Rickettsia rickettsii, Causitive Agent of Rocky Mountain Spotted Fever

The Bacteria

Rickettsia rickettsii is the small, aerobic gram-negative bacterium that is the cause Rocky Mountain spotted fever in humans (and other vertebrates). They are obligate, intracellular bacteria that range in size form 0.2x0.5 µm to 0.3x2.0µm. Rickettsia belong to the phylum alpha-protobacteria, which are capable of growing in low levels of nutrients, and have a long generation time relative to other gram negative bacteria such as Escherichia coli. In humans rickettsiae preferentially reside in the nucleus or cytoplasm of cells lining small to medium size blood vessels.
Rickettsia enter host cell by inducing phagocytosis, then immediately escape the phagosome to grow and reproduce within the cytoplasm (or nucleus)of the host cell. The host cell will normally lyse eventually, causing the release of new organisms. The host cell is also harmed by the toxic effect of the cell wall. (CDC)
Van Kirk et al have done studies showing that actin-based motility (ABM) is a mechanism for intercellular spread. R. rickettsii is the only Rickettsia species that utilizes ABM. Findings suggest that ABM is a major factor in causing infection; after a host cell becomes infected the polymerization of the actin of the host cell forms a filamentous actin comet tail. 


The actin tail aids the pathogen in moving through the cytosol and into membrane protrusions of the host cell, where it can be engulfed by neighboring cells and can initiate a new infectious cycle.

TEM picture of bacteria: http://iai.asm.org/cgi/content/full/68/8/4706/F3

This micrograph reveals the presence of intracellular Rocky Mountain spotted fever bacteria, Rickettsia rickettsii.


The Vector


Rocky Mountain spotted fever (RMSF) is classified as a zoonosis (diseases of animals that can be transmitted to humans), and requires a vector for transmission. The vectors (and reservoirs) for RMSF are ticks, specifically the American dog tick (Dermacentor variabilis) and the Rocky Mountain wood tick (Dermacentor andersoni). The bite of an infected tick is the primary mode of transmission to humans, or other vertebrate hosts, though it is also possible, but extremely rare for infection to result from exposure to crushed tick tissues, fluids, or feces. (http://www.cdc.gov/ncidod/dvrd/rmsf/Natural_Hx.htm)

Dorsal view of an American Dog Tick, Dermacentor variabilis, magnified 21x.

The Disease

“RMSF is the most common fatal human tick-borne disease in the United States, with a minimal average of 351 confirmed human cases occurring annually and undoubtedly many more going unreported” (Niebylski et al.). 
There are three major symptoms associated with RMSF which usually appear after an incubation period of five to ten days: fever, headache, and a rash. The rash is a diffuse macular (featuring elevated discoloration) rash, is most prominent on wrists and ankles, which will usually erupt three or four days after the onset of fever. 


All three of the primary symptoms are not always present in all cases of RMSF, making it much harder to correctly diagnose the disease. Other symptoms are general symptoms common with other types of infections such as vomiting, diarrhea, and abdominal pain. Despite the name the disease is endemic in the Carolinas, Oklahoma, and Tennessee, and is less common in the Rocky Mountain area. About 95% of reported cases occur between April and September. (Lyon and Kelsey). 
Case fatality rate can be as high as 30% for untreated patients. With quick diagnosis and proper treatment fatalities are extremely rare. The most common and preferred treatment is Doxycycline, dosage depending on age and weight. The best way to prevent RMSF is to reduce exposure to ticks, as there is no vaccine available. (CDC)

Child's right hand and wrist displaying the characteristic spotted rash of Rocky Mountain spotted fever. (Photo/CDC)

Case Report- from MMWR weekly (May 21, 2004)

In Oklahoma a female child, age seven had a fever of 102.7ºC, malaise, abdominal pain, nausea, and vomiting, the emergency department initially diagnosed her with Viral gastroenteritis. She returned four days later with persistent fever, anorexia, irritability, photophobia, cough, diffuse myalgias, nausea, vomiting, and a rash. Lab results indicated an elevated white blood cell count, thrombocytopenia, and elevated alanine aminotransferase. She was then treated with intravenous doxycycline, and after two days of declining mental status, metabolic acidosis, and respiratory failure she died. Spotted fever group rickettsiae were detected by immunohistochemical (IHC) staining at CDC in autopsy specimens form the brain, skin, heart, lung, spleen, and kidney.
Soon after the girl died, her sister (age 3) displayed similar symptoms, including the rash. RMSF was diagnosed, and treated with doxycycline and she was able to recover. 


