Abstract:
Rift Valley fever virus (RVFV) is a mosquito-borne Phlebovirus that causes
outbreaks such as witnessed in Kenya in 1997-1998 and 2006-2007. Vertebrate animals
become infected through bites of infected mosquito vectors while exposure to tissues or
body fluids of infected animals is the main mode of transmission to humans. Generally,
there is limited knowledge on the transmission dynamics and vertebrate reservoirs of
Rift Valley fever virus (RVFV) and other arboviruses. The effect of environmental
variables of the breeding habitats on the type of mosquito species has been studied
elsewhere but not in Kenya. Therefore this study was conducted in the Northeastern,
Garissa County, to evaluate the association between environmental variables and larval
composition and densities of key RVFV vectors, and the role of host-vector interaction
in the maintenance of the virus in El-Humow, an RVF epidemic region. The results of
this study showed a significant difference in species composition between Marey and
Wakab-Harey (p=0.005). However there was no association between species
composition and physical characteristics of dambos in both areas. There was also no
significant association between the water temperature and species composition, (Marey:
p=0.9; Wakab-Harey: p=0.100). On the other hand, there was a significant association
between species composition and the levels of pH (r=0.45, p=0.001). Transovarial
transmission (TOT) was demonstrated for Ndumu virus (NDUV) in Culex pipiens
mosquitoes collected as larvae. Both Aedes ochraceus and Aedes mcintoshi obtained
bloodmeals from the same vertebrate hosts in almost equal proportions with goats
(Capra hircus), cattle (Bos taurus) and donkeys (Equus asinus) being the most common
sources. The percentages of Ae. ochraceus and Ae. mcintoshi with human (Homo
sapiens) blood were nearly identical at 6% and 7.3% respectively. Among domestic
animals, camels (Camelus dromedarius) were the least fed upon. Midgut infection rates
(MIR) of 9.5% in Ae. ochraceus were highest in specimens that had fed on sheep. While
there was no RVFV isolate from bloodmeals from goats, 68.4% (13/19) of NDUV
isolates were from mosquitoes with bloodmeals from goats of which 46.2% (6/13) had
disseminated infection. This study demonstrated an association between physical and
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environmental characteristics of dambos and the composition and densities of secondary
vectors but not primary vectors of RVFV. It has also demonstrated that RVFV vectors
fed randomly on available vertebrate hosts, although preferably on goats, during the
2006-2007 RVF outbreak. Some viruses such as NDUV are also maintained in nature by
TOT. More studies on species succession during wet seasons should be conducted in
diverse RVFV epizootic regions. Specifically, the effect of physical and environmental
characteristics of breeding habitats on the diversity and density of mosquito spp. Studies
on TOT and the prevalence of RVFV and other arboviruses in bloodfed field-collected
mosquitoes during wet seasons should also be conducted further.