Rocky Mountain Spotted Fever
Rickettsia rickettsii - Rocky Mountain Spotted Fever
Clinical Background
Rickettsia rickettsii is a tick-borne illness (zoonosis) and the etiologic agent of Rocky Mountain Spotted Fever (RMSF).
Epidemiology
- Incidence – 3-5/1,000,000
- Age – peak incidence 5-9 years
- Transmission
- Via Dermacentor (variabilis, andersoni), and Amblyomma, and Rhipicephalus sanguineus ticks in the U.S.
- Geographical distribution is restricted to the western hemisphere
- Humans are accidental hosts
- 95% of the cases occur April through September
Organism
- Gram-negative coccobacilli of the Rickettsiaceae family – obligate intracellular organisms
- Characteristic feature of the Rickettsiae – life cycle requires multiplying in an arthropod
- Invertebrate hosts are both reservoirs and vectors
- Rickettsia are part of a family of organisms responsible for the following rickettsial diseases:
- Spotted fever and typhus (vector: tick, louse, flea or gamasid mite)
- Scrub typhus (vector: chigger)
- Ehrlichiosis (vector: tick)
- Neorickettsiosis
- Q-fever
Risk Factors
- Dog exposure to ticks
- Residence in a wooded area
- Residence in Central and Mid-Atlantic states
- Male sex
Clinical Presentation
- The incubation period between tick bite and onset of symptoms is 2 days-2 weeks
- RMSF is difficult to differentiate from viral illness
- Non-specific signs and symptoms
- Classic triad – fever, headache and rash
- Rash typically appears on the second or third day of illness
- Rash begins as macules on the wrists, palms, ankles and soles of feet, which then results in petechiae form
- Rash finally spreads to the trunk
- Rash is hallmark of infection but usually follows systemic symptoms; its absence should not rule out a possible rickettsial etiology
- Other symptoms include malaise, myalgias, vomiting and photophobia
- Mild pulmonary involvement, manifested by cough and infiltrates, is found in about one-third of patients with RMSF
- Neurologic (25% of patients)
- Meningitis
- Meningoencephalitis
- Seizures
- Confusion
- Focal neurologic deficits
- Cardiovascular
- Mortality is dependent on cardiac and central nervous system involvement or delay in treatment
Treatment
- Initiation of early antibiotic therapy is necessary to reduce mortality
- Decision to treat should not be delayed until lab confirmation of organism
Diagnosis
Indications for testing – flu-like illness with or without rash in association with tick bite exposure and proper epidemiologic setting
- Laboratory testing
- Diagnosis is made based upon clinical grounds, symptoms and/or serology with history of tick exposure
- Antibody testing
- The best evidence for infection is a significant change in 2 appropriately timed specimens where both tests are done in the same laboratory at the same time
- Appearance of an IgM antibody response by IFA or ELISA normally occurs 7-14 days after the onset of disease
- Not usually detectable by day 5, which is the day when fatality starts to rise dramatically
- Negative initial IgM does not exclude the disease
- Cannot distinguish between cross-reacting Rickettsiae
- Febrile antibody testing
- More specific than Weil-Felix but still has cross-reactivity with Brucella and Salmonella
- Must be used in conjunction with clinical presentation
- Weil-Felix – lacks sensitivity and specificity; outmoded; do not use
- Blood culture – not routinely available because of biosafety issues (research labs only)
- Highly sensitive and specific
- Immunohistochemistry
- Direct immunofluorescence or immunoperoxidase tests on skin biopsies
- Sensitivity ~70%; specificity 100%
- Testing not widely available
- Negative result does not rule out disease
Differential Diagnosis
- Rash
- Disseminated gonococcus, meningococcus
- Kawasaki disease
- Lyme disease
- Parvovirus B19
- Rickettsia typhi
- Secondary syphilis
- Dengue fever
- Human monotropic ehrlichiosis
- Pneumonia/influenza
- Meningoencephalitis
- Epstein-Barr virus
- Leptospirosis
- Lyme disease
- Vasculitis
- Human granulocytic anaplasmosis
- Human monotropic ehrlichiosis
- Dengue fever
- Myocarditis
- Enterovirus
- Epstein-Barr virus
- Mycoplasma pneumoniae
- Gastroenteritis
- Clostridium difficile
- Diarrhea, bacterial evaluation
- Diarrhea, viral evaluation
- Enterovirus
Pharmacogenetics and Therapeutic Drug Monitoring
Indications for Laboratory Testing
- Tests generally appear in the order most useful for common clinical situations
- Click on number for test-specific information in the ARUP Laboratory Test Directory
| Test Name and Number |
Recommended Use |
Limitations |
Follow Up |
| Rickettsia rickettsii (Rocky Mountain Spotted Fever) Antibodies, IgG & IgM by IFA 0050371 Method: Indirect Immunofluorescence Assay |
Determine if infection is due to Rickettsia rickettsii (Rocky Mountain Spotted Fever)
Acute and convalescent titers may be necessary
|
Low-positive results suggest past exposure or infection, while high-positive results may indicate recent or past infection, but are inconclusive for diagnosis Initial testing may not be helpful; determine treatment from clinical and other laboratory assessments Any antibody reactivity to Rickettsia rickettsii should also be considered group reactive for the Spotted Fever group (R. conorii, R. honei, R. akari, R. japonica, R. australis, and R. sibirica). |
|
| Febrile Antibodies Panel 2001789 Method: Direct Agglutination/Immunofluorescence Assay/ImmunoDOT |
Use to help confirm presence of disease; not recommended for initial testing
Panel includes IgM Rickettsia rickettsii antibody by ELISA testing; as well as testing for antibodies to Brucella, Rickettsia typhi, Salmonella O and H
|
Initial testing may not be helpful; determine treatment from clinical and other laboratory assessments |
|
Additional Tests Available
Click the plus sign to expand the table of additional tests.
| Test Name and Number | Comments |
| Rickettsia rickettsii (Rocky Mountain Spotted Fever) Antibody, IgG 0050369 Method: Indirect Immunofluorescence Assay |
|
| Rickettsia rickettsii (Rocky Mountain Spotted Fever) Antibody, IgM 0050372 Method: Indirect Immunofluorescence Assay |
|
| Weil-Felix Test, DA 0093143 Method: Direct Agglutination |
|
Guidelines
Chapman AS, Bakken JS, Folk SM, Paddock CD, Bloch KC, Krusell A, Sexton DJ, Buckingham SC, Marshall GS, Storch GA, Dasch GA, McQuiston JH, Swerdlow DL, Dumler SJ, Nicholson WL, Walker DH, Eremeeva ME, Ohl CA. Diagnosis and management of tickborne rickettsial diseases: Rocky Mountain spotted fever, ehrlichioses, and anaplasmosis--United States: a practical guide for physicians and other health-care and public health professionals. MMWR Recomm Rep. 2006; 55 (RR-4) :1-27.PubMed
General References
Comprehensive Review: November 2009
Last Update: November 2009