Rocky mountain spotted fever

Background

American dog tick

• Most frequent rickettsial disease reported in the US.
• Caused by Rickettsia rickettsii
  • Obligate intracellular, gram negative bacteria
  • Predilection for vascular endothelial cells
• Spread by ticks
  • American Dog Tick (Dermacentor variabilis) Most common vector
  • Rocky Mountain Wood Tick (Dermacentor andersoni)
  • Brown Dog Tick (Rhipicephalus sanquineus)
  • Cayenne Tick (Amblyomma cajennense)
• Seasonal distribution with most infections occurring in the summer months
• Widely distributed in the US
  • States with the highest incidence are Oklahoma, Nebraska, Arkansas, Tennessee and North Carolina
• Overall incidence is increasing while mortality is falling
  • Prior to the era of antibiotics, ~30% mortality
  • Currently mortality is 3-5%

Clinical Features

• Symptoms generally begin 2-14 days after inoculation from an infected tick

Early symptoms

• Non specific and highly variable
• Fever
• Nausea/Vomiting
• Abdominal pain
• Myalgias (severe calf pain)
• Headache
• Fatigue
• Conjunctivitis

Late symptoms

Rocky mountain spotted fever rash

• Rash
  • Begins as a blanching maculopapular rash that evolves to become a petechial rash
  • Usually 2-5 days after fever subsides
  • Starts on extremities and spreads inward (centripetally)
  • Can involve palms and soles (50% of cases)
• Arthralgias
• Positive Rumpel-Leede test
  • Development of petechiae at the site of blood pressure cuff and distally after compression

Complications

• Secondary to host response against infected endothelial cells
  • Vasculitis
  • Myocarditis
  • Interstitial pneumonitis
  • Encephalitis

Differential Diagnosis

• Other Tick Borne Illnesses
• Non-specific viral illness

Lower Respiratory Zoonotic Infections

• Psittacosis
• Anthrax (Bacillus anthracis)
• Brucellosis (Brucella species)
• Q fever (C. burnetti)
• Pasteurellosis (Pasteurella multocida)
• Melioidosis (Burkholderia pseudomallei)
• Rocky Mountain Spotted Fever (R. rickettsii)
• Pulmonic Plague (Yersinia pestis)
• Influenza A
• Hantavirus

Tick Borne Illnesses

• Babesiosis
• Colorado tick fever
• Ehrlichiosis
• Heartland virus
• Lyme
• Murine typhus
• Rocky mountain spotted fever
• Southern tick-associated rash illness (STARI)
• Tick paralysis
• Tularemia

Evaluation

• PCR (initially)
• Serial serologic examinations by indirect fluorescent antibody confirm the diagnosis
• Titers

Work-Up

• ECG (Myocarditis)
• Chem 7 (Hyponatremia)
• CBC (Thrombocytopenia)

Management

• Doxycycline 100 mg BID for 5-7 days
  • Indicated also in children at 2.2mg/kg BID
• Chloramphenicol (CAM) 50-100 mg/kg/day div Q6hr (Max dose = 4g/day)
  • Preferred agent in pregnancy. May cause aplastic anemia and Grey baby syndrome, more common in near term or 3rd trimester
  • Consideration should be made for doxycycline over CAM in the 3rd trimester

See Also

• Tick borne illnesses