Rickettsia Rickettsii

Rickettsia rickettsii is a form of bacteria, called a coccobacillus, which causes the group of symptoms generally called “Rocky Mountain spotted fever”. The bacteria is transmitted from the bite of an infected tick, which feeds on warm-blooded animals, and which manifests as Rocky Mountain spotted fever when spread to humans. Ticks are natural hosts for the rickettsia rickettsii bacteria, serving as both a reservoir for the parasite, and a vector, or entry point, for the disease to infect warm-blooded vertebrates, including humans.

This particular spotted fever can be misdiagnosed at first, since it commonly only shows as a rash at the onset of the disease. The rash from Rocky Mountain spotted fever caused by Rickettsia rickettsii occurs in the majority of cases 2 to 5 days after the onset of the fever, appearing as small flat patches of inflammation just below the skin, known as macules, and is not an itchy rash. These macules appear on peripheral body parts, such as the wrists, ankles, feet, and forearms, but as the disease continues, the rash will darken in appearance, becoming reddish and eventually purple, as well as spreading to the entire body.

Upon infection, the symptoms of fever, nausea, emesis, or vomiting, severe headache, muscle pain, lack of appetite will manifest. In some rare cases, parotitis can occur, which is the inflammation of the parotid salivary glands on either side of the face, located near the “hinge” of the jaw. When diagnosing Rocky Mountain spotted fever, a history of tick bite, combined with fever and rash, is found. Longer exposure to this fever can cause abdominal, joint, and muscle pain.

Generally, Rocky Mountain spotted fever is treated based on clinical and epidemiological, or skin-related, findings about the patient, and antibiotics are administered to wipe out the R. rickettsii within the body. Treatment is immediate, and should not be delayed for laboratory confirmation. Those with organ or internal damage from the disease can take longer to recover.

The lifecycle of rickettsia rickettsii involves the transmission of the bacteria from an infected female tick to her eggs, meaning that from a single infected tick, all generations descending from that tick can potentially carry the pathogen. Because the bacteria can live and replicate inside the tick, feeding off its host’s fluids, an infected tick can remain infected for life. However, the percentage of the population infected with R. rickettsii is only around 1%-3%. It is believed that humans are incidental in the life cycle of the bacteria, and that the infection of human hosts is not necessary to the existence of rickettsiae in nature.