I am back. I had
some business to take care of, and then of course there was another visit to spend time with my nephew and
fulfill my duties as a new aunt. But I am here and excited to
resume our conversation about Zika.
The first part of
the Zika and Wolbachia story morphed
into a biography of Wolbach, for whom Wolbachia
was named. I provided a teaser about recent research that suggests Wolbachia can reduce Zika transmission by mosquitoes. But I
didn’t actually talk about Zika at all, and even though it’s only been a short
time (~4 weeks) several new Zika stories have come out.
Lucky for us, scientists world-wide are ramping up their efforts to understand
Zika, and continue research on other diseases
like Ebola,
which remain important even when not in the spotlight (ideally congress would agree).
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| Aedes aegypti (Image credit: James Gathany, Center for Disease Control and Prevention Public Health Image Library) |
In 2009, a group of
17 scientists from Australia and Brazil published a study showing that when the
mosquito Aedes aegypti is infected with
particular strains of Wolbachia, the
mosquitoes were less competent disease vectors, and could reduce transmission
of diseases such as dengue, chikungunya,
and Plasmodium (malaria)
(Moreira et al. 2009).
Let's put this in perspective, 85 years after Wolbach and Hertig first
discovered Wolbachia in a mosquito in
Boston (Culex pipiens), and after
decades of Wolbachia research
resulting in hundreds of publications that have little to do with humans and
disease transmission, scientists discovered that Wolbachia could reduce the number of humans contracting deadly
viruses. If this is not a solid argument for exploratory research then go
fishing. Listen in, and we will see how it works. But first, there are several
pieces of information that will help you follow the story: 1) The three main
players are mosquitoes, Wolbachia bacteria,
and Drosophila fruit flies, 2) There
are over 3500 species of mosquitoes
and only a handful carry diseases (a.k.a disease vectors), 3) Some mosquito
species carry one disease, some carry many diseases, most carry no diseases, 4)
The main species of mosquito that carries dengue, chikungunya, and Zika is Aedes aegypti, which is not typically
infected with Wolbachia in the wild,
and 5) Wolbachia is a genus of
bacteria comprising many different types/strains that infect a variety of
different insects. The most well-studied strains of Wolbachia are from the model organism Drosophila melanogaster, a fruit fly we will inevitably
revisit in future posts.
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| Drosophila melanogaster: Illustration by Katy Wiedemann (Wiedemann Illustrations) |
Moving on. Methods
to reduce the number of humans contracting diseases carried by mosquitoes focus
on prevention, either through the development of vaccines or use of bug spray to
prevent mosquito bites. The other option is to try and get rid of mosquitoes.
Methods of reducing mosquito populations include spraying copious amounts of
DDT into
houses with growing children, and introducing copepods that eat mosquito
larvae.
Between 2000 and
2008, data from several labs coalesced on a different kind of
preventive method. By infecting A.
aegypti mosquitoes with certain Wolbachia
strains isolated from the fruit fly D.
melanogaster, the life span of A.
aegypti was reduced. From the perspective of limiting disease transmission,
this is good because many mosquitoes die before they can transmit disease. At
the same time, another group of scientists studying Wolbachia in D. melanogaster
found that certain strains were beneficial to D. melanogaster individuals that had harmful insect viruses
such as Drosophila C virus, Flock
House virus, and Cricket paralysis virus.
In 2009, Moreira and
colleagues put these two things together and hypothesized that Wolbachia infected mosquitoes would be
less effective disease vectors. The idea was to test whether A. aegypti mosquitoes infected with Wolbachia from D. melanogaster fruit flies would reduce disease transmission. The
answer was yes. The authors showed that A.
aegypti with Wolbachia are resistant to several diseases, which prevents the mosquito from being able to
pass the diseases onto the next human they bite. The mechanism for how this
works remains unclear. The authors suggest it has something to do with how the
mosquito immune system reacts to the Wolbachia
bacteria, but no direct link was found. The other possibility is that there is
competition for resources between the disease and Wolbachia in the cells of the mosquito, and Wolbachia outcompetes the disease. But no conclusive evidence has
been presented to support either hypothesis. Regardless of the mechanism, it is clear that Wolbachia infected mosquitoes offer a two-pronged approach for
controlling disease transmission: they have a shorter life-span and they show resistance to several different diseases. As a result, non-profit groups such as
Eliminate Dengue Program are using this
approach to slow the spread of dengue (and other diseases) in multiple
locations around the world.
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| Eliminate Dengue Program |
That was the status
of things before the spread of Zika skyrocketed last year,
and was declared a Public Heath Emergency of International Concern (PHEIC) on February 1st, 2016. Scientists
were quick to react and on June 8th, 2016, six Brazilian researchers published the results of a study demonstrating the effectiveness of Wolbachia to block two Zika
strains currently being transmitted by A.
aegypti mosquitoes in Brazil (Dutra et al. 2016).
The release of Wolbachia infected
mosquito eggs and adults is currently underway in Brazil
and scientists are optimistic about the spread of the Wolbachia in Brazilian A.
aegypti populations. All of this is very recent but at present it appears to be good news for humans, science, and exploratory research.
Exactly how Wolbachia spreads in insect
populations is an intriguing topic we will investigate next time. It is based
on decades of exploratory research covering many different Wolbachia strains in many different types of insects.
