eMedicine Specialties > Pediatrics: General Medicine > Infectious Disease

Rocky Mountain Spotted Fever

Author: Nicholas John Bennett, MB, BCh, PhD, Fellow in Pediatric Infectious Disease, Department of Pediatrics, State University of New York Upstate Medical University
Coauthor(s): Joseph Domachowske, MD, Professor of Pediatrics, Microbiology and Immunology, Department of Pediatrics, Division of Infectious Diseases, State University of New York-Upstate Medical University; Walid Abuhammour, MD, FAAP, Associate Professor of Pediatrics, Michigan State University; Director of Pediatric Infectious Disease, Department of Pediatrics, Hurley Medical Center
Contributor Information and Disclosures

Updated: Sep 10, 2009

Introduction

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Background

Rocky Mountain spotted fever (RMSF) is the most common rickettsial infection and the second most commonly reported tick-borne disease (after Lyme disease) in the United States.

The causative agent is Rickettsia rickettsii (named after Howard T. Ricketts, the discoverer of the organism), an Alphaproteobacteria and member of the spotted fever groups of rickettsial infections. Rocky Mountain spotted fever was first described in the late 1800s in the Bitterroot Valley of Idaho. For several decades, it was thought to be limited to the Rocky Mountain area; however, it now has a high documented prevalence in the eastern United States. The bacteria is spread mainly through the bites of infected ticks; the dog tick, wood tick and Lone Star tick are all potential carriers and are responsible for Rocky Mountain spotted fever in different parts of the United States.

The disease was so problematic because of its mortality of up to 30% that the Rocky Mountain Laboratory was established in Hamilton, Montana to help investigate it. This laboratory is now part of the National Institute of Allergy and Infectious Diseases (NIAID). Rocky Mountain spotted fever has the highest mortality of any tick-borne illness in the United States.

Rocky Mountain spotted fever is a reportable disease in the United States.

Pathophysiology

Rocky Mountain spotted fever is a diffuse small-vessel vasculitis. R rickettsii is a small, gram-negative, obligate intracellular coccobacillus with a tropism for human endothelial cells. This bacterium causes membrane disruption and increased permeability.

Rickettsiae can be demonstrated in the cytoplasm and the nucleus of cells. Possible mechanisms for cellular injury include injury to the cell membrane, depletion of adenosine 5-triphosphate (which leads to failure of the sodium pump), and damage to the cell caused by toxic products of rickettsial metabolism.

Vascular lesions are responsible for the clinical manifestations, including rash, headache, alteration in the level of consciousness, heart failure, and shock. Vascular lesions can be found everywhere, with highest predilection for the skin, gonads, and adrenal glands. Profound hyponatremia is common. Several mechanisms have been postulated, including (1) a shift in water from the intracellular spaces to the extracellular spaces, (2) increased loss of sodium in the urine, and (3) an exchange of sodium for potassium at the cellular level.

Edema of the medulla oblongata may contribute to fatality in some patients.

Concentrations of antidiuretic hormone and aldosterone are increased in some patients.

Frequency

United States

Rocky Mountain spotted fever has been reported in almost every state in the continental United States, with an age-related annual incidence of 0.5-3 cases per million population. In 1997-2002, the mean incidence of was 2.2 cases per million per year.¹

States reporting the highest rate of disease include North Carolina, South Carolina, Tennessee, Oklahoma, and Arkansas; these states accounted for more than half the total cases in recent years. The term Rocky Mountain spotted fever is a misnomer because it is relatively rare in the Rocky Mountain states. About 90% of cases occur between April and September, the time of the year when ticks have maximal activity and when people participate in outdoor recreational activities. See Media file 1.

International

The disease is also found in Canada, Mexico, Central America, and South America. However, the arthropod vector differs by location (see the table below). Other illnesses similar to Rocky Mountain spotted fever are also found worldwide.

Human Disease Around the World Caused by Spotted Fever Group Rickettsiae.

