Listeriosis in Sheep: Circling Disease, Abortion, and Treatment

Listeriosis is a serious bacterial disease caused most often by Listeria monocytogenes. In sheep, it is best known for the neurologic form called circling disease, but it can also cause abortion, septicemia, and sudden death. The bacteria are widespread in the environment and can survive in soil, feces, water, and decaying plant material.

In adult sheep, the most common form is infection of the brainstem. This leads to one-sided nerve damage, which is why affected sheep may circle, droop one ear or eyelid, salivate, or have trouble chewing and swallowing. Pregnant ewes may instead abort, often late in gestation, and some flocks see both neurologic cases and abortions around the same time.

This is an emergency because the disease can progress quickly. Sheep may move from mild depression and a subtle head tilt to recumbency and death in a day or two. Prompt veterinary care can improve the chance of survival, especially before the sheep becomes unable to stand or eat.

Listeriosis is caused by the bacteria Listeria monocytogenes. Sheep usually become infected by eating contaminated feed or forage. Poor-quality silage is the classic source, especially when the pH is not low enough to suppress bacterial growth. Moldy, spoiled, or visibly rotten feed also raises risk.

The bacteria are not limited to silage. They can also be found in soil, water, feces, bedding, and decaying vegetation. That means sheep can be exposed from the environment, especially in cool, wet conditions or where feed is offered on the ground and becomes contaminated.

Stressors can make disease more likely after exposure. Common examples include late pregnancy, overcrowding, sudden ration changes, confinement, poor sanitation, and harsh weather shifts. In many flocks, only a small number of sheep become obviously sick even when many animals have been exposed.

In the neurologic form, the bacteria are thought to enter through small injuries in the mouth and travel along cranial nerves to the brainstem. In pregnant ewes, the uterus and placenta can become infected, leading to placentitis, fetal infection, abortion, stillbirth, or weak newborn lambs.

Your vet often makes a presumptive diagnosis based on history and exam findings. A sheep with circling, a head tilt, one-sided facial paralysis, depression, and recent exposure to silage or spoiled feed strongly raises suspicion for listeriosis. Because the disease can move fast, treatment may begin before every test result is back.

Diagnosis is usually built from several pieces: the neurologic exam, flock history, feed history, pregnancy status, and whether abortions are occurring. Your vet may also consider other causes of neurologic disease in sheep, such as polioencephalomalacia, otitis, brain abscess, toxicity, or scrapie, depending on the case.

If abortion is part of the problem, the best samples are the placenta, fetus, and uterine discharge, along with history from the ewe. Laboratory confirmation may involve bacterial culture or PCR on placenta, fetal tissues, or brain tissue. In neurologic cases that die or are euthanized, necropsy and brainstem testing can confirm the diagnosis.

Because listeriosis can affect people, safe sample handling matters. Your vet may recommend gloves, separate lambing or sick pens, and careful disposal of aborted materials while the diagnosis is being worked up.

Prevention starts with feed management. Avoid spoiled, moldy, or visibly rotten silage, haylage, and stored forage. If silage is fed, good fermentation matters because more acidic silage is less likely to support Listeria growth. If a case appears in the flock, your vet may recommend removing the suspect feed source right away.

Keep feed bunks, troughs, and water sources as clean as possible. Feeding on the ground can increase contamination from soil and feces, so elevated feeders are often helpful. Regularly remove wet, dirty bedding and reduce crowding where sheep are confined.

Pregnant ewes deserve extra attention because stress and late gestation can increase risk. Minimize sudden ration changes, protect sheep from severe weather when possible, and watch closely for dullness, isolation, or abortion. Any ewe that aborts should be isolated promptly, and placentas and fetuses should be disposed of safely under your vet's guidance.

Listeriosis is also a human health concern. Wear gloves when handling sick sheep, aborted materials, or contaminated bedding. Wash hands well, keep children and immunocompromised people away from suspect materials, and talk with your vet about flock-level steps if more than one sheep is affected.