Ongoing
Research
Leishmaniases caused by parasitic protozoa of the genus Leishmania, that
are transmitted by bite of infected sand flies, present a large variety of
disease manifestations. These diseases are a serious public health problem in
many countries and they are difficult to control by conventional methods. In
addition to blood, sand flies feed on plants and this diet and its effect on
disease transmission is a subject of our investigations for several years. In
recent field and laboratory studies we demonstrated that some common garden
plants cause mortality of sand flies that feed on them. We have also
demonstrated in the wild that one of these plants (Bougainvillea glabra)
drastically reduces the sand fly population in its neighborhood. This plant and
apparently also other species can be used for biological control against
leishmaniasis. In an initial experiment in Kfar Adumim, coordinated by Dr. Laor
Orshan from the Ministry of Health, a planted hedge of Bougainvillea is expected
to serve as a barrier for sandflies that come uphill to the village.
The basic observation in
another line of studies was that the competence of Phlebotomus papatasi as a
host for Leishmania major is related to adult survival on sugar-poor foods.
Desert sand flies, provided with excess sugar, became progressively resistant to
infection. Selection for survival under sugar-poor conditions increased
susceptibility. About 85% of flies colonized from a desert habitat retain
experimental Leishmania infection compared to 25% from irrigated sites. It
appears that hunger tolerance of sand flies modulates the susceptibility to
leishmanial infection.
Sugar questing mosquitoes in arid areas gather on scarce blossoms
that can be used for control
Günter Müller and Yosef Schlein:
Biting female mosquitoes utilize the ingested blood primarily for egg
development while floral nectar sugars are the main source of energy for other
functions. Flowering plants are scarce in arid mosquito habitats including large
malaria endemic regions in Asia and Africa. We showed that such flowers are
highly attractive. Experiments in small isolated oases demonstrated that the few
flowering acacia trees were visited by most of the local mosquitoes and spraying
the trees with oral insecticide eliminated the mosquitoes. The study showed that
scarce sugar sources in desert areas are key elements for mosquitoes and can be
used for their control.
Most of the sugars for
the mosquito diet are obtained from flowers and our study pertains to an extreme
but widespread situation of floral sugar scarcity in a dry climate and arid
areas. We show that in this situation the flowers of some perennial plants are
highly attractive and even in the spring they are much more attractive than
seasonal flowers. We also show for the first time the dependence of mosquitoes
on defined and limited sugar sources.
An intriguing question raised by this study is the apparent influence that
sugar scarcity might have on the transmission of malaria, whether it limits
mosquito life span and thus fecundity and malaria transmission. A second
question is whether the definition of sugar sources as key elements in mosquito
habitats can be used for targeted control methods that would be less damaging to
the environment than the use of insecticides in wide areas. The mosquito control
approach may be useful in the pattern described by us and it may also or lead to
other ways of using attractive floral fragrance. Suitable areas for application
are primarily in arid regions such as the malaria endemic areas in the sub-
Saharan Sahel and other similar malaria foci in Africa and Asia.
