Also known as: North American tick typhus, New World spotted fever, RMSF, Tick fever
Hospital: Report by IDSS, facsimile, mail, or phone
Lab: Report by IDSS, facsimile, mail, or phone
Physician: Report by facsimile, mail, or phone
Local Public Health Agency (LPHA): Report by IDSS, facsimile, mail, or phone. Follow-up required
Iowa Department of Public Health
Disease Reporting Hotline: (800) 362-2736
Secure Fax: (515) 281-5698
Rocky Mountain spotted fever (RMSF) is caused by the bacterium Rickettsia rickettsii.
B. Clinical Description
Initial Symptoms include moderate to high fever, nausea, vomiting, significant malaise, muscle pain, severe headache, chills, and eye inflammation.
Later Signs and Symptoms include rash, abdominal pain, joint pain, and diarrhea. Over half of cases develop a rash or small bruises on the arms and legs, which typically appears 3-5 days after the onset of illness. The rash spreads to the palms and soles, and then to much of the body. Among untreated individuals, these signs and symptoms typically persist for 2 - 3 weeks. The classic triad of findings for this disease is fever, rash, and history of tick bite, but this combination is often not present when the patient initially seeks care.
Onset of RMSF is sudden.
Complications: More advanced manifestations include decrease in red blood cells (anemia) and platelets (thrombocytopenia), severe clotting disorders, involvement of the major organ systems, and shock. Severe cases can result in long-term neurological illness. If the disease is promptly recognized and treated, death is uncommon. For the United States, the reported case-fatality rate for RMSF has been less than 0.5% in recent years.
The primary vector for RMSF is the tick, which also serves as a reservoir. Only members of the tick family Ixodidae (hard ticks) are naturally infected with Rickettsia rickettsii. Among ticks, R. rickettsii is spread through eggs, and between life stages. This species is maintained in nature by a complex cycle involving ticks and mammals; several small wild animals and dogs may develop antibodies to R. rickettsii, but their role as possible reservoirs in the maintenance of RMSF is uncertain. Humans are considered accidental hosts, and are not involved in the natural transmission cycle of this pathogen.
D. Modes of Transmission
RMSF is acquired from a tick bite. Laboratory data suggest that the tick must remain attached for 4 - 6 hours before transmission of R. rickettsii to occur. Less commonly, infections may occur following exposure to crushed tick tissues, fluids, or tick feces when these fluids get into cuts or scratches.
E. Incubation Period
Signs of RMSF typically develop one week after exposure (range 3 - 14 days). The length of the incubation period is associated with the magnitude of exposure to R. rickettsii.
F. Period of Communicability or Infectious Period
RMSF is not communicable from person to person.
RMSF is a seasonal disease, occurring throughout the United States during the months of April through September, when the risk of contact with ticks is most likely. RMSF is uncommon in Iowa. The risk of mortality from RMSF is higher for men, people over the age of 40, non-whites, and individuals who do not develop (or recognize) the typical rash. Two-thirds of RMSF cases occur in children under the age of 15 years as they tend to spend more time in tick-infested areas. While rare, accidental transmission in the laboratory setting has been reported.
The incidence of RMSF has increased during the last decade, from less than 2 cases per million persons in 2000 to over 6 cases per million persons 2010. During the same time period, the proportion of RMSF cases resulting in death (case fatality) has declined to a low of less than 0.5%.
H. Bioterrorism Potential
I. Additional Information
The Council of State and Territorial Epidemiologists (CSTE) surveillance case definitions for Rocky Mountain Spotted Fever can be found at: www.cdc.gov/osels/ph_surveillance/nndss/phs/infdis.htm#top
CSTE case definitions should not affect the investigation or reporting of a case that fulfills the criteria in this chapter. (CSTE case definitions are used by the state health department and the CDC to maintain uniform standards for national reporting.)
American Academy of Pediatrics. 2006 Red Book: Report of the Committee on Infectious Diseases, 27th Edition. Illinois, American Academy of Pediatrics, 2006.
Beran, G.W. Handbook of Zoonoses, 2nd Edition, Section A: Bacterial, Rickettsial, Chlamydial, and Mycotic. Boca Raton, CRC Press, 1994.
Heymann, David L., ed., Control of Communicable Diseases Manual, 20th Edition. Washington, DC, American Public Health Association, 2015.