The parasite and its disease

Tiny black flies (Simulium spp.) transmit the parasite Onchocerca volvulus, a roundworm, which will cause onchocerciasis, also known as river blindness, a disease with severe skin and allergic reactions, and in many cases, the parasite blinds its victims.

When black flies bite and feed blood on man, the larvae of Onchocerca volvulus enter the body and develop to sexually mature adult worms that conglomerate under the skin to form a nodule, which can eventually swell to the size of a chicken egg. Two to three female and the same number of male worms can reside within such a node, and many more can be found. The adult females live for about thirteen years, and during this time, they give birth to 700 to 1500 larvae (microfilariae) daily, which colonize the whole body. The so-called microfilaria are responsible for the actual problems. Their migration under the skin will cause irritation and edema, and the almost unbearable itching is still a harmless symptom of the disease, the countless microfilariae become particularly dangerous when they migrate into the eye of the infected person. The patients go blind.
Adult filariae of Onchocerca volvulusFig. 1: Adult filariae of Onchocerca volvulus
Blind person due to onchocerciasis Fig. 2: Blind person due to onchocerciasis
Depigmentation of the skin caused by onchocerciasisFig. 3: Depigmentation of the skin caused by onchocerciasis

Onchocerciasis disease control and its elimination

The World Health Organization (WHO) began fighting onchocerciasis as early as 1974. First, the black flies were decimated in order to stop the transmission of the worms. Helicopters circled over the rivers where the black flies hatched and released insecticides - a costly and lengthy operation.

Survey and examination of an onchocerciasis-affected village Fig. 4: Survey and examination of an onchocerciasis-affected village
The only effective drug IVERMECTIN against the parasite O. volvulus has been introduced and is distributed on large scale for onchocerciasis disease control since 1987, and since then in West, Central and East Africa, the WHO reports that millions of children have been saved from infection and thus from going blind. There were hardly any new infections and hundred millions of people have been cured of the disease. But then and until now the critical phase of the project began. The national disease control programs themselves have to ensure that onchocerciasis does not return in the future. The national disease control programs have to fight the vector, distribute IVERMECTIN and organize a nationwide health surveillance - not an easy task in Africa.

In Togo, continuous epidemiological surveys conducted by the National Onchocerciasis Control Program (NOCP) have shown that after more than 2 decades of mass drug administration of IVERMECTIN in most of the onchocerciasis endemic districts the prevalence of O. volvulus infection has diminished below 5% in all age groups, and in children of less than 10 years of age below 1%; such epidemiological situations are criteria for consideration being close to elimination of onchocerciasis.
Examination of the eyes by ophthalmoscope Fig. 5: Examination of the eyes by ophthalmoscope
Examination of the eyes by lit lamp Fig. 6: Examination of the eyes by lit lamp
Treatment with ivermectin Fig. 7: Treatment with ivermectin

Above all and most important, regular annual ivermectin treatments will eliminate and prevent the migration of O. volvulus microfilariae into the anterior eye chamber and cornea; early-stage ocular onchocerciasis lesions will resolve completely and repeated IVERMECTIN treatments may prevent the emergence of ocular pathology in those populations still exposed to O. volvulus infection.

Our present surveys showed that O. volvulus infections still persisted in children of ≤ 10 years and also in adults, positive antibody responses indicated active O. volvulus infection, and parasite transmission by Simulium spp. vector black flies occurred close to the survey locations. Thus, the interruption of O. volvulus transmission for the purpose of stopping mass drug administration of IVERMECTIN has not yet been attained, but blinding onchocerciasis will not develop anymore.


Banla M, Tchalim S, Karabou PK, Gantin RG, Agba AI, Kére-Banla A, Helling-Giese G, Heuschkel C, Schulz-Key H, Soboslay PT. Sustainable control of onchocerciasis: ocular pathology in onchocerciasis patients treated annually with ivermectin for 23 years: a cohort study. PLoS One. 2014 Jun 2;9(6):e98411. doi: 10.1371/journal.pone.0098411.

Komlan K, Vossberg PS, Gantin RG, Solim T, Korbmacher F, Banla M, Padjoudoum K, Karabou P, Köhler C, Soboslay PT. Onchocerca volvulus infection and serological prevalence, ocular onchocerciasis and parasite transmission in northern and central Togo after decades of Simulium damnosum s.l. vector control and mass drug administration of ivermectin. PLoS Negl Trop Dis. 2018 Mar 1;12(3):e0006312. doi: 10.1371/journal.pntd.0006312.

A comprehensive version of the English text above is available in German under the following link: