Bacterial Nervous System Infections

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Bacteria can infect the brain and spinal cord, causing abscesses. Or in a single example, Hansen's disease, they can infect the peripheral nerves. More commonly, bacteria infect the meninges and cerebrospinal fluid, causing meningitis.

Meningitis provokes an intense inflammatory response, with a marked accumulation of white blood cells in the cerebrospinal fluid and swelling of brain tissue. The incidence of the disease is, on the average, fewer than 10 cases per 100,000 population per year. A large number of bacterial species can infect the meninges. In many cases, these organisms are carried by healthy people, are transmitted by inhalation, and only produce meningitis in a small percentage of people who are infected. Organisms in this category include three important causes of bacterial meningitis, Haemophilus influenzae, Neisseria meningitidis, and Streptococcus pneumoniae. ■ Streptococcus pneumoniae, p. 576

Meningitis in infants during the first month of life is usually due to bacteria that colonize the mother's birth canal. Streptococcus pneumoniae, H. influenzae, and N. meningitidis are uncommon causes of meningitis in newborn babies, because most mothers have antibodies against them that cross the placenta and protect the baby. Instead, newborn babies become infected during labor and delivery by Gram-negative rods such as certain strains ofE. coli originating from the mother's intestinal tract. The reason that the newborn is susceptible to Gram-negative rod infections is that its mother's antibodies to the organisms are of the IgM class, which do not cross the placenta and therefore cannot confer immunity to the infant. ■ IgM antibody, p. 400

Streptococcus agalactiae, a Lancefield group B streptococcus, which colonizes the vagina in 15% to 40% of pregnant women, exceeds E. coli as a cause of meningitis in newborn infants. Babies also acquire this infection from the mother's genital tract around the time of birth. The American Academy of Pediatrics recommends that cultures selective for group B streptococci be obtained from the vagina and rectum late in pregnancy. Women with positive cultures can then be treated with an appropriate antibacterial medication shortly before or during labor. Evidence indicates that by using this screening procedure

1987 1989 1991 1993 1995 1997

Year

Figure 26.3 Rate of Serious Haemophilus Influenzae Disease per 100,000 Children Less than Age Five, United States, 1987 through 1998

Before the availability of conjugate vaccines in late 1987, H. influenzae type b was the most common cause of bacterial meningitis in preschool children.

1987 1989 1991 1993 1995 1997

Year

Figure 26.3 Rate of Serious Haemophilus Influenzae Disease per 100,000 Children Less than Age Five, United States, 1987 through 1998

Before the availability of conjugate vaccines in late 1987, H. influenzae type b was the most common cause of bacterial meningitis in preschool children.

doctors can decrease the incidence of serious group B streptococcal disease by more than 75%. ■ Lancefield grouping, p. 561

Of the six antigenic types of Haemophilus influenzae, labeled a through f, type b is responsible for most cases of serious disease. Starting in the late 1980s, children in the United States have been routinely immunized using a T-cell-dependent antigen consisting of H. influenzae b polysaccharide joined covalently to a bacterial protein such as diphtheria toxoid. The result of using these conjugate vaccines has been a dramatic decline in meningitis and other serious infections caused by H. influenzae type b (figure 26.3). The decline in meningitis due to H. influenzae has resulted in a marked overall decrease in bacterial meningitis and a shift in the peak incidence to older age groups. ■ T-dependent antigen, p. 404

20 mm

Figure 26.4 Streptococcus pneumoniae (Pneumococci) in the Spinal Fluid of a Person with Pneumococcal Meningitis (Gram Stain) The large red objects are polymorphonuclear leukocytes that have entered the spinal fluid in response to the infection.

20 mm

Figure 26.4 Streptococcus pneumoniae (Pneumococci) in the Spinal Fluid of a Person with Pneumococcal Meningitis (Gram Stain) The large red objects are polymorphonuclear leukocytes that have entered the spinal fluid in response to the infection.

Streptococcus pneumoniae (figure 26.4) is the leading cause of meningitis in adults. The organism is a prominent cause of otitis media, sinusitis, and pneumonia, conditions that often precede pneumococcal meningitis. Neisseria meningitidis, the meningococcus, differs from the other causes in that it is often responsible for epidemics of meningitis. ■ otitis media, p. 571

The relationship between age and causative bacterium is shown in figure 26.5.

Meningococcal Meningitis

Meningococcal meningitis occurs most commonly in children aged 6 to 11 months, but it also occurs frequently in older children and adults. It is greatly feared because it can sometimes progress to death within a few hours, but most patients respond well to treatment and recover without permanent nervous system damage.

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