Ophthalmologic Manifestations of Onchocerciasis
- Author: Debora E Garcia-Zalisnak, MD; Chief Editor: Hampton Roy, Sr, MD more...
Onchocerciasis, commonly known as river blindness, is a vector-borne disease that affects millions of people in Africa, the Middle East, and South and Central America. This disease is caused by the filarial parasitic nematode Onchocerca volvulus, which is transmitted by the blackfly vector Simulium, which carries third-stage larvae.
Infection can lead to chronic skin disease, severe itching, and eye lesions that can progress to complete blindness. There are approximately 123 million people at risk for infection in 38 countries and at least 25.7 million people infected. Of infected persons, 1 million are blind or have severe visual impairments.
Advances in prevention and treatment have decreased the prevalence of this disease in localized areas of Africa and Latin America. Most cases are found in Africa, south of the Sahara, in a wide zone that lies along the fifteenth parallel from Senegal to Ethiopia.
Multiple organizations have created programs whose goal is to prevent and treat onchocerciasis. These include the Onchocerciasis Control Programme (OCP) in West Africa, a program created by the World Health Organization (WHO) that lasted for 28 years, ending in 2002; the program successfully eliminated onchocerciasis as a public health problem in 10 of the 11 African countries involved. The River Blindness Elimination Program, created by The Carter Center, works in Latin America and Africa to eliminate river blindness.
Although multiple programs are working extensively to eliminate this disease, onchocerciasis is still a very relevant world health problem.
A Simulium female black fly takes blood from an infected human and ingests microfilariae. The microfilariae then enter the fly’s gut and grow into the first larval stage. After approximately 7 days, the larvae mature and move to the fly’s saliva, where it is transmitted to another human during the fly’s next blood meal.
Development to the adult stage occurs in the human host. The adult worms pair and mate in the human host, and, unlike most nematodes that produce eggs, the female Onchocerca gives birth daily to thousands of microscopic larvae known as microfilariae. These larvae mature to adult worms in about 1 year. The life span of microfilariae is 6-30 months. Those adult worms that complete their life circle may survive a decade, during which time they release millions of microfilariae.
While alive, O volvulus causes little disease. The adults protect themselves in fibrous painless subcutaneous skin nodules called onchocercomas. These nodules are found predominantly on the head, face, and torso, although they can also be seen in the pelvic girdle, in the lower extremities, or near the joints. In most cases, 2-3 females and daughter microfilariae live inside the onchocercoma. Their death leads to an inflammatory cascade in the human host. Antigens released from the dead nematode cause a TH2 helper cell response, which then activates interleukins, neutrophils, eosinophils and antibodies released by plasma cells to create the inflammatory response, which, in turn, creates the manifestations of the disease.
O volvulus endosymbionts, such as the Wolbachia bacteria that are released upon its death, also cause an immunogenic response. Strains of O volvulus that carry Wolbachia DNA are associated with a higher incidence of ocular disease. It is also proposed that ocular inflammation, specifically in the posterior pole, is a consequence of antigen mimicry. There seems to be cross-reactivity between the Onchocerca antigen Ov39 and the retinal antigen hr44, which can explain the chorioretinitis seen in some humans despite proper treatment and evidence of decreased microfilariae in their blood.
No cases of onchocerciasis due to O volvulus have been reported in the United States.
More than 99% of African onchocerciasis cases occur in 27 sub-Saharan countries; 120 million people are at risk for infection on this continent. In West African savanna, the rate of infection has been as high as 80%-100% by age 20 years, with blindness peaking at age 40-50 years. In African forest areas, itching has affected 42% of the population older than 20 years, and skin lesions have appeared in 28% of persons aged 5 years.[2, 3]
Central and South America
Based on a report from the Centers for Disease Control and Prevention (CDC) in 2013, 231,467 people are at risk for onchocerciasis in Guatemala, making it the highest-risk country for river blindness in the Americas. Mexico is second, with 169,869 people at risk for infection. Venezuela has 119,358 people at risk. The Yanomami population in Brazil has 12,988 persons in 22 endemic areas at risk. Ecuador has 25,863 at risk. Although this is a relatively small number, they have the highest prevalence of microfilariae in the skin at baseline. Lastly, Colombia once had a single focus of infection, but, according to the WHO, the infection has been eradicated in Colombia as of 2012.
Microfilariae elicit the onchocerciasis syndrome that includes blindness, lymphadenitis, and dermatitis. O volvulus infection reduces immunity and resistance to other diseases. According to data published in the OCP, from 1971-2001, 1,283 (5.2%) deaths were due to onchocerciasis. Mortality was significantly associated with increased microfilarial burden but not blindness. Mortality rates peaked in the 1980s and decreased afterward.
No well-described racial differences in the incidence of onchocerciasis or susceptibility to the disease exist, although specific reports have documented higher, but not clinically significant, rates of infection in blacks.
Socioeconomic differences have been clearly identified as a contributing factor.
Per OCP data, females had a risk of death approximately 7.5% less than males. Men may be affected more often than women because of farm and field occupations.
As many as 50% of people older than 40 years may be blind in endemic areas, particularly in Africa.
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