;225(6):903-910.

3. McFarlane D, Hale GM, Johnson EM, Maxwell LK. Fecal egg counts after anthelmintic administration to aged horses and horses with pituitary pars intermedia dysfunction. J AM Vet Assoc. 2010; 236(3):331.

4. ZIMECTERIN Gold Product Label.

 

ZIMECTERIN® is a registered trademark of Boehringer Ingelheim Animal Health France, used under license. ©2025 Boehringer Ingelheim Animal Health USA Inc., Duluth, GA. All rights reserved. US-EQU-0111-2021-V2