Now a new study published this week in PLoS Medicine, an open access online journal (http://medicine.plosjournals.org), sheds light on the reason why the disease has spread suddenly and rapidly. The research provides evidence that the chikungunya virus has mutated in a way that makes it better at infecting the mosquitoes that spread the virus to people.
The French team leading the research identified the entire genetic sequence of virus samples from six patients in La Réunion and the Seychelles, as well sequencing a viral gene called E1 from samples taken from an additional 127 patients in Madagascar, Mauritius, Mayotte, Réunion and the Seychelles.
The analyses indicate that the Indian Ocean outbreak was caused by the same strain on La Réunion, Seychelles, Mayotte, Madagascar, and Mauritius islands, and that the outbreak strain is related to East-, Central-, and South-African ones. However as time went on the virus then developed into several distinct variants.
The now dominant strain differs genetically from those involved in earlier outbreaks and this, say the researchers, could explain why the virus has become more aggressive. In particular, they say, two changes to the structure of E1 could make the virus more likely to enter mosquito cells and replicate after the insect has fed on the blood of an infected person.
The researchers believe that that virus may no longer need cholesterol (which viruses normally need to infect the cells of their human and mosquito hosts). Because mosquitoes often do not have enough cholesterol for viruses to efficiently affect their cells, it is possible that the more recent version of the chikungunya virus could have survived and multiplied better in mosquitoes, which in turn could have contributed to its rapid spread.
Viruses have only a small amount of genetic material, and this material keeps changing rapidly (scientists call these changes mutations). The resulting changes in viruses' genetic sequence over relatively short times make it possible to distinguish different strains of the virus. Scientists had previously determined the entire genetic sequence of two chikungunya virus strains, one isolated from the first described outbreak in Tanzania, and the second one from an outbreak in 1983 in Senegal.
The scientists who conducted this new study say that the results will, on one hand, explain how the current outbreak started and why it affects many more people than previous ones, and on the other hand provide a picture of how the virus is changing over the course of the outbreak. In the absence of efficient vaccine or antiviral medicine, mosquito control is at present the only way to limit chikungunya transmission. The discoveries made by this study should contribute to bridge the gap of knowledge concerning this disease and will help to provide more specific and powerful tools to combat it.
By Nirmal Shah, Nature Seychelles' CEO