Onchocerca cervicalis in Horses (Equis) | Vetlexicon
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Onchocerca cervicalis

ISSN 2398-2977




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


  • Very common where high numbers of the vectors,Culicoidesspp are found.
  • High incidences reported in USA (25-100% horses are infested), Argentina, Australia.
  • Infections only occasionally diagnosed in Northern Europe.
  • Global warming and other changes in climate may increase prevalence.


  • 4th stage larve (microfilariae) cause dermatitis and conjunctivitis/blepharitis   Skin: onchocerciasis  .
  • Formerly implicated as a cause of 'fistulous withers'.

Active Forms

Active Form 1


  • Typical filarial nematode morphology.
  • Sexual dimorphism, males (2-6 centimeters long) smaller than females (20-30 centimeters long).


  • White.


  • Survive in nuchal ligament of definitive host (horses).


  • Host inflammatory response confines adults within fibrous tissue.
  • Females are viviparous and produce microfilariae.
  • Unknown, but thought to have a long life-span.

Active Form 2


  • Slender microfilariae 205-240 micrometers in length.


  • Microfilariae are active and are found in connective tissues of head and neck, and on occasion in the conjunctiva.
  • From these sites microfilariae are ingested by feedingCulicoidesspp.


  • Time taken for development of microfilariae in intermediate host to infective stage varies with environmental temperatures.
  • The lifespan of the adult fly.

Resting Forms

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.


Useful samples


  • Phosphate-buffered saline.
  • Fixation in formalin for preservation.

Field diagnosis

  • Clinical signs.
  • Area with large numbers ofCulicoidesspp.
  • Seasonality follows biting fly season.
  • Differentiation from cutaneous habronemiasis   Habronemiasis  , sweet itch   Insect hypersensitivity  , lesions due directly to biting flies or wounds.

Laboratory diagnosis

  • Incubate skin biopsies overnight in blanced salt solution of PBS, centrifuge medium, stain microfilariae in pellet with Giemsa or methylene blue.

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.