The role of Anopheles arabiensis (Diptera: Culicidae) in malaria transmission and control in Gokwe and Binga districts, Zimbabwe..
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Date
1996
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Abstract
Opportunistic feeding behaviour and partial exophily make An. arabiensis much more
difficult to control by indoor residual spraying than any other vector in the Afro-tropical
region. The persistent malaria outbreaks in Zimbabwe despite decades of indoor house
spraying prompted this investigation into the role of An. arabiensis in malaria transmission
and assessment of the possible impact of this control measure. The study was conducted in
the malaria endemic districts of Binga and Gokwe.
An. gambiae complex mosquitoes were collected from artificial outdoor resting sites,
and from human dwellings by i) daytime hut searches, ii) pyrethrum spray catches and iii)
exit window traps. Mosquito components were processed to enable: i) the distinction of An.
arabiensis from An. quadriannulatus and An. merus on the basis of the pale band at the
junction of the hind leg 3/4 tarsomeres; ii) species identification and scoring of inversion
polymorphism on the basis of the X chromosome and autosomes respectively; iii) the
determination of blood meal sources using the Ouchterlony precipitin test; and iv)
identification of An. gambiae s.l. using polymerase chain reaction (PCR) and enzyme
electrophoresis techniques. Entomological assessment of residual spraying included
determining: the vector resting densities indoors and outdoors, bioassay and insecticides
susceptibility tests. Data were also collected on hut profiles, knowledge-attitudes-practices
surveys, and household malaria prevalence . surveys.
An. arabiensis and An. quadriannulatus were found in sympatry in Binga and Gokwe,
and in addition, An. merus was found in Gokwe. Most species identifications were made
using PCR; which was found to have 7.5% and 41.6% levels of error for An. arabiensis and
An. quadriannulatus respectively, using the cytogenetic technique as benchmark. The pale
band technique yielded > 80% correct identification for An. arabiensis but the extent of
overlap in the pale band lengths between An. arabiensis and An. quadriannulatus renders the
method unsuitable for distinguishing these two species. Inversions 2Rb and 3Ra were found
floating in An. arabiensis, with 60% frequency in the former. The Wright's F statistic value
of -0.0416 indicated an excess of heterozygotes, and a state of panmixis in the vector
population. No significant differences were observed between 2Rb karyotypes in host choice.
Human blood indices among indoor (0.82), exit trap (0.98) and outdoor resting (0.30) specimens suggested exophilic behaviour. This was corroborated by the high fed:gravid ratios
of 6.8: 1 and 11.6: 1 in sprayed and non-sprayed dwellings respectively. This was worsened
by a high feeder-survivor index (FSI) of 93 % among exit trap specimens. The susceptibility
to deltamethrin coupled with residual efficacy nine weeks post-spray indicated the suitability
of the insecticide. Rural dwellings were suitably built for spraying but had no mosquito
proofing. Personal protective measures are hardly known; sleeping outdoors occurs in
Siabuwa.
While An. arabiensis bites humans indoors the partial exophily it exhibits is a threat
to indoor residual insecticide spraying. An integrated malaria control approach is
recommended.
Description
Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1996.
Keywords
Malaria--Zimbabwe., Mosquitoes--Behaviour., Malaria--Epidemiology., Anopheles arabiensis., Theses--Zoology.