Scrub typhus is an acute infectious illness caused by the bacterium Orientia tsutsugamushi. The infection is transmitted through the bite of an infected larval mite, a microscopic arthropod. If not recognized and treated promptly, the disease can lead to serious complications. Because initial symptoms resemble many common tropical fevers, scrub typhus is frequently overlooked or misdiagnosed, delaying necessary treatment.
The Cause and Transmission
Orientia tsutsugamushi is an obligate intracellular bacterium related to rickettsiae. It is transmitted to humans through the bite of an infected larval mite, commonly known as a chigger. These mites, primarily species of Leptotrombidium, are the vector responsible for carrying and passing the bacteria to a host.
Chiggers are tiny, less than a millimeter in size, and their bite is usually painless and unnoticed. The mites feed on skin fluids for several days, transmitting the bacteria into the human body. The infection cycle is maintained through these mites and small mammals, such as rodents, which serve as the primary reservoirs.
The geographical area where scrub typhus is historically prevalent is often referred to as the “tsutsugamushi triangle.” This region encompasses a vast area of the Asia-Pacific, extending from northern Japan, through Southeast Asia and the Indian subcontinent, and down to northern Australia. Vector mites thrive in areas of dense secondary vegetation, such as scrubland, forest edges, and tall grass. However, the disease’s range is expanding, with recent reports confirming its presence outside this traditional triangle, including parts of Africa and South America.
Recognizing the Symptoms
The incubation period typically ranges from 6 to 21 days, averaging 10 to 12 days before symptoms appear. The onset is usually sudden, characterized by a high-grade fever (39–40.5°C), severe headache, shaking chills, and generalized muscle aches. This non-specific presentation means the illness can easily be mistaken for other acute infections prevalent in the region, such as dengue fever or malaria.
A highly specific finding, though not always present, is the eschar, which forms at the chigger bite site. This lesion begins as a small, painless, red papule that gradually enlarges before ulcerating and becoming covered by a dark, dry, scab-like crust. While the eschar is a strong diagnostic clue, it is found in only about 50% of patients and can be located in concealed areas, making it easy to miss.
The infection often causes generalized lymphadenopathy (swollen and tender lymph nodes), particularly those draining the area near the eschar. A maculopapular rash, appearing as flat or slightly raised spots, may develop on the trunk between the fifth and eighth day of the fever. This rash can spread to the limbs, but it is often mild and may disappear quickly. In severe cases, the bacteria can affect the lining of the blood vessels, leading to organ involvement and complications like pneumonitis, altered mental status, and acute kidney injury.
Diagnosis and Medical Management
Diagnosing scrub typhus requires a high degree of clinical suspicion, especially in patients presenting with sudden fever and a history of exposure to endemic areas. Because initial signs mimic numerous other tropical illnesses, reliance on symptoms alone can be misleading. The presence of an eschar, combined with characteristic fever and exposure history, significantly raises the likelihood of the disease.
Laboratory confirmation primarily involves serological tests that detect the body’s antibody response to O. tsutsugamushi. The Indirect Fluorescent Antibody (IFA) test is the standard method for detection, though rapid diagnostic tests are increasingly used in field settings. A challenge with serology is that antibody levels may take several days to rise sufficiently, meaning a confirmed diagnosis may not be possible until the patient is well into recovery.
Treatment is generally started empirically based on clinical findings before laboratory results are finalized, due to the risk of progression. The first-line treatment is the antibiotic doxycycline, typically administered for 7 to 14 days. For vulnerable populations, such as pregnant women and young children, azithromycin is used as an effective alternative. With prompt administration of appropriate antibiotics, fever typically resolves within 36 hours, and the mortality rate drops significantly.
Prevention Strategies
Since no effective vaccine is available, prevention relies entirely on proactive measures to avoid chigger bites. Individuals living in or traveling to endemic regions must take precautions when venturing into areas of dense vegetation, tall grass, or scrubland, where infected larval mites are most likely to be found.
Effective methods include using EPA-registered insect repellents, such as those containing DEET, applied to exposed skin and clothing. An additional layer of protection is achieved by treating clothing, boots, and camping gear with permethrin, an insecticide that kills chiggers upon contact. Permethrin products are intended for materials and should not be applied directly to the skin.
Protective clothing minimizes exposed skin available for chigger attachment. Travelers and outdoor workers should also avoid sitting or lying directly on the ground or on vegetation. Specific clothing measures include:
- Wearing long-sleeved shirts, long pants, and closed shoes in high-risk environments.
- Tucking pant legs into socks or boots to create a physical barrier against mites.