Horse Wormer Chart and Rotational Schedule


Date: June 1st, 2016


One of the most important health conditions in your horse that needs to be monitored is the presence of unwanted parasites. While internal parasites (worms) are found in every horse, each horse is affected differently. Some horses may only suffer a slight weight loss, loss of blood, irrigation or growths, but in other cases, worms have proven to be fatal. 

Due to the differing effects, it is essential that horse owners understand worms and the medicine that treats them. It is recommended by most veterinarians and equine health experts that horses are put on a worming schedule as a regular preventative measure. The most commonly used approach to horse deworming involves a rotational schedule, as this prevents the development of parasite resistance to wormers.

Listed below are 4 simple steps that will help you treat your horse: identifying the parasite, learning about wormer active ingredients, comparing wormer medications, and setting up a wormer rotational schedule. Horse Grazing


Step 1 

How Horses Get Worms: Worms are usually acquired by horses in their normal living environment. Worms can live as eggs or larvae in pastures, feed, and mare's milk, or as flying insects on the horse's coat.


Worm/Parasite Types: The first step in properly treating your horse is to know exactly what you are treating for. Here is a quick overview of the most common parasites:


  • Pinworms: Oxyuris equi, pinworm eggs are acquired up by horses from contaminated feed, water, or bedding. Pinworms are found in the cecum, colon, large intestine, and rectum. These worms can be very irritating, causing the horse to rub against fence posts or other solid objects, resulting in hair loss and sometimes injury to the tail and rump. The irritation and rubbing can sometimes lead to a secondary infection. 


    • Pinworms are less damaging to the equine system than any other internal parasite group. However, the constant annoyance and irritation can alter the horse's temperance and appearance. 


  • Ascarids (large Roundworms): Parascaris equorum (large roundworms) are the largest internal parasite affecting horses, ranging in length from 5" to 15". These parasites are common in young horses and are usually not found in horses more than five years old. Immunity normally develops following exposure to these large roundworms during adolescence.


    • The adult life stages of the large roundworm are spent in the horse's small intestine. Here the female roundworm passes large amounts of eggs into the horse's manure. In about two weeks, these eggs become infective and the horse picks them up while grazing. The larvae migrate into the blood vessels and are carried to the liver and lungs. The immature worms are coughed up and swallowed, maturing in the small intestine to complete the life cycle.


  • Bots: Bots come in three types: Gastrophilus intestinalis, the most common; Gastrophilus haemorrhoidalis, the nose bot; and Gastrophilus nasalis, the throat bot. Bots are the immature maggot stages in the life cycle of the bot fly, the adult of which resembles the honeybee in general appearance. The females lay their eggs by attaching them to the hairs of the front legs, throat, and under line. As the horse licks itself, the larvae attach themselves to the lips and tongue and burrow into the tissues. After about three weeks they attach themselves to the lining of the stomach, where they may remain for several months causing additional damage.


  • Large Strongyles (Blood Worms): Strongyles range in length from 1/2" to 2". Adult strongyles are found firmly attached to the walls of the large intestine, where the females pass large numbers of eggs into the manure. These eggs hatch and the larvae climb blades of grass and are swallowed. The larvae then migrate to the large arteries which supply the intestines. As the artery walls are damaged, blood clots form and break away, causing colic.


  • Tapeworms: Difficult to diagnose, it is estimated that 40% of the horses in the U.S. are affected. Tapeworms are contracted during grazing, when the horse ingests the intermediate host, a mite found on plants. Once infected, tapeworms contribute to digestive problems, colic and malnourishment. Tapeworms are often undetected using normal fecal flotation methods.


Step 2

Once you have a general understanding of what parasites may be infesting your horse, the next step is understanding which active wormer ingredients are effective against each type of parasite. For further explanation, see the AskMax article titled What are the active ingredients in horse wormers?.


Step 3

Once you've gained a general understanding of the parasites and the active ingredients that treat them, tools such as dewormer comparison charts can help illustrate the differences between wormers and help you make schedule decisions.

Click here for a Horse Wormer Comparison Chart to see how the different wormers stack-up against each other. 


Step 4

All horses should be included in a regular parasite control program with particular attention being paid to mares, foals and yearlings. Foals should be treated initially at 6 to 8 weeks of age, and routine treatment repeated as appropriate. Consult your veterinarian for a control program to meet your specific needs.

Additionally, you can use sample rotational schedules as guidelines before you create your own tailored worming program for your horses.

Click here for a sample dewormer rotational schedule that can be used to base your own worming program off of. 



Horse wormer medications come in many forms - each with unique specifications. While at first it may seem overwhelming, a little research can go a long way. If you have any further questions regarding your horse's health, email, or visit your local CountryMax store today!



Additional Resources on Wormer Info:,,,

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Comments (1): Subscribe by Email

This is the best explanation and diagraming of wormers I have ever read. Wish I had a printer as I would make copies and give to all my confused friends that worm hit and miss and hope. You even give an easy schedule to follow with choices!!!!!Thank you soooo much. I was so very confused and jumping from site to site, getting more confused and going back and forth until I lucked on your site. Again thank you for clearing things up and making it soooo easy to follow.

cadam230 - July 5, 2014

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