Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (467 page)

Escherichia coli
is responsible for a wide spectrum of opportunistic and nosocomial infections. It is a major cause of nosocomial pneumonia, bloodstream infection, surgical site infection, and UTI. It is also responsible for a significant proportion of severe neonatal infections, including sepsis and meningitis.

   Laboratory Findings

Culture
: Recognition of
E. coli
strains that cause enterohemorrhagic gastroenteritis may be improved by the use of the differential sorbitol–MAC agar. These strains produce Shiga toxin 1 and/or toxin 2, which may be directly detected in stool specimens by antigen testing or NAAT.

Serotyping
: In the United States, most isolates are serotype O157:H7. Although there are tests that can be used to identify other types of diarrheagenic
E. coli
, testing is not widely available. Specific diagnosis is rarely needed for patient management.

FRANCISELLA TULARENSIS
INFECTION

   Definition

Tularemia is caused by
F. tularensis
, a fastidious, tiny gram-negative coccobacillus. Naturally acquired tularemia is a zoonotic, tick-transmitted infection. The normal host species include rabbits, rodents, squirrels and other small mammals, and deer. Domestic livestock, especially sheep, are also susceptible to infection. Human infection is transmitted by direct contact with an infected animal or through the bite of an intermediate arthropod vector.

Francisella tularensis
is highly infectious and poses a serious risk for laboratoryacquired infections; clinicians should alert the laboratory when tularemia is suspected so that appropriate precautions and culture techniques are used. The CDC has classified
F. tularensis
as a potential bioterror agent. Possible or confirmed
F. tularensis
infections must be reported to state departments of health.

   Who Should Be Suspected?

Disease usually occurs 2–10 days after exposure, with ulceration at the site of tick bite and painful regional adenopathy. Nonspecific symptoms are common, including fever, chills, headache, sweats, severe conjunctivitis, and regional adenopathy. Approximately 20% of patients present with acute onset of fever and abdominal symptoms, including nonbloody diarrhea, vomiting, pain, and tenderness.

   Laboratory Findings

Gram stain
: Tiny faintly staining coccobacilli.

Culture
: Samples of blood, bone marrow, primary ulcers, lymph node aspirates, or other infected tissue. Cysteine is required for growth.

HAEMOPHILUS
INFECTIONS

   Definition

Haemophilus
species are fastidious gram-negative coccobacilli and are responsible for a variety of infectious syndromes. They are common components of the endogenous flora of the mouth and upper respiratory tract. Most of the respiratory species have limited virulence and are able to cause disease only when normal host defenses are compromised. Strains of
Haemophilus influenzae
may be encapsulated (serotypes a, b, c, d, e, and f). The serotype b capsular material is a virulence factor and is responsible for the ability of
H. influenzae
type b (Hib) to cause severe, invasive infections.
Haemophilus ducreyi
causes the STD chancroid.

   Who Should Be Suspected?

Most
Haemophilus
infections present as localized infections of the pararespiratory structures, like sinusitis or otitis media. Acute sinusitis is usually manifested by nasal congestion with purulent discharge, which may be unilateral. See the discussion of sinusitis in Chapter
13
. Respiratory, Metabolic, and Acid–Base Disorder.

Haemophilus influenzae
may cause an acute lobar pneumonia, but lower respiratory disease is most commonly manifested as bronchitis in patients with underlying lung disease. These patients typically present with nonproductive cough, wheezing, and increasing shortness of breath. In these patients,
Haemophilus
infection may cause significant deterioration of pulmonary function tests, hypoxemia, and dyspnea. Low-grade fever may be seen.

Epiglottitis, cellulitis of the supraglottic structures, is a life-threatening manifestation of Hib infection. The tissue may be directly seeded by posterior pharyngeal organisms or as a result of bacteremia. There is typically an abrupt onset of fever, malaise, severe sore throat, and dysphagia. Dyspnea, inspiratory stridor, and drooling develop with progression to severe disease, caused by obstruction of the airway by the swelling of the supraglottic tissue. Attempts to collect swab specimens for culture may stimulate acute obstruction, so they are contraindicated prior to securing a protected airway. Lateral x-ray studies of the hypopharyngeal region demonstrate swelling of the epiglottis. Culture of blood commonly yields
H. influenzae
.

Encapsulated strains, especially type b, may cause meningitis or invasive disease. Culture and analysis of blood and CSF should be submitted to establish the diagnosis. Other localized infections associated with bacteremic disease include septic arthritis, osteomyelitis, and cellulitis. Buccal and periorbital cellulitis have been commonly, but not exclusively, associated with Hib. Buccal cellulitis presents with swelling of the cheek with deep red discoloration. Periorbital cellulitis presents with signs and symptoms of pus accumulation in the orbital tissues and a characteristic purple discoloration of the lids and skin surrounding the affected eye.
Haemophilus influenzae
may also cause acute conjunctivitis and endophthalmitis.
Haemophilus influenzae
biogroup
aegyptius
has been implicated in conjunctivitis and Brazilian purpuric fever, a bacteremia syndrome with fever and hypotension, purpuric rash, vomiting, and abdominal pain.

Haemophilus ducreyi
causes chancroid, an ulcerative STI that occurs primarily in tropical regions. Disease is manifested by multiple genital and perineal ulcers. Unlike the chancres of syphilis, the ulcers of chancroid are painful and have ragged borders with minimal induration. Inguinal adenopathy is common and may progress to draining buboes. Like other genital ulcerative diseases, chancroid increases the risk of transmission of HIV infection.

   Laboratory Findings

Gram stain
: Diagnosis of
Haemophilus
infection depends primarily of Gram stain and culture of infected specimens. Gram staining shows small, pleomorphic, faintly staining gram-negative rods; some end-to-end pairing or small filamentous forms may be present.

Culture
:
Haemophilus
species are fastidious but are efficiently isolated on chocolate agar and in routine blood culture media. Positive cultures from the upper respiratory tract must be interpreted with caution because
Haemophilus
species, including encapsulated strains, are common components of the endogenous flora. Specimens for the diagnosis of chancroid are collected from the margin and undermined base of fresh ulcers.
Haemophilus ducreyi
is difficult to isolate by culture, requiring specialized enriched media that should be inoculated at bedside.