| dc.description.abstract | Five populations of Anopheles vectors of malaria from different altitudes along a transect in
Nyanza Province of western Kenya were studied over a period of two years, January, 1989
through to December, 1990. Each population was sampled by two methods: the
pyrethrum-spray-sheet collection (PSC) technique for the day-resting, and the human-bait
technique for the biting population (WHO 1975; Service, 1976). The following parameters were
then determined:
(1) species composition,
(2) the relative density of indoor day-resting
populations in inhabited houses,
(3) the relative density of the biting populations
(4) the man-biting rates and biting cycles between 19.00 and 07.00 hours, and for a full
24 hour day cycle, ,
(5) Blood feeding preferences,
(6) parity rates, (7) sporozoite rates, and
(8) entomological inoculation rates, i.e. man-biting rate x sporozoite rate.
In the PSC technique, a. total of 192 house searches, were carried out by sampling once
monthly from eight human-inhabited houses at each altitude. The anophelines were identified
morphologically with the aid of keys. Siblings of the An. gambiae complex were separated by
the cytogenetic identification of polytene chromosomes as described by Coluzzi & Sabatini
(1967). Bloodmeal types were identified by the bloodmeal enzyme-linked
immunosorbent assay (ELISA) techniquetxervice, 1986). .
The human-bait catches were performed at three different altitudinal sites located at 1219
m (Ahero), 1350 m (Rota) and 1524' m (Oriwo). The protocol involved hourly catches for
12 hours of the night, and on six occasions throughout the 24 hours of day and night, by a
team comprising two collectors seated indoors and two others seated outdoors with their
legs exposed, and collecting mosquitoes from themselves using test-tubes with the aid of
torch lights (WHO, 1975). The data was used for the analysis of the man-biting rates, biting
cycles, longevity and parity rates. Sporozoite rates were determined by the ELISA
technique described by Burkot et al. 1984 and Wirtz et al. 1985 and also by dissection for
comparison; and were used to calculate the entomological inoculation rates. All the parameters
were correlated to altitude, seasonal and climatic changes.
The anopheline composition along the transect varied in species diversity and reduced in
population abundance with rising altitude. Member species of the Anopheles gambiae complex
An. gambiae s.s. and An. arabiensis were caught at all altitudes. An. funestus was not found
in collections above 2100 m above sea level. At the lower altitude of 1219 m (Ahero), An.
arabiensis existed as a homogeneous population, whereas at higher altitudes this species and
An. gambiae s.s. existed as sympatric (mixed) populations, with the latter tending to be
predominant. Small numbers of An. zeimanni, An. pharaoensis and An. coustani were also
captured.
The man-biting rates reduced with rising altitude from apeak of 108 bites/man/night for An.
arabiensis at 1219 m, through 28.3 bites/man/night in a sympatric population, to less than 0.1
bites/man/night in a homogeneous population of An. gambiae s.s. at 1524 m. The man-biting
rate for An. junestus, also reduced from' 69.3 bites/man/night to 65.1 bites/man/night and less
than 0.1 bites/man/night at the respective altitudes .
Altitude and seasonality affected the parity rates and age composition. An. arabiensis at 1219
m had a lower parity rate than An. gambiae s.s. at higher altitudes. However, the parity rate
for An. funestus at 1350 m was higher than that of the same species at 1219 m.
Age-composition studies showed that the percentage of age-groups at each gonotrophic
cycle differed with altitude, with the duration of the gonotrophic cycle of the three vectors
being longer at higher altitude.
Although sporozoite rates were bound to differ with species, it was evident that altitude
affected sporozoite development in a given species. While the mean sporozoite rate in An.
arabiensis from 1219 m was 0.3%, it was significantly higher - 5.3% at 1350 m and 5.5% at
1524 m in its closest relative, An. gambiae s.s. The sporozoite rates in An. funestus also
showed an increasing trend, from 1.9% at 1219 m to 4.2% at 1350 m and 4.0% at 1524 m.
These differences occurred also in day resting populations, notably An. gambiae s.s. whose
sporozoite rates increased from 3.7%',at 1524 m to 5.3% at 1829 m and 12.5% at 2134 m.
Irrespective of altitude, no sporozoite infections were detected in An. zeimanni, An. pharoensis
and An. coustani.
In contrast to the sporozoite rates, the entomological- inoculation rate (EIR) reduced with
increasing altitude. The EIR was intermittent and unstable at 1524 m, medium at 1350 m and
intense at 1219 m. This phenomenon was consistent with parasitemia rates observed in school
children during the same period. A comparison of the three vectors revealed that An. funestus
consistently had a higher entomological inoculation rate at each altitude than the other two
vectors.
Bloodmeal analysis for pooled samples of the three vectors showed that at 1524 m feeding was
predominantly on humans in contrast' to both human and bovid feeding exibited at 1350 m and
1219 m. Human feeding by An. arabiensis was only 28.8% in contrast to 63.5-72% for bovid
feeding. However, human feeding was above 90% in both An. gambiae s.s. and An. funestus
without significant variation in altitude.
It is concluded that altitude, besides seasonal, ecological and climatic factors, appeared to have
significant effect on malaria epidemiology. In particular, malaria transmission albeit very low
and intermittent, occurred also at the high altitudes, formerly known popularly as the "white
highlands", contrary to the long held notion that malaria did not occur in these "white
highlands" of Kenya. The practical implications of these observations are that malaria
control strategies against the mosquito vectors must be carefully selected in relation to
altitude and their ecological diversity. | en |
