Causative Agent: Listeriosis, or circling disease, is a central nervous system and digestive system infection caused by the bacterium Listeria monocytogenes. This bacterium is resistant to a wide range of temperatures. It can survive in temperatures ranging from 39 degrees Fahrenheit to 111 degrees Fahrenheit (40 C to 440 C). Listeria monocytogenes lives in the soil and in animal intestinal tracts. It may also live in spoiled silage where the pH is less acidic than in good quality silage.
Clinical Signs: Many species of animals are susceptible to listeriosis including cattle, sheep, poultry, pigs, dogs, cats, rabbits, and other small mammals. Each species may display varying signs. In general, ruminants (cattle, sheep, and goats) exhibit central nervous system signs and monogastric animals (pigs, dogs, cats, and rabbits) show signs of septicemia (infection in the blood and several organs). Poultry also show signs of septicemia including damage to the liver and heart. Abortion, stillbirths, and neonatal death occur in all species. In sheep and goats, the infection localizes in the brain stem causing encephalitis. Small ruminants are more susceptible to the disease than are cattle. Some of the first signs observed in sheep and goats include depression, anorexia (not eating), disorientation, head tilt, and circling. As the disease progresses, facial paralysis may develop. The paralysis usually affects only one side of the face, causing the ear and eyelid to droop, the muzzle to be pulled to one side, and a lack of muscle tone in the lip of the effected side. Profuse salivation may also be observed. Sick animals may also be unable to stand or fall when standing. They may also exhibit involuntary running movements while lying down.
Disease Transmission: Listeriosis can be contagious between animals through inhalation or ingestion (eating) of the bacteria. Ingestion could occur by eating pasture, hay, or silage that was contaminated with feces containing Listeria monocytogenes. Lesions or breaks in the membranes of the mouth can allow the organisms to enter the body.
Diagnosis: Listeriosis may be diagnosed by analyzing cerebrospinal fluid (CSF) collected from the lumbosacral spinal region. The CSF will have an elevated protein concentration, and large mononuclear cells may also be present microscopically. The diagnosis can be confirmed by isolation and identification of the bacteria. This can be accomplished by submitting specific organs or specimens to a veterinary diagnostic laboratory. The brain should be submitted if there has been nervous system involvement. If abortion has occurred, the placenta and fetus should be submitted. Specimens that may help confirm the diagnosis include spinal fluid, nasal discharge, urine, feces, or milk. These specimens are often only helpful if the animal has septicemia (organisms in the blood).
Treatment: The drug of choice for treating listeriosis is penicillin G. The recommended dose is 40,000 IU/kg of body weight intramuscularly (IM) 3-4 times daily for 7 days, followed by 22,000 IU/kg of body weight once daily for an additional 14-21 days. Intramuscular penicillin treatment may be supplemented for the first 3-5 days with potassium penicillin G administered intravenously (IV) at 44,000 IU/kg of body weight 3-4 times daily. Oxytetracycline has also been used at a dose of 10 mg/kg IM twice daily. Other antibiotics that may be effective are ceftiofur, florfenicol, and trimethoprim/sulfonamide. Supportive care such as intravenous (IV) fluids and electrolytes are necessary if the infected animal is having difficulty eating or drinking. Any feed (spoiled silage) suspected to be the cause should be removed.
Recovery: Recovery depends primarily on early intervention with high doses of antibiotics. In cases of severe encephalitis, death may occur despite antibiotic treatment.
Prevention: To prevent listeriosis, care should be taken to avoid feeding livestock spoiled silage. Ensiling corn before it becomes too mature and feeding grass silage that contains additives may also help prevent listeriosis. Crops prepared under these conditions create silage with a more acidic pH which inhibits multiplication of Listeria monocytogenes. Avoid pastures that are boggy and areas where the soil has a high pH. Rough, coarse feeds that have the tendency to injure the mouth should also be avoided. Additionally, if an outbreak of listeriosis occurs, ill animals should be isolated from healthy animals to prevent the spread of disease between animals.
Zoonotic Risk: Listeriosis can occur in humans and is potentially life-threatening. To prevent human illness, all suspicious material should be handled with caution. This includes all dead animals and any aborted fetuses. Extra caution should be used when performing a necropsy on any animal suspected of having listeriosis. Infected female animals may harbor the bacteria in their milk. Listeriosis is a public health concern in these animals since the bacteria may survive certain forms of pasteurization.