Equine onchocerciasis

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Equine Onchocerciasis

River Blindness

Equine onchocerciasis is a parasitic skin disease of horses, caused by infection with Onchocerca spp. Onchocerca are filarial nematodes that affect horses, donkeys, cattle, and other mammals. The presence of Onchocerca on horses doesn't necessarily mean that it is the cause of disease in horses, as Onchocerca are frequently found on normal, healthy horses without causing any problems. Only certain horses have been shown to develop clinical signs. This is why horses that develop a skin reaction to the presence of Onchocerca, are thought to be caused by a type I or type III hypersensitivity reaction to the antigens of dead and dying microfilaria or as a result of the arthropod bites. Each species of Onchocerca has a preference for certain areas of the body, and differs in it's mature adult size from 30 to 60 cm in length.
Where to look for Equine onchocerciasis

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 itching
Thinning hair which progresses to hair loss
Scaling and/or crusting
Small nodules under the skin
Swelling of legs


  • History
  • Clinical signs
  • Physical exam
  • Skin biopsy


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


  • Implement insect control strategies into stable management program, in order to reduce populations of blackflies and sand flies.

Scientific Research References

Good 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