Introduction
Rickettsiae are an unusual type of bacteria that cause several diseases, including Rocky Mountain spotted fever and epidemic typhus. Rickettsiae differ from most other bacteria in that they can live and multiply only inside the cells of another organism (host) and cannot survive on their own in the environment. Ehrlichiae are similar to rickettsiae and cause similar diseases.
People are the main host for some rickettsiae and ehrlichiae. For most species, however, animals are the usual host. This population of animals is called the reservoir of infection. Animals in the reservoir may or may not be ill from the infection. Rickettsiae are usually spread to people through the bites of ticks, mites, fleas, and lice (vectors) that previously fed on an infected animal. Q fever can be spread through the air or in food. Each of the rickettsiae and ehrlichiae has its own hosts and vectors.
In people, rickettsiae infect the cells lining small blood vessels, causing the blood vessels to become inflamed or blocked or to bleed into the surrounding tissue. The part of the body where this occurs determines what symptoms develop.
Symptoms and Diagnosis
The different rickettsial infections tend to produce similar symptoms. Fever, severe headache, a characteristic skin rash, and a general feeling of illness (malaise) typically develop. Because the rash often does not appear for several days, early rickettsial infection is often mistaken for a common viral infection, such as influenza.
As rickettsial disease progresses, a person typically experiences confusion and severe weakness--often with cough, difficulty breathing, and sometimes vomiting and diarrhea. In some people, the liver or spleen enlarges, the kidneys fail, and blood pressure falls dangerously low. Death can occur.
Because rickettsiae are transmitted by ticks, mites, fleas, and lice, a history of a bite from one or more of these vectors is a helpful clue--particularly in geographic areas where rickettsial infection is common. However, many people do not recall such a bite.
The diagnosis of a rickettsial infection is difficult for a doctor to confirm quickly, because rickettsiae cannot be identified using commonly available laboratory tests. Special cultures and blood tests for rickettsiae are not routinely available and take so long to process that the person usually needs to be treated before the test results are available. The doctor's suspicion of rickettsial infection remains the most important factor in diagnosis.
Treatment
Rickettsial infections respond promptly to early treatment with the antibiotics tetracycline, doxycycline, or chloramphenicol. These antibiotics are given by mouth unless the person is very sick, in which case they are given intravenously. Noticeable improvement usually takes place in 1 or 2 days, and fever usually disappears in 2 to 3 days. The person continues to take antibiotics for a minimum of 1 week--longer if the fever persists. When treatment begins late, improvement is slower and the fever lasts longer. Death can occur if the infection is untreated, or if treatment is begun too late.
 See the table Some Other Rickettsial Infections.
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