Niebylski, Peacock and Schwan: “Lethal Effect of Rickettsia rickettsii on Its Tick Vector (Dermacentor andersoni)”

Studies done by infecting ticks at different life stages with different strains, to test the effect the infection caused in the tick. The study showed that, 96.9% of the dead nymphs form the infected groups tested positive for R. rickettsii. Only 30.0% of the infected females successfully fed, where as 90.7% of the uninfected were successfully fed. The overall conclusions made from the experiments was that a high mortality rate was associated with infected ticks, so they commonly die before they are able to transmit the rickettsiae and cause disease. Other findings from the studies are that very few offspring are developed from infected ticks, so though vertical transmission is possible it is relatively rare, and horizontal transmission is the more common mode of transmission. Temperature effects were also observed and it was noticed that infected adult ticks incubated at 4 °C survived better compared to those incubated at temperatures around 20°C indicating that temperature changes may be responsible for increased incidence of the disease (as mentioned above 95% of cases occur between April and September). The article suggested possible future studies to be done on the temperature causing heightened virulence or shorter reproduction periods.

Rahman, Simser, Macaluso, and Azad: “Molecular and Funcitonal Analysis of the lepB Gene, Encoding a Type I Signal Peptidase from Rickettsia rickettsii and Rickettsii typhi

Their studies were aimed primarily at determining the mechanism of protein secretion. They performed DNA sequencing of the lep B operon, which codes for genes involved in protein secretion pathways and found the operon to be short in length, an ORF of only 801 nucleotides for R. rickettsii. The Type I signal peptidases for the rickettsiae are smaller in size relative to the Type I signal peptidases commonly found in many gram negative bacteria. Their studies are important for future research on how that protein secretion system affects the pathogenicity, and its role in causing RMSF.

Eremeeva, Dasch, Silverman: “Evaluation of a PCR Assay for Quantitationof Rickettsia rickettsii and Closely Related Spotted Fever Group Rickettsiae
SQ-PCR assay was used to quantify growth, by looking at the number of DNA copies. They found that the peak number of DNA copies per flask was independent of the inoculation dose, but was dependant on the number of host cells available for replication. They also used the SQ-PCR tests to quantify rickettsemia by calculating the number of DNA copies. They found that the quantities ofrickettsiae detected in the brains and lungs of acutely infected voles were 8.7x10^4 and 3.9x10^4 DNA copies per mg, respectively; those numbers were two to five times higher in tissues of animals that died from infection.



CDC Viral and Ricketsial Zoonoses Branch. Rocky Mountain spotted Fever. “The Organism.” <http://www.cdc.gov/ncidod/dvrd/rmsf/Organism.htm>, “Natural History” <http://www.cdc.gov/ncidod/dvrd/rmsf/Natural_Hx.htm> (19 September 2004).

Eremeeve, Marina e., Gregory A. Dasch, and David J. Silverman. “Evaluation of a PCR Assay for Quantitation of Rickettsia rickettsii and Closely Related Spotted Fever Group Rickettsiae.” Journal of Clinical Microbiology Vol. 41, No. 12, Dec. 2003: 5466-5472. (2003 American Society for Microbiology).

Lyon, G. Marshall, Anita M Kelsey, MD. “What’s Your Diagnosis?” Consultant August 1996:1729-1735

Niebylski, Mark L., Mort G. Peacock, and Tom G. Schwan. “Lethal Effect of Rickettsia rickettsii on Its Tick Vector (Dermacentor andersoni)” Applied and Environmental Microbiology Feb 1999: p 773-338.

Rahman, M. Sayeedur, Jason A. Simser, Kein R. Macaluso, and Abdu F. Azad. “Molecular and Funcional Analysis of the lepB Gene, Encoding a Type I Signal Peptidase from Rickettsia rickettsiiand Rickettsia typhi.” Journal of Bacteriology August 2003 (2003 American Society for Microbiology):4578-4584.

Van kirk, Levi S., Stanley F. Hayes, and Robert A. Heinzen. “Ultrastructure of Rickettsia rickettsii Actin Tails and Localization of Cytoskeletal Proteins.” Infection and Immunity (August 2000). Abstract. 16 September 2004. <http://iai.asm.org/cgi/content/abstract/68/8/4706>.