Section 22. Rickettsial Infection

Rocky Mountain spotted fever (RMSF), the most severe of the rickettsial infections, is characterized by sudden onset of fever, severe headache, myalgia, and a characteristic acral exanthem; it is associated with significant mortality.

Epidemiology

Age Any age.

Gender M > F.

Incidence Uncommon.

Etiology Caused by Rickettsia rickettsii gram-negative obligate intracellular bacteria.

Season April to September in the United States (95% of patients).

Geography Endemic in Virginia, North Carolina, South Carolina, Georgia, Kansas, Oklahoma, Texas, New York, northern Mexico, central, and South America.

Pathophysiology

Occurs through bite of an infected tick or inoculation through abrasions contaminated with tick feces or tissue juices. The reservoirs and vectors are the wood tick (Dermacentor andersoni) in the western United States, the dog tick (D. variabilis) in the eastern United States, and the lone star tick (Amblyomma americanum) in the southern United States. Patient either lives in or has recently visited an endemic area; however, only 62% have knowledge of a recent tick bite. Following inoculation, there is an initial local replication of the organism in endothelial cells followed by hematogenous dissemination. Focal infection of vascular smooth muscle causes a generalized vasculitis. Hypotension, local necrosis, gangrene, and DIC may follow. The petechial rash results from extravasated blood after vascular necrosis.

History

The incubation period for RMSF after tick exposure ranges from 5 to 7 days. The rash, present in 90% of cases, begins on day 3 or 4 on the extremities and spreads proximally. By day 6 or 7, the rash is generalized. Systemic symptoms include abrupt onset of fever (94%); severe headache (94%); generalized myalgia, especially the back and leg muscles (87%); a sudden shaking rigor; photophobia; prostration; and nausea with occasional vomiting.

Physical Examination

Skin Findings

Type Macule, papule, eschar, infarcts, gangrene.

Color Pink, deep red, violaceous (Fig. 22-1A).

Size 2 to 6 mm.

Distribution Distal extremities, palms, soles to arms, thighs, trunk, face (Fig. 22-1B). Gangrene (4%) occurs in acral digits, extremities, ears, and prepuce.

General Findings

Fever Up to 40°C.

Rheumatologic Myalgias.

Pulmonary Cough, pneumonitis, lung infections, pulmonary edema, ARDS.

GI Nausea, abdominal pain, hepatosplenomegaly, GI hemorrhages.

Renal Incontinence, oliguria, acute renal failure.

CNS Altered consciousness, headache, meningoencephalitis, stupor, coma.

Other Hypotensive, deafness, middle ear/ parotid gland infections, septic shock.

Variants

• Spotless Fever (13%) Higher mortality because diagnosis is missed.
• Abdominal Syndrome Mimics acute abdomen, acute cholecystitis, acute appendicitis.

Differential Diagnosis

The differential diagnosis for RMSF includes meningococcemia, Staphylococcus aureus septicemia, other rickettsioses (ehrlichiosis, murine typhus, epidemic typhus, rickettsialpox), leptospirosis, typhoid fever, viral exanthem (measles, varicella, rubella, enterovirus, parvovirus, adenovirus), hemorrhagic fevers (Ebola, Marburg, Lassa), meningococcemia, chancroid, tularemia, syphilis, anthrax, ITP, drug reaction, or an immune–complex-mediated vasculitis.

Laboratory Examinations

Dermatopathology Necrotizing vasculitis. Rickettsia can at times be demonstrated within the endothelial cells.

Direct Immunofluorescence Stains for R. rickettsii antigen within endothelial cells.

Serology Immunofluorescent antibody test can be used to measure both IgG and IgM antibodies against R. rickettsii. A fourfold rise in titer between acute and convalescent stages of the disease is diagnostic.

Course and Prognosis

Untreated, the fatality rate for RMSF is 20%. With adequate therapy, the mortality decreases to 3%. Severely affected patients may experience DIC, purpura fulminans, permanent cardiac, and/or neurologic sequelae. The most important prognostic factor is early diagnosis and treatment even if this means starting antibiotics before confirmatory study results are available.

Management

The drug of choice for RMSF in older children and adults is doxycycline, tetracycline, or chloramphenicol for 7 to 10 days. In younger children, azithromycin or clarithromycin may be used.