Ophthalmologic Manifestations of Onchocerciasis 

Onchocerciasis is an insidious nonfatal filarial disease that has caused blindness, lifelong human suffering, and grave socioeconomic problems. Onchocerciasis is a cause of clinical and epidemiological burden of skin disease in Africa. An estimated 18-40 million people are afflicted worldwide. Approximately 2 million people are blind because of this disease. About 85.5 million people in 35 countries live in endemic areas. In 1875, O'Neill first reported the presence of filaria in "craw-craw" as onchocerciasis is called in West Africa. In 1919, Robles described in the French literature an anterior uveitis and keratitis associated with acute and chronic skin changes.

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. The endemic area extends from south of the equator to Angola in the west and Tanzania in the east. Localized foci exist in Sudan and Yemen. The geographic distribution of onchocerciasis in Latin America is sporadic, with important foci in Guatemala, Ecuador, Venezuela, Mexico, Colombia, and the state of Amazonas in northern Brazil.

The parasite, Onchocerca volvulus, is a nematode that belongs to the family Filariidae. O volvulus is the only Onchocerca with a human host, although an infected spider monkey and a gorilla have been recorded. Blackflies of the genus Simulium are the only vectors of O volvulus. They are tiny ferocious biters. At least 15 different species of blackfly simuliids can transmit onchocerciasis; they vary by terrain and continent (eg, Simulium damnosum in Africa). Their eggs require fast-running rivers for breeding grounds. As a result, the numbers of flies produced fluctuate with the season. The adults emerge after 8-12 days following egg production with the ability to travel hundreds of kilometers in flight on wind currents. Their life span is about 4 weeks.

Onchocercal larvae, in the skin of infected individuals, are ingested with a blood meal. They mature to the infective stage within the fly between 6-10 days and are transmitted by the female blackflies. Development to the adult stage occurs in humans. 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.

The classic lesion of onchocerciasis is the onchocercoma, a firm, painless nodule in the subcutaneous tissue. Onchocercomata are formed predominantly on the head, face, and torso, but they may be found on the pelvic girdle and lower extremities deep-seated against the bones or near the joints. The nodule usually is composed of 2-3 females and daughter microfilariae encapsulated in a fibrous coat. Dead worms may calcify within the nodules. The ocular tissues are involved via migration of the microfilariae from the neighboring tissues, through the bloodstream, or along the nerves. Intraocular organisms are evident early in the disease by direct invasion from the conjunctiva, through the sclera, or through the cornea.

Most microfilariae die as immature worms in the host. Their death causes an intense inflammatory reaction that is responsible for most of the morbidity of onchocerciasis. Antigens of the infective larvae seem to induce cell-mediated and humoral responses. Circulating immune complexes have been identified and implicated in the inflammatory response to infection. Perivascular deposits of immune complexes have been shown in various tissues. Immunoglobulin E (IgE) levels are also very high, thus implicating all of the known mechanisms of pathologic immune destruction. The lymph nodes that drain infected areas show granulomatous inflammation, fibrosis, and atrophy on histologic examination.

Frequency

United States

No current report exists of onchocerciasis due to O volvulus in the United States.

International

Onchocerciasis was considered to be one of the world's most formidable public health problems. Approximately 85.5 million people live in endemic areas. One half of the cases in the world are located in Nigeria. Ocular onchocerciasis has been found in more than 1 million individuals. Variation exists in the blindness rate in different geographical areas, possibly because of distinct strains or biological variants.

Onchocerciasis is more likely to lead to blindness in Africa than in Latin America, and it is 7 times less frequently blinding in the forested areas than in the Savannah (nonforested) areas. More than 90% of certain village populations are infected with this disease, and about 35-50% have ocular onchocerciasis.

Mortality/Morbidity

Microfilariae elicit the onchocerciasis syndrome that includes blindness, lymphadenitis, and dermatitis. O volvulus infection reduces immunity and resistance to other diseases, resulting in a reduction of the life expectancy of infected individuals by approximately 13 years.

Race

• No well-described racial differences in the incidence of onchocerciasis or susceptibility to the disease exist.
• Socioeconomic differences have been clearly identified as a contributing factor.

Sex

• Although no reported differences of exposure exist between men and women, men may be afflicted more often than women because of farm and field occupations.

Age

• As many as 50% of people older than 40 years may be blind in endemic areas.