Human Onchocerciasis


Adult Worms which live in nodules under the skin


The Microfilaria are the first stage lava which dwell in the skin and wait to be taken up by a vector black fly. Only in the vector do they continue their development and grow up to infective third stage lava.

What is onchocerciasis?

Onchocerciasis is a parasitic disease with an insect vector that breeds in water. It is the world’s second leading infectious cause of blindness. Controlling insect breeding sites in rivers is one of the pillars of prevention. Onchocerciasis is often called “river blindness” because in extreme cases, it can cause blindness, and because the blackfly, which carries it, abounds in fertile riverside areas. More than 120 million people in 37 countries in Africa, Latin America, and Yemen are at risk of the disease. Over 37 million people have been infected (source WHO), often affecting those living in poor, rural African communities. There is currently no official data about the levels of blindness in Cameroon, but it is estimated that about 1% of the population are blind (Sightsavers International), with the principal causes being cataract, river blindness, corneal scarring and refractive error.

design by P. Wenk & A. Renz

Where are the high risk destinations for infection?

Onchocerciasis is present in 36 countries, 29 of them in sub-Saharan Africa, Yemen in the Arabian peninsula and six in the Americas. As a public health problem the disease is most closely associated with Africa, where it constitutes a serious obstacle to socio-economic development.

Out of some 120 million people world-wide who are at risk of onchocerciasis, 96% are in Africa. A total of 18 million people are infected with the disease and have tiny worms under the skin (dermal microfilariae), of whom 99% are in Africa. The worst affected countries are Angola, Burundi, Cameroon, Chad, the Central African Republic (CAR), the Congo, the Democratic Republic of the Congo, Ethiopia, Equatorial-Guinea, Gabon, Kenya, Liberia, Malawi, Mozambique, Nigeria, Rwanda, Uganda, Sudan and Tanzania.

How is onchocerciasis transmitted?

Onchocerciasis is caused by Onchocerca volvulus, a parasitic worm that lives for up to 14 years in the human body. It is spread through the bite of a blackfly of the Simulium damnosum species complex, which breeds in fast-flowing rivers and streams.

When the fly bites, it deposits the larvae of a parasitic worm, which matures to adulthood and produces millions of tiny worms, called microfilaria. Adults emerge after 8-12 days and live for up to four weeks, during which they can cover hundreds of kilometres in flight.

Each adult female worm, thin but more than 0.5 metre in length, produces millions of microfilariae (microscopic larvae) that migrate throughout the body.

After mating, the female blackfly seeks a bloodmeal and may ingest microfilariae if the meal is taken from a person infected with onchocerciasis.

A few of these microfilariae may transform into infective larvae within the blackfly, which are then injected into the person from whom the next meal is taken and subsequently develop into adult parasites, thus completing the life cycle of the parasite.

Crosstransmission of animal and human onchocercoses (A. Renz)

What are the symptoms of onchocerciasis?

When the microfilariae migrate throughout the body and give rise to a variety of symptoms rashes, lesions, intense itching and depigmentation of the skin; lymphadenitis, which results in hanging groins and elephantiasis of the genitals, general debilitation and serious visual impairment, including blindness.

from Wenk & Renz: Parasitologie, Thieme-Verlag, 2003

How soon do onchocerciasis symptoms occur?

Onchocerciasis manifestations begin to occur in persons one to three years after the injection of infective larvae.

How is onchocerciasis treated?

The development of ivermectin in the 1980’s provided for the first time a safe, effective drug capable of reducing the numbers of skin microfilariae in infected people and resulting in clinical improvement and decreased transmission of infection.

Tetracycline kills Onchocerca worms! (A. Renz)

At that time this idea – very revolutionary at this time – did not interest many people, when Alfons Renz promoted it first: No reaction from WHO, and our veterinarian colleague from the Liverpool School of Tropical Medicine even argued “if this really worked, than we would have known this since longtime, because tetracycline is in use since may years”. Nevertheless, together with the project veterinarian Dr. Mark Bronsvoort, we decided to repeat the trial and added 6 oxytetracycline-treated cattle in the next trial round, aimed to test Moxidectin (1997). Again, all worms died (Langworthy et al. 2000). At the same time, Prof. Dieter Büttner and Dr. Achim Hoerauf from the Bernhard Nocht Institute in Hamburg, who had learned from our success with tetracycline during a filariasis meeting in Woods Hole, repeated the experiment with a rodent filaria, Litomosoides sigmodontis. Again, these filarial worms, which also harbour endobacteria, were killed by tetracycline treatment. Now, it became clear, these these ‘Rickettsia-like-bacteria’ (since known to be Wolbachia species) must be truely endosymbiotic partners of the filaria, which cannot survive without them

What can I do?

