Rickettsial infections and related infections (such as ehrlichiosis and Q fever) are caused by an unusual type of bacteria that can live only in another organism.

• Most of these infections are spread through ticks, mites, fleas, or lice.
• A fever, a severe headache, and usually a rash develop, and people feel generally ill.
• Symptoms suggest the diagnosis, and doctors use special cultures and blood tests to confirm it.
• Antibiotics are given as soon as doctors suspect one of these infections.

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. Ehrlichia bacteria and Coxiella burnetii bacteria are similar to rickettsiae and cause similar diseases.

People are the main host for some of these bacteria. However, for most species, animals are the usual host. These animals are 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, or lice (vectors) that previously fed on an infected animal. Q fever, caused by Coxiella burnetii, can be spread through the air or in food. Each species of rickettsiae and related bacteria 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. Where this damage occurs and how the body responds determine which symptoms develop.

Symptoms

Different rickettsial infections tend to cause similar symptoms:

• Fever
• Severe headache
• A characteristic rash
• A general feeling of illness (malaise)

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 severe rickettsial diseases progress, people typically experience 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 result.

Diagnosis

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.

Often, doctors cannot confirm a rickettsial infection 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 people usually need to be treated before test results are available. Doctors base their decision to treat on the patient's symptoms and the likelihood of possible exposure.

Useful tests include immunofluorescence assay and polymerase chain reaction (PCR) testing, which use a sample from the rash or blood. These tests make the organism easier to identify. Immunofluorescence assays label foreign substances produced by the bacteria (antigens) with a fluorescent dye that makes them easier to detect. PCR increases the amount of the bacteria's DNA.

Treatment

Rickettsial infections respond promptly to early treatment with the antibiotics doxycycline (preferred), chloramphenicol, or tetracycline. These antibiotics are given by mouth unless people are very sick, in which case 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, death can occur, especially in people with epidemic typhus or Rocky Mountain spotted fever.

Ciprofloxacin and other similar antibiotics may be used to treat some rickettsial infections.

Some Rickettsial and Related Infections Infection Infecting Organism Host Areas Where Infection Description Occurs Epidemic typhus Rickettsia prowazekii, transmitted by lice People Throughout the world (uncommon in the United States, but occasionally occurs in homeless people) After an incubation of 7 to 14 days, symptoms begin suddenly, with fever, headache, and extreme fatigue (prostration). A rash appears on the 4th to 6th day. Untreated, the infection may be fatal, especially in people older than 50. Murine typhus Rickettsia typhi, transmitted by fleas Cats, rodents, and opossums Throughout the world Symptoms are very similar to those of epidemic typhus but are less severe. People also have shaking chills. Scrub typhus Rickettsia tsutsugamushi, transmitted by mites and mite larvae (chiggers) Rodents Asiatic-Pacific area, bounded by Japan, India, Australia, and Thailand After an incubation of 6 to 21 days, symptoms begin suddenly, with fever, chills, headache, and swollen lymph nodes. A black scab may develop at the site of the chigger bite. A rash appears on the 5th to 8th day. Rickettsialpox Rickettsia akari, transmitted by mites and chiggers Rodents First observed in New York City Other areas of the United States and Russia, Korea, and Africa About 1 week before the fever develops, a small buttonlike sore (ulcer) with a black center appears on the skin. Fever comes and goes, lasts about a week, and is accompanied by chills, profuse sweating, headache, sensitivity to the sun, and muscle pains. Q fever Coxiella burnetii, transmitted by inhaling infected droplets containing the bacteria or by consuming contaminated raw milk Sheep, cattle, and goats Throughout the world After an incubation of 9 to 28 days, symptoms begin suddenly, with fever, severe headache, chills, extreme weakness, muscle aches, loss of appetite, sweating, an unproductive cough, chest pain, and inflammation of the airways (pneumonitis), but no rash.

Last full review/revision November 2007 by William A. Petri, Jr., MD, PhD