The circulation of mammarenaviruses in rodent populations of the Mekong region has recently been established, with a genetic variant of Wēnzhōu virus, Cardamones virus, detected in two Rattus species. This study tests the potential teratogenic effects of Wēnzhōu infection on the development of a Murid rodent, Rattus exulans. Using direct virus detection, morphological records and comparative analyses, a link was demonstrated between host infection status and host morphologies (the spleen irrespective of weight, the skull shape and the cranial cavity volume) at the level of the individual (females only). This study demonstrates that mammarenavirus infections can impact natural host physiology and/or affect developmental processes. The presence of an infecting micro-parasite during the development of the rat may lead to a physiological trade-off between immunity and brain size. Alternatively, replication of virus in specialized organs can result in selective morphologic abnormalities and lesions.