eMedicine Specialties > Emergency Medicine > Infectious Diseases

Tick-Borne Diseases, Rocky Mountain Spotted Fever

Author: Allon Amitai, MD, International Emergency Medicine Fellow, Rhode Island Hospital; Consulting Staff, Memorial Hospital of Rhode Island; Doctoring Preceptor, Brown University Medical School
Coauthor(s): Richard H Sinert, DO, Associate Professor of Emergency Medicine, Clinical Assistant Professor of Medicine, Research Director, State University of New York College of Medicine; Consulting Staff, Department of Emergency Medicine, Kings County Hospital Center
Contributor Information and Disclosures

Updated: Nov 11, 2009

Introduction

Background

Rocky Mountain spotted fever (RMSF), classically characterized by fever, myalgias, headache, and a petechial rash, is the most common fatal tick-borne disease in the United States. As one of the spotted fevers, it belongs to a large group of tick- and mite-borne infections caused by closely related rickettsiae. These organisms are small, gram-negative bacteria that grow strictly in eukaryotic cells. Rickettsia rickettsii is the organism responsible for RMSF.

The tick that functions as the vector and the reservoir in RMSF is usually the American dog tick (Dermacentor variabilis) in the eastern United States or the Rocky Mountain wood tick (Dermacentor andersoni) in the western United States. However, a 2002-2004 outbreak in Arizona was traced to the common brown dog tick (Rhipicephalus sanguineus), a tick common worldwide but not previously recognized as a vector of disease. 

The chance for an individual tick to harbor Rickettsia rickettsii is slight. Although an estimated 4% of the American dog ticks are infected with Rickettsia species, the vast majority of these organisms are nonpathogenic Rickettsia. Therefore, prophylactic treatment of a tick bite is not indicated.

Mortality rates as high as 30% were reported for RMSF in the preantibiotic era. The current mortality rate is 1.4%. A significant portion of this persistent mortality likely is due to delay in diagnosis and treatment.

Pathophysiology

Rickettsia are introduced into humans after an infected tick feeds for more than 6 hours. The tick bite is painless and frequently goes unnoticed. Rickettsia enter the skin and spread via lymphatics to the bloodstream and attach to their target cells, vascular endothelial cells.

Rickettsia replicate intracellularly and after an average of 1 week (range, 3-12 d), the patient develops clinical manifestations of infection. Illness is characterized by increased vascular permeability with a subsequent host mononuclear cell response. Systemic increase in vascular permeability leads to edema, hypovolemia, and hypoalbuminemia.

In skin, vascular injury initially appears as blanchable erythematous macules (1-5 mm in diameter). Eventually, progression to the classic petechial rash of Rocky Mountain spotted fever (RMSF) usually occurs, although from 10-15% of patients will not have any clinically apparent dermatologic involvement (Rocky Mountain spotless fever).

Central nervous system (CNS) manifestations include encephalitis and meningoencephalitis secondary to vascular injury. Seizures, cranial nerve damage, and permanent blindness and deafness have been documented.

Pulmonary involvement may lead to noncardiogenic pulmonary edema, interstitial pneumonia, and adult respiratory distress syndrome (ARDS), contributing significantly to mortality.

Myocarditis may occur secondary to microcirculatory vasculitis. 

Renal manifestations include decreased glomerular filtration rates (GFRs) and prerenal azotemia from hypovolemia.

Focal hepatocellular necrosis occurs in 38% of patients, as evidenced by moderately increased serum aminotransferase levels. Autopsies of patients with RMSF have revealed portal triaditis and vasculitis. However, hepatic failure does not typically occur.

Gastrointestinal (GI) endothelial cell injury leads to abdominal pain, nausea, vomiting, and diarrhea. Many patients have guaiac positive stools. Thirty percent of patients are anemic, and death has been reported from massive GI bleeding.

Remember that RMSF is a multisystem disease. In any particular patient, one organ system may be more affected than the others.

Frequency

United States

In 2004, 1514 cases were reported—more than 4 times the 365 cases reported in 1998. The reasons for this increase are not known, but wide swings in the incidence of Rocky Mountain spotted fever (RMSF) have occurred since 1920.

Seasonal outbreaks of RMSF parallel the activity of the tick; 90% of cases are reported from April 1 to September 30, with peaks in May and June.

Cases are geographically distributed; North Carolina and Oklahoma account for one third of total cases reported. South Carolina, Tennessee, and Georgia accounted for the third, fourth, and fifth highest number of cases. Less than 2% of the total number of cases are found in the Rocky Mountain states.

Asymptomatic infection may be common; in one study, 12% of children living in high-risk zones had positive serology test results, indicating past exposure to RMSF.¹

International

Rocky Mountain spotted fever has been extensively documented in Canada, Mexico, Central America, Colombia, and Brazil. No reports document RMSF infection outside of the Americas. However, a wide range of related spotted fever group (SFG) rickettsioses has been described across Europe, Africa, Asia, and Oceania. The true incidence of spotted fever infections internationally is not known.

