Microbe of the Week: Rickettsia rickettsi

All the small, reddish, rod-shaped dots are Rickettsia rickettsi. Photo Credit: CDC Public Health Image Library.

Microbe of the Week is written by undergraduate researcher, Maria Bergquist

Microbe of the Week: Rickettsia rickettsi

 

Welcome to microbe of the week, where we break down the different disease-causing microbes that lurk inside our tick vectors! This week’s microbe is Rickettsia rickettsi which causes Rocky Mountain Spotted Fever (RMSF).

What is Rickettsia rickettsi?

Rickettsia rickettsi is a bacterial species that infects the cells on the inner wall of blood vessels. Despite the disease name, Rocky Mountain Spotted Fever is endemic throughout the United States. The three species of tick that are known to commonly carry R. rickettsi are the American Dog Tick (Dermacentor variabilis), the Rocky Mountain Wood Tick (Dermacentor andersoni), and the Brown Dog Tick (Rhipicephalus sanguine). When ticks become infected, either from feeding on an infected mammal or in mating, they are infected for life and female ticks can pass the infection on to her eggs.

A) Male and Female American Dog Ticks B) Male Brown Dog Tick C) Rocky Mountain Wood Tick

All images photo credit: CDC Public Health Image Library

More data and statistics on RMSF available at: https://www.cdc.gov/rmsf/stats/index.html

Symptoms

Symptoms of RMSF are initially non-specific to the disease, but the CDC warns that cases can rapidly become serious and potentially life threatening. This makes treatment post-tick bite critical. Initial symptoms include fever, headache, nausea, vomiting, stomach pain, muscle pain, and lack of appetite.

The CDC notes that while the rash appears in almost all cases as red splotches with some pinpoint dots, rashes may not appear until later in illness.

RMSF may result in severe long-term health issues including amputation due to blood vessel damage, hearing loss, paralysis, and mental disability.        

Image showing RMSF rash

Photo Credit: CDC Public Health Image Library

More information on symptoms of RMSF is available at: https://www.cdc.gov/rmsf/symptoms/index.html

Treatment

The CDC recommends doxycycline to treat RMSF.  https://www.cdc.gov/rmsf/treatment/index.html

A Condensed History

• 1896 — First cases of RMSF recorded in Snake River Valley, Idaho. Originally called “black measles” due to the skin turning black in the late stages of the illness.
• 1899 — First clinical case of RMSF recorded by Edward E. Maxey.
• 1906 — Rickettsia rickettsi discovered by H. T. Ricketts.
• 1920 — National record of RMSF cases begins.
• 1921 — First test available for RMSF.
• 1940 to 1950 — Highest fatality rate per case of RMSF (between 25-30%).
• 1954 — First isolation of Rickettsia rickettsi from naturally infected mammal by Gould and Miesse.
• 2010 — RMSF changed to Spotted Fever Rickettsiosis (SFR) which encompasses cases of RMSF, Rickettsia parkeri rickettsiosis, Pacific Coast tick fever, and rickettsial pox.
• 2018 — 5,544 cases of RMSF reported to the CDC with a fatality rate of about 0.5%.