by Meredith Maxey [Honors Tutorial College/BIOS]    

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Onchocerciasis: River Blindness

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Worldwide Magnitude

17.7 million are infected with O. volvulus (worm that causes the disease)

99% in Africa

500,000 are visually impaired

270,000 are blind

123 million more are at risk in the 37 endemic countries       
 Picture: Countries where onchocerciasis is found Source

Symptoms

Visible “microfilariae” or tiny worms in the eye area

Migration of worms to upper layers of skin

• Blindness                                                   

• Skin rashes/lesions

• Skin atrophy

• Intense itching

• Skin depigmentation

• Palpable nodules

• Picture: Man afflicted with skin nodules caused by onchocerciasis Source

How does it cause blindness?

Microfilariae migrate to surface of cornea where they are attacked by the immune system.  Punctate keratitis, or inflammation of the cornea, then follows.  This condition can clear up.  However, if the infection is chronic, sclerosing keratitis will result.  Sclerosing keratitis makes the affected area become opaque.  Blindness results if the entire cornea becomes opaque.

Treatment

• 1980s: Merck & Co. Inc developed Mectizan®                      

• Derived from Streptomyces avermitilis

• Late 1980s: Merck & Co agreed to make donations to all who need the drug, for as long as needed.  This was called the Mectizan Donation Program (MDP).

• The MDP is widely considered the stimulus for the current global initiative that is centered on providing community-based treatment.

• Picture: Lady dispensing doses of Mectizan. Source

Ivermectin

This drug paralyzes microfilariae by interfering with their nervous system, but not the human hosts’.  This in turn will prevent the itching caused by the microfilariae.  The drug does not kill the adult worm, however.  It DOES prevent the production of offspring.  This in turn prevents morbidity and transmission.  Ivermectin is commonly known as Mectizan® and Stromectol®.

Mectizan ®

Dosing every 3 to 12 months

Merck has provided enough drugs for over 250 million doses

Current estimates = 30 million receiving Mectizan® annually

Carter Center involved with administration of over 78 million doses…

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Onchocerciasis Prevents People From…

Working

Harvesting their crops

Receiving an education

Taking care of their children

Using fertile lands near rivers

Who is most at risk?

Americas

• Guatemala and Mexico                    

• Coffee plantations

• Picture: A coffee plantation in Central America  Source
  

• Ecuador and Colombia

• Brazil and Venezuela

• Yanomami population (Amazon)     

• Picture: Yanomami People Source

Africa

Poor, those who can’t escape/protect themselves from contaminated water

Brief History

1875: John O’Neill first observed microfilariae in West Africa

1925: In Sierra Leone, Donald Blacklock discovered the black fly as the transmission vector of O. volvulus

1990s: O. volvulus eradication efforts begin

Diagnostic Lab Studies

• Skin snips (3 to 5-mg) to count emerging microfilariae

• Mazzotti test (DEC side effects, seldom used)

• ELISAs (finger-stick sample)                                  

• Picture: An example of an ELISA plate Source

• More sensitive, less invasive than snips

• Can’t distinguish past/current infections (main problem with them)

• Polymerase Chain Reactions

• Amplifies repetitive parasite DNA sequence

• However, they are expensive and invasive

New Developments in Diagnostic Methods

--Rapid-format antibody card tests

--Serum samples used to detect antibodies (IgG4) to antigen Ov16

--Dipstick assay

   -Detect oncho-C27 antigen in tears and urine

   -Only takes 3 hours

   -Strips remain usable for up to 8 months

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