Onchocerciasis / River Blindness
Onchocerciasis (pronounced /ˈɒnkɵsɜrˈsaɪ.əsɨs/ or /ˈɒnkɵsɜrˈkaɪ.əsɨs/),[1] also known as river blindness, is the world's third leading infectious cause of blindness. It is caused by Onchocerca volvulus, a nematode that can live for up to fifteen years in the human body[2] though it can also live in other mammals. It is transmitted to humans through the bite of a black fly. The worms spread throughout the body, and when they die, they cause intense itching and a strong immune system response that can destroy nearby tissue, such as the eye.[3]
The primary treatment is a drug, ivermectin. For best effect, entire communities are treated at the same time. A single dose may kill first-stage larvae (microfilariae) in infected people and prevent transmission for many months in the remaining population.[4]
Classification
- Onchocerciasis may be divided into the following phases or types:[5]:440-441
- Erisipela de la costa
- Mal morando
- Sowda
Additionally, the various skin changes associated with onchocerciasis may be described as follows:[5]:440
- Leopard skin
- Elephant skin
- Lizard skin
Epidemiology
99% of onchocerciasis cases occur in Africa.[6] About 18 million people are currently infected with this parasite; approximately 300,000 have been permanently blinded.[7] Onchocerciasis is currently endemic in 30 African countries, Yemen, and isolated regions of South America. Travelers who do not stay long in those areas have little risk of developing the disease as it requires prolonged exposure to the fly bites and parasite introduction
Causes of morbidity
Adult worms remain in subcutaneous nodules, limiting access to the host's immune system. Microfilariae, in contrast, are able to induce intense inflammatory responses, especially upon their death. Dying microfilariae have been recently discovered to release Wolbachia-derived antigens, triggering innate immune responses and producing the inflammation and its associated morbidity. Wolbachia species have been found to be endosymbionts of O. volvulus adults and microfilariae, and are thought to be the driving force behind most of O. volvulus morbidity. Severity of illness is directly proportional to the number of microfilariae and the power of the resultant inflammatory response.
Skin involvement typically consists of intense itching, swelling, and inflammation. A grading system has developed to categorize the degree of skin involvement:
- Acute papular dermatitis - scattered pruritic papules;
- Chronic papular dermatitis - larger papule, resulting in hyperpigmentation;
- Lichenified dermatitis - hyperpigmented papules and plaques, with edema, lymphadenopathy, pruritus and common secondary bacterial infections;
- Skin atrophy - loss of elasticity, skin resembles tissue paper, 'lizard skin' appearance;
- Depigmentation - 'leopard skin' appearance, usually on anterior lower leg.
Ocular involvement provides the common name associated with onchocerciasis, river blindness. The microfilariae migrate to the surface of the cornea. Punctate keratitis occurs in the infected area. This clears up as the inflammation subsides. However, if the infection is chronic, sclerosing keratitis can occur, making the affected area become opaque. Over time the entire cornea may become opaque, thus leading to blindness. There is some evidence to suggest that the effect on the cornea is caused by an immune response to bacteria present in the worms.