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.
Etiology
Members of the genus Listeria are facultatively anaerobic, non–spore-forming, motile, gram-positive bacilli that are catalase positive. The 6 Listeria species are divided into 2 genomically distinct groups on the basis of DNA-DNA hybridization studies. One group contains the species Listeria grayi, considered nonpathogenic. The 2nd group contains 5 species: the nonhemolytic species Listeria innocua and Listeria welshimeri and the hemolytic species L. monocytogenes, Listeria seeligeri, and Listeria ivanovii. L. ivanovii is pathogenic primarily in animals, and the vast majority of both human and animal disease is due to L. monocytogenes.
Subtyping of L. monocytogenes isolates for epidemiologic purposes has been attempted with the use of heat-stable somatic O and heat-labile flagellar H antigens, phage typing, ribotyping, and multilocus enzyme electrophoresis. Electrophoretic typing demonstrates the clonal structure of populations of L. monocytogenes as well as the sharing of populations between human and animal sources.
Selected biochemical tests together with the demonstration of tumbling motility, umbrella-type formation below the surface in semisolid medium, hemolysis, and a typical cyclic adenosine monophosphate (cAMP) test are usually sufficient to establish a presumptive identification of L. monocytogenes.
Epidemiology
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).
Table 181-1 TYPES OF LISTERIA MONOCYTOGENES INFECTIONS
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).
Pathology
One of the major concepts of Listeria pathology and pathogenesis is its ability to survive as an intracellular pathogen. Listeria incites a mononuclear response and elaboration of cytokines, producing multisystem disease, particularly pyogenic meningitis. Granulomatous reactions and microabscess formation develop in many organs, including liver, lungs, adrenals, kidneys, central nervous system (CNS), and, notably, the placenta. Animal models demonstrate translocation, the transfer of intraluminal organisms across intact intestinal mucosa. Histologic examination of tissues including the placenta shows granulomatous inflammation and microabscess formation. Intracellular organisms can often be demonstrated with special stains.
Pathogenesis
Listeria organisms usually enter the host through the gastrointestinal tract. Gastric acidity provides some protection, and drugs that raise gastric pH may promote infection. Studies of intracellular and intercellular spread of L. monocytogenes have revealed a complex pathogenesis. Four pathogenic steps are described: internalization by phagocytosis, escape from the phagocytic vacuole, nucleation of actin filaments, and cell-to-cell spread. Listeriolysin,
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