Intestinal Health

Caring For Your Horse’s Intestinal Health

Interval/ Rotational Deworming vs. Strategic Deworming
All horses have parasites. A practical approach to internal parasites is not to eliminate them, but to keep the level of parasites low so they do not cause disease.

Interval deworming is defined as dosing horses with different classes of dewormers (anthelmintics) in rotation at a given interval (8-12 weeks)

Strategic deworming takes advantage of the seasonal threat of parasites which are the greatest risk to a horse. This is tailored to individual horses in the herd based on Fecal Egg Counts (FEC’s) which keeps selection pressure for resistance down and reduces the total number of treatments per year. Parasites are developing drug resistance to the current deworming products (anthelmintics) available. Strategic deworming programs may be the most effective way to slow resistance. It is very important owners work with their veterinarian in setting up any deworming program and use current anthelmintics responsibly.

There are 3 classes of anthelmintic/dewormers:
– Benzamidazole – Fenbendazole, Oxibendazole (i.e. Panacur/Anthelcide)
– Tetrahydropyrimidines – Pyrantel Pamotae, Pyrantel Tartrate (i.e. Strongid/Rotectin & Strongid C daily dewormer)
– Macrocyclic Lactones – Ivermectin, Moxidectin (i.e. Eqvalan, Zimecterin, Quest)

Note: Praziquantel is used in combination with some of these anthelmintics to control tapeworms

Use of daily dewormers is not recommended because it likely interferes with acquired immunity in young horses and promotes resistant parasites.

Alternative/Natural dewormers do not work. You must use an FDA approved dewormer to get results.

Anthelmintics must be properly dosed based on weight. Consult with a veterinarian to determine the best deworming program for your horse. Always read the label before administering.


Interval/Rotational Deworming Program – every 8-12 weeks



Moxidectin (Quest)


Best:    Power Pack (2 x Fenbendazole for 5 consecutive days)

Good:  2 x Fenbendazole (Panacur, Safe-Guard)


Pyrantel Pamoate (Strongid)


Ivermectin with Praziquantel (Equimax, Zimecterin Gold)


Horses less than 2 years old only:  2 x Fenbendazole (Panacur, Safe-Guard)

Strategic Deworming Program
This deworming schedule demands education of the client on methods.  There should be a close working relationship with the veterinarian to choose correct dewormers based on Fecal Egg Counts (FEC’s).  The first deworming will occur in May and the last deworming in December.

Fecal Egg Counts are absolutely necessary to optimize control of parasites in the herd. Fewer treatments lead to less drug resistance.  Checking FEC’s on a regular basis is the best way to improve parasite control.  Primarily small Strongyle eggs counted in the feces are the greatest concern. Internal parasite numbers are related to the level of exposure because they do not multiply within the horse.  These counts will identify the horses that most need deworming. Individuals within a herd can vary greatly with 20% of the horses carrying 80% of the parasites. Individual horses differ in their ability to handle parasites because of variation in their immunity.  The FEC’s monitor the success of the program; determine efficacy of anthelmintics and what interval is needed between treatments.  Horses with higher FEC’s can be dewormed more often than those with low FEC’s.  A few fecal balls are collected in zip lock bags with as much air as possible removed before sealing bag and refrigerating.  Fecal samples should be collected before deworming and again 2 weeks after deworming for the first year to determine resistance.  The client will need to work with a veterinarian to interpret laboratory results of the fecal samples.

Other Deworming Recommendations

Foals – begin deworming at 2 months of age. Do not use Moxidecin (i.e.Quest) in foals.

2 months


4 months


6 months

2 x Fenbendazole

8 months


10 months


12 months


New Horses to a herd – Keep off pasture for 7 days after deworming.
Deworm adult horses (older than 1 ½ years) with Moxidectin. Repeat in 10-12 weeks.
Deworm young horses (less than 1 ½ years) with 2x Fenbendazole for 5 consecutive days.

When deworming you horse, make sure their mouth is empty or dewormer can easily be spit out.

Environmental Management Recommendations

Lawns are areas that horses like to graze in a pasture and roughs are areas that horses prefer to defecate. The infective strongyle parasite is 15 times higher in roughs. Horses will avoid these rough areas for grazing as long as the pasture is not overgrazed. Hay can be supplemented to avoid overgrazing of a pasture. If your pasture does not have lawns and roughs, it is overgrazed and will have a higher parasite concentration.

Drag your pasture in the summer – it takes temperatures of 90 degrees Fahrenheit to kill parasite larvae. Keep horses off of dragged pastures for 4 weeks. Pasture rotation is recommended.
Do not drag your pasture in the spring or fall. This practice only aides in spreading the larvae which can over winter and be infective the following spring.

Removal of manure from pastures and paddocks is beneficial. Manure should be composted. The temperature must reach 90-140 degrees Fahrenheit to kill infective eggs and larvae. Only composted manure should be spread on pastures.

Delay turnout to pastures in the spring/summer. A few weeks at 90 degrees kills larvae. Limited pasture exposure minimizes parasite exposure.

Interesting Facts about Parasites in Horses

Strongyles – Eggs hatch in the manure at temperatures of 45– 85 degrees Fahrenheit. The larvae can be infective in as little as 3 days in warm temperatures and can remain infective for several weeks in cooler temperatures. No hatching of strongyle eggs or development of larvae occurs from November to March in Minnesota. The eggs are killed by freezing temperatures. The larvae on the other hand can over winter and be infective the following spring. Infective larvae can persist over winter and are especially adapted for our weather in Minnesota.

Large Strongyles are well controlled in most deworming programs because of their simpler life cycle.

Small Strongyles invade the mucosa lining of the large intestine where they form a protective shell and can remain for several years before emerging. They are now the most important equine parasite because of their ability to produce disease and survive deworming programs. Transmission of strongyles is almost totally through pastures. Fecal egg counts are primarily concerned with small strongyle eggs.

Tapeworms – Horses get these from eating mites on pasture. Minnesota has a very high prevalence of tapeworms in horses at 98%. Tapeworms can cause colic because of inflammation of the intestinal mucosa.

Bots – The larvae are infective 7 days after the eggs are laid on the horse’s hair by the Bot fly. The oral stage is about one month. The larvae migrate and spend 8-10 months in the stomach and intestine after being swallowed.

Roundworms – These are a problem in horses less than 1 ½ years old. Eggs can remain infective for 10 years in the environment. Horses develop immunity after 2 years of age.

Pinworms, Threadworms (Strongyloides) – these and many other internal parasites are controlled with the dewormers that we use in our effort to control the more dangerous parasites.

Parasites are becoming increasingly resistant to deworming. The most effective way to slow resistance is through responsible use of current deworming (anthelmintic) products. Please work with your veterinarian to set up the best deworming program for your horse.

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