Why it can be useful to attract the enemy: leading mosquitoes around by the nose.

Full text

A little more than a hun-
dred years have passed
since mosquitoes were
found not just to be an
irritating pest – they can
actually be deadly. In India
in 1897, a British army
doctor, Ronald Ross, con-
firmed the hypothesis that
malaria was borne by the
Anopheles mosquito. Three
years later, in October
1900, a working group led
by his American colleague,
Walter Reed, confirmed
the role played by another
mosquito species – Aedes
aegypti (fig. 1), in trans-
mitting the yellow fever
pathogen.
But things did not stop
there: as time went by
mosquitoes turned out to
be vectors of a broad spec-
trum of diseases. Tab. 1
shows the most important
mosquito-borne diseases
worldwide. Fig. 2 shows
the approximate distributi-
on of a selection of viruses
transmitted to humans by
mosquitoes. For some of
the pathogens transmitted
by mosquitoes, such as
malaria or dengue fever,
we have still yet to find an
effective vaccine or
method for warding off
disease. All too often, the
only things standing in the
way of widespread vacci-
nation programs are
poverty and over-populati-
on. Sudden outbreaks can
even catch well-prepared
health programs off guard.
Sometimes mosquitoes
wreak havoc in places
where people otherwise
only consider them to be
an irritating pest, as was
the case in the US when it
was struck by West Nile
fever. This particular
disease is also transmitted
by mosquitoes, and is
most common in birds.
But by changing its host it
can also transfer itself to
Why it can be useful
to attract the enemy:
leading mosquitoes
around by the nose
64
Mosquito Control
Mosquitoes as disease vectors
Fig. 1: Aedes aegypti, vector of yellow fever and dengue.
(Source: E A Goeldi (1905) Os Mosquitoes no Pará.)
Fig. 2: The distribution of the main mosquito-borne viral diseases
affecting man. DEN:Dengue Fever, EEE: Eastern Equine Encepha-
litis, JE:Japanese Encephalitis, LAC:La Crosse Encephalitis, MVE:
Murray Valley Encephalitis, SLE: St Louis Encephalitis,VEE: Vene-
zuelan Equine Encephalitis,WEE: Western Equine Encephalitis,
WNV:West Nile Virus, YF:Yellow Fever. Derived from various
sources

people. Once considered only an
„old-world“ virus, West Nile first
appeared in America in 1999, when
it was discovered in the corpses of
dead birds. But soon it claimed its
first human victims. In its first year,
the epidemic boasted 62 seriously
ill and 7 deaths. In the following 2
years, serious infections numbered
21 and 66 respectively, there were
2 deaths and then 9. In 2002 the
number of reported cases rose to
4156, with 284 deaths. By 2003 as
many as 9858 instances of the
disease were counted in 40 States,
with 262 deaths. (Source: Centers
for Disease Control and Preventi-
on, USA, www.cdc.gov.)
Whether a mosquito has the po-
tential to transmit disease, and
which diseases it can transmit,
depend primarily on its preferred
host. Some mosquito species do
not really care who their host is,
they tap into the first vertebrate
they can find. These are the ones
most likely to transfer diseases
from one type of host to another,
as was the case with the West Nile
fever virus. We face more serious
problems with the mosquito spe-
cies that prefer humans or will only
seek out humans for a tasty top-up
of blood – even if they could
actually turn to other hosts. It is
mainly this selective type of beha-
viour that causes the rapid prolife-
ration of epidemics. Some of the
best known cases of this were with
Anopheles gambiae, the mosquito
transmitting malaria, or Aedes aegypti,
the transmitter of yellow fever and
dengue; despite intensive efforts to
treat and eradicate these diseases,
infection levels continue to rise.
Worldwide, more than 500 million
people are infected every year by
diseases passed on by mosquitoes.
The damage this inflicts on coun-
tries’ economies is put in the tens
of billions.
There are a number of ways to wage
wholesale war on mosquitoes. The
most common technique is to use
chemical and biological insecticides,
or destroy the waters in which they
lay their eggs. But the ecological
side-effects, the often astronomical
costs involved and the danger of the
mosquito developing insecticide
resistance make it increasingly
essential for us to simply become
more efficient at what we do.
