Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (474 page)

This disorder is an infectious vasculitis due to
Rickettsia rickettsii
, an intracellular bacterium. RMSF is transmitted by infected ticks, primarily of the
Dermacentor
genus in the United States.

   Who Should Be Suspected?

Most patients present approximately 7 days after exposure with nonspecific symptoms, including fever, headache, malaise, and muscle and joint pains. Nausea and abdominal pain may be significant. Rash appears in approximately 90% of patients, usually 3–7 days after onset of illness. Rash typically starts on the wrists and ankles and then spreads widely, including the palms and soles. The rash becomes petechial; itching is not characteristic. Disease may progress to involve multiple organ systems, including gangrene, CNS manifestations, and other organ dysfunction.

   Laboratory Findings

Culture
: Requires special conditions and is rarely performed.

Histology
: DFA of skin biopsy for antigen has S/S of approximately 70%/100% and is the only specific test in early stages of disease. Sensitivity declines after the initiation of antimicrobial therapy.

Molecular tests
: PCR has been used to detect
R. rickettsii
DNA in blood and tissues.

Serology
: Sera should be collected during acute infection and then 2–4 weeks later for both IgG and IgM. A ≥4 times increase in IgG or total antibody, or specific IgM, is evidence of recent infection. IgM appears by days 3–8, peaks at 1 month, and lasts 3–4 months. IgG appears within 3 weeks, peaks at 1–3 months, and lasts for >12 months.

Core laboratory
: WBC is mildly elevated; thrombocytopenia may be severe.

SALMONELLA
AND
SHIGELLA
INFECTIONS

See Chapter
5
, Digestive Diseases.

STAPHYLOCOCCUS AUREUS
INFECTION

   Definition

Staphylococci are nonfastidious, aerobic GPCs that form clusters. The genus
Staphylococcus
is composed of several species that are implicated in human infection.
Staphylococcus aureus
is a frequent cause of pyogenic infection. Staphylococcal disease may also be caused by the elaboration of several potent toxins.

   Who Should Be Suspected?

Staphylococcus aureus
is able to cause disease in virtually all organ systems. The many clinical presentations of
S. aureus
infection include the following:

   
Pneumonia
: Pulmonary infections may be caused by aspiration of organisms from the upper respiratory tract or by hematogenous seeding from another primary site of infection.
Staphylococcus aureus
pneumonia may represent a severe complication of viral infection (e.g., measles, influenza), CF, or debilitating underlying disease.

   
Acute osteomyelitis, septic arthritis
: Osteomyelitis in adults is usually a result of a direct extension of local infection, often at the site of a surgical or traumatic wound. The vertebral column is a common site of infection of hematogenous origin. Septic arthritis in adults is usually of hematogenous origin.

   
Pyomyositis
:
Staphylococcus aureus
infection of skeletal muscle is usually caused by trauma or direct extension from an adjacent site.

   
Bacteremia and endocarditis
: Bacteremia may occur as a complication of localized pyogenic infection. Metastatic foci of infection are common. Patients generally present with acute sepsis syndromes, often with signs and symptoms due to localized infections. Endocarditis may be caused by seeding of valves during a primary bacteremia or by organisms directly introduced into the blood stream (e.g., intravascular catheter, injection drug use). Patients with endocarditis may present with subacute or acute symptoms. Normal cardiac valves are commonly affected.
Staphylococcus aureus
endocarditis causes rapid, severe damage to valves, producing acute mechanical heart failure (e.g., rupture of chordae tendineae, perforation of valve, valvular insufficiency) in addition to the physiologic effects of severe infection. Typical stigmata of endocarditis (e.g., Janeway lesions, splinter hemorrhages, Roth spots) are common.

   
Food poisoning
: Staphylococcal food poisoning is caused by ingestion of food tainted by enterotoxin-producing strains of
S. aureus
. Symptoms, including crampy abdominal pain, nausea and vomiting, and diarrhea, occur early (2–6 hours after ingestion). Patients are symptomatic for 8–10 hours after onset of illness. Aggressive fluid management forms the mainstay of therapy. Suspected clusters of foodborne gastroenteritis must be reported to state boards of public health.

   
Impetigo
: A superficial skin infection commonly affecting the face: seen most commonly in infants. The rash of impetigo presents with red macules that mature into vesicles, which may shed honey-colored serous fluid prior to drying. Most cases of impetigo are caused by
S. aureus
.