Equine onchocerciasis

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Equine Onchocerciasis

River Blindness

Equine onchocerciasis is a parasitic disease of horses worldwide, caused by a filarial worm of the genus Onchocerca. The three most common species known to infect horses are: O. cervivalis, O. reticulata, and O. railliti.

O. cervivalis is the most common parasite associated with uveitis in horses. It has also been known to cause fistulous withers, poll evil, and dermatitis. As adults, O. cervivalis live within the tissue of the horse's neck, where they will travel along the body causing sores to break out--often along the midline of the stomach, base of the mane and withers, and uveitis within their eyes. The uveitis occurs when there is an accumulation of dead worms in their eye, because they give off antigens which cause eye irritation and inflammation.
Where to look for Equine onchocerciasis

How the Parasites are Transmitted to Horses

Onchocerca spp. are transmitted to horses through bites from infected insects (black flies, gnats, biting midges), which act as vectors. As the insect bites down on the horse, it deposits Onchocerca infective larvae into the host's skin. Once the larvae are in the horse's body, they mature into adults between 3 to 12 months.


Mild to moderate rubbing
Thinning hair which progresses to hair loss
Scaling and/or crusting skin
Small sores along the stomach and base of the mane and withers


  • History
  • Clinical signs
  • Physical exam
  • Eye exam
  • Laboratory tests



Ivermectin0.2 mg/kg administered orally. Caution 10-25% of horses with this condition have an adverse reaction to ivermectin treatment within 7 to 10 days.
CorticosteroidsUsually administered before or during ivermectin therapy, used to decrease the likelihood or severity of adverse reactions.
Diethylcarbamazine5 mg/kg administered orally for a minimum of 5 consecutive days
Levamisole10 mg/kg administered orally for 7 to 10 days


Scientific Research

General Overviews

Risk Factors

  • History of inconsistent or minimal deworming
  • Horses living in or recently imported from tropical climates
  • Living nearby rivers or streams where there are Simulium spp. (blackflies) present