A key issue in combating mosquito-
borne disease is therefore how
quickly we identify rapidly devel-
oping mosquito hot-spots. Pockets of
dangerous mosquitoes can be moni-
tored in a number of ways. One is to
examine the biotopes and waters
populated by larvae, pupae and eggs
(fig. 3). One drawback with this
method is that you already have to
know where the breeding grounds
are (or look for them), and the ana-
lysis requires specialist knowledge.
Another approach is to try and catch
adult mosquitoes. But using cap-
turing devices in large areas is costly
and findings are not representative:
so far, you can only use generic baits
such as lights and carbon dioxide.
For this reason, to gather sufficiently
representative samples of the
overall mosquito population for
health assessment reasons, scientists
often still turn to volunteers –
human guinea-pigs, who catch
approaching mosquitoes and count
them. The ethics of this technique
are somewhat dubious due to the
risk of infection. It is also unreliable
(mosquito catcher motivation varies
widely). And it is expensive. Much
better solutions would be to find
simple, inexpensive, highly targeted
traps for adult mosquitoes which are
as appealing as human hosts or the
natural breeding grounds. Another
significant advantage associated with
the capturing of adult mosquitoes is
that these can then be examined
using molecular biological testing to
ascertain whether they are already
infected with pathogens.
Scientists at the Institute of Zool-
ogy at the University of Regensburg
(Germany) have now joined forces
with the University of Minas
Gerais in Belo Horizonte (Brazil)
to pool research findings and
develop new types of monitoring
systems. The aim is to fight diseases-
transmitting mosquitoes more effi-
ciently. The Regensburg resear-
chers identified the types of aroma
mosquitoes recognise on human
beings and how they actually zoom
in on them. Their colleagues in
Brazil investigated the aromas used
by pregnant mosquitoes to find a
suitable place to lay their eggs.
The role of efficient
monitoring systems
Tempting aroma cocktails
65
Mosquito Control
Fig. 3: Monitoring of mosquito populati-
ons: the hunt in Brazil for water recept-
acles infested by eggs, larvae or pupae
(photo: Á.E. Eiras, University of Minas
Gerais, Belo Horizonte, Brazil.)
Disease Cases / Year Deaths / Year
Malaria 300.000.000 1.000.000
Yellow Fever 200.000 30.000
Dengue Fever/
Dengue Hemorrhagic Fever 20.000.000 24.000
Japanese Encephalitis 50.000 15.000
Lymphatic Filariasis
(Elephantiasis) 120.000.000 -
Tab . 1: The currently most important diseases transmitted by mosquitoes. Sources:World
Health Organization (www.who.int) and the Centers for Disease Control and Prevention
in the US (www.cdc.gov)

Both baited cocktails are now being
introduced to improved monitor
systems, making it easier to track
and influence mosquito population
developments quickly, economi-
cally and accurately.
How mosquitoes spot their
hosts
Workers in the Regensburg labora-
tory focussed primarily on odours
emitted by the human body. These
are attractive to female mosquitoes
who roam around looking for
human hosts.
As with all insects, mosquitoes
mainly pick up smells with their
antennae. Under the microscope
you can make out elongated
mechano-sensitive bristles and
short olfactory hairs (sensillae)
covering the whole antenna (fig. 4).
Tiny pores on the outside of each
hair allow scents to pass through to
the sensory cells inside. There are
different types of hairs, distin-
guished by morphological features.
Generally, each contains characte-
ristic sets of sensory cells. In total,
each antenna boast more than 2000
olfactory cells on around 1000
scent-detecting hairs. You can
record the neuronal activity of indi-
vidual hairs by using electrophysio-
logical techniques, measuring how
they react to scents. Depending on
the stimulus, the olfactory cells res-
pond by increasing or inhibiting
neuronal activity. It is not until in-
sects start recognising patterns of
activity across many cells that they
can piece together the properties
of individual scent stimuli, as well
as their concentration and duration.
To break down the parts of the
human bouquet with the most
appeal, scientists isolated aromatic
components on human skin and
measured their electrophysiological
effects, as well their effect in attrac-
ting mosquitoes in behavioural
assays. According to their findings,
only certain aromas are needed to
identify the right host. But when
they examined each component in
isolation they hardly had any effect.
It is only when they are brought
together in certain combinations
that the attractiveness of each
aroma unfolds, tempting the mos-
quito in a similar fashion to their
natural host. So mosquitoes re-
cognise their favourite hosts by their
characteristic aroma patterns. This
aroma cocktail has been registered
for patent. The substances involved
have no adverse effects on health,
are easy to handle and economical,
side-stepping expensive carbon
dioxides used to date. Laboratory
and field testing shows that they
can entice several species of mos-
quito attracted to humans.