Take precautions to avoid blackfly bites and seek medical advice if you feel you have been bitten.

Zooprophylaxis by divertion of bloodseeking Simulium damnosum s.l. from man to a non-human source of blood considerably reduces the vectorial capacity of a vector population. Cattle, which are almost twice as attractant for S. damnosum s.s. and S. squamosum as is the human bait, can therefore strongly reduce the risk of onchocerciasis transmission, if they are staying between the human settlements and the Simulium breeding river.:

modified from Renz et al. 1994, mathematical model by K. Dietz and M. Eichner

The epidemiology of human onchocerciasis in Cameroon

Starting in 1976 and in collaboration with Drs John Anderson and Harald Fuglsang, we studied the prevalence of human onchocerciasis in various regions of Cameroon and collected entomological, parasitological and ophthalmological date to follow-up the longitudinal evolution of the endemic disease:

Endemicity of human onchocerciasis in Cameroon (A. Renz)

There is a clear correlation between intensity of transmission (ATP, Annual Transmission Potential; infective larvae per man and year) and the mean microfilarial density in the village population:

Correlation between intensity of transmission of O. volvulus L3 and endemicity of human onchocerciasis in the such exposed villages (average number of microfilariae per mg of skin in persons aged above 5 years). Data of several studies in the rain-forest and savanna of West-Africa are combined.

From these data, a critical level for a tolerable Annual Transmisson Potential was developed, i.e. an entomological criterium for a level of endemicity of onchocerciasis, which might still be considered tolerable, as there are but few eye-lesions. This criterium was set to 100 L3 of O. volvulus per man and year and 1,000 flies/man, year (Renz, WHO-SAP meeting in 1977). It was adapted by the Onchocerciasis Control Programme (OCP-WHO) in West-Africa as the criteriun for successful control of transmission by treating the Simulium breeding rives with insecticides. It is still valid today. The treshold level for elimination of the parasite is much lower, however, and is probably as low as an ATP of 10 L3/man,year. Such a very low transmission is almost impossible to measure in the field, as it would mean, that only 10 L3s would be expected to be found, if a fly-collector was working every day from dawn to dusk throughout every day of the year, and all attacking flies were dissected for O. volvulus L3s. Still the ATP measured at the river banc, where transmission is highest, might not be representative for the average person in the village, who usually is much less exposed. We therefore developed methods to estimate the Effective Annual Transmission Potential (EATP, Renz et al. 1987, Jacobi et al. 2010) to which subgroups of the village population are exposed. Boys are much more exposed than girls, and this explains the much higher prevalence of parasites and blindness in males.

Control of human onchocerciasis in the Vina valley in Northern Cameroon:

In 1987, we proposed to WHO to carry out the first mass-treatment by ivermectin in the hyperendemic villages along the river Vina du Nord and to study the effects of eliminating the microfilariae on the level of O. volvulus L3 transmission:

Figure from Seidenfaden et al. 2002, design A. Renz

Ivermectin, given at an annual regime to the local village populations (ca. 70% treatment coverage), can very successfully reduce onchocerciasis endemicity, but could not eliminate the reservoir in former hyperendemic villages, so that transmission still continues at a level, high enough to maintain endemicity:

photo Renz, data Seidenfaden et al. 2001

Now, over 20 years after the start of the treatment, infective larvae of O. volvulus are still present in man-biting Simulium damnosum s.l. flies and transmission still continues:

In conclusion, ivermectin mass-treatments have efficiently cleared the human population from the skin-dwelling microfilariae and have strongly reduced the intensity of onchocerciasis. There is little risk of eye-lesions in the thus exposed populations, but as long, as the parasite has not finaly been eliminated, there is a risk of recrudescence of the disease, as soon as resistance to ivermectin shall develop.

One very simple and easy to adapt method is to protect the lower limbs from the attacks of the Simulium flies: They tend to bite below the knees, mainly at the ancles:

sources: onchocerciasis:  WHO; bovine model and transmission A. Renz; illustrations by Alfons Renz and Peter Wenk, publications cited => see publications