Virulence evaluation of the Argentinean Eq7229 West Nile Virus strain in C57BL/6 mice

Abstract

West Nile virus (WNV, Flaviviridae, Flavivirus) is a mosquito-borne virus amplified by a wide variety of bird species. In the last decades it has been recognized as an etiological agent of veterinary and public health concern, causing encephalitis outbreaks in birds, horses and humans. Therefore, studies focused to characterize its biological features are important. In Argentina, WNV has been circulated since 2004, but aspects regarding its ecology and virulence of autochthonous strain are still unknown. The purpose of this study was to characterize the virulence of the WNV strain Eq7229, isolated in Argentina. Three, six and thirteen weeks old C57BL/6 mice were intraperitoneally inoculated with increasing doses ranging from 10 to 10⁵ PFU suspended in 100µl of cell culture medium. For the thirteen-week-old adults, only the two higher doses were administrated. A total of 8 mice per treatment were used. Groups of six mice per age were inoculated with cell culture medium and maintained as control groups. Mortality rates were obtained and used to calculate LD50 using the Reed-Muench method. Survival curves for each age were calculated using the Kaplan-Mayer test. The experiment was approved by the ethical committee at Instituto de Investigaciones Biológicas y Tecnológicas (Acta nº 13, 20/08/2020). For the three-weeks-old mice, the calculated LD50 was <10 UFP, for the six-week-old mice 50,11 PFU, and for the thirteen-week-old mice was >10⁵ PFU. No significative differences were observed regarding survival curves for the three and six-week-old mice (log rank, p= 0.46; p= 0.095 respectively). Thirteen-week-old mice showed no mortality nor morbidity to WNV exposure. These results show a high virulence of the strain under study. In addition, from these results we will be able to establish the initial parameters to deepen the study of this strain in the murine model.