Listeriosis, also known as circling disease, is caused by bacteria called Listeria monocytogenes. It is most often seen in temperate climates and affects ruminants, camelids, and, in rare instances, horses, with wide-ranging symptoms listed below. This bacterium thrives where there is moisture and organic material and the pH is in the 5-10 range.

During damp winters with above-freezing days and nights Listeria will flourish. There will be build-up of bacteria in poor quality wet forages, and in areas such as water troughs, feed troughs, hay racks, or around forages fed on the ground. It can survive for long periods in the environment and in asymptomatic carriers, and it proliferates in rotting vegetation. Common sources are spoiled silage, decaying forage at bottom of feed bunks, rotting hay at periphery of hay stacks.

Clinical signs:

  • Fever, which occurs early in the course of the disease and often disappears in 3-5 days.
  • Off feed
  • Depression
  • Dropped jaw, droop ear, decreased lip tone
  • Paralysis of the muscles that control the eyelid, which leads to exposure keratitis (inflammation of the cornea)
  • Drooling
  • Head tilt causing animal to circle or fall
  • Paralysis of the tongue
  • Blindness
  • Spontaneous abortion

Listeria causes microabcesses in the brainstem which causes all of these neurologic clinical signs. The theory is that the infection of the brain occurs due to ascent up cranial nerve rootlets.

In untreated cases the fatality rate is almost 100%. The disease in sheep and goats tends to be more acute and results in a higher case fatality rate than in cattle. Recovery rate is best if treatment is administered early in the course of disease. Animals that are recumbent, comatose, or convulsive rarely survive despite aggressive treatment. Recommended treatment is oxytetracycline (LA-200) or penicillin G. Supportive care may also be needed, especially if the tongue is paralyzed and the animal is not able to eat.