Rickettsial infections and related infections (such as anaplasmosis, ehrlichiosis, and Q fever) are caused by an unusual type of bacteria that can live only in another organism.
Rickettsiae and rickettsia-like bacteria are an unusual type of bacteria that cause several diseases, including Rocky Mountain spotted fever and epidemic typhus. These bacteria 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. Ehrlichia, Anaplasma, and Coxiella burnetii bacteria are similar to rickettsiae and cause similar diseases.
For many species of these bacteria, small animals (such as rats and mice) are the usual host. Cattle, sheep, or goats are the hosts for Coxiella burnetii, which causes Q fever. Humans are the usual host for Rickettsia prowazekii, which causes epidemic typhus. These animals and humans—the hosts—are called the reservoir of infection. Host animals may or may not be ill from the infection. Rickettsiae and rickettsia-like bacteria are usually spread to people through the bites of ticks, mites, fleas, or lice that previously fed on an infected animal. Ticks, mites, fleas, and lice are called vectors because they convey (transmit) organisms that cause disease. Q fever, caused by Coxiella burnetii, can be spread through the air or in contaminated food and water and do not require a vector. Each species of rickettsiae and rickettsia-like bacteria has its own hosts and usually vectors.
Some of these bacteria (and the diseases they cause) occur worldwide. Others occur only in certain geographic regions.
A sore covered by black scab (eschar) often forms at the site of the bite. Nearby lymph nodes may be swollen. Some of these bacteria infect the cells lining small blood vessels, causing the blood vessels to become inflamed or blocked or to bleed into the surrounding tissue. Other bacteria (Ehrlichia and Anaplasma) enter white blood cells. Where damage occurs and how the body responds determine which symptoms develop.
Different rickettsial infections tend to cause similar symptoms:
Because the rash often does not appear for several days, early rickettsial infection is often mistaken for a common viral infection, such as influenza. People may have swollen lymph nodes.
As the infection progresses, people typically experience confusion and severe weakness—often with cough, difficulty breathing, and sometimes vomiting. When the infection is advanced, gangrene may develop, the liver or spleen may enlarge, the kidneys may malfunction, and blood pressure may fall dangerously low (causing shock). Death can result.
Because rickettsiae and rickettsia-like bacteria are transmitted by ticks, mites, fleas, and lice, doctors ask people whether they have been bitten by a tick or another vector and whether they have traveled to an area where these infections are common. Being bitten is a helpful clue—particularly in geographic areas where rickettsial or a related infection is common. However, many people do not recall such a bite. If doctors suspect Q fever, they may also ask whether people were at or near a farm (because cattle, sheep, and goats are the host for the bacteria that cause this infection).
Symptoms also help doctors diagnose these infections. Doctors ask people how long it took for the rash to appear after they were bitten (if known) and whether they have other symptoms. A physical examination is done to determine which parts of the body are affected and what the rash looks like. Doctors also look for an eschar that people may not have noticed and for swollen lymph nodes.
Testing is usually needed to confirm the diagnosis. Often, doctors cannot confirm an infection with rickettsiae or rickettsia-like bacteria quickly because these bacteria cannot be identified using commonly available laboratory tests. Special blood tests for these bacteria are not routinely available and take so long to process that people usually need to be treated before test results are available. Doctors base their decision to treat on the person's symptoms and the likelihood of possible exposure.
Useful tests include blood tests that detect antibodies to rickettsiae or rickettsia-like bacteria. If people have a rash, doctors sometimes remove a small sample of affected skin for testing. The polymerase chain reaction (PCR) technique can be used to increase the amount of the bacteria's DNA, so that the bacteria can be detected more rapidly.
Rickettsial infections respond promptly to early treatment with the antibiotics doxycycline (preferred) or chloramphenicol. These antibiotics are given by mouth unless people are very sick. In such cases, antibiotics are given intravenously. Most people noticeably improve in 1 or 2 days, and fever usually disappears in 2 to 3 days. People take the antibiotic for a minimum of 1 week—longer if the fever persists. When treatment begins late, improvement is slower and the fever lasts longer. If the infection is untreated or if treatment is begun too late, people may die, especially if they have epidemic typhus, scrub typhus, or Rocky Mountain spotted fever.
Ciprofloxacin and other similar antibiotics may be used to treat Mediterranean spotted fever but are usually not used to treat other rickettsial or related infections.
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Last full review/revision March 2013 by William A. Petri, Jr., MD, PhD