Onchocerciasis is a filarial infection caused by the filarial nematode Onchocerca volvulus. Also known as river blindness, it is transmitted by black flies (genus Simulium) that breed near fast-flowing rivers.
According to the 2017 Global Burden of Disease study, at least 20.9 million people worldwide are infected, of whom 14.6 million have skin disease and 1.15 million have vision loss1). It is the second leading infectious cause of blindness after trachoma1). More than 99% of infected individuals are concentrated in 31 countries in sub-Saharan Africa.
As of 2024, at least 249.5 million people in 28 countries require interventions aimed at elimination. In 2023, a total of 172.2 million people received treatment, achieving a global coverage rate of 69.0%.
Four countries—Colombia (2013), Ecuador (2014), Mexico (2015), and Guatemala (2016)—have been verified by WHO as having eliminated onchocerciasis5). In Mexico, ivermectin was administered 2–4 times per year from 1994 to 2011, achieving interruption of transmission in all three foci5).
In Ethiopia, a meta-analysis using the skin snip method reported a pooled prevalence of 31.8% 2). In highly endemic areas, the infection rate reaches 80–100% by age 20, and the prevalence in males (28.4%) is significantly higher than in females (19.3%) 2).
In Ghana, control programs initiated in 1974 reduced the microfilaria prevalence from 69.13% in 1975 to 0.72% in 2015 3). Treatment coverage increased from 58.5% in 1997 to 83.8% in 2016, and approximately 100 million tablets of ivermectin were distributed 3).
In Gabon, the establishment of control programs has been delayed, and prevalence varies widely from 0% to over 80% depending on the region 6). Co-infection with Loa loa is common, and the risk of severe adverse reactions to ivermectin poses a barrier to community-directed treatment with ivermectin (CDTI) 6).
In hyperendemic areas, all-cause mortality is 3–4 times higher compared to uninfected populations, and life expectancy is shortened by 7–12 years.
More than 99% of infections are concentrated in sub-Saharan Africa. In South America, transmission persists only in the border region between Brazil and Venezuela. In the Middle East, endemic areas also exist in Yemen.
Skin symptoms usually appear before eye symptoms. Eye symptoms become apparent several years after infection, peaking in the 40s and 50s.
The roundworm can involve all tissues of the eye. Punctate superficial keratitis, sclerosing keratitis, anterior chamber larvae, anterior uveitis, chorioretinitis, chorioretinal atrophy, and optic neuritis may be observed.
Anterior Segment Findings
Snowflake opacities: Subepithelial punctate lesions between the eyelids. Appear in the early stage.
Sclerosing keratitis: Scarring and neovascularization of the corneal stroma due to chronic inflammation. A major cause of permanent blindness.
Anterior chamber microfilariae: Observed as fine, S-shaped or C-shaped motile elements using slit-lamp transillumination.
Iridocyclitis: Causes pupillary deviation, iris atrophy, and extensive iris adhesions.
Posterior Segment Findings
Chorioretinitis: Begins around the optic disc and progresses to extensive chorioretinal atrophy.
Optic neuritis: Starts with optic disc edema and eventually leads to optic atrophy.
Secondary glaucoma: Often angle-closure type due to iris adhesion. Even without adhesion, it is an independent risk factor for glaucoma.
Cataract: Forms early secondary to iridocyclitis.
Sclerosing keratitis and chorioretinitis are the main causes of permanent blindness. Secondary glaucoma and optic atrophy also cause irreversible visual impairment. For details, see the “Pathophysiology/Detailed Pathogenesis” section.
The pathogen is Onchocerca volvulus. Blackflies bite infected individuals and ingest microfilariae, which develop over one week into infective third-stage larvae (L3) inside the blackfly. L3 enter the skin of a new human host and mature into adult worms over 6 to 12 months.
Adult female worms migrate to subcutaneous or deep fascial tissues and become encapsulated in fibrous capsules (subcutaneous nodules). Inside these capsules, fertilized females produce millions of microfilariae. The reproductive lifespan of adult worms is estimated to be up to 15 years1). Microfilariae migrate to the dermis and various tissues, including the eyes.
Transmission via transplacental infection has also been reported.
Diagnosis is based on clinical findings and a history of residence in endemic areas. The skin snip method is used for definitive diagnosis.
The bloodless skin snip method is the standard definitive diagnostic method. Samples are taken from the scapula, each iliac crest, and each calf. The samples are cultured in physiological saline for up to 24 hours, and motile elements are stained and identified. Onchocerca volvulus can be distinguished from other nematodes because it lacks a sheath and nuclei in the tail.
Specificity is very high, but sensitivity is low in early infection or when the worm burden is low. Detection value increases after 18 months post-infection.
Enzyme-linked immunosorbent assay (ELISA) and Western blot detect antibodies against Onchocerca volvulus antigens in skin, tears, and urine. Measurement of the IgG4 subclass is also used. Antibody testing for the Ov16 antigen is useful in the later stages of elimination programs 4). However, it should be noted that the Ov16 antibody test cannot distinguish between current infection and past exposure 4).
PCR is more sensitive than skin snip and can detect even low parasite loads. The O-150 PCR is also used for molecular xenomonitoring (MX) of black flies 4). Molecular xenomonitoring can detect communities with microfilaria prevalence ≥1% with high sensitivity and is recommended for determining transmission interruption 4).
Rosa et al. (2023) reported a method using proteomic analysis to directly detect O. volvulus-derived proteins in plasma and urine of infected individuals 9). Nineteen candidate biomarkers were identified and prioritized; notably, OVOC11613 (major antigen) was detected in plasma from 5 cases and urine from 1 case 9). This approach is expected to be applied for diagnosing active infection and monitoring treatment efficacy.
