Rev 10/27/2026
Nematoda
Chromadorea
Chromadoria
Filariidae or Onchocercidae
Onchocerca cervicalis (Diesling, 1842) Railliet & Henry, 1910
Synonyms:
Adults up to 30 cm long.
Reported median body size for this species (Length mm; width micrometers; weight micrograms) - Click:
Parasitic in horses worldwide.
Onchocerca cervicalis causes a nonseasonal dermatitis in horses. Clinical disease occurs year-round but may be worse in the spring and summer.
The adult nematode is approximately 30 cm in length and resides in the ligamentum nuchae. The estimated prevalence of O. cervicalis infestation in clinically normal horse populations in the United States varies from 25% to 100%.91,101 There are no apparent breed or gender predilections for onchocerciasis, but affected horses are usually older than 4 years.91 Because O. cervicalis is the most common member of the Onchocerca genus to affect horses, the remainder of this discussion focuses on this parasite unless otherwise indicated.
The female nematode lays eggs in the host blood stream and these enter the vector fly when it bites the horse or other definitive host. The eggs hatch to the larval stage (the microfilariae) within 24 hours after fly has the first blood meal. The microfilariae spreads throughout the intermediate hosts muscles. Microfilariae undergo development to infective larval stages over a 25-day period within the Culicoides vector.
In the definitive host, microfilariae migrate to the dermis of the ventral midline, pectoral area, withers, inguinal region, and eyelids, with highest numbers in the spring, when Culicoides spp., the predominant intermediate host, are most active. Mosquitoes may also serve as vectors in some geographic areas. Microfilariae undergo development to infective larval stages over a 25-day period within the Culicoides vector (Sellon, 2014).
Ecophysiological Parameters:
The estimated prevalence of O. cervicalis infestation in clinically normal horse populations in the United States varies from 25% to 100%.
There are no apparent breed or gender predilections for onchocerciasis, but affected horses are usually older than 4 years.
The severity of the reaction varies between horses, and many horses have dermal microfilariae without any evidence of reaction.
Ocular lesions are the result of microfilariae in the eyelids, conjunctiva, cornea, anterior/chamber, uvea, and fundus. Concurrent uveitis, presumably an immune-mediated event, may occur.
In either cutaneous or ocular lesions, sudden death of parasites resulting from chemotherapy may produce severe exacerbation of disease, presumably as a reaction to antigens released by dead and dying microfilariae (Sellon, 2014).
Ivermectin at a dose of 200 ug/kg orally is the treatment of choice for elimination of microfilariae in the skin or ocular tissues. A single dose may result in improvement in clinical signs within 2 to 3 weeks, with complete resolution in 2 months.
Some horses may remain free of cutaneous microfilariae for 4 to 5 months or longer after treatment with injectable ivermectin. Some horses may require two or three monthly treatments before clinical signs resolve.
Approximately 10% to 25% of horses with onchocerciasis have an adverse reaction (e.g., ventral midline edema or pruritus) to ivermectin treatment within 7 to 10 days (Sellon, 2014).
Dzimianski, M.T., Marchiondo, A.A., Cruthers, L.R. 2020. Nematoda, Filarioidea. Chapter 2 in Marchiondo, Cruthers and Fourie (eds) Parasiticide Screening Vol 2. Academic Press.
Peters, W. and Gilles, H.M. 1991. A Colour Atlas of Tropical Medicine and Parasitology. Third edn., Wolfe, London.
Sellon, D.C. 2014. Nonenteric nematodes. . Chapter 57 in Sellon, D.C. and Long, M.T.(eds) Equine Infectios Diseases. W.B. Saunders.