How mosquitoes decide
where to lay their eggs
On the other side of the Atlantic,
colleagues in Brazil tackled the
problem from a different angle,
putting the particularly malicious
66 Mosquito Control
Fig. 4 : Images of a yellow fever mosquito (Aedes aegypti) taken with a scanning electron
microscope. A) head with antennae,proboscis with stylet and sheath, and two palps
(paired extensions at the base of the proboscis). B) A segment of the antenna with
olfactory hairs containing the olfactory cells. C) Tip of the stylet which is injected into the
skin of the host to feed on blood. (Images: A.Kühn,University of Regensburg)

dengue and yellow fever mosqui-
toes under the microscope. They
were interested in female mosqui-
toes after they had sucked blood
from humans. After an incubation
period lasting several days, the
female mosquito goes on the hunt
for a suitable place to lay its eggs.
We mainly still have the Aedes
aegypti mosquito to thank for pas-
sing on yellow fever and dengue.
But we have also had to start loo-
king out for its close relative, the
Asian Tiger Mosquito, Aedes albo-
pictus, which has joined its cousin
in spreading these diseases world-
wide – and not only in tropical and
sub-tropical regions, since they can
also survive at cooler latitudes: in
Europe, the Asian Tiger Mosquito
has already reached Greece, the
Balkans, northern Italy and southern
France. In the US, this species was
first documented in Texas in 1985
and is currently established in more
than 25 states.
One thing both species have in
common is the way they lay their
eggs. Originally, both mosquitoes
used natural water retaining cavities
in tree holes and herbaceous plants
as a breeding ground. But in an
urban environment, there are also
plenty of attractive locations provi-
ded by humans to bring their
young into the world; they are
quite happy to use even small
quantities of water, in discarded
plastic beakers, bottles, broken
drainpipes, tyres left out in the
open, plants using hydroponic
(soil-less) growing media, watering
cans, rainwater collection tubs, etc.
Mosquitoes go about identifying
good oviposition sites the same
way they look for a host – by
using their sense of smell. Water
rich in the nutrients needed by
mosquito larvae has a distinctive
smell, so this is what mosquitoes
look for to lay their eggs in. In the
laboratory in Belo Horizonte,
scientists therefore started by
examining the behaviour of preg-
nant mosquitoes to see which blends
of water and nutrients they found
most attractive. They then used a
combined gas chromatography and
electrophysiological measuring de-
vice to identify which substances set
off a reaction in the antennae. In a
sequence of experiments they then
observed mosquito behaviour to
work out the ideal mixture to attract
their attention ( fig. 5). In field
testing, this mixture was found to be
just as effective as their natural bree-
ding grounds and it has now been
registered for patents.
If used in the right trap, both
aroma cocktails can significantly
improve the efficiency of monito-
ring systems. The synthetic host
aroma works particularly well in a
special ventilator trap (also devel-
oped in Regensburg and currently
being patented) which optimises
the emission of odours. By way of
contrast, the oviposition attractant
requires a plastic container filled
with water, coated on the inside
with a special adhesive which grabs
onto the mosquitoes. This captu-
ring device is also registered for
patent and is highly economical to
produce. Outdoor testing in Brazil
shows that the best strategy to
monitor mosquitoes is to use both
systems together.
The ventilator trap uses a synthetic
human odour. It is a highly sensi-
tive monitoring system and can
capture up to ten times more mos-
quitoes per day than the oviposition
trap. The latter offers significant
cost benefits and, unlike the venti-
lator trap, does not need power.
The straightforward oviposition
trap can be used for on-going
monitoring purposes across wide
areas. The ventilator trap can be
used on a more targeted basis in
mosquito hot spots when you need
to find out the precise density of
the mosquito population quickly.
