Listeriosis
Basic Information 
Epidemiology
Risk Factors
• Found in soil, water, vegetation, and silage. In horses, infection is highly associated with feeding of fermented feeds. Shed in feces of carrier animals, including horses. Shed in urine, uterine discharge, aborted fetal tissues, and milk. Abortions tend to occur in the winter months.
• Listeriosis infections are highly associated with horses located on farms where disease has occurred previously.
Contagion and Zoonosis
• Listeria monocytogenes is considered a zoonotic agent. Clinical listeriosis in humans occurs most often in pregnant women and immunocompromised patients.
• Possible sources for human infections include exposure to contaminated soil and food or to human and animal carriers. Most human epidemics have been traced to food sources of animal origin. Horse meat used for human consumption has been contaminated with L. monocytogenes.
Clinical Presentation
Physical Exam Findings
• Encephalitis: Characterized as a meningoencephalitis with the pathology in horses demonstrating a granulomatous encephalitis with microabscessation on histopathology. Animals may exhibit normal mentation with circling similar to ewes; however, severe mental depression is the most frequent presentation. Animals become unresponsive to external stimuli and usually die within 3 to 5 days.
• In Icelandic horses, many are febrile and inappetent, with diffuse diarrhea and death.
• Septicemia: Clinical signs include depression, anorexia, fever, diarrhea, and abdominal pain. Weakness, seizures, ataxia, jaundice, and respiratory distress have been reported as well.
• Conjunctivitis and keratitis: Range from purulent conjunctivitis to deep ulcerative keratitis and ophthalmitis
• Abortion: Sudden spontaneous abortion about 48 to 72 hours after infection in a dam. Fetuses are infected late in gestation and acquire septicemia. The fetus will be fairly fresh and is either dead at expulsion or dies shortly thereafter. Dams are usually normal and do not exhibit problems after abortion.
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