Management Strategies to Decrease Parasite Exposure

(Based on a Northern Climate)

Spring will come, eventually, and that means manure clean-up in the paddocks, sheds, and pastures that the horses have used all winter.  The following strategies can help decrease the parasite burden and exposure through out the year.

  • Make your manure pile outside of the horses paddock or pasture.
  • Feed horses away from potentially contaminated areas, or use feeders to avoid feeding on the ground.
  • Manure should be picked from paddocks, lean- tos or any areas where horses are eating regularly (ideally every 2-3 days a week)

Pasture management is very important.  Ideally horses should be rotated to a new pasture manure happenswhen they have eaten the grass down to 4 inches high.  The tall clumps should be mowed down to this height.  Keeping the pastures mowed allows manure to properly desiccate.  Horses should be allowed back out to graze when the grass has grown up to 8 inches high.  This may mean the horses are in a designated dry lot until the pasture has grown back.

During very hot dry weather, pastures can be mowed and then harrowed to disperse manure and facilitate desiccation of parasites.    Horses should be kept off the harrowed pasture for 7 days to ensure all parasites are dead.

Do not spread manure on the pasture while horses are using the pasture.  If you are going to spread manure it should be done during the very hot dry season, after the pasture has been mowed.  It should then be harrowed so that it can desiccate properly.  The horses should be kept off that pasture for 7 days.

Horses should be grouped together by the type of shedder into separate paddocks/pastures if possible.  This decreases contamination levels overall and each group (example yearlings) can be dewormed similarly.  Do you know what type of shedder your horse is?

The use of routine fecal testing allows veterinarians to decide which horses to deworm, how often, and which class of dewormer is appropriate.  Additionally, they can help identify anthelmintic resistance on individual farms.

It is important that the fecal sample is collected and handled properly for accurate results.

Sample Collection

It is ideal to collect the fecal sample from the horse immediately after the horse has defecated.  The sample should be placed in a Ziplock bag with minimal air inside, labeled with the date, horse’s name, and promptly placed in a refrigerator or cooler with an ice pack.  The sample should remain refrigerated until delivered to the clinic (within 36 hours from sample collection).

McMasterThe fecal sample is evaluated under the microscope and the species of parasite eggs are noted as well as the overall count.  A McMaster’s fecal egg count* is performed and the sample is also checked for the presence of sand.

Horses are categorized into 3 groups depending on the number of strongyle eggs per gram as determined by the fecal egg count using the McMasters technique:

<200 eggs per gram               low shedder

200-500 eggs per gram         moderate shedder

>500 eggs per gram               high shedder

It is not necessary to completely remove all internal parasites each time you deworm your horse.  Moderate and high shedders should have a Fecal test repeated 2 weeks after deworming.  In addition, assessing risk factors including age, horse density, movement on/off the property, manure management, and pasture rotation can help decrease parasite exposure and minimize future infections.

Young horses(<5years) and older horses(>15 years) along with any horses diagnosed or displaying signs of Cushings Disease should be monitored more closely as they are  often the high shedders.

Uncontrolled parasitic infections can cause problems, especially in foals, yearlings, older, or debilitated horses.  Internal parasites can cause poor hair coat, ill thrift, pneumonia (secondary to the presence of migrating larvae through the lungs), colic, and diarrhea.

*NOTE-only strongyle type eggs are counted in the FEC.  Ascarid, strongyloides, or tapeworm eggs are clinically relevant and therefore considered when determining the deworming program but not included in the Fecal Egg Count.

For a schedule for deworming based on the level of shedder that your horse it. 2015 Deworming Schedule.


Types of Parasites

Large Strongyles

strongyleEffects: Fever, loss of appetite, weight loss, progressive weakness, anemia, recurring colic, diarrhea, hepatitis, blood clots and organ damage from the larval migration. Young horses are particularly susceptible to strongyles.

Source: From manure; Eggs survive winter on pasture.

Small Stronglyes

Effects: Fever, loss of appetite, weight loss, progressive weakness, anemia, recurring colic and diarrhea. Young horses are particularly susceptible to strongyles.

Source: From manure; Eggs survive winter on pasture.

Ascarids (Roundworms)

Effects: Primary Damage: impaction, colic and possible rupture of the small intestine. Lung migration can produce respiratory disease; common cause of slow growth & unthriftiness in young horses.

Source: From eggs in manure that survive winter on pasture.


Effects: Cause unthriftiness and irritation of the stomach.

Source: Yellow eggs of bot fly laid on hair => licked by horse => hatch in mouth => create oral ulcers and larvae that attach to stomach lining.


Effects: Excessive tail rubbing, digestive disturbances, retarded growth, loss of condition and irritability. Young horses are more susceptible.

Source: Eggs in manure and found on places where the horse rubs it’s tail; eggs die quickly on pasture.


Effects: Colic, marked digestive disturbances, unthriftiness.

Source: Eggs in manure; pasture mites as intermediate hosts.

Intestinal Threadworms

Effects: Erode small intestine lining. Produce weight loss, anemia, diarrhea, & lack of appetite.

Source: Passed to foal via mare’s milk; possible contributing factor “foal heat” diarrhea.

Stomach Worms

Effects: Digestive disorders, summer sores, inflammation of stomach wall, & skin ulcers.

Source: Eggs in Manure with fly maggots as the intermediate host that deposits the larvae on the horse.