CHAPTER 179

Rickettsial and Related Infections

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 species 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 that previously fed on an infected animal. These organisms 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 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 malfunction, 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 person’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 bacteria 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 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, 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.

Rocky Mountain Spotted Fever

Rocky Mountain spotted fever (spotted fever, tick fever, tick typhus) is a rickettsial infection that is transmitted by dog ticks and wood ticks. It causes a rash, headache, and high fever.

People become infected when a tick carrying the infection bites them.

A severe headache, chills, extreme exhaustion, and muscle pains develop, followed a few days later by a rash.

Avoiding tick bites is the best way to prevent the infection.

People are given antibiotics immediately if they have been bitten by a tick and have typical symptoms.

Rocky Mountain spotted fever (RMSF) is caused by the bacteria Rickettsia rickettsii and is probably the most common rickettsial infection in the United States. It was first recognized in the Rocky Mountain states but occurs throughout most of the continental United States. It is most common in the Midwest and on the southern Atlantic seaboard. It also occurs in Central and South America. The infection occurs mainly from March to September, when adult ticks are active and people are likely to be in tick-infested areas. In the southern states, the disease may occur throughout the year. People who spend a lot of time in tick-infested areas—such as children younger than 15—have an increased risk of infection.

Ticks acquire rickettsiae by feeding on infected mammals. Infected female ticks can also transmit rickettsiae to their offspring. Infection is spread to people through bites by wood ticks or dog ticks. Rickettsial infection is probably not transmitted directly from person to person.

Rickettsiae live and multiply in the cells lining blood vessels. Blood vessels in and under the skin and in the brain, lungs, heart, kidneys, liver, and spleen are commonly infected. Small infected blood vessels may become blocked by blood clots.

Did You Know…

Almost three fourths of people with symptoms remember having a tick bite.

Symptoms

Typically, symptoms include a severe headache, chills, extreme exhaustion (prostration), and muscle pains. Symptoms begin suddenly 3 to 12 days after a tick bite. The more quickly symptoms begin, the more severe the infection. A high fever develops within several days and, in severe infections, persists for 1 to 3 weeks. A hacking, dry cough may also develop. Nausea and vomiting are common.

SOME RICKETTSIAL AND RELATED INFECTIONS

On about the fourth day of the fever, a rash appears on the wrists and ankles and rapidly extends to the palms, soles, forearms, neck, face, armpits, buttocks, and trunk. At first, the rash is flat and pink but later darkens and becomes slightly raised. It does not itch. Warm water—for example, in a bath—makes the rash more evident. In about 4 days, small purplish areas (petechiae) develop because of bleeding in the skin. A sore may form where these areas merge.

As this infection progresses, it may cause other symptoms:

Restlessness, insomnia, delirium, or sometimes coma if the blood vessels in the brain are affected

Abdominal pain

Inflammation of the airways (pneumonitis) and pneumonia

Heart damage

Anemia

Severe low blood pressure and death (uncommonly, when the infection is severe)

Prevention

There is no vaccine against this infection, so avoiding tick bites is the best prevention (see box on page 1166). The following measures can help:

Tucking trousers into boots or socks and applying insecticide that contains permethrin to clothing limits tick access to skin.

Tick repellents such as DEET (diethyltoluamide) may be applied to the skin. These repellents are effective but, rarely, cause toxic reactions, such as seizures, in small children.

Searching frequently for ticks helps prevent infection because the tick must be attached for 24 hours on average to transmit infection.

Attached ticks should be removed carefully with tweezers. The head of the tick should be grasped as close to the skin as possible. Care must be taken when removing a tick because rickettsiae may be transmitted if an infected tick that is engorged with blood is crushed while being removed.

Treatment

Doctors immediately prescribe antibiotics if they suspect the infection based on symptoms and the potential for exposure to infected ticks—even if laboratory test results are not yet available. Early treatment with antibiotics has reduced the death rate from about 20% to 5%.

Doxycycline (preferred), chloramphenicol, and tetracycline are effective. They are given by mouth when the infection is mild and intravenously when it is more severe. However, a doctor usually does not prescribe antibiotics for people who have had a tick bite but have no symptoms. Instead, the doctor may ask them to immediately report symptoms.

Ehrlichioses

Ehrlichioses are tick-borne infections that cause fever, chills, headache, and a general feeling of illness (malaise). These symptoms begin suddenly.

Ehrlichia bacteria, like rickettsiae, can live only inside the cells of an animal or a person. However, unlike rickettsiae, Ehrlichia bacteria inhabit white blood cells (such as granulocytes and monocytes). Different species inhabit different types of white blood cells.

Ehrlichioses are most common in the southeastern and south central United States. They also occur in Europe. They are most likely to develop between spring and late fall, when ticks are most active. Infection is spread to people through tick bites, sometimes resulting from contact with animals that carry the brown dog tick or deer tick.

Symptoms

Symptoms usually begin 1 to 2 weeks after a tick bite. The first symptoms are fever, chills, severe headache, body aches, and malaise. As the infection progresses, other symptoms may develop:

Vomiting

Diarrhea

Seizures

Confusion

Coma

Cough

Difficulty breathing

Skin rash is much less common than in rickettsial infections. Death is uncommon but can occur in people with a weakened immune system or who are not treated soon enough.

Diagnosis and Treatment

Doctors do blood tests, which may detect a low white blood cell count, a low platelet count (thrombocytopenia), and abnormal blood clotting. But these findings occur in many other disorders. Blood tests to check for antibodies to these bacteria may be helpful, but results are usually not positive until several weeks after the illness begins. Polymerase chain reaction (PCR) testing may be more useful. It increases the amount of the bacteria’s DNA and thus makes the bacteria easier to identify. Sometimes white blood cells contain characteristic spots (morulae) that can be seen under a microscope. The presence of morulae confirms the diagnosis of ehrlichiosis.

If people who may have been exposed to infected ticks have typical symptoms, treatment is usually started based on the person’s symptoms before test results are available. Doxycycline, chloramphenicol, and tetracycline are all effective. When treatment is started early, most people respond rapidly and well. A delay in treatment may lead to serious complications, including death in 2 to 5% of people.