eMedicine Specialties > Ophthalmology > Infectious Disease

Rocky Mountain Spotted Fever

Author: Byron L Lam, MD, Professor, Department of Ophthalmology, Bascom Palmer Eye Institute, University of Miami School of Medicine
Contributor Information and Disclosures

Updated: Oct 30, 2009

Introduction

Background

Rocky Mountain spotted fever (RMSF) is the most common rickettsial disease in the United States; it also occurs throughout the Western hemisphere. RMSF is caused by Rickettsia rickettsii, an obligate intracellular gram-negative coccobacilli that contains both DNA and RNA. Ticks serve as both vectors and reservoirs for RMSF. The organism usually is harbored by the wood tick Dermacentor andersoni in the Rocky Mountain states; the American dog tick Dermacentor variabilis in the eastern, central, southern, and Pacific coastal states; the cayenne tick Amblyomma americanum in Texas and in Central and South America; and the brown dog tick Rhipicephalus sanguineus in Arizona and in Mexico.

RMSF is characterized by fever, myalgias, headache, and a petechial rash. Early symptoms are nonspecific. Ocular manifestations include petechial conjunctivitis, anterior uveitis, retinal hemorrhages, cotton-wool spots, retinal vascular engorgement and tortuosity, branch retinal arteriolar occlusion, and optic disc edema.

RMSF is a potentially fatal disease with a mortality rate as high as 30% in the preantibiotic era. Early treatment with appropriate antibiotics is the key prognostic factor. Therapy should be instituted as soon as the disease is suspected clinically. Further, RMSF should be considered in family members and contacts who have febrile illness and share environmental exposures with the patient.¹

An ophthalmologist rarely participates in the treatment of patients with RMSF where fulminant systemic symptoms overwhelm mild ocular manifestations. The ocular changes probably are underestimated and underdiagnosed, usually resolving within 3 weeks of systemic antibiotic therapy.

Pathophysiology

The disease is transmitted to humans through tick bites, which often occurs unnoticed. The organism invades the endothelial and smooth muscle cells of the blood vessels, producing a systemic vasculitis with increased vascular permeability. Loss of serum proteins, decreased blood volume, and thrombi result in edema, hypovolemia, hypoperfusion, and circulatory failure. Ocular manifestations are due to ischemia and increased vascular permeability.

Frequency

United States

According to the Centers for Disease Control and Prevention (CDC), 3908 cases of RMSF were reported in 2002-2004.² The numbers of reported cases are increasing in the United States with declining mortality rates. This may be because other rickettsial diseases are being grouped under the general term of Rocky Mountain spotted fever. 

Seasonal outbreaks parallel tick activity. Most cases occur during the spring and summer with rare sporadic cases throughout the year. Risk factors include exposure to wooded areas and to dogs.³

Geographic distribution of RMSF shows that more than one half of reported cases are from Oklahoma, Tennessee, Arkansas, Maryland, Virginia, North Carolina, and South Carolina.⁴

International

RMSF is endemic in Central and South America.

Mortality/Morbidity

• Untreated cases may result in death within 15 days of symptom onset.
• Even with treatment, the hospitalization rate is as high as 72%.
• Mortality increases if treatment is delayed. 
• The overall case-fatality rate in the United States was 0.7% in 2002-2004.²
• The recent declining mortality rates in the United States may be because of improved early treatment or in part to other rickettsial diseases being grouped under the general term of Rocky Mountain spotted fever.

Race

No racial predilection exists for RMSF.

Sex

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

Age

In a survey of children, the findings from immunofluorescence antibody assays suggest infection with R rickettsii or the related spotted fever group rickettsiae may be subclinical and occur more commonly than previously thought. 
 
According to the CDC, the incidence was highest among persons aged 5-9 years and in those aged 40-64 years.

Clinical

History

Early diagnosis is based on clinical and epidemiologic grounds. The clinician must always have a high index of suspicion, because the early signs and symptoms are nonspecific. A history of tick bite or tick exposure and recent travel to endemic regions are risk factors.¹ The incubation period is 2-14 days following a tick bite.

• High fever (>102°F), headaches, and myalgias occur in greater than 85% of patients. 
• Central nervous system (CNS): Of patients with RMSF, 25% develop signs of encephalitis, including lethargy and confusion. 
• Gastrointestinal symptoms include abdominal pain, diarrhea, nausea, and vomiting.

Physical

• High fever (>102°F)
• Skin
  • Ninety percent of patients develop a maculopapular rash between days 3-5 of the illness.
  • The rash gradually becomes petechial and progresses to ecchymoses.
  • The rash may have a variable distribution, although classically it first involves the distal extremities (including the palms and soles) and subsequently spreads toward the trunk.
• Central nervous system
  • Confusion and lethargy occur in about 25% of patients.
  • Encephalitis also may produce ataxia, seizures, cranial nerve palsies, hearing loss, photophobia, severe vertigo, dysarthria, aphasia, paralysis, and nystagmus.
• Lungs
  • Findings consistent with pulmonary edema and interstitial pneumonitis may be present.
  • Patients may be short of breath, or develop respiratory compromise.
• Abdomen
  • Signs and symptoms of acute abdomen, splenomegaly, and hepatomegaly may occur.
  • RMSF is included in the differential diagnosis of the acute surgical abdomen.
• Eyes
  • Petechial conjunctivitis occurs as part of the generalized rash.
  • Anterior uveitis has been reported.
  • Retinal vascular dysfunction may result in retinal hemorrhages, retinal ischemia manifested by cotton-wool spots and nerve fiber layer hemorrhages, retinal vascular engorgement and tortuosity, and branch retinal arteriolar occlusion.
  • Optic disc edema due to ischemia and inflammation and orbital edema from increased extravascular volume may be present. Optic disc edema may be associated with peripapillary subretinal fluid extending into the macula (neuroretinitis).
  • The incidence of ocular changes is considered low but probably is underestimated.

Causes

R rickettsii causes RMSF.

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