that typically have high morbidity but low mortality and affect the poorest sections of
society, mainly in tropical countries. Consequently, they have low visibility compared
with diseases such as HIV/AIDS, malaria, or tuberculosis, and, thus, they do not
attract the attention of most governments and almost all large foundations that claim
to focus on disease in the global South. Few treatments are available for them, and
efforts at control are often sporadic and ineffective.
The Aedes mosquitoes that spread Zika also spread two better-known neglected
tropical diseases: dengue and chikungunya. They also spread yellow fever, which
periodically decimated populations throughout the southern U.S. until the end
of the nineteenth century (Crosby 2007). But yellow fever never established itself
permanently in North America, and by the early twentieth century, it was restricted
to parts of Africa and Latin America. By and large, this control was achieved
through active vaccination programs. Most countries continue to require vaccination
certification as a condition of admission for travelers from regions in which yellow
fever still exists. Vector control was never properly achieved, but, because of its
containment, yellow fever has largely disappeared from public attention.
However, no such excuse can be offered for the neglect of dengue, which has been
the most rapidly spreading arthropod-borne disease for the past decade (Gardner
and Sarkar 2013). Dengue, chikungunya, and Zika are all caused by closely related
flaviviruses; in fact, the first two are so closely related that they are believed to
have switched names during the last century (Halstead 2015). This means that, if
appropriate resources had been deployed for the control of dengue and chikungunya,
we would have been much better prepared to face the public health crisis posed by
Zika. There is no specific treatment for either dengue or chikungunya. Had these
existed, they may have provided a basis for Zika treatment. There is no vaccine for
chikungunya; the first vaccine for dengue has just been licensed after decades of
development (Pitisuttithum and Bouckenooghem 2016). There have been many
recent promises of rapid vaccine development for Zika, but the stories of dengue and
chikungunya give ample reason for caution.
Most importantly, effective vector control regimes for Aedes mosquitoes have not
been devised. Traditional methods of vector control consist of the use of insecticides
and larvicides, and destruction of larval breeding sites. Insecticides and larvicides,
even when they have been systematically used (for instance, in Brazil), have been
used at such low volumes that they have been ineffective (Yakob and Walker 2016).
Moreover, widespread insecticide resistance (Lima et al. 2011), similar to what was
seen in attempts at malaria vector control, befuddles this strategy. It is probably
impractical to attempt to destroy all breeding sites (pools or even containers of
water) at a regional level in wet tropical or subtropical regions. As was noted earlier,
insecticides may also decimate populations of species that are critical to ecosystem
function, and there is no known technological fix to this problem.
Consequently, in recent years,
attention has shifted to the use of three
novel approaches: the release of insects
carrying dominant lethal genes (Phuc
et al. 2007); Wolbachia infection, which
works for at least Ae. aegypti (
Iturbe-Ormaetxe et al. 2011); and gene drives
(Adelman and Zu 2016). The first two
of these methods have been available
for several years, and because of the
continuing spread of dengue, have
even been field-tested (Carvalho et al.
2015). Yet, because of lack of attention
to NTDs, they have not been readied
to the extent required to respond
to the Zika crisis. Had dengue and
chikungunya not been neglected, we
would have been better prepared to
deploy these techniques.
The third approach, gene drives,
constitutes the most important
technological advance in molecular
biology since the advent of the
polymerase chain reaction in 1983.
They allow an introduced gene to
be propagated through a population
at a much faster rate than ordinary
inheritance based on Mendel’s rules.
If this gene causes carriers to become
sterile, the population is driven to
extinction. If, through migration
or other factors, the gene also is
introduced in other populations of the
species, they also become extinct. In
many cases, the species would itself be
driven to extinction.
Should we do this? For a species such
as Ae. aegypti, forced extinction through
the introduction of a gene that turns
all carriers male is a realistic possibility