Moran de Bustos, S.; Sanchez del Pozo, I.; Pedrera, M.; Ceron Madrigal, J. J.; Fuentes, E.; Sardon, D.; Rodriguez-Temporal, D.; Borrego, B.; Brun, A.; Rodriguez-Sanchez, B.; Sanchez-Cordon, P. J.

Rift Valley fever (RVF) is a zoonotic arboviral disease that causes adverse pregnancy outcomes and high mortality in domestic and wild ruminants. The disease is caused by the RVF virus (RVFV), which is transmitted by mosquitoes from several genera, mainly Aedes and Culex. However, whether ruminants can become infected by horizontal virus transmission remains unclear. In addition, how the route of RVFV inoculation may influence RVF pathogenesis and the host immune response in animals is still largely unknown. With this aim, we conducted a comparative experimental study in which young sheep were either inoculated subcutaneously (SC) or intranasally (IN) with the virulent RVFV 56/74 strain. We then evaluated disease dynamics, viremia, virus excretion, tissue damage, and the humoral immune response. We also aimed to determine whether RVFV can be transmitted from infected to in-contact animals, and to assess whether the inoculation route may influence virus excretion and the likelihood of subsequent horizontal transmission. The results showed that SC inoculated sheep had a shorter incubation period, an earlier onset of viremia, and an earlier seroconversion. In contrast, IN inoculated animals developed higher rectal temperatures, reached higher peak viremia, and developed a more robust neutralizing antibody response. They also exhibited increased concentrations of analytes indicative of moderate but more severe hepatic injury compared with the subcutaneous group, along with more pronounced histopathological damage in the central nervous system. These results demonstrate the influence of the route of inoculation on RVF pathogenesis and the host immune response. Our results also confirmed the horizontal transmission of RVFV between SC inoculated sheep and in-contact animals housed in the same room, a phenomenon not observed in the IN inoculated group. This finding underscores the influence of the inoculation route on virus transmission and the potentially significant role of horizontal transmission in RVF epidemiology and disease control.