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Island-hopping virus' ferocity exposed

May 23, 2006 By Helen Pearson This article courtesy of Nature News.

Joint-crippling disease threatens to spread across the globe.

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Scientists have found clues as to why a little-known virus is disabling hundreds of thousands of Indian Ocean island dwellers, in an outbreak that threatens to spread further around the world. It seems the virus has adopted a genetic change that may make it more efficient at invading the mosquitoes that carry it from person to person.

The chikungunya virus has infected around one-third of the population (about 250,000 people) on the French island of Réunion since early 2005. It has also hit the neighbouring islands of Mauritius, Seychelles, Madagascar and Mayotte, and reared its head in India.

The outbreak seems to be more severe than the sporadic occurrences of chikungunya that have hit Africa and Asia in the past: it has spread like wildfire, infecting some people's nervous systems and even killing a fraction of its victims. The fever strikes so fast that "you're driving your car and suddenly you have to stop", says Sylvain Brisse at the Institut Pasteur in Paris, France.

Genetic key

Now Brisse and his colleagues have revealed some key aspects of the virus' genetic makeup that might explain its newfound ferocity.

The team worked out the genetic sequence of six samples of chikungunya virus taken from patients throughout the current outbreak, and scrutinized one key coat protein from another 121 patients.

The sequences suggest that the virus is most similar to strains found in Africa, indicating it came from that continent. The analysis also revealed that the virus had acquired a change in the genetic sequence that alters the virus' coat protein during the course of this outbreak. This could allow the virus to more easily invade and multiply in mosquito cells, Brisse says, so that an insect spreads it more quickly from one person's blood to another.

The researchers also found three protein changes that were present in a virus that had successfully infected a patient's nervous system, but were missing from others. These differences might help the virus to attack humans more fiercely, they say, although it is unclear how.

It could take off and cause an epidemic.
Scott Weaver,
University of Texas, Galveston
It is also possible that the islanders are badly affected simply because they have not been exposed to the virus before, and so lack any natural immunity. "I don't think anyone knows why it exploded," says Stephen Higgs who studies chikungunya virus at the University of Texas Medical Branch, Galveston.

Brisse's work, reported in PLoS Medicine1, is part of a swathe of new research initiated by the French government in an effort to understand why and how chikungunya fever is spreading so fast.

Sweeping epidemic

Until now, chikungunya virus and its relatives were studied by only a handful of scientists. "There was the attitude that if it doesn't kill people it's not important," says Scott Weaver, who studies this family of viruses, also at the University of Texas.

But scientists warn there is a real risk that the virus will spread to susceptible populations in Europe or the Americas, perhaps carried by tourists in the early stage of infection. Some cases have already been reported in Europe. "It's probably just a matter of time," warns Weaver. "It could take off and cause an epidemic."

The first outbreak of chikungunya fever was reported in 1952. The name is Swahili for 'that which bends up', referring to the bent posture of those suffering with the disease. It is thought to cause sporadic outbreaks in Africa and Asia, but the cases have not been documented carefully before and some are probably misdiagnosed as dengue fever, says Weaver. There is no treatment, and only an experimental vaccine. Painkillers offer the only relief.

Chikungunya adds to a growing list of little-known viruses that have risen to attention over recent years, such as Ebola virus, Marburg virus and SARS, probably due to improved diagnosis and communication. "There are undoubtedly a huge number of viruses that infect people all the time that go unnoticed," Weaver says.

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  1. Schuffenecker I., et al. PLoS Medicine, 3. e263 (2006).


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