Overall annual incidence rose from 1.7 to 7 cases per million in the United States between 2000 and 2007 (Openshaw 2010)
Annual incidence is highest in children aged 5-9 years of age (Amsden 2005)
Age:
Median age is 42 years of age with greater than 87% of cases reported in Caucasian patients (Openshaw 2010)
Gender:
Slight male to female predominance (Openshaw 2010)
Morbidity/Mortality: RMSF is the most common fatal rickettsial illness in the United States
Overall hospitalization rates are noted be at 23.4% based on aggregated reviews of case reports (Openshaw 2010)
Case fatality rate is estimated to be 5-10% overall (Biggs 2016)
If treatment is delayed, case-fatality rates of 40-50% have been described for patients treated on day 8 or 9 of their illness
Case fatality rate is highest in those over the age of 70 (Amsden 2005)
Without treatment, the case fatality rate is over 25% (Lacz 2006)
Geography and Seasonality (Openshaw 2010): Cases have been found in all continuous 48 states
Five States account for over 64% of all reported cases:
North Carolina
Oklahoma
Arkansas
Tennessee
Missouri
Seasonal distribution is concentrated in June (38 %) and July (38%).
Second smaller peak is noted in October and November when adult ticks are active
Poor Prognostic Factors (Biggs 2016):
Age < 10 or > 40 years of age
Alcohol abuse
Glucose-6-phosphate dehydrogenase deficiency
Delays in diagnosis and treatment
Immunocompromised states
Use of sulfonamide antimicrobial
Pathogenesis (Lacz 2006):
Rickettsia has tropism for endothelial cells and is able to spread centripetally via filopodia propulsion.
Invasion of endothelial and smooth muscle cells of various organs leads to dysfunction of microcirculation, host immune response, vascular tone, angiogenesis, and normal hemostasis.
Multiple organ systems are infected including the brain, liver, skin, lungs, kidneys, as well as the gastrointestinal system.
History and Physical:
Symptoms (Biggs 2016):
Symptoms typically appear 3-12 days after bite of an infected tick
Classic Triad: Tick Bite, Rash, Fever
Present in only a minority of patients
Rash: Absence of rash does not rule out this diagnosis
Timeline:
Typically appears 2-4 days after fever onset
Distribution:
Begin on the ankles, wrists, and forearms and subsequently spreads to the palms, soles, arms, legs, and trunk
Typically spares the face
Morphology:
Day 1: Initially seen as small, blanching macules
Over next several days, the rash becomes maculopapular, sometimes exhibiting central petechiae
Day 5 and 6: Rash becomes primarily petechial with involvement of the palms and soles
Initial Symptoms (< 5 days):
Fever
Headache
Chills
Malaise
Myalgia
Photophobia
Conjunctival Suffusion
Periorbital and Peripheral Edema
Calf Pain
Acute Transient Hearting Loss
Gastrointestinal Symptoms
Severe Manifestations: Most cases are self-limiting, but RMSF has been associated with the following severe disease manifestations:
Meningoencephalitis, Coma, Cerebral Edema
Renal Failure
ARDS
Seizures
Shock
Cutaneous Necrosis and Gangrene
Arrhythmias, Myocarditis
Diagnosis:
Labs: Characteristic laboratory findings are seen in RMSF along fever and non-specific flu-like illness:
Thrombocytopenia
Hyponatremia
Mild Elevated Hepatic Transaminase
Increased Number of Immature Neutrophils
Confirmatory Testing (Dantas-Torres 2007):
Serological testing revealing four-fold change in IgG antibodies specific to R rickettsii using indirect immunofluorescence assay (IFA) in paired samples
Antibodies are not detected until 7-10 days after disease onset
RT-PCR assays are also available for diagnosis
ELISA testing is also available for diagnosis
Immunohistochemical staining from skin or tissue biopsies has also been used for diagnosis
Guest Post By:
Akash Ray, DO
PGY-2 Emergency Medicine Resident Inspira Medical Center
Vineland, NJ Twitter:@_kashray
References:
Biggs H et al. Diagnosis and Management of Tickborne Rickettsial Diseases: Rocky Mountain Spotted Fever and Other Spotted Fever Group Rickettsioses, Ehrlichioses, and Anaplasmosis – United States. MMWR Recomm Rep. 2016. PMID: 27172113
Openshaw J et al. Rocky mountain spotted fever in the United States, 2000-2007: interpreting contemporary increases in incidence. Am J Trop Med Hyg. 2010. PMID: 20595498
Amsden J et al. Tick-borne bacterial, rickettsial, spirochetal, and protozoal infectious diseases in the United States: a comprehensive review. Pharmacotherapy. 2005. PMID: 15767235
Lacz N et al. Rocky Mountain spotted fever. J Eur Acad Dermatol Venereol. 2006. PMID: 16643138
