Rev 07/12/2022

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Onchocerca (Diesing 1841) Railliet and Henry 1910

The genus Onchocerca consists of 28 parasitic species  which, with one exception, infect ungulate mammals (Morales-Hojas et al, 2006). The exception is O. volvulus, the causative agent of human onchocerciasis or river blindness, which has no known wild animal reservoir.

The parasites of ungulates cause lesions which can affect animal health and diminish the value of carcasses (Muller, 1979).

Onchocerca volvulus has, for many years, been regarded as the causative agent of River Blindness.  Reports in 2002 indicate that the bacterium Wolbachia, which is associated with the nematodes and which may be required for their growth and reproduction, has a major role in the pathology of the disease. Wolbachia incites a severe inflammatory response, leading to blindness and serious skin disorders.

The immune response of the host is also involved in the disease.  Mice lacking an immune cell receptor molecule called TLR4 showed fewer signs of eye inflammation when exposed to Wolbachia-laden worm extract (Hoerauf and Volkmann, 2002).

The programs of the Carter Center and the development of avermectin-based antheminthics has dramatically decreased the incidences of river blindness and lymphatic filariasis (elephantiasis).

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Morphology and Anatomy:

Posterior rregion of Onchocerca gutturosa female (left), male (right).  Note different sizes of spicules in male.

Drawings from Eberhard, 1979


Body size range for the species of this genus in the database - Click:
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Human onchocerciasis is found in both the Old and New World but about 96% of all cases are in Africa and mostly in Western Africa. Of the 36 countries where the disease is endemic, 30 are in sub-Sahara Africa (plus Yemen) and six are in the Americas. Important foci exist also in Mexico, Guatemala, Venezuela and Ecuador. A total of 18 million people are infected with the disease and have dermal microfilariae, of whom 99% are in Africa. (WHO's Fact Sheets #95)

River blindness is the second leading infectious cause of blindness in the world. It is spread to humans by the bite of black flies infected with O. volvulus.


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Economic Importance:


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Biology and Epidemiology


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One early sign of infection with Onchocerca is the raised nodules that can be seen under the skin. These are most often seen in areas over bony prominence.  It is suggested that this phenomenon occurs because the larvae are immobilized in these locations (while the host is sleeping) long enough for them to be trapped by the body's cellular defense mechanisms.
Reactions to dead microfilariae around these nodules can lead to several unpleasant conditions. In the skin there is destruction of the elastic tissues and the formation of redundant folds. There is also often a loss of pigmentation and the histological appearance of advanced cases often resembles the skin of very old normal subjects.
The microfilariae can also enter the eye by passing along the sheaths of the ciliary vessels and nerves from under the bulbar conjunctiva directly into the cornea, via the nutrient vessels into the optic nerve, and via the posterior perforating ciliary vessels into the choroid. Dead microfilariae in the eye lead to an inflammatory immune response and the eventual formation of secondary cataracts and ocular lesions. Because of this, heavy infections often lead to progressive blindness.

Recent research (Hoerauf and Volkmann, 2002) shows that Wolbachia bacteria associated with the nematode provoke a severe inflammatory response, leading to blindness and serious skin disorders.

The microfilariae can also cause inflammation of regional lymph glands which remove foreign material from the distal skin. This inflammation along with the loss of tissue elasticity can lead to protruding lymph glands enfolded in pockets of skin. This condition is especially prominent in the areas around the scrotum (often called the 'hanging groin' effect) and in severe cases is classified as minor elephantiasis.
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Life Cycle:


In filarial nematodes, Two hosts to complete the life cycle, an intermediate host (often an arthropd) and a primary host (usually a vertebrate). The juvenile stages occur in the intermediate host and the reproductive adult in the definitive (primary) host.

