Listeriosis in Deer: Circling Disease and Brainstem Infection

Listeriosis is a bacterial infection caused by Listeria monocytogenes. In deer and other ruminants, the best-known form is a brainstem infection called encephalitis or meningoencephalitis. Because affected animals often walk in tight circles or lean into objects, it is commonly called circling disease.

In many cases, the bacteria enter through the mouth and travel along nerves to the brainstem rather than spreading only through the bloodstream. That is why deer can develop one-sided facial paralysis, drooling, head tilt, trouble chewing, and balance problems. The disease is most often recognized in adult ruminants, while septicemia is less common in adult deer.

Listeriosis tends to show up in cooler weather and is strongly linked with spoiled or poorly fermented silage in housed or farmed ruminants. Deer kept in captivity, rehabilitation settings, or mixed-species farm environments may be at higher risk if feed storage and hygiene are not ideal.

This is also a zoonotic organism, meaning it can infect people. Pet parents and handlers should use gloves, avoid contact with saliva or aborted tissues, and follow your vet's biosecurity advice.

Listeria monocytogenes is widespread in soil, water, decaying vegetation, feces, and the intestinal tract of healthy animals. Deer are usually exposed by eating contaminated feed or forage. In captive cervids and other ruminants, spoiled silage is the classic source because the organism can multiply when fermentation is poor and the pH stays too high.

The disease is seen most often in winter and spring, especially when animals are being fed stored forage. Outbreaks may appear about 10 days or more after poor-quality silage is introduced. Even if many animals are exposed, only a few may develop obvious neurologic disease.

Small abrasions in the mouth from coarse feed, stemmy forage, or erupting teeth may help the bacteria invade local tissues and travel up cranial nerves to the brainstem. Stress, crowding, poor feed hygiene, pregnancy, and concurrent illness may also increase risk.

Although adult deer most often show the neurologic form, Listeria can also cause septicemia, abortion, stillbirth, or neonatal infection in ruminants. If one deer is affected, your vet may recommend reviewing feed management for the whole group.

Your vet usually starts with history and a focused neurologic exam. The combination of circling, depression, cranial nerve deficits, facial asymmetry, drooling, and recent exposure to silage or other questionable feed can make listeriosis highly suspicious in a deer.

Diagnosis during life is often presumptive, meaning it is based on the pattern of signs and risk factors rather than a single perfect test. Your vet may recommend bloodwork, evaluation for dehydration and metabolic problems, and testing to rule out other causes of neurologic disease. In herd or farm settings, feed assessment is also important.

Definitive diagnosis is most often made after death by testing brain tissue, especially the brainstem, with culture, PCR, or histopathology. This matters because other diseases can also cause circling or ataxia in deer, including brain abscesses, parasitic migration, otitis, trauma, toxic plants, and chronic wasting disease.

Because listeriosis is zoonotic and neurologic deer can be difficult to handle safely, diagnostics should be planned with both animal welfare and human safety in mind. Your vet may advise isolation, gloves, dedicated equipment, and careful disposal of contaminated bedding or feed.

Prevention starts with feed management. Do not feed moldy, spoiled, visibly contaminated, or poorly fermented silage, haylage, or baleage. Good silage fermentation matters because Listeria grows best when preserved forage does not acidify adequately. Clean bunks regularly, discard spoiled outer layers, and store feed to reduce moisture intrusion and contamination from soil or feces.

Work with your vet and nutrition team to review forage quality, especially in winter and spring when stored feed makes up more of the diet. If one deer develops suspected listeriosis, remove the suspect feed source immediately and monitor the rest of the group for subtle neurologic changes, reduced appetite, or abortions.

Reduce mouth trauma when possible by avoiding overly coarse or stemmy feed and maintaining feeders that do not splinter or injure oral tissues. Good stocking density, clean water, and stress reduction may also help lower disease risk.

Because Listeria monocytogenes is zoonotic, handlers should wear gloves when dealing with sick deer, aborted tissues, contaminated bedding, or suspect feed. Wash hands well, keep food-prep areas separate from animal areas, and follow your vet's advice on carcass handling, necropsy submission, and environmental cleanup.