Abstract:
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Background: Understanding malaria vector mosquitoes and their infectivity dynamics is of importance in setting
up intervention and control programmes. Patterns of malaria transmission have been shown to differ between
non-irrigated and irrigated semi-arid areas of eastern Sudan. However, very little information is available regarding
malaria transmission dynamics along the seasonal river’s basin. Such information is required for the design of
effective vector control strategies.
Methods: A longitudinal study for mosquito sampling using pyrethrum spray catch (PSC) was conducted in two
villages (Koka & Um Salala) along the Rahad River basin from December 2005 to October 2006. The Plasmodium
falciparum circumsporozoite (CSP) and human blood index (HBI) were detected by ELISA. Three seasons were
considered and the surveys represented cool dry, hot dry and rainy seasons were November - February, March -
June, July - October, respectively. The CSP was compared between the seasons and populations using Chisquare
test. The differences between the seasons and the populations in the other entomological indices,
including Entomological Inoculation Rates (EIR), were measured using Tukey-Kramer HSD and Student T-test,
respectively. The association between An. arabiensis density and monthly total rainfall was examined using
regression analysis.
Results: A total of 1,402 adult female anopheline mosquitoes were sampled, of which 98% were An. gambiae
complex; the rest were An. rufipes. All specimens of An. gambiae complex identified by the PCR were An. arabiensis.
Bimodal annual peaks of An. arabiensis densities were observed following the peak of rainfall and recess of the
Rahad River after a time- lag of two months (Koka r = 0.79, d.f. = 1, P = 0.05; Um Salala, r = 0.88, d.f. = 1, P = 0.02).
The CSP differed significantly among the seasons only in Koka (P = 0.0009) where the mean was nine times higher
than in Um Salala (P = 0.0014). Active transmission was observed in Koka during the hot, dry season (CSP = 6.25%)
and the EIR was observed to be 0.01 ib/p/n during this time. The EIR peaked to 0.71 ib/p/n during the rainy
season and decreased to 0.18 ib/p/n during the minor peak of the cool dry season (P = 0.54). The combined
annual average of the EIR for both populations was 55.48 ib/p/y and, typically, it would take approximately 192.7
days for an individual to receive an infective bite from An. arabiensis.
Conclusion: The bimodal annual peaks and the active transmission observed during the hot dry season suggested
low to moderate perennial malaria transmission pattern. Infectivity and transmission rates increased with proximity
to the river following the peak of rainfall and the subsequent recession in the flow of the Rahad River. Current
vector interventions can be integrated with larval control and should be formatted in accordance with targeted
according to the time and space. |