Onchocerciasis: River Blindness

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Onchocerca volvulus

--Second leading infectious cause of blindness worldwide           

--O. volvulus causes the disease onchocerciasis      

--Vector of O. volvulus is the black fly (genus: Simulium)

Picture: O. volvulus, the worm that causes onchocerciasis Source

Life Cycle

Person typically needs to be bitten 100s of times by infected black flies before getting the disease.  This makes it an uncommon disease among visitors; they are not around the flies long enough to receive that many bites.  In Ethiopia, individuals have been found with over 20,000 bites. Female black fly takes blood meal.  Onchocerca volvulus larvae enter body through bite and the larvae migrate to subcutaneous tissue. Life cycle continued below. Map Right: Ethiopia Source

Picture above: Life Cycle Source

Form nodules and develop into adult worms

–New worms = new nodules (2 to 3 worms per nodule)

–Males can travel amongst nodules to mate

After mating, the resultant eggs develop into microfilariae (which are the larvae form of the nematode)

Microfilariae leave female one by one, up to 1000 per day

Microfilariae migrate around subcutaneous tissue

–Their death causes characteristic rash/lesions/itching/depigmentation

–Migration to eye causes blindness

When host is bitten by another fly, microfilariae transferred to fly

Develop into larvae in fly…as described below

 Simulium 

 –Genus of black fly

 –O. volvulus vector

 –Associated with fast-flowing bodies of water

Picture: Prime example of where the black fly resides Source

In Africa, the vector species is Simulium damnosum

In the Americas, on the other hand, there are three other species identified as vectors.  Those species are as follows: Simulium metallicum, Simulium ochraceum, and Simulium exiguum.

Picture: Simulium damnosum Source

O. volvulus in Simulium

Microfilariae advance to thoracic muscles

–Differentiation in to L1 larvae within 28 hours            

First molt producing L2 larvae

–within 96 hours

Second molt producing infective L3

--Within 7 days

–Migration to head and mouth

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How big are these worms?

Picture: O. volvulus worms Source

• Adult females 45-50cm by 300 um

• Adult males 20-40mm by 200um

• Microfilariae 250um by 7um  

O. Volvulus Incubation Period

Microfilariae usually found about 1 year after bite

–In Guatemala, found in infants as young as 6 months of age    

    Map Source

–Over 100 million microfilariae found in people suffering the most severe cases

–Vector can be infectious 7 days after blood meal

–Can’t transfer person to person

O. volvulus Survival Rates in Host

–Adults can live for 8-10 years in nodules (onchocercomas*)

*concentric bands of fibrous tissue 

Picture: Microscopic view of onchocercomas Source

–Not directly harmful

–Microfilariae can survive for about 6 months

O. volvulus & Immune System

Adults in nodules; immune system has limited access       

–Surrounded by eosinophils and lymphocytes

Microfilariae induce inflammatory response              

–Especially after their death

–Wolbachia-derived antigens as a trigger (more on Wolbachia below)

Recent Research                                                            

Symbiotic relationship between Wolbachia bacteria and filarial nematodes (including O. volvulus).  Recent research has shown that Wolbachia might be essential to the fertility of these nematodes.

Picture: Black arrow is pointing to Wolbachia Source

The endotoxins released by the Wolbachia upon the death of the microfilariae are now suspected as the cause of most of the symptoms associated with onchocerciasis including blindness and corneal inflammation

Antibiotic therapy effective?  Researchers studying whether giving antibiotics to patients (and thus killing the Wolbachia) would relieve symptoms…

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