Clinical Overview of Listeriosis

Etiology

Listeria monocytogenes is a facultatively anaerobic, rod-shaped, gram-positive bacterium. It can be readily isolated in standard bacterial culture of normally sterile body sites. It is widespread in the environment and can be isolated from soil, water, and decaying vegetation.

Serotyping differentiates isolates of Listeria below the species level. Listeria serotypes are designated based on the immunoreactivity of two cell surface structures, the O and H antigens. Twelve serotypes of L. monocytogenes (1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4a, 4b, 4c, 4d, 4e, and 7) are recognized, three of which (1/2a, 1/2b, and 4b) cause most (95%) human illness; serotype 4b is most commonly associated with outbreaks.

Serotyping differentiates isolates of Listeria below the species level. Listeria serotypes are designated based on the immunoreactivity of two cell surface structures, the O and H antigens. Twelve serotypes of L. monocytogenes (1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4a, 4b, 4c, 4d, 4e, and 7) are recognized. Three of these (1/2a, 1/2b, and 4b) cause most (95%) human illness. Serotype 4b is most commonly associated with outbreaks.

Who is at risk

Older adults, people with weakened immune systems, and pregnant people and their newborns are at highest risk of listeriosis. Otherwise healthy young people only rarely develop invasive listeriosis.

Risk factors

Older adults, people with weakened immune systems, and pregnant women and their newborns are at highest risk of listeriosis. Otherwise healthy young people only rarely develop invasive listeriosis.

How it spreads

Listeriosis is usually acquired through foodborne transmission, except for fetal and neonatal infection, which is usually acquired in utero.

Cutaneous infections have been reported very rarely among veterinarians and farmers following direct animal contact. Many of these cases have involved particularly livestock products of conception.

Incidence

The annual incidence of laboratory-confirmed listeriosis in the United States is about 0.24 cases per 100,000 population, based on active surveillance in 10 FoodNet sites. The U.S. Healthy People 2020 target for listeriosis is 0.2 cases per 100,000.

Approximately 800 laboratory-confirmed cases are reported annually to CDC's National Notifiable Disease Surveillance System. However, many cases are not detected or reported, and CDC's 2011 Estimates of Foodborne Illness, which includes estimates of cases not diagnosed or reported, indicates that approximately 1,600 cases occur annually in the United States.

Mortality rates

CDC estimates that listeriosis is the third leading cause of death from foodborne illness with about 260 deaths per year.

Nearly everyone with listeriosis is hospitalized. The case-fatality rate is about 20%. Nearly 25% of pregnancy-associated cases result in fetal loss or death of the newborn.

Clinical features

The clinical features of listeriosis depend on the patient.

In older adults and people with immunocompromising conditions, the most common clinical presentations are invasive infections, such as sepsis, meningitis, and meningoencephalitis. People can also experience focal infections, including septic arthritis, osteomyelitis, prosthetic graft infections, and infections of sites inside the chest and abdomen or of the skin and eye. Less commonly, otherwise healthy young people may also develop invasive listeriosis.

Listeriosis during pregnancy is typically a relatively mild "flu-like" illness. Some pregnant people with Listeria infection have no symptoms. Although severe disease in the pregnant person is rare, infection during pregnancy can result in miscarriage, stillbirth, preterm labor, and sepsis or meningitis in the neonate.

• Some neonates with listeriosis develop granulomatosis infantiseptica, a severe disorder involving the internal organs and skin.
• Neonatal listeriosis is classified as early (within 6 days of birth) or late onset (7­–28 days after birth).
  • Early-onset neonatal listeriosis is usually acquired through transplacental transmission.
  • The sources of late-onset listeriosis are less clear; they may involve exposure during delivery or nosocomial exposure.

People with normal immune systems rarely develop invasive infection. However, they may experience a self-limited acute febrile gastroenteritis following high-dose Listeria exposure. Because Listeria cannot be detected by routine stool culture, febrile gastroenteritis from Listeria infection is rarely diagnosed outside of outbreak settings.

Trends

Listeria was first recognized as a foodborne pathogen in the 1980s. The incidence was first estimated at about 0.8 cases per 100,000 in the late 1980s. By the early 2000s, listeriosis incidence had declined to about 0.3 cases per 100,000. Since then, listeriosis incidence has remained largely unchanged.