In non-immune sows the course on the infection is radically different depending on whether the sow is pregnant or not.
In non gestating adult sows, clinical signs are usually mild or absent and limited in any case to slight fever, loss of appetite for some days. There is no evidence for the replication of the virus in the ova although PRRSV can be transiently found in macrophages residing in the atretic ovarian follicles.
In gestating sows, the outcome of the infection depends on the gestational period at which the infection takes place.
This is because PRRSV is unable to cross the placental barrier before day 72 of gestation. When the infection takes place after day 72 of gestation, fetuses are infected and abortions may occur. Mummies are typically seen within the aborted litters. Additionally premature or delayed farrowing as well as the delivery of a mix of live piglets (usually viraemic pigs with low viability), mummies and stillborn piglets may occur.
When the infection of the sow occurs at earlier gestation phases, the outcome of the infection is unclear. However, in early studies it was already shown that inoculation of pregnant sows before day 90 of gestation resulted in a significantly lower impairment of the reproductive performance even when using a highly virulent strain. It is assumed that infections before implantation of the embryos have no impact on the reproductive performance because PRRSV is unable to cross the zona pellucida.
No definitive evidence exist regarding the implication of PRRSV in inducing early embryonic or fetal deaths that could be observed as acyclical returns to oestrus or early abortions (less than 72 days) although there is evidence regarding the potential of the virus for causing placentitis.