Listeriosis is caused by the gram-positive bacillus Listeria monocytogenes. The organism infects farm and laboratory animals worldwide and is rarely directly transferred to humans by handling infected animals. Ingestion of contaminated milk, cheese, and poultry is the main source of human infection. Vegan cheeses have also been associated with Listeria. While infection due to contaminated produce has been an uncommon occurrence, there has been an increase in produce-related illnesses in the past few years.

Although most adult infections appear to be minimally symptomatic, clinical presentations can range from mild to severe illness. Most symptomatic adult infections occur in immunocompromised patients and in those at the extremes of age, but infection in immunocompetent hosts does occur, particularly in farmers and veterinarians. Listeriosis is associated with increased susceptibility among pregnant individuals, who account for 1 in 4 reported cases in the United States. Infections during pregnancy can lead to stillbirth.

Severe infections present with meningitis, gastrointestinal symptoms and signs, or a glandular fever-like illness. Hematogenous spread and widespread dissemination is uncommon.

Cutaneous involvement outside of the neonatal period is rare and usually occurs from direct inoculation of the skin in farmers or veterinarians who are exposed to animal products of conception.

Pediatric patient considerations: As with adults, in children, infection and illness are more likely in the immunocompromised. Infection can occur by ingestion of contaminated or unpasteurized milk or ingestion of cheeses from unpasteurized milk.

The highest incidence of disease is in the neonatal period. See neonatal listeriosis for more information.