Differential diagnoses include other microfilarial infections (Mansonella perstans, Loa loa, Dracunculus medinensis, etc.), systemic inflammatory diseases such as sarcoidosis, and corneal degenerative or sclerotic diseases.
| Test method | Characteristics | Indication |
|---|---|---|
| Skin snip | High specificity; sensitivity depends on load | Definitive diagnosis |
| Ov16 antibody | Non-invasive, reflects exposure history | Post-elimination surveillance |
| PCR | High sensitivity, detects even low parasite load | Elimination programs |
In early infection or when the parasite burden is low, the skin snip test may yield false-negative results. If clinically suspicious, additional serological tests or PCR are recommended.
Oral ivermectin (Stromectol®) is the standard treatment. Developed by Satoshi Ōmura and William C. Campbell, who won the 2015 Nobel Prize in Physiology or Medicine, it is the cornerstone of mass drug administration programs.
Ivermectin has no effect on adult worms. However, early treatment has been shown to reduce the development of optic atrophy and decrease the severity of visual field defects and keratitis. It is ineffective for advanced chorioretinal lesions or secondary glaucoma.
A 6-week course of doxycycline depletes the symbiotic bacteria Wolbachia, suppressing microfilarial production by adult worms for up to 18 months and reducing corneal opacification.
For iridocyclitis, steroid eye drops and cycloplegic agents (mydriatics) are used. Cataract surgery is indicated for cataracts. Intraocular pressure-lowering treatment is performed for glaucoma.
Ivermectin reduces microfilariae but does not kill adult worms. Since the reproductive lifespan of adult worms can be up to 15 years, long-term repeated administration is necessary. Development of new drugs targeting adult worms is currently underway. For details, see the section “Latest Research and Future Prospects”.
Living Onchocerca volvulus rarely cause inflammation. Adult worms are protected by fibrous nodules, and microfilariae lack immunogenicity through unknown mechanisms. The main cause of ocular lesions is a helper T cell (Th2) response to antigens released from dead microfilariae.
This response induces interleukin release, neutrophil and eosinophil influx, and antibody production. Sclerosing keratitis is thought to result from modification of intercellular adhesion molecule 1 (ICAM-1) expression and production of interleukin-4 and interleukin-14.
The symbiotic bacterium Wolbachia released upon death of microfilariae is suggested to be a major cause of inflammation. In Africa, there are two main strains (savanna and rainforest types); the savanna type has higher Wolbachia DNA content and is more likely to cause ocular disease even with moderate parasite loads. The rainforest type rarely causes blindness even with high loads.
Some of the posterior pole intraocular inflammation may be due to antigen mimicry. Cross-reactivity between the roundworm antigen Ov39 and the retinal antigen hr44 is thought to contribute to the continued progression of chorioretinitis even when microfilarial load decreases.
Glaucoma associated with onchocerciasis is often of the angle-closure type due to iris adhesions. However, even without adhesions, it is an independent risk factor for glaucoma. Microscopic examination reveals abnormal structures posterior to the trabecular meshwork that affect the downstream outflow system while maintaining normal trabecular meshwork structure.
Emodepside is a cyclic octadepsipeptide developed as a veterinary anthelmintic that acts on calcium-activated potassium channels (SLO-1) in nematodes 1). It has activity against multiple life stages, including adult worms, and is effective against ivermectin-resistant strains 1).
Phase I clinical trials showed good safety and tolerability in healthy adults, with dose-proportional increases in plasma concentrations up to a single dose of 40 mg 1). The half-life was approximately 11 hours in the first 24 hours and over 500 hours in the terminal phase 1). In 2014, Bayer and DNDi (Drugs for Neglected Diseases initiative) initiated joint development, and a Phase II clinical trial in Ghana is planned 1).
In a model of O. ochengi (a closely related species parasitic in cattle), repeated administration of emodepside for 7 days resulted in death or sterilization of adult worms in 5 out of 7 animals 1).
Zhan et al. (2022) reported the development status of an onchocerciasis vaccine 8). Key candidate antigens include the cysteine protease inhibitor Ov-CPI-2, essential for L3 larval molting, the secreted protein Ov-RAL-2, and the surface-associated antigen Ov-103 8).
In the O. ochengi bovine model, immunization with irradiated L3 provided protection against experimental challenge and natural infection 8). Putative immunity is observed in 1–5% of residents in endemic areas, characterized by elevated IL-5, IFN-γ, GM-CSF production, and high IgG3 levels 8).
Pryce et al. (2021) evaluated the diagnostic accuracy of molecular xenomonitoring (MX) for black flies and showed that it can detect positive flies with high sensitivity in communities with a microfilaria prevalence above 1% 4). A significant linear relationship (R² = 0.50, p < 0.001) was found between MX rate and human prevalence 4).
Rosa et al. (2023) identified 19 biomarkers in infected plasma using mass spectrometry proteomics, with OVOC11613 (major antigen) as the most promising candidate 9). Verification with isotope-labeled peptides confirmed 11 proteins and 15 peptides 9). This may pave the way for the first non-invasive diagnostic method that directly detects active adult worm infection.
In Ghana, despite over 40 years of control programs, microfilaria prevalence remains above 1% in some communities, and low response to ivermectin has been reported 3). In Gabon, co-infection with Loa loa poses a barrier to CDTI implementation, and even prevalence mapping remains incomplete 6).
Achieving elimination requires addressing ivermectin resistance, developing safe treatment strategies in co-endemic areas, securing funding, and strengthening surveillance systems 7).
