Bovis ISSN 2398-2993

Onchocerca spp

Synonym(s): Onchocerciasis, Onchocerca gutturosa, Onchocerca lienalis, Onchocerca gibsoni, Onchocerca armillata, Onchocerca ochengi, Onchocerca dukei, Onchocerca cebei

Contributor(s): Tracy Anderton , Mike Taylor

Introduction

Classification

Taxonomy

  • Phylum: Nematoda.
  • Class: Secernentea.
  • Superfamily: Filariodea.
  • Genus: Onchocerca.
  • Species: gutturosa (synonym lienalis), armillata, gibsoni, ochengi (synonym dermatata), dukei, cebei (synonym sweetae).

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Clinical Effects

Epidemiology

Habitat

  • Adult - in bovine host.
  • Larvae - in bovine and insect host.

Lifecycle

  • Indirect lifecycle involving insect vectors, the biting midge Culicoides spp or the biting fly Simulium spp.
  • Adults in the bovine host produce and release L1 larvae which migrate and reach the interstitial tissue spaces of the host dermis.
  • L1 are picked up by the intermediate host insects when they have a blood feed.
  • Microfilariae develop and mature within the insect vector to the mature, infective L3 stage.
  • L3 larvae are deposited onto a subsequent host during feeding activity.
  • Microfilariae migrate to various sites within the host where they develop to the adult form.

Transmission

  • O.guttoros and possibly O’dukei/ochengi are transmitted by the vector Simulium spp (the biting fly).
  • O.gibsoni/armillata/cebei transmitted by the vector Culicoides spp (the biting midge).

Pathological effects

  • The main pathological findings are the formation of fibrous, diffuse, painless nodules caused by the adult worms. These occasionally become a focus for bacteria and form an abscess. It may be possible to palpate these nodules (in some species) in superficial areas such as the inguinal region.
  • The site of nodules is dependent upon the feeding site of the intermediate host and differs between species:
    • O.gutturosa - nuchal ligament and gastrosplenic ligament (note- nodules not formd with this species).
    • O.gibsoni - subcutaneous and intermuscular tissue of the chest and hindlimbs.
    • O.armillata  - aorta. (In a quarter of cases aortic aneurysms are noted but do not seem to cause clinical problems).
    • O.cebei - brisket and hindlimbs.
    • O.ochengi - subcutaneous tissue, scrotum, udder.
    • O. dukei - abdomen, thorax, thighs.

Other Host Effects

  • Onchocerca infection does not usually cause clinical signs in the host.
  • Possible pruritis due to the presence of microfilariae.
  • Incidence and adult worm numbers do not increase with age of the host. This suggests that there is a strong immunity to superinfection.

Control

Control via animal

  • Control is not usually necessary as infections are generally asymptomatic. 

Control via chemotherapies

  • In the UK, specific treatment for Onchocerca spp is not usually required as it does not cause clinical disease.
  • Treatments with macrocyclic lactones used to target other nematode infections are likely to be effective.
  • Ivermectin Ivermectin (200 microgram/kg) has been shown to be effective against Onchocerca spp.  It has a long term supressive effect against microfilariae and a will decrease the release of microfilariae by adult worms but is ineffective against adult worms. 

Control via environment

  • Measures to reduce the number of biting flies/midges are extremely difficult due to the extensive breeding habits of these vectors.

Vaccination

  • No vaccine available.

Diagnosis

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Further Reading

Publications

Refereed Papers

  • Recent references from PubMed and VetMed Resource.
  • Eichler D A & Nelson G S (1971) Studies on Onchocerca gutturosa (Neumann, 1910) and its Developement in Simulium ornatum (Meigen, 1818). I. Observations on O. gutturosa in cattle in South-East England. Journal of Helminthology 45 (2) pp 245-58 PubMed.

Other sources of information

  • Taylor M A, Coop R M & Wall R (2016) Parasites of Cattle. In: Veterinary Parasitology. 4th Edition. Wiley-Blackwell Publishing, Oxford, pp 417-418.

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