Any time a sow farrows prematurely, there is the fear that additional litters will be lost. An abortion rate of less than 2 percent is considered an acceptable level in most herds. Premature farrowing usually results from nutritional, genetic or environmental stresses, but the exact cause is often unknown.
Abortion is the most dramatic expression of production loss.
When the incidence increases for a given herd, an infectious agent is usually suspected. This increase may follow the addition of replacement animals or the mingling of previously isolated groups of animals. Approximately 38 percent of diagnosed abortions are attributed to infectious causes. In some cases the infectious agent will directly affect developing fetuses and placentas, causing fetal compromise and death. Any severe illness in a pregnant sow may result in fetal death due to a disruption in the normal uterine environment.