Friday, 21 January 2011

Onchocerca volvulus and Wolbachia

In my book, Parasites: Tales of Humanity’s Most Unwelcome Guests, I discuss the efforts to treat people for river blindness - the difficulty of treating enough people for long enough to eradicate the disease. I also explore the fascinating relationship between the worm Onchocerca volvulus, and a genus of bacteria, Wolbachia. Wolbachia lives literally inside the cells of the worm, even within the embryos.

[caption id="attachment_231" align="alignleft" width="300" caption="Wolbachia inside a cell, Creative Commons Attribution 2.5 Generic"][/caption]

We know that O. volvulus can’t live without Wolbachia - that if you kill the bacteria with antibiotics, the worms die as well. Obviously Wolbachia does something for O. volvulus that it can’t do for itself. But what? This seems counterintuitive to our ideas of germs: a bacterial infection you can’t live without? But it is not so foreign really: even humans have bacteria living in their intestines that help to digest food and provide nutrients, and protect us from infection caused by less friendly species. Perhaps Wolbachia produces some vital nutrient for the worm that the worm can’t produce alone.

But here’s where the relationship gets more complex and more fascinating. Research indicates that the symptoms of river blindness are actually caused by the response of the human immune system to Wolbachia, not O. volvulus. While most of the bacteria are inside the worm, and therefore protected from the immune system’s attack, enough are exposed to keep the attack going, causing long term damage to host tissues but never wiping out the bacteria.

New evidence reveals that, meanwhile, our own immune cells targeted at Wolbachia shield the worm from the immune system like an invisibility cloak. The immune system doesn’t see the worm for the bacteria. So in essence, this is Wolbachia’s game: it uses Onchocerca to evade our immune defenses and causes river blindness. What we have here is not a horrible worm that uses human bodies and bacteria to provide its every need while unleashing dreadful disease on millions. What we have is a bacterium that uses a worm like a fortress to protect it while IT causes dreadful disease. It looks like the worm might be innocent.

University of Liverpool “Study sheds new light on river blindness parasite” January 12, 2011

Welsh, Jennifer. “River Blindness Parasite Relies on Bacteria to Fool Host” LiveScience Jan 19, 2010

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