Listeria monocytogenes, the bacteria that causes listeriosi,s is widely distributed in nature and is found in soil, feedstuffs, and feces from healthy animals. It is most commonly associated with the feeding of moldy silage or spoiled hay, but because the organism lives naturally in the environment, listeriosis may occur sporadically.
Listeriosis usually presents itself as encephalitis (inflammation of the brain), but may also cause abortion in ewes. Sheep with the neurological form of the disease become depressed and disoriented. They may walk in circles with a head tilt and facial paralysis. Mortality is high and treatment (high doses of antibiotics) is generally not effective.