All pictures presented in this page have been taken from Peters and Gilles 1991
Human onchocerciasis is caused by the filarial parasite Onchocerca volvulus whose life cycle occurs in two different hosts: black flies, and human. The infective larvae (a: stage L3) are normally transmitted by the bite of Simulium flies (see picture below). Once in the human body, the larvae undergo molting to stage L4 (e), to then reach the adult stage in about one year (f). Adult females produce millions of microfilariae (h) that they shed in the blood of their human host. When female blackflies take a bloodmeal they ingest microfilariae.  The microfilariae in the fly host transition to L2 life stage (j). L2 larvae then molt to L3 (a), the infective stage for humans.
Simulium flies breed in fast flowing rivers i.e well oxygenated water, because their larvae have an obligatory aquatic stage during which they require high oxygen tension (see picture below). Hence, Onchocerciasis is associated with fast flowing rivers including rapids. That's why the blindness Onchocerciasis can lead to is often referred to as 'river blindness'.

The infective larvae of Onchocerca (stage L3) enter the body through the wound made by the bite of its host fly. The larvae then move to the subcutaneous tissues where they become encapsulated within nodules and mature into adults in approximately one year (cross section of worms in a subcutaneous nodule right).


After mating the female sheds microfilariae 300 mm in length and 0.8 mm in diameter. The microfilariae are sheathless with sharply pointed, curved tails.

The microfilariae can be found free in the fluid within the nodules and in the dermal layers of the skin spreading away from the nodules containing the adults. Microfilariae also can be found in the blood and eye during heavy infections. They infect their fly vectors while the flies are feeding on the human host and mature into third stage infective larvae in the flies' flight muscles (about 10 days total).



For Ecophysiological Parameters for this genus, click 
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Surgical removal of nematode from eye; chemotherapy.

The pinpointing of Wolbachia bacteria as the direct factor behind the virulence suggests new therapies for combating river blindness, especially since recent studies in infected humans have shown that the bacteria can be killed by the common antibiotic doxycycline.

The battle against river blindness is taking place on two fronts at the moment:

Antibiotic treatment of Wolbachia may help reduce the severity of the symptoms of river blindness in already-infected individuals.

Targeting Wolbachia could prevent the spread of Onchocerca.  Recent studies have shown that doxycycline treatments in infected humans kills Wolbachia and also sterilize the nematodes, breaking the life cycle.  In contrast, ivermectin treatments only reduce numbers of microfilariae for a few months and repeated treatments are necessary.


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"Synopsis and Classification of Living Organisms"; McGraw-Hill 1982, pg. 919

"Human Filariasis: A Global Survey of Epidemiology and Control"; University Park Press; 1976, pgs. 149-189

" Onchocerciasis in Zaire: A New Approach to the Problem of River Blindness"; F.D. Rodger, Pergamon Press 1977, 2-5

"Animal Diversity"; Hickman-Roberts Wm. C. Brown Publishers1955, pg. 134-135

  • Peters, W. and Gilles, H.M. (1991) A Colour Atlas of Tropical Medicine and Parasitology. Third edn., Wolfe, London.
  • Material from Laura Langenburg, 1995.

    Extensively From the Filarial Genome Network (http://math.smith.edu/~sawlab/fgn/pnb/filbio.html)

    Source:  Lisa Onaga (202-326-7088) American Association for the Advancement of Science.

    Hoerauf, A. and L. Volkmann, March 2002. Science.

    Morales-Hojas, R., Cheke, R.A., Post, R.J. 2006. Molecular systematics of five Onchocerca species (Nematoda: Filarioidea) including the human parasite, O. volvulus, suggest sympatric speciation. J. Helminthol. 80:281-290.

    Eberhard, M.L. 1979. Studies on the Onchocerca (Nematode Filarioidea)found in cattle in the United States. I. Systematics of O. gutturosa and O. linealis with a description of O. stilesi n.sp.J. Parasitol., 65:379-388


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    Copyright © 1999 by Howard Ferris.
    Revised: July 12, 2022.