O El Niño mais severo de 2015,
provocado provavelmente pelas mudanças climáticas, ajudou na rápida infestação
do vírus da Zika na América do Sul. O estudo da Universidade de Liverpool
analisou a proliferação dos mosquitos Aedes aegypti e Aedes albopictus, seus
principais transmissores, em anos mais quentes e mais frios. O verão de 2015
ofereceu as condições ideais para que a distribuição do vírus ocorresse,
principalmente no Brasil e na Colômbia. Por conta das mudanças climáticas,
novas doenças do tipo deverão aparecer com mais frequência.
A female Aedes aegypti
mosquito in the process of acquiring blood from a human host. (James
Gathany/Centers for Disease Control and Prevention via AP)
In a world characterized by rising temperatures,
deforestation and other human influences on the environment, the spread of
infectious disease is a hot topic. Many recent studies suggest that
environmental changes can affect the transmission of everything from malaria to
the Zika virus — and it’s increasingly important to understand these links,
scientists say.
This week, a new study has provided
new evidence that environmental changes can increase the threat of
disease. It concludes that unusually warm temperatures caused by 2015’s severe
El Niño event — probably compounded by ongoing climate change — may have
aided in the rapid spread of the Zika virus in South America that year. And
while there are many complex factors at play in the spread of mosquito-borne
diseases, the study may help scientists better prepare for the kinds of future
effects we might see in our warming world.
“The start of the mission was simple — trying to
address where the risk will be, where is it going to move next, where could
Zika happen on the planet on a global scale,” said Cyril Caminade, a research fellow at the University of Liverpool and
the new study’s lead author. To that end, the authors designed a study that
would help them determine how climatic changes have impacted the mosquito-borne
transmission of Zika.
There are two main species of mosquito known to carry
the Zika virus — Aedes aegypti, or the yellow fever mosquito, which is
widespread in the tropics; and Aedes albopictus, or the Asian tiger mosquito,
which lives in both tropical and temperate regions of the world. Scientists
also believe Zika can be sexually transmitted, but the new study focused only
on mosquito transmission.
For the study, the researchers collected published
information on the distribution of these two mosquito species and how
temperature variations can affect them. Studies suggest, for instance, that up
to a certain point, rising temperatures can cause mosquitoes to bite more
frequently. The researchers also collected global historical climate data from
the past few decades and used all the information to build a model of Zika
transmission worldwide.
The model produced an unusually high disease
transmission potential in the tropics for the year 2015, including in Colombia
and Brazil, the countries hit hardest by Zika. Similar results occurred between
1997 and 1998, one of the only other times on record to experience such a
brutal El Niño event.
“[O]ur model indicates that the 2015 El Niño event,
superimposed on the long-term global warming trend, has had an important
amplification effect,” the researchers note in the paper.
The model also helped the researchers identify the
ideal seasonal climate conditions for Zika transmission around the world. In
South America, for instance, the model suggests that the potential for
transmission should peak in the winter and spring.
In the southeastern U.S., on the other hand, summer is
ideal. In fact, the model suggests this region has a high potential for disease
transmission during this time, due partly to the high temperatures and partly
to the fact that both mosquito species are found there.
That said, reports of Zika have been limited in the
U.S. so far — and this speaks to the complexity of vector-borne disease
transmission, Caminade said. Climate can certainly play a significant role in
setting up the right conditions for an outbreak, but epidemics also depend on
many other factors, including population density, access to healthcare and the
use of pesticides and other anti-mosquito interventions in any given location.
Some of these factors — which were not accounted for in the new study — can
probably explain why there hasn’t been much Zika transmission in the U.S. so
far.
Caminade also pointed out that after a population has
been exposed to a mosquito-borne disease like Zika, a phenomenon called “herd
immunity” often occurs — this happens when so many people have already been
exposed, and developed an immunity, that there aren’t enough new people left to
infect to continue the epidemic. This is the probably part of the reason we
didn’t see Zika epidemics in other tropical parts of the world in 2015, despite
the new study’s results. And some experts have suggested that herd immunity
will likely cause the current situation in South America to burn itself out
within a few more years.
But Caminade cautions that there’s still the potential
for Zika outbreaks in other parts of the world where the conditions are right,
including the United States and even southern Europe. The result would likely
be milder than what’s been experienced in South America in the past year, but
there’s “still risk,” he said.
According to Caminade, one of the study’s major
takeaway points is that extreme climate conditions can lead to all kinds of
unusual events — droughts, floods and wildfires are only a few examples — and
disease outbreak is just one more potential disaster scientists should be
looking out for when these conditions occur.
And such events may only be exacerbated by future
climate change. Some studies have suggested that the kinds of “monster” El Niño
events seen in 1997 and 2015 may be more likely in a warmer world, Caminade
pointed out. But he added that scientists wishing to make more precise
predictions about the future spread of disease must take a wide variety of
factors into account — climate is just one of them.
“I won’t overplay the role of climate for the future,”
he said. “It’s still a disease, and there are still parameters which are going
to be very important.”
By Chelsea Harvey, The Washington Post
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