I’ve noticed when the weather turns warm every spring my horse tends to lose hair in a circular pattern on his face and neck. This slight balding is accompanied by severe itchiness on his stomach and chest area. I spoke with my vet and he is attributing this to a reaction from the Onchocerca parasite. Can you recommend any preventive measures I can perform so my horse doesn’t suffer every spring? Thanks, LC, Massachusetts
Good for your vet for recognizing this! I had to look in my veterinary parasitology textbooks for information on Onchocerca and even then there was less than one page on this particular parasite. But I do think I may have found some information that will help you, so let me share what I learned.
Turns out this is a nematode, or roundworm, and in its adult state it prefers connective tissue, specifically the nuchal ligament in the neck. But it’s the immature stage, called microfilariae, that’s causing your horse problems. These microfilariae don’t circulate in the blood like other parasite species (heartworms, for example), they migrate through the skin of the face, neck, chest, withers, forelegs and abdomen. That’s why you see hair loss, itchiness and probably scales, crusts and ulcers. Technically it’s called “onchocerca-associated dermatitis.” While Onchocerca has been blamed for such things as fistulous withers, poll evil, and even equine recurrent uveitis (“moon blindness”), the association has never been proven.
Many vets (like yours!) diagnose it based on response to treatment. That is, ivermectin or moxidectin is given, a mild inflammatory response may be seen, then the problem goes away. However, it can be diagnosed by a reasonably quick and simple procedure called a skin biopsy. Then you know for sure that you’re dealing with Onchocerca.
Since you’re interested in preventing this parasite from ever setting up house, though, and because it’s spread by Culicoides gnats, also called biting midges or “no-see-ums,” you might not see Onchocerca next spring if you take steps early to keep these insects away from your horse.
• Keep your horse in his stall when these gnats are most active, dawn and dusk
• Use screens, fans or misters to protect your horse when he’s inside the barn
• Cover him with fly sheets that wrap around the belly, fly masks and fly leggings
• Regularly apply fly repellent that contains permethrin
Culicoides ideal breeding ground is standing or slow moving water so do what you can to eliminate these sources where you keep your horse.
Finally, you may want to try supplements designed to improve the overall health of the skin, such as omega 3 fatty acids, and supplements that have natural anti-inflammatory properties, such as MSM. I hope some of these suggestions help you and horse eliminate these pesky parasite.