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.

Keywords: Usutu virus; encephalitis; immunocompetent; infection; mice.

Figure 1

The brain of a wild-type 129/Sv mouse injected with the Usutu virus via the intradermal route. Massive neuronal death demonstrated by karyorrhexis and karyolysis (a,b) in correlation with intense immunohistochemical labeling of USUV antigens (c,d). (a,b) Hematoxylin and eosin staining, (b,c) hematoxylin counterstain. Scale bars a and c = 200 µm, magnification 100×; Scale bars b and d = 50 µm; magnification 200×.

Figure 1

The brain of a wild-type 129/Sv mouse injected with the Usutu virus via the intradermal route. Massive neuronal death demonstrated by karyorrhexis and karyolysis (a,b) in correlation with intense immunohistochemical labeling of USUV antigens (c,d). (a,b) Hematoxylin and eosin staining, (b,c) hematoxylin counterstain. Scale bars a and c = 200 µm, magnification 100×; Scale bars b and d = 50 µm; magnification 200×.

Figure 2

Spinal cord (gray matter) of a wild-type 129/Sv mouse injected with the Usutu virus via the intradermal route. Abundant neuronal death with neuronophagia and moderate satellitosis and gliosis (a) and lymphoplasmacytic perivascular cuffs (b). Hematoxylin and eosin staining. Scale bars = 50 µm, magnification 200×.

Figure 2

Spinal cord (gray matter) of a wild-type 129/Sv mouse injected with the Usutu virus via the intradermal route. Abundant neuronal death with neuronophagia and moderate satellitosis and gliosis (a) and lymphoplasmacytic perivascular cuffs (b). Hematoxylin and eosin staining. Scale bars = 50 µm, magnification 200×.

Figure 3

Viral RNA loads measured by RT-qPCR in brain samples (n = 6 per condition) collected from mice 15 days after their challenge with the Usutu virus via different routes. * p-value < 0.05. A: USU-BE-Seraing/2017, B: USU-BE-Grivegnee/2017, i.d.: intradermal, i.n.: intranasal, i.p.: intraperitoneal.