For decades, people have blamed a parasitic nematode worm for a disease that has blinded at least 250,000 people now living in Africa and South America. But the real culprit may be the ubiquitous Wolbachia, bacteria that colonize many hundreds of species, including the worm indicted in river blindness. Researchers now report that Wolbachia stimulate the severe immune system response that slowly robs people of their vision.
River blindness begins with repeated bites from black flies that are common along rivers and streams in tropical areas. The insects transmit nematode larvae that settle under the skin, mature, and produce millions of young larvae. Those of the species Onchocerca volvulus travel through the skin to the eyes, where they remain in the larval stage and die after about a year. A victim of the disease can have hundreds of worms wiggling in the eye. Parasitologists have long assumed that the nematodes cause the inflammation that damages the eyes and cornea, probably by releasing proteins when they die that spark an immune reaction.
Although researchers have known for some 30 years that Wolbachia live inside the worms, they paid the bugs little attention. To determine whether Wolbachia play a role in river blindness, a team led by Eric Pearlman, an immunologist at Case Western Reserve University in Cleveland, Ohio, obtained extracts of worms taken either from untreated patients or those who had received antibiotics. In the latter group, the antibiotic had killed most of the Wolbachia, leaving a solution of worm proteins. When the researchers injected the extracts into the eyes of mice, they found that the worm-plus-Wolbachia extract caused much more damage, judging by how hazy the eyes became, than worm proteins alone. They also tested extracts from two other nematodes, one that doesn't carry Wolbachia and one that does. Only the latter clouded the mice's eyes, the team reports in the 8 March issue of Science.
The work "sheds a different light on the pathology of this disease," comments Jan Bradley, a parasitologist at the University of Nottingham, United Kingdom. The study raises the possibility that treating patients with antibiotics could block the inflammatory response, says Thomas Nutman, a parasitologist at the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland. Still, Nutman cautions that nematode proteins may also play a role--in which case antibiotics wouldn't be the perfect cure.