|dc.description.abstract||Anopheles gambiae sensu lato and Anopheles funestus were collected
from villages with permethrin impregnated bednets or curtains (0.5gm/m2
villages with no intervention measures and from the Ahero Rice irrigation
Scheme by two methods: the resting collection (RC) and human bait
collection (HBC). These mosquitoes were assayed for susceptibility to
permethrin and other insecticides using the conventional WHO test kit and
cone. An. gambiae s.l. mosquitoes were speciated by the Polymerase chain
reaction method (PCR). A colony of permethrin resistant An. gambiae sensu
stricto was selected for and reared from mosquitoes captured in villages
where permethrin treated bednets or curtains were being used. This colony
was used to study the biochemical basis of permethrin resistance and its
All the mosquitoes from the villages where permethrin treated nets
were present and the control villages speciated by PCR were found to be
An. gambiae s.s
Use of permethrin impregnated nets for one year, raised An. gambiae
s.s permethrin tolerance (PT) to 2.5 times its' baseline value as measured by
the WHO kit. In th-e second year of this studv, An. gambiae s.s permethrin
tolerance showed no further significant rise in .comparison to the results of
the first year. The results of the WHO cone test compared favourably with
- those of the WHO kit with An.qembiee s.s from the bednet or curtain
villages having a longer exposure time to 50 per cent mortality than
mosquitoes from non intervention villages (10 minutes versus 5 minutes).
Mosquitoes which were found to be tolerant to permethrin remained
susceptible to other insecticides when tested in the WHO kit test.
An.gambiae s.I. and An. funestus from Ahero were susceptible to all
the insecticides they were tested against. All the An.gambiae s./ from Ahero
which were speciated by normal taxonomic techniques using common visible
features were found to be An. arabiensis.
An. gambiae s.s. which were selected for resistance to permethrin
had higher levels of mixed function oxidase (MFO) enzymes when compared
to a susceptible strain. Resistance to permethrin in this colony was
abolished by the use of piperonyl butoxide, which is a known oxidase
inhibitor, and a permethrin synergist.
Mosquitoes which were resistant to permethrin were more successful
in feeding across a treated bennet (permethrin O.5gm/m2) compared to those
which were susceptible. Most of the mosquitoes that were able to obtain a
full blood meal across CI•• treated net successfully oviposited.
It is concluded that village-wide use of permethrin impregnated nets
can lead to selection for permethrin resistance in An. gambiae s.s. Those
mosquitoes which were resistant to perrnetqrin were, however, not resistant
to deltamethrin and propoxur. These insecticides can therefore be used as
alternatives to permethrin in the event that permethrin resistance
- compromises the efficacy of perrnethrin impregnated nets. Since the colony
of An. gambiae s.s. selected for, resistance to perrnethrin was found to have
higher levels of mixed function oxidases, the use of suitable synergists
known to inhibit oxidase activity (such as piperonyl butoxide) can be used to
forestall the development of oxidase related resistance.
There was no further significant rise in permethrin tolerance in An.
gambiaes.s. which were captured from villages where permethrin treated
nets were present, during the second year of the study, in comparison to the
rise in permethrin tolerance observed in the first year of the study. This
could be attributed to immigration of susceptible mosquitoes from
surrounding villages which had no treated nets, and also the presence of
alternative blood meal sources such as cattle. These factors mini mise the
selection for genes responsible for increased permethrin tolerance.
It is also concluded that the WHO tube test and the WHO cone test
can be easily used to monitor for the development of permethrin resistance
in areas where permethrin impregnated nets are being used. Biochemical
assays such as the oxidase microplate assay, can also be used to monitor
for the development of. permethrin resistance in An. gambiae s.s. Some of
the advantages of biochemical assays are that fewer numbers of mosquitoes
are needed to perform the test and secondly, they can be carried out more
rapidly than the conventional WHO assays.
In the Ahero rice irrigation scheme, aqricultural pesticides are normally
in use. The two main vectors of malaria; An. arabiensis and An. funestus
were found to be susceptible.to all the major classes of insecticides. It
would appear that the use of agricultural pesticides has not resulted in the
development of resistance in the above vectors.
The significance of these findings is discussed||en