New Insights into the Susceptibility of Immunocompetent Mice to Usutu Virus

Abstract

Usutu virus (USUV) is a mosquito-borne flavivirus that shares many similarities with the closely related West Nile virus (WNV) in terms of ecology and clinical manifestations. Initially distributed in Africa, USUV emerged in Italy in 1996 and managed to co-circulate with WNV in many European countries in a similar mosquito–bird life cycle. The rapid geographic spread of USUV, the seasonal mass mortalities it causes in the European avifauna, and the increasing number of infections with neurological disease both in healthy and immunocompromised humans has stimulated interest in infection studies to delineate USUV pathogenesis. Here, we assessed the pathogenicity of two USUV isolates from a recent Belgian outbreak in immunocompetent mice. The intradermal injection of USUV gave rise to disorientation and paraplegia and was associated with neuronal death in the brain and spinal cord in a single mouse. Intranasal inoculation of USUV could also establish the infection; viral RNA was detected in the brain 15 days post-infection. Overall, this pilot study probes the suitability of this murine model for the study of USUV neuroinvasiveness and the possibility of direct transmission in mammals.

Differential effects of Usutu and West Nile viruses on neuroinflammation, immune cell recruitment and blood–brain barrier integrity

“…Because we detected differential responses and effects of BBB infection by USUV and WNV, we decided to monitor the systemic and CNS inflammation, as well as the potential effects on key BBB proteins in vivo . As immunocompetent mice are poorly infected by USUV and manifest low symptoms [ 39 ], mice lacking the interferon α/β receptor ( Ifnar -/- ) were used to investigate USUV and WNV infections. In infected mice, clinical signs and mortality rate were similar between USUV and WNV but WNV-infected mice die more rapidly after infection (around 3 dpi compared to around 5 dpi for USUV) (Supplemental Figure 2b and c).…”

In Vitro and In Vivo Models to Study the Zoonotic Mosquito-Borne Usutu Virus

“…Likewise, in the same study, the IP inoculation of a mouse-derived USUV strain induced a 10% mortality 10 days after infection [ 46 ]. USUV infection failed to elicit pathogenicity in wild-type 129/Sv mice via the IP [ 118 , 119 ] and IN routes [ 118 ] but induced a typical neurological disease in a single 129/Sv mouse infected via the ID route [ 118 ]. These findings indicate that the outcome of USUV infection in immunocompetent mice depends on several factors, such as the strain of virus or mouse used.…”

“…In immunocompetent mice, USUV infection induced clinical signs, such as disorientation, depression, paraplegia, and paralysis, associated with extensive neuronal death, including both necrosis and apoptosis in the brain [ 118 , 122 ]. Alternatively, no trace of viral infection [ 120 ] or a simple detection of the USUV genome in brain portions of USUV-infected mice were described after 15 days post-infection, without the induction of specific clinical signs [ 118 ]. These models reflect the infection in humans, in which most individuals show subclinical infections but rare cases can develop clinical disease.…”

“…The study of the antibody response to USUV showed the induction of detectable IgG against the virus by 15 days post-infection [ 118 , 120 , 121 ]. The neutralizing capacity of these specific antibodies was assessed in one study and none of the USUV-IgG-positive samples efficiently neutralized USUV [ 120 ].…”

Modelling West Nile Virus and Usutu Virus Pathogenicity in Human Neural Stem Cells

“…In this study, 6 week-old mice deficient of the interferon type I receptor ( ifnar -/- 129SvEv mice) were infected intraperitoneally (ip) with 10 4 pfu of USUV SAAR strain to model lethal USUV infection [ 21 ]. At variance, wild type 129SvEv mice infected with USUV neither showed signs of USUV infection nor died [ 21 , 22 ], with the exception of a single animal showing neurological symptoms associated with neural death in the brain and spinal cord following infection with a recent USUV isolate from Belgium [ 22 ]. However, virulence of USUV could vary among different strains [ 23 ], such as in the case of WNV, for which motifs associated with neuroinvasion have been identified [ 24 , 25 , 26 ].…”