Abstract:
BACKGROUND : West Nile virus (WNV) has a wide geographical distribution and has been associated to cause
neurological disease in humans and horses. Mosquitoes are the traditional vectors for WNV; however, the virus has
also been isolated from tick species in North Africa and Europe which could be a means of introduction and spread
of the virus over long distances through migratory birds. Although WNV has been isolated in mosquitoes in Kenya,
paucity of genetic and pathogenicity data exists. We previously reported the isolation of WNV from ticks collected
from livestock and wildlife in Ijara District of Kenya, a hotspot for arbovirus activity. Here we report the full genome
sequence and phylogenetic investigation of their origin and relation to strains from other regions.
METHODS : A total of 10,488 ticks were sampled from animal hosts, classified to species and processed in pools of
up to eight ticks per pool. Virus screening was performed by cell culture, RT-PCR and sequencing. Phylogenetic
analysis was carried out to determine the evolutionary relationships of our isolate.
RESULTS : Among other viruses, WNV was isolated from a pool of Rhipicephalus pulchellus sampled from cattle,
sequenced and submitted to GenBank (Accession number: KC243146). Comparative analysis with 27 different strains
revealed that our isolate belongs to lineage 1 and clustered relatively closely to isolates from North Africa and
Europe, Russia and the United States. Overall, Bayesian analysis based on nucleotide sequences showed that
lineage 1 strains including the Kenyan strain had diverged 200 years ago from lineage 2 strains of southern Africa.
Ijara strain collected from a tick sampled on livestock was closest to another Kenyan strain and had diverged
20 years ago from strains detected in Morocco and Europe and 30 years ago from strains identified in the USA.
CONCLUSION : To our knowledge, this is the first characterized WNV strain isolated from R. pulchellus. The
epidemiological role of this tick in WNV transmission and dissemination remains equivocal but presents tick verses
mosquito virus transmission has been neglected. Genetic data of this strain suggest that lineage 1 strains from
Africa could be dispersed through tick vectors by wild migratory birds to Europe and beyond.
