Symptoms associated with listeriosis may vary widely. Nonpregnant adults most often present with bacteremia (bacteria in the blood), meningitis (infection of the meninges, the membrane surrounding the brain and spinal cord), or meningoencephalitis (infection of the meninges and brain). Common presenting symptoms in patients with bacteremia, meningitis, or meningoencephalitis include fever, muscle aches, headache, stiff neck, malaise, ataxia (loss of balance), convulsions, and mental status changes, such as confusion. Less common clinical syndromes include endocarditis (infection of the lining of the heart), pneumonia, osteomyelitis (infection of the bone), septic arthritis (infection of the joint), abscesses of the brain or liver, peritonitis (infection of the membranes lining the abdominal cavity), and endopthalmitis (infection of structures within the eye). Symptoms secondary to these conditions depend on the anatomical site of infection and may include abdominal pain, bone pain, respiratory distress, pneumonia, and heart failure. Outbreak investigations have demonstrated that listeriosis can cause febrile gastroenteritis, a noninvasive syndrome, in healthy individuals. Patients with febrile gastroenteritis typically experience diarrhea, fever/chills, fatigue, headache, abdominal pain, and nausea.
Listeriosis during pregnancy can result in fetal loss (spontaneous abortion and stillbirth), premature delivery, and neonatal bacteremia or meningitis. Listeriosis may occur at any time during pregnancy; however, it is most often detected in the third trimester. Among pregnant women diagnosed with listeriosis, most reported only mild flu-like symptoms including fever, muscle aches (myalgias), headaches, joint pain (arthralgia), and gastrointestinal symptoms. Some pregnant women do not recall having had any symptoms before the diagnosis of listeriosis in their infant. In contrast to maternal illness, fetal and newborn infections are serious and frequently (~30%) fatal. Infants with listeriosis (neonatal listeriosis) may present with fever, lethargy, irritability, diarrhea, poor feeding, vomiting, respiratory distress, or a characteristic skin rash consisting of widely spread, small, pale nodules (granulomatosis infantiseptica). Neonatal listeriosis may be classified as early-onset or late-onset. Early-onset neonatal listeriosis, which occurs in the first 7 days of life, is most often associated with bacteremia or sepsis. Early-onset infections occur following maternal bacteremia with transplacental transmission before birth. Late-onset neonatal listeriosis is most often associated with meningitis. The mode of transmission is less clear in late-onset listeriosis; environmental sources may be involved in some cases, and outbreaks of hospital-acquired, neonatal listeriosis have been reported.
Listeriosis is caused by the bacterium L. monocytogenes. Nearly all cases of listeriosis (other than neonatal listeriosis) occur from eating contaminated food products, especially ready-to-eat, refrigerated foods. Unlike most foodborne bacterial pathogens, L. monocytogenes can grow at refrigeration temperatures. Listeria organisms are commonly found in the environment, including in water, soil, mud, and decaying vegetation. L. monocytogenes can be introduced into abattoirs and food manufacturing plants, which results in the contamination of food products such as meat or dairy products, raw produce, and processed foods. Numerous outbreaks have been traced to contaminated food products such as unpasteurized or improperly pasteurized milk, soft cheeses, and dairy products, hot dogs, turkey delicatessen meats, and cantaloupe.
Although L. monocytogenes exposures are common, the interaction of several factors is thought to play a role in the development of invasive disease. These factors include the number of bacteria a person consumes (dose), the virulence of the bacterial strain, and the functioning of the patient’s immune system.
Invasive listeriosis usually affects pregnant women and their unborn fetuses, newborn infants, older adults, and individuals with weakened immune systems (immunocompromised). Medical risk factors for invasive listeriosis include hematologic malignancy, organ transplantation, HIV infection or AIDS, alcoholism, and liver and kidney disease. In the United States, L. monocytogenes accounts for ~5% of all cases of bacterial meningitis in infants <2 months of age and ~10% of all cases of bacterial meningitis in people ≥65years of age. Invasive listeriosis occurs very infrequently in young, healthy adults. Overall rates of invasive listeriosis in the United States have remained relatively constant over the past decade (~3 cases per million population), as measured where active surveillance for listeriosis has been conducted.
A diagnosis of invasive listeriosis is confirmed through laboratory tests called cultures, which confirm the presence of L. monocytogenes in the body by isolating the bacteria from a clinical specimen. Cultures of blood, amniotic fluid, cerebrospinal fluid, placenta, or specimens from any affected organ systems may be performed to determine whether the bacterium L. monocytogenes is present. Certain x-ray tests such as computed tomography (CT) scan or magnetic resonance imaging (MRI) might be used to detect abscesses that may form on internal organs, especially the brain or liver.
Listeriosis is treated with antibiotics. The most commonly prescribed treatment is intravenous ampicillin. Many physicians also recommend treatment with the antibiotic gentamicin in combination with ampicillin. For affected individuals who cannot tolerate B-lactam antibiotics (such as ampicillin), trimethoprim-sulfamethoxazole is recommended. Erythromycin and vancomycin may also be used. Antibiotic treatment of pregnant women with documented listeriosis may prevent infection of the fetus. L. monocytogenes is resistant to all third generation cephalosporins. Other treatment is symptomatic and supportive.
Knowing what foods to avoid and how to safely prepare and store foods is the best way to reduce the risk of listeriosis. This information is most important for those persons in groups at higher risk for invasive listeriosis, especially pregnant women, older adults, and persons who have weakened immune systems. To prevent listeriosis and other foodborne diseases, thoroughly cook all raw foods of animal origin, such as meat, because heat kills L. monocytogenes. Reheat leftovers or prepackaged foods, especially deli meats, until steaming hot. Wash fruits and vegetables thoroughly if eating raw. Do not eat or drink raw (unpasteurized) milk or other dairy products. Do not eat soft cheeses such as Brie, feta, and Mexican-style cheeses, unless they are labeled as made from pasteurized milk. Additionally, be aware that Mexican-style cheeses and other Latin-style soft cheeses made from pasteurized milk have also caused listeriosis. Do not eat refrigerated smoked seafood, unless it is contained in a cooked dish, such as a casserole, or unless it is a canned or shelf-stable product. Follow recommended “sell by” or “best if used by” labels on processed foods. Keep unwashed and uncooked foods separate from foods that have been already prepared for eating.
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