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The Zika virus has undergone "significant" genetic changes since it was first discovered in 1947, according to a study published Friday by U.S. and Chinese researchers that traced how a relatively unknown pathogen led to the current fast-spreading outbreak in the Americas.
The mosquito-borne virus had caused sporadic disease throughout Africa and Asia until the 2007 Micronesia and 2013 French Polynesia outbreaks.
Scientists previously believed that infection caused only mild disease but the latest epidemic has linked it to microcephaly and other birth defects.
New modes of transmission, including infection through sex and from mother to newborn, have also surfaced.
"Zika virus is changing all the paradigms of arbovirus biology -- no other arbovirus has been so infectious by alternative means, and no other arbovirus has been shown to cause significant birth defects," senior study author Genhong Cheng, a professor of the University of California, Los Angeles (UCLA), told Xinhua.
In the new study, Cheng's laboratory collaborated with researchers at the Chinese Academy of Medical Sciences and Peking Union Medical College in Beijing to compare individual genetic differences between more than 40 strains of Zika virus, 30 of which originated from humans, 10 from mosquitoes and one from monkeys.
"We were surprised to find a significant divergence between the Asian and African lineages, as well as between mosquito and human isolates," said Cheng, whose paper was published in the U.S. journal Cell Host & Microbe.
He said the current Zika virus strains are "mostly similar" to that from the mosquito identified in Malaysia in 1966, rather than the 1968 Nigerian strain, suggesting the current strains evolved from the Asian lineage.
The researchers also found all human strains identified in the current epidemic appear to be more closely related to the 2013 French Polynesia strain than the 2007 Micronesia strain.
This implied that the strains in the current outbreak may have come directly from French Polynesia, a conclusion similar to a Science paper published three weeks ago.
In addition, they found a key viral protein that varied the most between the Asian human strain and the African mosquito strain.
"We believe these changes may, at least partially, explain why the virus has demonstrated the capacity to spread exponentially in the human population in the Americas," said Cheng.
"These mutations could enable the virus to replicate more efficiently, invade new tissues that provide protective niches for viral propagation, or evade the immune system, leading to viral persistence."
Cheng and his colleagues' next step will be to analyze the viral strains causing the current epidemic, and look for potential targets for drug and vaccine development.
"We hope that our work provides a strong basis that will help the larger scientific community in accelerating Zika virus research," he said.
