The infection has prompted
the World Health Organization to declare a global health emergency due to the
link to thousands of suspected cases of babies born with small brains - or
microcephaly - in Brazil.
But there are still many,
crucial, unanswered questions.
How many people
have been infected in the Americas?
The best estimate of Zika infections
is between 500,000 and 1.5 million - which is quite a wide margin of error.
What percentage of people in an affected area are getting infected? Is it
everyone? We don't know.
Why the explosive
outbreak?
One theory is that the virus has
mutated to become more infectious. Alternatively, some experts argue it could
simply be a case of the virus reaching areas where people are densely packed
together and there are huge numbers of mosquitoes.
Who is infectious?
Around 80% of people have no symptoms
when they get the virus - although this figure needs further investigation.
It's not known if they can also spread the virus or even why they are
asymptomatic.
Does it cause
microcephaly?
It's the biggest health concern in
the outbreak, yet the link with Zika and birth defects is still only
"strongly suspected". Parts of Brazil that have seen cases of Zika
have, several months later, also seen a surge in microcephaly. However, the
trials to prove the link have not finished.
How risky is
infection?
If the virus does cause microcephaly,
how often does this happen? Does every infection lead to birth defects? Or is
it one-in-100? One-in-10,000 perhaps? At the moment it's not clear how worried
pregnant women should be.
Is there a risky
period in pregnancy?
If the virus causes microcephaly,
does it matter when you are infected? There have been some suggestions that the
first trimester (the first 12 weeks) is key, but other doctors have hinted
there might be risks as late as 29 weeks. And those risks could change over
time.
How could it damage
the brain?
Some infections, such as rubella, can
damage the brains of developing babies during pregnancy. But it is not known
how Zika could be crossing the placenta and damaging brain growth.
Does developing
symptoms change the risk?
Around four-in-five people infected
will not develop symptoms. Do silent infections carry the same risks of
microcephaly as those which result in a fever or a rash? There is also the rare
neurological disease Guillain-Barre syndrome that has been linked to Zika
infection and we don't know which patients are most a risk.
What's going on in
Africa and Asia?
The virus was first detected in
Africa and then parts of Asia until it reached Brazil and then spread. So do
these continents represent giant vulnerable populations susceptible to Zika
outbreaks? Or has Zika been around and undetected there for years, so that most
of the population are immune? It is hard to establish the global threat without
knowing the answer.
How big is the
surge in microcephaly really?
There are big questions about the
quality of the data, both before the outbreak of Zika and now. The figures for
previous years may be underestimates, and the number of suspected cases is an
overestimate. Of the 4,783 reported cases of microcephaly - 404 have been
confirmed, 709 have been disproved and 3,670 are still being investigated.
Can it be spread by
other mosquitoes?
The Zika virus is
spread by the Aedes aegypti mosquito, but it
is confined to tropical and sub-tropical countries. There is concern that Zika
could be spread by other mosquitoes too such as the Asian tiger mosquito. This
species prefers more temperate climes such as parts of Europe.
How big a risk is
sex?
It seems likely
that the overwhelming majority of cases are spread by the Aedesmosquito where the insect bites a person with the
virus and passes it onto the next person it bites. But sexual transmission has
been implicated in a couple of infections. It's unknown how common this is.
What about
immunity?
Are you infected once and then
protected for life, like measles? Or does it take multiple infections to
achieve immunity? How long does immunity last? These answers will tell us how
long the outbreak could last and indicate whether a vaccine would be effective.
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