Equis ISSN 2398-2977

Onchocerca cervicalis

Contributor(s): Rosanna Marsella, Grace Mulcahy




  • Phylum:Nematoda.
  • Superfamily:Filaroidea.
  • Genus:Onchocerca.
  • Species:cervicalis.

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Clinical Effects



  • Microfilariae in dermis.
  • Ingested by biting flies(Culicoides spp).
  • Develop to infective stage.
  • Deposited by fly on to skin during feeding.
  • Adult worms found in tendons/ligaments, typically nuchal ligament.


  • Through feeding activity ofCulicoidesspp.

Pathological effects

  • The host response to dying worms may cause dermatitis with pruritus, alopecia, depigmentation and ulceration.
  • This immunopathologic response (mazzotti reaction) is characteristic of hypersensitivity responses to helminth parasites.
  • Parasite cuticle collagen components have been implicated in inducing such responsesviaaggregation of platelets and production of arachidonic acid metabolites.
  • Skin lesions (alopecia, scaling, crusting, inflammatory plaques, erythema, ulceration, lichenification, pruritus) are most usually seen on the forehead, ventral mid-line and pectoral region.
  • Sporadic cases involving the conjunctiva also occur.
  • Adult parasites in nodules are sometimes apparent as painless swellings in the nuchal ligament.

Other Host Effects

  • In general, infections are tolerated well with little pathology.
  • Horses that have clinical lesions have developed hypersensitivity.


Control via animal

  • Fly avoidance measures - stabling of horses at dawn and dusk.
  • Fly control measures - repellents (eg in USA 2% permethrin products are very effective as repellents), insecticides.

Control via chemotherapies

  • Ivermectin   Ivermectin  is effective against microfilariae 200 micrograms/kg one dose (PO or IM), occasionally second and third dose may be necessary at 1 month interval.
  • Does not kill adults.
  • Repeated treatments advisable in areas where high rates of transmission occur.
    Treatment may precipitate clinical signs due to intense response to dying parasites
  • In clinical cases non-steroidal anti-inflammatory agents   Therapeutics: anti-inflammatory drugs  may be beneficial in reducing the pathology associated with the host inflammatory/immune response.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Mancebo O A, Verdi J H and Bulman G M (1997) Comparative efficacy of moxidectin 2% equine oral gel and ivermectin 2% equine oral paste against Onchocerca cervicalis (Raillet and Henry, 1910) microfilariae in horses with naturally acquired infections in Formosa (Argentina). Vet Parasitol 73 243-248 PubMed.
  • Klei T R, Torbert B, Chapman M R & Foil L (1983) Prevalence of Onchocerca cervicalis in equids in the Gulf Coast Region. Am J Vet Res 45 (8) 1646-1647 PubMed.
  • Lloyd S and Soulsby E J L (1978) Survey for infection with Onchocerca cervicalis in horses in eastern United States. Am J Vet Res 39, 1962-1963 PubMed.