Parasite Control in Horses

Dr. Janice Sojka

Common Signs of Internal Parasites

  • Poor growth
  • Weight Loss
  • Decreased feed efficiency
  • Colic
  • Diarrhea
  • Pneumonia
  • Death

The amount of disease a horse will show due to parasites depends on three factors:

  • Kind of parasite involved
  • Number of parasites involved
  • Host defenses. Young and debilitated horses are generally most susceptible to parasites

Do current management styles lead to increased parasite problems?

  • High stocking density increases number of parasites a horse ingests
  • Poor manure handling may increase exposure by spreading infective parasites over pasture

Parasite Control - General considerations

  • Parasites are most successfully prevented in horses through a combination of management and therapeutic strategies
  • Management
    • Decreasing infective parasites in environment
  • Therapeutic
    • Deworming with anthelmintics at proper intervals

Parasite Prevention

  • Adequate pasture acreage
  • Composting Manure
  • Cleanliness
  • Pasture Rotation
  • Deep plowing and reseeding
  • Mixed grazing (cattle and horses)

Pasture Rotation

  • Moving horses from old, infested pasture to one with a minimal number of infective parasite larvae
  • Should deworm prior to moving animals
  • Foals and young horses should go on the cleanest available pasture

Deworming -- Types of dewormers

  • Purge dewormers
  • Extended action dewormers
  • Continously fed dewormers
    • Kill infective larvae as they are eaten. Can prevent larvae from entering horse’s system

Most Important Parasites of the Horse

  • Large strongyle
  • Small strongyle
  • Ascarids
  • Bots
  • Stongyloides
  • Pin worms

Large Strongyle - Stongylus vulgaris

  • "Blood worm"
  • Causes thromboembolic colic
  • Most significant parasite of horses
  • Direct life cycle
  • Larvae live in artery supplying the intestines. Blood clots form which cut off the blood supply to the gut

Diagnosis of strongyles

  • Fecal flotation
  • Necropsy

Small Strongyles

  • Strongylus endentatus and Strongylus equinus
  • Direct life cycle
  • Live in gut wall of large intestine
  • Cause damage to gut wall resulting in G.I. upset

Treatment of Strongyles

  • Many products are available, nearly all horse wormers are effective against adults in intestinal track.
  • Ivermectin, mixodectin, and fenbendazole effective against migrating larvae

Control of Strongyles

  • Use effective wormers routinely
  • Avoid overgrazing pastures
  • Use clean pastures for young horses
  • Pile and compost manure

Ascarids - Roundworms

  • Parascaris equorum
  • Disease of horses up to 2 years of age
  • Direct lifecycle
  • Larvea migrate through lungs where they can cause damage
  • Build up in large numbers in small intestine

Ascarids - Clinical Signs

  • Impaction colic - death
  • Pneumonia
  • Pot belly
  • Unthrifty appearance
  • Poor hair coat

Control of Ascarids

  • Good sanitation
  • Eggs live in environment for many years,
  • Avoid pasturing foals in the same pastures year after year

Treatment of Ascarids

  • Most common dewormers are effective against ascarids
  • If a foals has a very heavy infection it should be wormed with less effective wormers to prevent colic

Stomach Bots

  • Insects- Adult is a fly, larvae live in the horse stomach
  • Flies lay eggs on hair, they hatch and migrate to stomach
  • May cause stomach irritation and colic

Diagnosis of Horse Bots

  • See eggs on hair and mane
  • Gastroscopy
  • Necropsy
  • Knowing flies are in the area

Treatment of Bots

  • Because bots are insects, only products which have activity against insects will be effective
  • Ivermectin and Moxidectin are effective
  • Nits can be removed from hair before they hatch


  • Adult pinworms adult lay their eggs around the anus
  • These eggs cause irritation and horses will rub their tails causing broken tail hairs
  • Controlled by all common dewormers

Diagnosis of Pin Worms

  • Egg masses in perineal region
  • Tail rubbing
  • Eggs in feces (rarely)
  • Adults in feces

Control of Pinworms

  • Thorough cleaning of stalls
  • Fresh feed and water

Threadworms - Strongyloides

  • Stronglyoides westeri
  • Infects young foals
  • Larvae passed in mare’s milk to foals
  • May cause diarrhea in young foals
  • DOES NOT cause foal heat diarrhea

Diagnosis of Strongyloides

  • Fecal exam for larvae
  • Fecal culture

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