I will never forget the first time that I incised an episcleral nodule in a dog and found coiled white nematodes. It was 2011 and Onchocerca lupi was not even on my list of differentials for the dog referred to me for oral antibiotic-unresponsive orbital swelling with episcleral nodules. I have been a veterinary ophthalmologist since 1994, so I had purged my parasitology textbook from my library sometime during the Clinton years. It was just as well, because there was no information about Onchocerca lupi in any of the parasitology textbooks from that time. Like me, you probably have not learned much about canine eye worms.
Life Cycle. Since 1991, case series reports from clinical veterinarians, pathologists and parasitologists have detailed canine Onchocerca lupi infestation characterized by variable orbital swelling and pain. Endemic areas have been identified in Greece, Turkey, Portugal and more recently in the Four Corners of the Southwestern United States. Male and female adult worms find each other in a dog’s episcleral tissues, mate and produce microfilaria that leave the orbit and travel through the skin primarily of the head and back. A currently unidentified fly ingests the microfilaria during a blood meal. Inside the fly, O. lupi transforms through larval stages L1-L3. After biting another dog, O. lupi larvae differentiate into male and female nematodes and migrate to the episcleral tissues to complete the life cycle. The life cycle is similar to canine heartworm probably because Dirofilaria immitis and Onchocerca lupi belong to the same family of filaria.
Clinical Disease. The adult O. lupi is a coiled white thread-like nematode with the larger female worms suspected to be up to 20 cm in length. This is an estimate given that no one has extracted a large female worm intact. Clinical disease is related to inflammation around adult worms in the orbit. For reasons that are poorly understood, some dogs have no apparent orbital inflammation while others have severe orbital swelling, pain, decreased ocular retropulsion and exophthalmos. Many occult Onchocerca infestations are revealed only after a dog is treated with melarsamine for Dirofilaria immitus. Older dogs have more severe orbital infestations with nematodes located deep in the orbit sometimes adherent to the fascial tissues around the optic disk.
Prevalence. Since that first dog with Onchocerca presented to me in 2011, we have definitively diagnosed or suspected O. lupi infestation in 15 dogs. All of the dogs originated in the Farmington, New Mexico area and were transported to the Denver area for adoption. Earlier this year, I examined 29 young dogs presented to shelters for routine spay/neuter in Farmington and Aztec, New Mexico and found episcleral granulomas with nematodes in 3 dogs and microfilaria in a skin snip biopsy from one of the dogs with episcleral nematodes.
Zoonotic Potential. Sporadic cases of human infestation with O. lupi have been reported worldwide from all areas where O. lupi is endemic in dogs. In 2013, a 22-month-old child from Arizona recovered after surgical extraction of a gravid female O. lupi nematode from around her cervical spinal cord.
Diagnosis. Currently, episcleral biopsy is necessary for definitive diagnosis. Serologic testing is unavailable, skin snip biopsy for microfilaria is unreliable and microfilaria are not found in the bloodstream.
Treatment. There is no universally accepted treatment. We are currently treating dogs with daily doxycycline for 3 months, monthly ivermectin for 6 months and oral and topical corticosteroids as needed. Because of the zoonotic potential, lifelong heartworm preventative is indicated. Melarsamine injection and surgical excision may be considered, but carry a complication risk that is unnecessary for the majority of patients.
Episcleral nodules due to Onchocerca lupi
Onchocerca lupi adult worms in an episceral nodule