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Mortality/Morbidity

The mortality rate during the preantibiotic era was as high as 30%; however, the mortality rate now ranges from approximately 2% in children to 9% in elderly persons. The case-fatality rate is higher (6.2%) for persons whose treatment begins more than 3 days after onset of symptoms than for those treated within the first 3 days of illness (1.3%). Patients with glucose-6-phosphate dehydrogenase (G-6-PD) deficiency tend to have a severe course of Rocky Mountain spotted fever.

Race

People of African descent have reportedly had a high mortality rate partly due to the 12% rate of G-6-PD deficiency in male blacks. Rocky Mountain spotted fever may also be diagnosed later in blacks than in others because of the difficulty in detecting the early macular rash.

Prior to 2000, American Indians had similar rates of Rocky Mountain spotted fever to other races in the United States.²  From 2001-2005, rates increased disproportionately (16.8 cases per million vs 0.5-4.2 cases per million for other races). The highest rates were in Oklahoma (113.1 cases per million).³

Sex

The incidence is higher in male individuals than in female individuals, with a male-to-female ratio of 1.7:1.

Age

Children are at greater risk of acquiring Rocky Mountain spotted fever than are adults. The highest incidence occurs in children aged 5-9 years. However, the highest mortality is in those aged 50 years or older.

Clinical

History

• The incubation period for Rocky Mountain spotted fever (RMSF) is 2-8 days after the tick bite.
• History of tick bite is only present in two thirds of cases.
• Symptoms can begin gradually or abruptly.
• Fever, headache, rash, toxicity, myalgia, and mental confusion are the major clinical manifestations.
  • The patient's body temperature is usually higher than 38.8°C (101.8°F)
  • Headache is the most common neurologic manifestation. In older children and adults, the headache may be intractable and may be ongoing day and night. Young children may not complain of headache.
  • Nausea, vomiting, and abdominal pain may occur.
  • Conjunctival hyperemia and photophobia may be observed.
• The rash of Rocky Mountain spotted fever is an important pathognomonic feature of the disease and is present in 80-90% of patients.
  • Rash begins as blanching maculopapular lesions. These lesions become petechial or purpuric in approximately one half of patients, accounting for its former name of black measles.
  • The rash first appears peripherally on the wrists and ankles. It spreads centripetally over the next 2-3 days.
  • Involvement of the palms and soles is an important diagnostic feature.
  • In most patients, rash usually appears by the second or third day. However, it may be delayed until the sixth day.
  • Early recognition of the blanching macular eruption is vital, because the classic petechial rash does not typically appear until 6 days or so after the initial symptoms become apparent.

Physical

Physical signs vary and include the following:

• Body temperature is higher than 38.8°C (101.8°F).
• Patient might have a toxic appearance.
• A characteristic skin rash appears. It may be absent in 10-20% of infected individuals.
• Hepatomegaly and splenomegaly are present in approximately 33% of patients.
• Signs of meningoencephalitis include restlessness, irritability, mental confusion, and delirium.
• Meningismus may occur. Findings may include neck stiffness, photophobia, a positive Kernig sign (pain on knee extension when hips flexed to 90°), and a positive Brudzinski sign (knee and hip flexion when the is neck flexed).
• Ataxia may be present.
• Spastic paralysis may occur.
• Sixth nerve palsy may be observed.
• Muscle tenderness is a common feature.

Causes

Ticks are the natural hosts, reservoirs, and vectors of R rickettsii. The species of tick acting as the vector varies by geographic location. R rickettsii is transmitted to humans by the bite of an infected tick. Adult ticks transmit the disease to humans during feeding. At least 6 hours of tick attachment is needed for the transmission of R rickettsii.

• Primary hosts of R rickettsii
  • Dermacentor variabilis (dog tick) in the eastern United States and eastern Canada
  • Dermacentor andersoni (wood tick) in the western United States and western Canada
  • Amblyomma americanum (Lone Star tick) in the southwestern United States
• Transmissions
  • Humans usually acquire infection through the bite of an infected tick.
  • On occasion, transmission occurs by scratching or rubbing infectious tick feces into the skin.
  • Laboratory personnel can be infected by inoculation or inhalation of aerosolized infectious specimens. For this reason, only specially equipped laboratories should attempt to culture and isolate Rickettsia species. Detection by other means (eg, serology) is more readily available than culture and isolation.

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