Epidemiology and Statistics

  • Rocky Mountain spotted fever (RMSF) has been a nationally notifiable condition since the 1920s.
  • As of January 1, 2010, cases of RMSF are reported under a new category called Spotted Fever Rickettsiosis (SFR).
    • This category captures cases of RMSF, Rickettsia parkeri rickettsiosis, Pacific Coast tick fever, and rickettsialpox.
    • The change reflects the inability to differentiate between spotted fever group Rickettsia species using commonly available serologic tests.

At a glance

  • The number of SFR cases has risen in the last two decades, from 495 cases in 2000, to a peak of 6,248 in 2017. However, cases reported in 2018 were slightly lower.
  • Because of the inability to differentiate between spotted fever group Rickettsia species using commonly available serologic tests, it is unclear how many of those cases are RMSF, and how many result from other, less severe spotted fevers.

Epidemiology Figure 1 – Reported cases of SFR in the United States, 2000–2018

Graph showing number of US SFR cases reported to CDC, 2000-2018. See table below for data.

 

Epidemiology Figure 1 – Number of U.S. SFR cases reported to CDC, 2000–2016 
Year of report Number of cases
 2000 495
2001 695
2002 1104
2003 1091
2004 1713
2005 1936
2006 2288
2007 2221
2008 2563
2009 1815
2010 1985
2011 2802
2012 4470
2013 3359
2014 3757
2015 4198
2016 4269
2017 6248
2018 5,544

Historical trends

  • The number of SFR cases reported to CDC per year have generally increased over time with distinct increases since the mid-1990s.
  • Notably, while the number of cases and incidence rose, the case fatality rate (i.e., the proportion of SFR patients that died as a result of infection) has declined since the 1940s when tetracycline antibiotics became available.
  • The current case fatality rate for SFRs using surveillance data is still roughly 0.5% of cases.
    • In clinical reviews of RMSF cases, about 5-10% of cases are fatal.
    • Inclusion of less severe spotted fevers, such as R. parkeri rickettsiosis, likely leads to the lower case fatality rate observed in recent decades.

Epidemiology Figure 2 – Reported incidence and case fatality of SFR in the United States, 1920–2015

Cases of SFR have been recorded since the 1920s. Trends in SFR incidence vary over time, but are generally increasing. Periods of increased incidence can be seen between 1930 and 1950 and 1968 through 1990. More recently, there has been a dramatic increase in incidence of SFR increasing from 1.7 cases per million persons in 2000 to an all-time high of 14.2 cases per million persons in 2012. Case fatality rates vary from year to year, but have had an overall decreasing trend from 28% case fatality in 1944 to less than 1% case fatality beginning in 2001.

 

Epidemiology Figure 2 – Reported incidence and case fatality of SFR in the United States, 1920–2015
Year of Report  Cases Incidence (per Million)  Case Fatality Rate (%)
1920 60 0.56
1921 80 0.74
1922 180 1.64
1923 115 1.03
1924 110 0.96
1925 112 0.97
1926 200 1.7
1927 215 1.81
1928 180 1.49
1929 290 2.38
1930 195 1.58
1931 260 2.1
1932 440 3.52
1933 465 3.7
1934 450 3.56
1935 490 3.85
1936 360 2.81
1937 430 3.34
1938 431 3.32
1939 555 4.24
1940 460 3.47 23
1941 510 3.54 22.5
1942 490 3.64 25
1943 468 3.46 27.5
1944 460 3.44 28
1945 465 3.48 27
1946 580 4.12 23
1947 595 4.13 17.5
1948 548 3.73 17
1949 560 3.75 6
1950 455 2.99 7
1951 350 2.27 7.3
1952 328 2.1 6
1953 306 1.93 7
1954 284 1.75 3.5
1955 290 1.76 3
1956 288  1.71 5
1957 248 1.45 7.5
1958 250 1.44 6
1959 198 1.12 4.9
1960 202 1.12 5.1
1961 221 1.21 5
1962 240 1.29 4.8
1963 220 1.17 7
1964 280 1.46 6
1965 285 1.47 6.5
1966 275 1.41 7.5
1967 310 1.57 8.5
1968 300 1.5 7
1969 490 2.43 7.2
1970 380 1.86 7.6
1971 432 2.09 8.3
1972 523 2.5 9.6
1973 668 3.16 5.7
1974 754 3.53 5.8
1975 844 3.92 3.4
1976 937 4.31 4.4
1977 1115 5.1 3.7
1978 1063 4.79 2.8
1979 1070 4.76 5.5
1980 1163 5.12 5.1
1981 1192 5.19 3.1
1982 976 4.2 4.9
1983 1126 4.82 4.1
1984 847 3.59 4.2
1985 700 2.94 4.6
1986 755 3.14 2.8
1987 592 2.44 2.7
1988 615 2.51 4.4
1989 603 2.44 1.3
1990 649 2.6 4.3
1991 635 2.52 2.7
1992 502 1.98 3
1993 424 1.9 4.7
1994 427 1.6 3.5
1995 510 1.9 1.5
1996 701 2.6 1.2
1997 396 1.5 2
1998 352 1.3 3.6
1999 572 2.1 2.1
2000 487 1.7 2.2
2001 695 2.4 0.6
2002 1102 3.8 0.9
2003 1083 3.7 0.3
2004 1713 5.8 0.6
2005 1936 6.5 0.3
2006 2288 7.7 0
2007 2222 7.4 0.3
2008 2563 8.4 0.3
2009 1815 5.9 0.4
2010 1985 6.4 0.4
2011 2802 8.9 0.6
2012 4470 14.2 0.5
2013 3359 10.6 0.2
2014 3757 11.9 0.4
2015 4198 13.1 0.1

