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
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
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).
Posterior rregion of Onchocerca gutturosa female (left), male
(right). Note different sizes of spicules in male.
Drawings from Eberhard, 1979
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
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 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).
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
Antibiotic treatment of Wolbachia
may help reduce the severity of the symptoms of river blindness in
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.
"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
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.
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.
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