- Researchers at the French Institut Pasteur have managed to retrace the origin and evolution of the Chikungunya virus that is causing a major epidemic in the Indian Ocean. Viral strains isolated from Réunion and the Seychelles clearly demonstrated a small mutation from its East African origin while passing from Comoros to Réunion, aiding the virus to reproduce more aggressively in mosquitoes. Around 275,000 persons have so far been infected during the current Chikungunya outbreak.
The Chikungunya outbreak that started late last year on the French-administered tourist paradise Réunion caught authorities totally by surprise. Not only was the mosquito-borne disease alien to the Indian Ocean island, but the speed of its spread was also unheard of. The outbreak has caused 248,000 cases in Réunion since March 2005.
Now, researchers from the prestigious Pasteur Institute have shed light on the unprecedented Chikungunya epidemic. Indeed, the outbreak did not start on Réunion but on the Comoros archipelago in early 2005. The French-administered island of Mayotte belongs to this archipelago, and Mayotte is well connected with Réunion, the only other major French island in this part of the Indian Ocean.
Analysing the genome of six viral strains isolated from patients from Réunion and Seychelles - an island state close to Comoros that by now has registered 8,976 cases of Chikungunya - the Parisian scientists were able to trace the spread and "microevolution" of the virus. A partial sequencing of one viral protein was also made from 127 patient samples from Reunion, Seychelles, Madagascar, Mauritius (6,000 cases until now) and Mayotte (5,834 cases).
The study had shown that "the Indian Ocean viral strains are closely related to each other and are related to East, Central and South African strains isolated between 1952 and 2000," the researchers concluded yesterday. "The viruses that emerged in the Indian Ocean were therefore probably imported from the African continent." East Africa is also the region where the virus is most known, and the name Chikungunya origins in Tanzania.
"This scenario is compatible with the human population exchanges between East Africa and the Comoros where the epidemic started in early 2005," the Institute holds. "Moreover, changes that occurred in the viral genome as the epidemic progressed, especially the emergence and predominance of a particular genotype since September 2005, suggest an adaptive evolution of the viral strains."
Sequencing of the complete genome of a viral strain isolated from a patient in Réunion had revealed "several mutations causing amino acid substitutions specific to this clinical isolate." Studies are currently underway to establish whether these substitutions are linked to the neuro-virulence of the Chikungunya virus, on the one hand, and to greater efficiency of viral multiplication, on the other hand.
One of the "molecular signatures" of the virus, which constitute its genetic fingerprints, is not found early on in the epidemic, as the milder outbreak still was defined to the Comoros archipelago. However, this "signature" had become "predominant" in Réunion strains from September 2005, and therefore shortly before the epidemic explosion.
The authors of the study suggest that "this signature may confer an advantage and favour the multiplication of the virus" in the mosquito Aedes albopictus, which then infects humans. The mutated protein (E1) is in fact involved in attaching the virus to the mosquito's cell membranes, the researchers found.
The magnitude of the epidemic in the Indian Ocean and the description of new clinical forms of the disease has underlined "the critical lack of understanding of the physiology of the disease and the biology of the virus," the authors point out. The molecular data presented in the study was said to be "a first step to bridge this gap of knowledge and opens the way to functional studies." The study was supervised by Sylvain Brisse and Isabelle Schuffenecker.
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