Usutu Virus Isolated from Rodents in Senegal

Abstract

Usutu virus (USUV) is a Culex-associated mosquito-borne flavivirus of the Flaviviridae family. Since its discovery in 1959, the virus has been isolated from birds, arthropods and humans in Europe and Africa. An increasing number of Usutu virus infections in humans with neurological presentations have been reported. Recently, the virus has been detected in bats and horses, which deviates from the currently proposed enzootic cycle of USUV involving several different avian and mosquito species. Despite this increasing number of viral detections in different mammalian hosts, the existence of a non-avian reservoir remains unresolved. In Kedougou, a tropical region in the southeast corner of Senegal, Usutu virus was detected, isolated and sequenced from five asymptomatic small mammals: Two different rodent species and a single species of shrew. Additional molecular characterization and in vivo growth dynamics showed that these rodents/shrew-derived viruses are closely related to the reference strain (accession number: AF013412) and are as pathogenic as other characterized strains associated with neurological invasions in human. This is the first evidence of Usutu virus isolation from rodents or shrews. Our findings emphasize the need to consider a closer monitoring of terrestrial small mammals in future active surveillance, public health, and epidemiological efforts in response to USUV in both Africa and Europe.

Keywords: Senegal; Usutu virus; arbovirus; in vivo experiment; rodents.

Figure 1

A map of Kedougou with the locations of the five captured Usutu virus-positive mammals.

Figure 2

The comparative genome organization and mapping of amino acid differences between Usutu virus (USUV) sequences, characterized from different hosts. Positions are annotated and aligned to their loci in the USUV polyprotein. Host of origin and strain names can be found to the left and right of the polyprotein sequence maps, respectively. The shared non-synonymous changes from the SAAR-1776 to the rodent USUV strains are shown in the bottom right corner.

Figure 3

A maximum-likelihood phylogenetic tree estimated using 28 USUV polyprotein sequences. Rodent taxon labels are in bold. The colored circles indicate the host where the USUV sequence originated. The asterisks (*) at major nodes indicate an SH-like support value > 70%. The tree branches are scaled by the nucleotide substitutions per site.

Figure 4

The in vivo evaluation of mouse mortality and virulence of three Usutu virus strains inoculated in 3-to-4-week-old Swiss mice. Survival (left panels) and changes in body weight (right panels) were observed after three different inoculation methods: intracerebral (IC, panels (A,B)), intraperitoneal (IP, panels (C,D)), and subcutaneous (SC, panels (E,F)) for 20 days, post-infection period. The control mice (red) were injected with PBS in duplicate groups using the same inoculation methods. Error bars show the 95% confidence intervals for each sampled time point. The days of disease-specific symptom presentations (SSP) is shown in the top left corner of panels (B,D,F). The number of days (D) post-infection when disease-specific symptoms are shown for each strain, as well as the proportion of symptomatic mice in each group.