Onchocerciasis is a type of dermatitis in equids and ruminants caused by the microfilariae of Onchocerca nematodes. These parasites are transmitted by various biting flies, and infection is prevented by topical repellent. Diagnosis is based on histological examination or identification of the microfilariae in skin scrapings.
The taxonomic status of the three species of Onchocerca currently recognized in the US, and other previously recognized species, is under debate. Onchocerca cervicalis is found in the ligamentum nuchae and possibly other sites of Equidae. In cattle, Onchocerca gutturosa locates in the ligamentum nuchae, and Onchocerca lienalis, in the gastrosplenic ligament.
Onchocerca adults are associated with connective tissues; they are very thin and 3–60 cm long. Microfilariae are found in the dermis and, on rare occasions, circulating in peripheral blood. Microfilariae lack a sheath and are 200–250 mcm long, with a short, sharply pointed tail.
The intermediate hosts for O cervicalis are Culicoides midges; and for O gutturosa and O lienalis, Simulium black flies.
Clinical Findings of Onchocerciasis in Animals
O cervicalis has been associated with fistulous withers, poll evil, dermatitis (see dermatitis image), and uveitis in horses. However, because O cervicalis in large numbers is common in horses without these diseases, there is some debate about its role in the pathogenesis of these conditions.
Courtesy of Dr. Stephen White.
In the ligamentum nuchae, O cervicalis adults induce inflammatory reactions ranging from acute edematous necrosis to chronic granulomatous changes, resulting in marked fibrosis and mineralization. Mineralized nodules are more common in older horses. Although lesions occur in these areas, presumably associated with dead parasites, it is generally agreed that fistulous withers and poll evil are not caused by O cervicalis infection.
O cervicalis microfilariae concentrate in the skin of the ventral midline. Large numbers can be found both in horses without dermatitis and in horses with dermatitis of the face, neck, chest, withers, forelegs, and abdomen. These lesions can be pruritic and are often scaly, crusty, ulcerated, alopecic, and depigmented.
Dermatitis caused by O cervicalis can be associated with an immunological reaction to dead and dying microfilariae. Although the pathogenesis of these lesions is unclear, treatment with microfilaricidal drugs—eg, ivermectin (200 mcg/kg, PO, once); moxidectin (400 mcg/kg, PO, once)—can result in dramatic improvement.
Allergic reactions to the bites of small flies can produce similar lesions or exacerbate microfilaria-associated dermatitis. Thus, diagnosis of Onchocerca-associated dermatitis can be based on responsiveness to microfilaricidal treatment.
O cervicalis microfilariae also accumulate in horses' eyes; however, no clear association has been made between microfilariae and equine uveitis or other ocular lesions in horses.
Diagnosis of Onchocerciasis in Animals
Clinical evaluation, in combination with histological examination
PCR assay
The most effective method of diagnosing onchocerciasis is skin biopsy, preferably full-thickness ≥ 6 mm. The tissue is minced and macerated in isotonic saline solution for several hours. Microfilariae are concentrated and stained with new methylene blue after removal of skin pieces. O cervicalis microfilariae can be differentiated microscopically from Setaria spp, found in the blood of cattle and Equidae, by the presence of a sheath around Setaria.
Treatment of Onchocerciasis in Animals
Macrocyclic lactones
No treatment is effective against infection by Onchocerca adults. Against microfilariae, ivermectin (200 mcg/kg, PO, once) and moxidectin (400 mcg/kg, PO, once) are efficacious (> 99%), producing marked clinical improvement in horses with Onchocerca-associated dermatitis.
A small percentage of horses infected with O cervicalis react to treatment with marked, edematous ventral midline swelling 1–3 days after drug administration. Ocular lesions have also been reported. These reactions usually resolve spontaneously; however, supportive care could be necessary. Systemic corticosteroids, often applied topically, can help decrease skin lesions.
Key Points
Onchocerca-associated dermatitis is a vector-borne disease that occurs in mid- to late summer into autumn.
Lesions consist primarily of dermatitis and alopecia, and the disease is diagnosed via biopsy of affected tissue.
Treatment with macrocyclic lactones (ivermectin, moxidectin) has successfully resolved the dermatitis.
For More Information
Mansell SE, Behnke MC. Onchocerca cervicalis: a survey into awareness and knowledge of the parasite amongst UK equine veterinarians. J Equine Vet Sci. 2022;114:103942.
Dagnaw M, Zemene M, Getaneh G, Tibebu S. A review on equine onchocerciasis. Afr J Basic Appl Sci. 2016;8:27-33.
