Listeria monocytogenes

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Chapter 181 Listeria monocytogenes

Listeriosis in humans is caused principally by Listeria monocytogenes, 1 of 6 species of the genus Listeria that are widely distributed in the environment and throughout the food chain. Human infections can usually be traced to an animal reservoir. Infection occurs most commonly at the extremes of age. In the pediatric population, perinatal infections predominate and usually occur secondary to maternal infection or colonization. Outside the newborn period, disease is most commonly encountered in immunosuppressed (T-cell deficiencies) children and adults and in the elderly. In the USA, food-borne outbreaks are caused by improperly processed dairy products and contaminated vegetables, and principally affect the same individuals at risk for sporadic disease.


L. monocytogenes is widespread in nature, has been isolated throughout the environment, and is associated with epizootic disease and asymptomatic carriage in more than 42 species of wild and domestic animals and 22 avian species. Epizootic disease in large animals such as sheep and cattle is associated with abortion and “circling disease,” a form of basilar meningitis. L. monocytogenes is isolated from sewage, silage, and soil, where it survives for >295 days. Human-to-human transmission does not occur except in maternal-fetal transmission. The annual incidence of listeriosis decreased by 36% between 1996 and 2004 and has remained level since then. However, outbreaks continue to occur. In 2002, an outbreak that resulted in 54 illnesses, 8 deaths, and 3 fetal deaths in 9 states was traced to consumption of contaminated turkey meat. The rate varies among states. Epidemic human listeriosis has been associated with food-borne transmission in several large outbreaks, especially in association with aged soft cheeses; improperly pasteurized milk and milk products; contaminated raw and ready-to-eat beef, pork, and poultry, and packaged meats; and vegetables grown on farms where the ground is contaminated with the feces of colonized animals. The incidence of Listeria infections in the USA in 2008 was 0.29 cases per 100,000 population, being highest in children <4 yr old and next highest in adults >60 yr. The ability of L. monocytogenes to grow at temperatures as low as 4°C increases the risk for transmission from aged soft cheeses and stored contaminated food. Small clusters of nosocomial person-to-person transmission have occurred in hospital nurseries and obstetric suites. Sporadic endemic listeriosis is less well characterized. Likely routes include food-borne infection and zoonotic spread. Zoonotic transmission with cutaneous infections occurs in veterinarians and farmers who handle sick animals.

Reported cases of listeriosis are clustered at the extremes of age. Some studies have shown higher rates in males and a seasonal predominance in the late summer and fall in the Northern hemisphere. Outside the newborn period and during pregnancy, disease is usually reported in patients with underlying immunosuppression, with a 100-300 times increased risk in HIV-infected persons and in the elderly (Table 181-1).


Listeriosis in pregnancy

Food-borne outbreaks/febrile gastroenteritis

Listeriosis in normal children and adults (rare)

Focal listeria infections (e.g., meningitis, endocarditis, pneumonia, liver abscess, osteomyelitis, septic arthritis)

Listeriosis in the elderly

The incubation period, which is defined only for common-source food-borne disease, is 21-30 days but in some cases may be longer. Asymptomatic carriage and fecal excretion are reported in 1-5% of healthy persons and 5% of abattoir workers, but duration of excretion, when studied, is short (<1 mo).