Seasonality

  • Although SFR cases can occur during any month of the year, most cases reported illness in May–August.
  • This period coincides with the season when adult Dermacentor ticks are most active.
  • Seasonal trends may vary depending on the area of the country and tick species involved.
    • In Arizona, the majority of SFR cases are spread by the brown dog tick (Rhipicephalus sanguineus sensu lato) and cases occur year-round with peak months of illness onset April through October.

Epidemiology Figure 3 –Reported SFR cases in the United States by month of onset, 2000–2018

Graph showing number of reported SFR cases by month of onset, 2000-2018. See table below for data.

 

Epidemiology Figure 3 – Number of reported SFR cases by month of onset, 2000–2016
 Month of onset Number of cases
1 791
2 769
3 1,534
4 3,972
5 7,855
6 10,615
7 9,701
8 7,501
9 4,924
10 2,577
11 1,274
12 852

Geography

  • SFR cases have been reported throughout the contiguous United States, although five states (Arkansas, Missouri, North Carolina, Tennessee, and Virginia) account for over 50% of SFR cases.
  • In Arizona, RMSF cases have recently been identified in an area where the disease had not been previously seen. From 2003 to 2018, nearly 430 cases were reported with a case-fatality rate of about 5%.
    • The tick responsible for transmission of R. rickettsii in Arizona is the brown dog tick, which is found on dogs and in and around people’s homes.
    • Almost all of the cases occurred within communities with large numbers of free-roaming dogs.

Epidemiology Figure 4 – Annual incidence (per million persons) of SFR in the United States, 2018

Map of the US showing annual incidence (per million persons) for SFR in 2018. See table below for data.

 

Epidemiology Figure 4 – Annual incidence (per million persons) for SFR in the United States, 2016
State of Residence Cases per Million
Alabama 137.7
Alaska NN
Arizona 5.4
Arkansas 353.9
California 0.4
Colorado 1.2
Connecticut 6.7
Delaware 51.8
District of Columbia 7.1
Florida 1
Georgia 0.1
Hawaii NN
Idaho 6.3
Illinois 11.9
Indiana 11.9
Iowa 7
Kansas 62.2
Kentucky 48.9
Louisiana 6.4
Maine 7.5
Maryland 17.6
Massachusetts 4.2
Michigan 1.6
Minnesota 3
Mississippi 51.7
Missouri 97.4
Montana 8.5
Nebraska 24.9
Nevada 3
New Hampshire 5.2
New Jersey 16.5
New Mexico 1.4
New York 2.6
North Carolina 47.8
North Dakota 13.2
Ohio 3.3
Oklahoma 20.3
Oregon 1
Pennsylvania 2
Rhode Island 8.5
South Carolina 10.6
South Dakota 15.9
Tennessee 79.4
Texas 2.7
Utah 2.5
Vermont 8
Virginia 39.9
Washington 0.4
West Virginia 11.1
Wisconsin 4.8
Wyoming 3.5

People at Risk

  • SFR cases are more frequently reported in men than in women.
  • People over the age of 40 years account for the highest number of reported cases, however, children under 10 years old represent the highest number of reported deaths.
  • Persons with glucose-6-phosphate dehydrogenase (G6PD) deficiency.
  • Surveillance data shows higher risk for hospitalization in people with compromised immune systems (e.g., resulting from cancer treatments, advanced HIV infection, prior organ transplants, or some medications).

Epidemiology Figure 5 – Average annual incidence of SFR in the United States by age group, 2000–2018

graph showing average annual incidence of SFR by age group, 2000-2018. See table below for data.

 

Epidemiology Figure 5 – Average annual incidence of SFR by age group, 2000–2017
Age Group Cases per Million
Under 1 0.1
1-4 2.5
5-9 4.5
10-14 4.2
15-19 5.6
20-24 5.6
25-29 7
30-34 8.7
35-39 9.4
40-44 10.1
45-49 11.2
50-54 12.5
55-59 14
60-64 14.5
65 and older 13.2