Mortality/Morbidity

The overall case-fatality rate of Rocky Mountain spotted fever (RMSF) has been reported as 1.4%.² Children younger than 5 years have a reported fatality rate of 5%.² Adults older than 70 years have a reported case-fatality rate of 9%.³ Death occurs on average 8 days after onset of symptoms.⁴ Long-term scarring, blindness, and deafness have been documented.⁵

• Patients treated 5 or more days after onset of symptoms experienced 3 times the mortality rate of patients treated earlier. Long-term morbidity is most usually seen in patients in whom treatment has been delayed.
• Patients who died received antibiotics an average of 2 days later than patients who lived.

Race

Whites have twice the incidence of African Americans. However, African Americans have a higher case-fatality rate, likely due to the greater difficulty of appreciating a rash in highly pigmented individuals.

American Indians are at greater risk for RMSF than the general population.⁶ From 2001-2005, the average annual incidence of RMSF reported among American Indians was 16.8 per 1,000,000 persons compared with 4.2 for whites, 2.6 for blacks, and 0.5 for Asian/Pacific Islanders. The incidence of RMSF in American Indians increased at a disproportionate rate during this period, though the rate was comparable to those for other races from 1990-2000.⁷

Sex

The male-to-female ratio is 1.7:1.

Age

• Incidence of Rocky Mountain spotted fever is highest among adults aged 60-69 years (3.1 cases/million persons) and children aged 5-9 years (estimated 3.3 cases/million persons).
• The highest case-fatality rate (5%) occurs among children younger than 5 years.

Clinical

History

A high index of suspicion is the most important aspect of diagnosing Rocky Mountain spotted fever (RMSF). Diagnosis is typically delayed. In the case series by Buckingham et al, of 92 patients eventually diagnosed with RMSF, the median delay between first visiting a health care provider and starting antirickettsial therapy was 5 days; only 49% reported a tick bite.⁸

In other studies, 66% of reported cases had a history of tick attachment 14 days prior to illness. An additional 26% of patients reported being in a tick-infested area.

Children may often present without a known history of tick exposure.

• Fever greater than 102°F - 94% of reported cases
• Fever within 3 days after tick bite - 66% of reported cases
• Headache, frequently severe - 86% of reported cases
• Myalgias - 85% of reported cases
• CNS - 25% of patients develop signs of encephalitis (ie, confusion, lethargy). This may progress to stupor, delirium, seizures, or coma.
• GI - Some patients present with anorexia, nausea, vomiting, diarrhea, and abdominal pain.

Physical

• Temperature usually is greater than 102°F.
• Seventeen percent of patients are hypotensive on presentation.
• Rash
  • Rash affects 85-90% of patients overall, usually after onset of fever, headache, myalgias, and GI symptoms.
  • Fifteen percent have a rash on the first day.
  • Fifty percent have a rash by the third day.
  • Approximately 10-15% of patients have Rocky Mountain spotless fever. This more often is reported in older patients and African American patients. Spotless fever is not synonymous with mild or early illness because substantial proportions of the deaths occur in patients without a rash.
  • Rash typically begins around the wrists and ankles, but it may start on the trunk or be diffuse at the onset.
  • Classic distribution of RMSF rash on the palms and soles occurs relatively late in the course, in 43% of patients only after the fifth day of symptoms.
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    The palm of a patient with Rocky Mountain spotted fever exhibiting the classic petechial rash associated with the disease. Courtesy of Sadhana Sathe, MD, PhD.

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    The palm of a patient with Rocky Mountain spotted fever exhibiting the classic petechial rash associated with the disease. Courtesy of Sadhana Sathe, MD, PhD.

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    The petechial rash of Rocky Mountain spotted fever affecting the sole and the dorsum of the patient's foot. Courtesy of Sadhana Sathe, MD, PhD.

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    The petechial rash of Rocky Mountain spotted fever affecting the sole and the dorsum of the patient's foot. Courtesy of Sadhana Sathe, MD, PhD.

  • Some reports have documented 36-80% of patients with RMSF lack the classic distribution of rash on the palms and soles.
  • Four percent have skin necrosis or gangrene secondary to hypoperfusion.
• Jaundice occurs in 8-9% of patients.
• CNS
  • Confusion and lethargy are present in 26-28% of cases.
  • Ataxia is present in 18% of cases.
  • Coma occurs in 9-10% of cases.
  • Seizures are present in 8% of cases.
  • Many other findings have been reported, including cranial nerve palsies, hearing loss, meningismus, photophobia, severe vertigo, dysarthria, aphasia, hemiplegia, paraplegia, complete paralysis, nystagmus, hyperreflexia, spasticity, fasciculations, and neurogenic bladder.
• Pulmonary findings can be consistent with pulmonary edema or focal infiltrates.
• Patients may present with an acute abdomen leading to an exploratory laparotomy. Ten percent of patients have heme-positive stools.

Causes

R rickettsii is the organism responsible for Rocky Mountain spotted fever (RMSF).

• Vectors
  • D variabilis - American dog tick in the eastern United States
  • D andersoni - Rocky Mountain wood tick in the western United States
  • Rhipicephalus sanguineus - Common brown dog tick in Mexico and Arizona (R rickettsii has been detected in this tick in California.)
  • Amblyomma cajennense - Central America and South America

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