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 Early-stage laboratory findings by researchers in Britain France and
Thailand suggest Zika uses the body's own defenses as a "Trojan
horse", allowing it to enter a human cell undetected. Once inside
the cell, it replicates rapidly.
The scientists said their results, published in the journal Nature
Immunology, suggested that some dengue antibodies can recognize and
bind to Zika due to the similarities between the two viruses, but
that these antibodies may also amplify Zika infection in a
phenomenon called antibody-dependent enhancement.
This effect is already known with dengue, they said, and is thought
to explain why, when a person gets dengue fever a second time, the
infection is often more serious than the first.
"Although this work is at a very early stage, it suggests previous
exposure to dengue virus may enhance Zika infection," said Gavin
Screaton, a professor at Britain's Imperial College who led the
research.
"This may be why the current outbreak has been so severe, and why it
has been in areas where dengue is prevalent."
Dengue infections have increased dramatically over recent decades.
The virus causes around 390 million infections a year globally -
with 40 percent of the world's population living in areas of risk.
Dengue is common in Brazil, and the health ministry there reported a
leap in cases this year.
Zika is spread by the same mosquitoes and has been causing alarm
throughout the Americas since cases of the birth defect microcephaly
were reported in Brazil, the country hardest hit by the current
outbreak.
The rare birth defect is marked by unusually small head size and
potentially severe developmental problems. Brazilian authorities in
Brazil have confirmed more than 1,400 cases of microcephaly in
babies whose mothers were exposed to Zika during pregnancy.
People from all over the world are already beginning to converge on
Brazil for the Olympics in Rio in August.
INVADERS
Antibodies are large proteins that latch onto invading bacteria or
viruses, neutralizing them and enabling the immune system to destroy
the pathogens. The antibodies are then primed to recognize the same
invaders should another attack occur.
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The studies found that existing dengue virus antibodies latch onto
Zika when it invades. But because the two viruses are not exactly
the same, they cannot latch on securely, and instead the antibodies
actually help Zika to get into the human immune cells. Here, the
Zika virus replicates and causes disease.
Jeremy Farrar, an infectious disease specialist and director of the
Wellcome Trust global health charity which part-funded the research,
said the results offered potential clues about the current outbreak
and about how to make progress on vaccines.
"We know that Zika has been present in Southeast Asia and Africa for
many years and yet has not taken off there as it has in South
America. This is what the international research effort needs to
work out, and quickly," he said.
In a second study by the same team and published in a sister
journal, Nature, the scientists found that one of several antibodies
that work against dengue can also neutralize Zika - providing a
potential target for a vaccine.
"We now need further studies to confirm these findings, and to
progress towards a vaccine," Screaton said.
(Reporting by Kate Kelland; editing by Andrew Roche)
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