Rickettsial Diseases, including Typhus and Rocky Mountain Spotted Fever (page 4)
(This chapter has 6 pages)
© 2009 Kenneth Todar, PhD
Epidemiology
Rocky Mountain spotted fever has been a notifiable disease in the
United
States since the 1920s. In the last 50 years, approximately 250-2288
cases
of Rocky Mountain spotted fever have been reported annually, although
it
is likely that many more cases go unreported. Between 2002 and 2006,
the
number of reported cases doubled.
Figure 9a. Reported
cases
of Rocky Mountain spotted fever in the United States, 1942-1996. CDC
compiles
the number of cases reported by the state health departments. (CDC)
Figure 9b. Reported
cases
of Rocky Mountain spotted fever in the United States, 1997-2002.According to CDC's annual
Summary of Notifiable Diseases, in 2002 - 1104 cases were
reported;
2003 - 1091 cases; 2004 - 1713 cases; 2005 - 1936 cases; 2006 - 2288
cases. The number of reported cases by county in the United
States in
2006 is shown in Figure 9c below.
Figure 9c. Reported
cases
of Rocky Mountain spotted fever in the United States, 2006. (CDC)
Over 90% of patients with Rocky Mountain spotted fever are infected
during April through September. This period is the season for
increased
numbers of adult and nymphal Dermacentor ticks. A history of
tick bite
or exposure to tick-infested habitats is reported in approximately 60%
of all cases of Rocky Mountain spotted fever.
Figure 10. Seasonal
distribution
of reported cases of Rocky Mountain spotted fever, 1993-1996. (CDC)
Over half of Rocky Mountain spotted fever infections are reported
from
the south-Atlantic region of the United States (Delaware, Maryland,
Washington
D.C., Virginia, West Virginia, North Carolina, South Carolina, Georgia,
and Florida). Infection also occurs in other parts of the United
States,
namely the Pacific region (Washington, Oregon, and California) and west
south-central (Arkansas, Louisiana, Oklahoma, and Texas) region.
The states with the highest incidences of Rocky Mountain spotted
fever
are North Carolina and Oklahoma. These two states combined
accounted
for 35% of the total number of U.S. cases reported to CDC during 1993
through
1996. Although Rocky Mountain spotted fever was first identified
in the Rocky Mountain states, actually less than 3% of the U.S. cases
were
reported from that area during the same interval (1993-1996).
Figure 11. Number of
reported
cases of Rocky Mountain spotted fever by state and region, 1994-1998.
(CDC)
Certain individuals are at higher risk of disease. The frequency of
reported cases of Rocky Mountain spotted fever is highest among males,
Caucasians, and children. Two-thirds of the Rocky Mountain spotted
fever
cases occur in children under the age of 15 years, with the peak age
being
5 to 9 years old (see Figure 12). Individuals with frequent
exposure
to dogs and who reside near wooded areas or areas with high grass may
also
be at increased risk of infection.
Figure 12. Average
annual
incidence of Rocky Mountain spotted fever by age group,
1993-1996.
(CDC)
