By Jeffrey R. Powell
The mosquito Aedes aegypti rose to global attention around 1900 when it was shown to be the vector of yellow fever, a viral disease that was ravag- ing the New World. After World War II, a partly successful program was
mounted to eliminate this invader from
the New World through the use of DDT.
By the late 1960s, however, the urgency
for eliminating Ae. aegypti receded after
the widespread use of an effective yellow
fever vaccine. Eradication efforts were suspended, and the mosquito reestablished
itself in its previous, or even a greater,
range. The mosquito continues to be a substantial public health threat that requires
urgent attention, particularly given recent
evidence for hybridization of previously
We cannot say that we were not warned.
In 1970, Fred Lowe Soper wrote that Ae.
Ae. aegypti is permitted to remain” (1).
aegypti “should be studied as a long-term
national, regional, and world problem
rather than as a temporary local threat to
the communities suffering at any given mo-
ment from yellow fever, dengue or other
aegypti-borne disease. No one can foresee
the extent of the future threat of Aedes ae-
gypti to mankind as a vector of known virus
diseases, and none can foretell what other
virus diseases may yet affect regions where
These words turned out to be prescient.
In the 1980s, the dengue fever virus, also
transmitted by Ae. aegypti, began to cause
increasingly severe epidemics wherever
the mosquito bred. A third Ae. aegypti–
borne virus was also rising in importance,
especially in Asia and Indian Ocean islands: the virus causing chikungunya fever. Chikungunya virus has caused disease
outbreaks in Europe and was first reported
in the New World in 2013 (2). More recently, the Zika virus, also transmitted by
Ae. aegypti, has risen to considerable noto-riety worldwide. In the past 9 months, yellow fever has reemerged in Africa (3). The
geographic range of human populations at
risk for these diseases coincides with the
geographic distribution of the mosquito.
With the exception of yellow fever, no
aegypti-borne diseases have a proven effective vaccine, nor are there medications
that specifically treat human infections.
Control efforts to limit these diseases thus
focus on the mosquito.
Ae. aegypti is a native of Africa, where its
Mosquitoes on the move
ancestral form can still be found. This an-
cestor, called Ae. aegypti formosus (Aaf), is
a black mosquito (see the photo, left) found
in forests and ecotones in sub-Saharan
Africa. It breeds in tree holes and prefers
nonhuman animals for blood meals. A “do-
mesticated” form evolved from this sylvan
ancestor, possibly as a response to the ex-
pansion of the Sahara desert between 4000
and 6000 years ago. This form, closely asso-
ciated with and spread by humans, is called
Ae. aegypti aegypti (Aaa). It is a brownish
mosquito (see the photo, right) with white
scaling on its abdomen, found throughout
the tropics and subtropics outside Africa,
that breeds in human-generated containers
and prefers humans for blood meals.
Because of their very distinctive habitats
and behaviors, the two forms or subspecies
have historically presented different health
risks. Aaf was relatively benign, being found
in forests with low human densities and
preferring nonhumans for blood meals.
Human-loving Aaa has been responsible
for most of the human disease transmitted
by this species. The distribution of the two
types is thus of considerable importance.
Several recent observations have upset
the simple picture of Aaf residing in sub-
Saharan Africa and Aaa residing outside of
Africa (see the figure). In sub-Saharan Af-
rica, mosquitoes that would be considered
Aaf based on morphology and genetics are
now breeding in human habitats, includ-
ing cities like Dakar, Yaounde, Luanda, and
Libreville (4–6). This is likely a response
to expanding urban centers in Africa that
encroach on Aaf’s native forests. The larval
breeding habitat has shifted from tree holes
to discarded items like tires, indicating that
females have changed their egg-laying be-
havior. Outbreaks of dengue in African cit-
Spread and hybridization of Aedes aegypti mosquitoes raise
the risk of Zika, dengue, and other viral epidemics
Yale University, Ne w Haven, CT 06520, USA.
Aedes aegypti females of the African
subspecies formosus (left) and the
cosmopolitan subspecies aegypti (right).