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 ...