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Tag words: Rickettsia, Rickettsia rickettsiae, R. rickettsiae, typhus, scrub typhus, Rocky Mountain Spotted fever, RMSF, Hans Zinsser, tick-borne disease, dog ticks, Ixodes ticks, zoonoses, Dermacentor ticks, doxycycline, tick control, DEET, Boutonneuse fever, Rickettsialpox, cat flea tyuhus


Kingdom: Bacteria
Phylum: Proteobacteria
Class: Alpha Proteobacteria
Order: Rickettsiales
Family: Rickettsiacae
Genus: Rickettsia
Species: e.g. R. rickettsii

Common References: Rickettsia, typhus, scrub typhus, Rocky Mountain Spotted fever, RMSF, tick-borne disease, dog ticks, Ixodes ticks, Dermacentor ticks Rickettsial diseases

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Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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Rickettsial Diseases, including Typhus and Rocky Mountain Spotted Fever (page 4)

(This chapter has 6 pages)

© Kenneth Todar, PhD

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)

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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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