Salmonella is an important cause of abortion in cattle and sheep. The organism may be carried by the host without clinical disease and is excreted in the saliva, milk, feces, urine, and in fluids discharged at abortion. Animals can become infected through feed, water, or bedding. The proposed pathogenesis of placental infection by Salmonella begins with an enteric infection followed by bacteremia and localization in lymph nodes, spleen, and lung. A second bacteremia occurs and results in infection of the placentome.
Bacterial growth causes destruction of the fetal villi and abortion of an autolyzed fetus.
The placenta is often retained after abortion. Grossly, the chorioallantois is thickened by amber, fibrin-containing fluid. Portions of the caruncle may remain adherent to the cotyledon. Histologically, there is mineralization of the trophoblast cells, interstitium of the villi, and chorioallantoic arcade. The villi are often expanded by large numbers of Salmonella , and, in some villi, neutrophils.