Infection occurs primarily by eating or inhaling the organism. The infected cow may have a brief fever and depression but abortion will only follow after a stressful event, so a long time may pass before an abortion occurs. After L. monocytogenes enters the body, it travels through the blood stream to the reproductive tract. Once there, it infects the placenta. If large numbers of L. monocytogenes bacteria are present, they may invade the fetus and cause it to die.
Abortion usually follows within two to three days.
The diagnosis is made when the bacterium is cultured from fetal or placental tissues. Culturing the organism from silage or spoiled forages is often unsuccessful because it tends to be unevenly distributed within the feed, and the sample that is collected may not be from the right spot. Abortion due to L. monocytogenes is common in Alberta. Treatment with antibiotics is the usual practice after an abortion has occurred. No vaccine is available for prevention.