Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (469 page)

Cultures
:
Klebsiella pneumoniae
isolates often produce mucoid colonies due to production of capsular material.

Susceptibility
: All
Klebsiella
species are intrinsically resistant to ampicillin and ticarcillin. Many hospital isolates have additional resistance through acquisition of plasmids that carry resistance genes. Extended-spectrum beta-lactamases confer resistance to third-generation cephalosporins and most other beta-lactam antibiotics.
Klebsiella pneumoniae
carbapenemases confer resistance to imipenem, ertapenem, and meropenem in addition to most beta-lactam antibiotics.

LISTERIA
INFECTION

   Definition

Listeriosis is caused by infection with
Listeria monocytogenes
, an aerobic, pleomorphic gram-positive bacillus. This organism is widely distributed in nature, and up to 5% of asymptomatic, healthy adults carry
L. monocytogenes
as a component of their endogenous fecal flora. CNS and placental tissue are predisposed to
Listeria
infection. Most infections are thought to occur as a result of oral ingestion, followed by invasion across the gut mucosa and systemic spread. Disease may occur in a sporadic or epidemic pattern.

   Who Should Be Suspected?

   
Listeria
is responsible for a small proportion of foodborne infections, and most cases are sporadic, but the case–fatality rate is relatively high. Outbreaks have been caused by a variety of types of food, including delicatessen meats, unpasteurized cheeses, smoked seafood, and processed meat spreads. Ingestion of contaminated food may cause self-limited gastroenteritis in normal hosts, with onset typically several days after exposure. Symptoms include fever, nausea, vomiting, and diarrhea. Flu-like symptoms are common.

   Risk factors associated with increased infection risk and severity include immunocompromise, age ≥70 years, alcoholism, glucocorticoid therapy, kidney disease, nonhematologic malignancy, neonatal infection, and pregnancy.

   In normal hosts, complete recovery is typical after several days of illness. During pregnancy, listeriosis usually presents with flu-like symptoms and may resolve spontaneously. Severe listeriosis may occur in the third trimester where placental infection and transmission to the fetus or neonate may occur. The signs and symptoms of
Listeria
sepsis are not distinctive, and diagnostic cultures are critical for specific diagnosis. Patients present with fever and malaise that may progress to shock and sepsis. Symptoms of meningoencephalitis are nonspecific and may include meningeal signs, mental status changes, or focal neurologic defects (e.g., ataxia, cranial nerve abnormalities, and deafness). Direct hematogenous seeding of the brain parenchyma may result in cerebritis or brain abscess, most typically manifested by stroke-like symptoms or focal neurologic defects.

   Laboratory Findings

Culture
(
blood
): Most reliable diagnostic test; culture of CSF and other infected tissue is indicated on the basis of clinical presentation. Isolation of
Listeria
from suspected food samples requires special techniques performed at reference laboratories.

Gram stain
: CSF Gram stain is only positive in about one third of patients with meningoencephalitis and lower in localized CNS infections.
Listeria
may be misidentified as
Streptococcus pneumoniae
, diphtheroids, or even
H. influenzae
.

CSF findings
: Pleocytosis is typical (100 to 10,000 WBCs/μL). Significant CSF lymphocytosis (>25%) may be seen on CSF WBC differential prior to antibiotic therapy. CSF protein concentration is typically moderately elevated; CSF glucose is reduced in only approximately 40% of patients with CNS infection. CSF findings may lead to misdiagnosis as viral infection, syphilis, Lyme disease, or TB.

Serology
: Not usually useful for diagnosis of acute listeriosis.

LYME DISEASE

   Definition

Lyme disease is a systemic, chronic borreliosis caused by
Borrelia burgdorferi
, a fastidious spiral bacterium. Infection is transmitted by the bite of
Ixodes
ticks. A variety of clinical manifestations are seen. Recurrent clinical disease may be caused by reinfection. Lyme disease is reportable in the Nationally Notifiable Infectious Diseases Surveillance System. Criteria for case definition may be seen on the CDC Web site: (
http://www.cdc.gov/ncphi/disss/nndss/casedef/lyme_disease_2008.htm
).

   Who Should Be Suspected?

   Acute disease occurs about 1–4 weeks after tick bite, manifested by nonspecific febrile symptoms that may be confused with a “viral syndrome.” Erythema migrans (EM) is characteristic for Lyme disease and occurs in 60–80% of infected patients. EM typically begins as a red papule with surrounding erythema that expands over days to weeks. The central region commonly clears, resulting in a bull’s-eye appearance. Secondary EM lesions may appear. Other common acute symptoms include fever, headache, and fatigue. Myalgias, arthralgias, and mild meningeal signs may occur. EM is diagnostic for Lyme disease in patients at epidemiologic risk, but its absence does not exclude this diagnosis. Laboratory confirmation is recommended for patients with EM with no known exposure or for patients with nonspecific signs and symptoms of Lyme disease.