Rickettsioses are infectious diseases caused by bacteria from the genus Rickettsia and related genera like Orientia, Ehrlichia, and Anaplasma. Found globally, these diseases range in severity from mild to life-threatening.
What Are Rickettsioses?
These bacteria are obligate intracellular parasites, meaning they can only survive and multiply inside the living cells of a host organism. They typically infect the endothelial cells that line blood vessels, causing damage and inflammation.
The diseases are broadly categorized into three main groups. The Spotted Fever Group Rickettsioses, which includes Rocky Mountain spotted fever, are often associated with a rash that can appear on the extremities and spread inward, and sometimes a dark scab called an eschar at the bite site. The Typhus Group Rickettsioses, such as epidemic typhus and murine typhus, may present with a rash that starts on the trunk and spreads outwards. The Scrub Typhus Group, caused by Orientia tsutsugamushi, frequently involves an eschar and is prevalent in the Asia-Pacific region.
How Rickettsioses Are Transmitted
Rickettsial bacteria are primarily transmitted to humans through the bites of infected arthropods. These vectors include ticks, fleas, lice, and mites, which acquire the bacteria from animal reservoirs. The specific arthropod vector varies depending on the type of rickettsiosis.
Ticks are common vectors for Spotted Fever Group rickettsioses, with different tick species transmitting different Rickettsia species. For instance, American dog ticks, Rocky Mountain wood ticks, and brown dog ticks can transmit Rocky Mountain spotted fever. Fleas are known to transmit Typhus Group rickettsioses, such as murine typhus carried by rat fleas. Body lice are responsible for transmitting epidemic typhus. Mites, specifically chiggers, transmit scrub typhus.
Symptoms and Diagnosis
The early symptoms of rickettsioses are often non-specific, making them challenging to distinguish from other common illnesses. Patients typically experience a sudden onset of fever, headache, and body aches. A rash is also a common symptom, though its appearance can vary depending on the specific rickettsial disease.
For some rickettsioses, including Spotted Fever Group infections and scrub typhus, an eschar may develop at the arthropod bite site. Rocky Mountain spotted fever can cause a maculopapular rash that progresses to petechiae, often starting on the wrists and ankles and spreading to the trunk. In contrast, typhus group rashes may begin on the trunk and spread to the extremities. Given the general nature of early symptoms, a healthcare provider considers the patient’s exposure history, such as recent travel or outdoor activities, when suspecting a rickettsial infection.
Diagnosis involves clinical suspicion based on symptoms and exposure, followed by confirmatory laboratory tests. Rapid tests are not available for early diagnosis. Serologic assays, which detect antibodies against the bacteria, typically become positive 10 to 12 days after symptom onset. Polymerase Chain Reaction (PCR) tests can also detect bacterial DNA in blood or tissue samples, including swabs from eschars, providing direct confirmation.
Treatment and Prevention
Early intervention with antibiotics is important for treating rickettsioses. Doxycycline is the standard treatment and is effective for people of all ages. Administering this antibiotic early can reduce the risk of severe complications and improve outcomes.
Preventing rickettsioses primarily involves minimizing exposure to the arthropod vectors that transmit these bacteria. Wearing protective clothing, such as long-sleeved shirts and pants, when in areas where ticks or mites are prevalent can help reduce bites. Using insect repellents containing DEET or picaridin on skin and permethrin on clothing provides an additional layer of protection. Regularly checking for and promptly removing ticks after outdoor activities is also a simple yet effective preventive measure. Managing environments by clearing brush and tall grass can help reduce vector populations around homes.