Pregnancy increases the risk of mortality in West Nile virus-infected mice.

West Nile fever outbreaks in the USA have caused over 700 human deaths, primarily due to neurological disease. The usual transmission route of West Nile virus (WNV) involves mosquito bites; however, alternative routes, including intrauterine infection, have also been reported. Here, the pathogenicity of WNV in mice during gestation has been investigated. An extremely high mortality rate was observed in pregnant mice (98 %, 60/61) compared with non-pregnant mice (52 %, 28/53; P<0.001), independent of the infecting dose or the week of pregnancy. Antibody titres were similar between pregnant and non-pregnant mice and between surviving and non-surviving animals. WNV RNA titres in brains were also similar between pregnant and non-pregnant mice. WNV RNA could be detected in placentas and fetuses. These observations suggest strongly that, in the mouse model, pregnancy increases the risk of severe WNV infection and may help to understand the pathogenic mechanisms involved in WNV infection during pregnancy.