To process data gathered by the
traps as quickly as possible, it is
transferred to a so-called GIS
(geographical information system)
using a standard electronic data-
gathering system. This makes it
possible to identify high-risk areas
quickly and accurately and make
highly localised battle plans. You
can then use the ventilator traps to
assess how successful each cam-
paign was. After a series of success
studies in Belo Horizonte, scien-
tists are currently carrying out
broad-scale tests in Brazilian cities
67
Mosquito Control
Innovative monitoring
systems for combating
disease
Fig. 5: Development of a synthetic oviposition attractant. Left: The schematic output of a
combined gas chromatography and electrophysiological measurement device.Electrical
nerve signals are taken from the antenna of a pregnant yellow fever mosquito positio-
ned directly at exit of a gas chromatograph.The gas chromatogram is shown in red
(GC), the electrophysiological responses are in white (EAG).Peaks can be seen on the EAG
curve only when certain substances are emitted from the GC. Middle: A comparison of
synthetic and natural attractants in release and capture testing. Right: The stability of
the synthetic oviposition attractant . (Á E Eiras, University of Minas Gerais, Belo Horizon-
te, Brazil)

on an early warning system de-
signed to pinpoint dengue fever,
using the same type of technology.
Further testing is in the pipeline
later in the year in North America,
Europe, Africa and Asia. These
tests will be looking at the first
batch of a new type of monitoring
trap.
To update, refine and market the
new capturing devices efficiently,
two university spin-off companies
were set up at the beginning of
2003 – one in Belo Horizonte, the
other in Regensburg, and each lin-
ked up commercially to the other.
The Brazilian company, Ecovec
Ltda focuses on the public health
sector in South America; the Ger-
man company, BioGents GmbH,
manages Europe, North America,
Asia and the remaining internatio-
nal markets. The combination of
laboratory and outdoor testing, the
complementary blend of expertise
provided by the two research
teams, and their tried-and-tested
ability to work together closely
(despite the long distances in-
volved) point to a promising future
for the joint venture.
It is also a good example of an
attractive and inexpensive alterna-
tive to the types of preventative
medicines used in vaccination pro-
grams, or the proposed release of
genetically engineered vectors. For
decades, bodies have channelled
colossal research funds into the
development of vaccinations to
fight malaria and dengue. We have
still yet to find one that works. In
recent years, we have also wit-
nessed intensified efforts to find a
solution for combating these di-
seases with genetically engineered
mosquitoes. This has been a simi-
lar drain on technology research
funding, even though many experts
have expressed their doubts about
the technology. This contrasts to
targeted research in the field of
chemical ecology and the sensory
ecology of mosquitoes, which – for
relatively small amounts of money
– can help us move a significant
way forward in the battle against
disease.
Highly effective lures and mosquito
traps are not only useful instruments
in the battle against disease. They
can prove interesting to anybody
trying to shoo away the irritating
airborne pest without damaging the
environment. In the United States
in particular, there have been a
variety of mosquito traps launched
in recent years, targeted at the con-
sumer market. Apart from a num-
ber of totally ineffective electro-
cution devices with light sources,
most consumers plump for traps
using propane gas to attract mos-
quitoes by generating carbon di-
oxide, heat and moisture. These
traps cost anywhere between 300
and 1300 US dollars; last year the
US market for such capturing de-
vices was worth 650 million dollars.
The consumer can clearly see the
benefit of the methods used by
these inventions: to catch the little
bloodsuckers before they sting you,
thus helping control the overall
population.
As with the monitoring trap descri-
bed above, these traps use carbon
dioxide and other more generic
lures, mainly because there are no
alternatives. As a result, the initial
outlay and running costs are high.
They are also limited to areas out-
side the home because they use
explosive and poisonous gases. In
laboratory settings and outdoor
testing, scientists have found that
the mosquito capturing device
from Regensburg with the human
aroma frequently outperforms – or
at least matches – propane gas dri-
ven traps. With the new lure, the
manufacturing costs of capturing
devices can be slashed to one tenth
of current levels. They are also
much easier to handle. Based on
the current capturing device and
scientists’ experience in the devel-
opment process, a number of dif-
ferent types of trap are being devel-
oped for launching on consumer
markets. Each will be targeted at
different price segments and sub-
markets. From straightforward ad-
hesive traps with time-release aro-
mas, to high performance devices
for professional mosquito hunters
in hotel grounds and parks, the new
technology opens up completely
new avenues in capturing unwan-
ted mosquitoes – without dam-
aging the environment, when and
where it is needed.
Contact:
BioGents GmbH
Universitätsstraße 31
D-93053 Regensburg/Germany
Tel./Fax +49 (0)941 / 943 3064
biogents@biogents.com
Dr. Andreas Rose
Dr. Martin Geier
68 Mosquito Control
New products in the fight
against mosquitoes –
in business and consumer
markets