worldwide blindness

I recently went on an overseas medical mission trip, and it got me thinking.  Thinking about a lot of things, but as it pertains to this blog: worldwide blindness.  So this post will unpack some of the major diseases affecting vision globally.  

The leading causes of preventable blindness world-wide are cataracts, glaucoma, trachoma, and onchocerciasis.  We do not see the latter two much, if at all, in the United States; they are prevalent primarily in less developed areas, where there are also specific environmental hazards (1). 

TRACHOMA
Trachoma is the world's leading infectious cause of preventable blindness. Eight million world-wide are visually impaired as a result of trachoma (2). It is caused by a bacteria, Chlamydia trachomatis, and typically spreads by contact with an infected person’s hands or clothing, or can be transmitted by flies who have had contact with discharge from an infected person's nose or eyes. Trachoma is endemic in the poorest, most rural parts of Africa, Asia, the Middle East, South and Central America, and Australia (3), where lack of access to water and sanitation play a role in the disease's proliferation.  

Trachoma spreads easily among children, and from child to caregiver.  Blindness typically occurs in adulthood, and affects women 3 times more than men (4).  Left untreated, the repeated trachoma infections cause scarring of the conjunctival tissue of the eyelid, which in turn causes the eyelashes to turn in (trichiasis). The lashes then rub against the cornea, which causes scarring that leads to visual impairment and blindness.

Trachoma grading card (image: WHO)

Control:  The SAFE strategy is a public health initiative developed by the World Health Organization (WHO), combining treatment with prevention to eliminate blinding trachoma by the year 2020.  Trachoma is transmitted through close contact, so it is a community problem and all intervention must be community-targeted.

• Surgery to correct trichiasis.  As discussed earlier, trichiasis is the precursor to blindness.
• Antibiotics to treat active disease.  Single dose azithromycin, donated by Pfizer in the 1990s, is the treatment of choice (5).
• Facial cleanliness and Environmental improvement. While the former two points of the SAFE strategy target treatment of the disease, the latter two focus on prevention.  Facial cleanliness reduces disease transmission. Better environmental sanitation and proper waste disposal helps control the fly population (6).

ONCHOCERCIASIS

Onchocerciasis, also known as river blindness, is the second leading infectious cause of blindness globally.  It accounts for at least one million cases of blindness or severe visual disability.  The majority of those infected live in sub-Saharan African countries.  Onchocerciasis is caused by a parasitic nematode, Onchocerca volvulus.  The worms induce intense inflammatory responses of the skin and eyes, especially when they die (7). The disease is transmitted from person to person by a black fly of the genus Simulium. Repeated bites are usually necessary for infection.  The fly breeds near rivers and streams.    

Onchocerciasis causes inflammation in the eyes (uveitis, keratitis, chorioretinitis, optic neuritis (8)) and the skin (itchy rash, nodules under the skin).  The disease's toll extends into the social and economic realm as well.  Once blind, affected individuals have a life expectancy of only one third that of the sighted, and most die within 10 years (9). There is also a social stigma attached to the skin lesions and disfigurement associated with this disease.

Sclerosing keratitis in Onchocerciasis (image: CEH Journal)

Control:  Onchocerciasis elimination strategy involves vector control and/or large scale ivermectin chemotherapy (10). 

• In terms of vector control, regular aerial spraying of Similium breeding sites for at least 14 years is recommended (until the infection has died out of the human population).  
• A single dose of ivermectin (Mectizan), donated by Merck in the 1980s, is effective for up to a year.  It has the added benefit of helping with intestinal worms, scabies, and head lice, which are common problems in the areas where this disease is endemic. Ivermectin must be given yearly for 12-15 years.

CliffsNotes:  The elimination of preventable worldwide blindness is within sight!

Additional Resources:

• Vision2020
• Unite For Sight: Eye Stats
• WHO: Global Data on Visual Impairment