In 2015, Brazil began experiencing a major public health crisis: a large
number of babies were being born of microcephaly, a birth defect that results
in abnormally small head and underdeveloped brain. At the same time,
researchers discovered that Zika virus was spreading around the population. Zika
virus is spread by mosquitoes, and can cause fever-like symptoms. Researchers
began to wonder: could Zika virus also be responsible for the uptick in
microcephaly? You may have heard the phrase "correlation does not equal
causation." This is a problem that plagued epidemiologists when they were trying to
figure out what the symptoms of Zika actually were. Zika and microcephaly
were occurring at the same time and in the same place, meaning that they were
"correlated". But it is possible that microcephaly was being caused by
something completely unrelated to Zika-- in fact, during the outbreak in Brazil,
some people believed that chemicals, not a virus, was causing the uptick in
microcephaly. In order to properly understand whether Zika was causing
microcephaly epidemiologists engaged a variety of methods they had to rule out
other causes, conduct "case controls" comparing microcephaly within Zika
affected areas to non affected areas, and monitor Zika-infected women during
pregnancy to determine if and when microcephaly occurred during development.
epidemiologists also tested the disease on animal models and analyzed the
molecular mechanisms of the virus. In 2016 the World Health Organization
confirmed that there was scientific consensus that Zika causes microcephaly.
In addition to understanding the symptoms of a disease, epidemiology
involves tracing the origin and route of a disease to determine its evolution.
How is it that microcephaly came to be a symptom of Zika when it wasn't before?
The Zika virus originated in the Zika forest of Uganda where it started out as
a relatively mild disease. Its symptoms were fever, rash, headache, and achiness.
Researchers discovered that it was spread via the Aedes mosquito, and that it
could infect both humans and monkeys. Further research in Zika transmission in
humans showed that it could be sexually transmitted as well. From the 1951 to
2006 Zika, was identified fourteen separate times throughout Africa and
South Asia. Until 2007, however, there were no large-scale outbreaks. At this point,
there was an outbreak on the island of Yap.
The CDC conducted studies and collected samples, and eventually determined that
almost 75% of the population had Zika. A second epidemic occurred in 2013 in
French Polynesia. At this point, epidemiologists noticed a new symptom
associated with Zika: Guilin Barre symptom, an autoimmune disease that can
cause temporary paralysis. By 2014 zika had arrived in brazil probably by plane.
But microcephaly had never been a symptom of Zika before, and people
outside of South America who contracted Zika were not experiencing the same
abnormally high rates of microcephaly. Epidemiologists developed a hypothesis
that there are two main strains of Zika virus. An "African" lineage and an "Asian"
lineage. At some point, the "Asian" lineage split off from the "African" lineage and
acquired the ability to cause microcephaly through a mutation to the
virus's genome. Therefore, only populations infected with the "Asian"
lineage of Zika would experience microcephaly as a symptom of Zika while
those infected but the "African" lineage would not. Microcephaly is a public
health crisis, once epidemiologists confirmed that microcephaly is a symptom
of Zika, the epidemic was declared an international emergency in 2015 leading
to the influx of funds to help fight the disease. Because Zika only causes
microcephaly in unborn infants, it became most important to protect pregnant
mothers from being infected with the virus. In 2015, Brazil urged the
population to avoid pregnancy, and many countries such as the United States
advised their citizens not to travel to zika-infested countries while pregnant.
Because Zika is spread by mosquitoes, mosquito eradication efforts were also
effective mechanisms of reducing the number of infected individuals
Additionally, various vaccines have been in development since the crisis first
broke out. Hopefully, learning from the Zika crisis, we will be more prepared for
not only the next Zika epidemic but also any emerging disease.