Equis ISSN 2398-2977

Onchocerca spp

Contributor(s): Maggie Fisher, Sheelagh Lloyd, Craig Reinemeyer




  • Phylum: Nematoda.
  • Superfamily: Filarioidea.
  • Genus:Onchocerca.
  • Species:cervicalis/reticulata.

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Clinical Effects



  • Adult worm in tendinous/ligamentous tissue.
  • Microfilariae migrate to dermis.
  • Picked up by biting flies when flies feeding.
  • Microfilariae develop to infective stage inside fly.
  • Infective microfilariae deposited onto skin when fly feeds.


  • ByCulicoidesspp.

Pathological effects

  • Live microfilariae well tolerated, host reaction occurs to dying microfilariae.
  • Pruritus caused by presence of microfilariae leads to excoriation - alopecia, skin scaling, scab formation on forehead, pectoral region and ventrum.
  • Areas of leukoderma and alopecia   Skin: onchocerciasis  .
  • Occasional inflammation of conjunctiva.
  • Diffuse, painless swelling occurs in the area of the developing adult parasite.
  • Previously believed that adult worm caused fistulous withers, no longer believed to be the case, in fact adult worms seem to be well tolerated.


Control via animal

  • Repel/kill biting flies that approach horse.

Control via chemotherapies

  • Ivermectin   Ivermectin  has licensed claim against microfilariae.
  • Ivermectin known to be ineffective against adult worms causing human onchocerciasis, therefore unlikely to be effective against adult worms in the equine.
  • Repeated treatment necessary for infected horses every 2-12 months to maintain reduction in the number of microfilariae in the dermis and other locations.
  • Normally subsides after this time.
  • Concurrent treatment with glucocorticoids has been advocated to prevent dermatitis.
    In a heavily infected animal, ivermectin   Ivermectin  treatment could induce a host response to dying larvae with ventral edema, dermatitis and eye lesions

Control via environment

  • Reduce number of biting flies.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • French D D et al (1988) Efficacy of ivermectin in paste and injectable formulations against microfilariae of Onchocerca cervicalis and resolution of associated dermatitis in horses. Am J Vet Res 49 (9), 1550-1554 PubMed.
  • Moran C T & James E R (1987) Equine ocular pathology ascribed to Onchocerca cervicalis infection: a re-examination. Trop Med Parasitol 38 (4), 287-288 PubMed.
  • Cummings E et al (1985) Prevalence of eqiune onchocerciasis in southeastern and midwestern United States. JAVMA 186 (11), 1202-1203 PubMed.
  • Schmidt G M et al (1985) Onchocerca cervicalis in horses: dermal histopathology. Acta Trop 42 (1), 55-61 PubMed.