Scrub typhus is an acute infectious disease caused by the bacterium Orientia tsutsugamushi. Also known as bush typhus, this illness can range from mild to potentially fatal if left untreated. Recognizing its visual signs is important for early detection and medical intervention.
The Distinctive Eschar
A distinctive, though not universally present, sign of scrub typhus is the eschar, a dark, scab-like lesion that forms at the chigger mite bite site. This lesion typically begins as a small, red papule that gradually enlarges over several days. The papule then vesiculates, ruptures, and progresses to an ulcer, eventually becoming covered with a black scab, giving it the appearance of a cigarette burn. The surrounding skin often exhibits some redness, known as erythema.
The eschar typically measures around 1 to 1.5 centimeters in diameter. While its presence can vary, appearing in about 50% of primary infections and 30% of recurrent cases, it is a diagnostic clue. These lesions are frequently found in areas where skin surfaces meet, such as the armpits (axillae), under the breasts, in the groin, or on the neck. Multiple eschars have also been reported in some patients.
The development of the eschar often coincides with the onset of fever. Its unique morphology and location can guide healthcare professionals toward a diagnosis of scrub typhus. Its occurrence can depend on the specific strain of Orientia tsutsugamushi and may be easier to detect in individuals with lighter skin tones.
Additional Visual Signs
Beyond the eschar, other visible symptoms may appear, providing additional diagnostic clues. A maculopapular rash develops in 25% to 50% of patients. This rash typically emerges on the trunk around the fifth to eighth day of fever, and it may then spread to the arms and legs. The rash can vary in intensity, sometimes appearing rapidly and fading quickly.
In some instances, the rash may be macular, characterized by flat, red spots, or maculopapular, involving both flat spots and small, raised bumps. While petechiae, which are tiny red or purple spots caused by bleeding under the skin, are uncommon, the presence and distribution of the rash can support a clinical suspicion of scrub typhus. Swollen lymph nodes (lymphadenopathy) are another common visual sign, occurring in 40% to 97% of cases. They often affect regional nodes near the bite site before becoming generalized. These nodes can be tender to the touch.
The Mite and Its Habitats
Scrub typhus is transmitted to humans through the bite of infected chigger mites, the larval stage of trombiculid mites. These tiny arthropods are typically between 0.2 and 0.4 millimeters, often requiring magnification to be seen. While the adult mites possess eight legs, the infective larval stage, or chigger, has six legs.
Chigger mites are both the vector and natural reservoir for Orientia tsutsugamushi. These mites do not burrow into the skin but attach to the host and inject digestive enzymes, forming a feeding tube called a stylostome, through which the bacteria are transmitted. Humans typically encounter these mites when spending time outdoors in rural environments.
The preferred habitats of trombiculid mites include areas with dense vegetation, such as scrub brush, grassy fields, forest clearings, riverbanks, and abandoned plantations. These mites are closely associated with small mammals, particularly rodents, which serve as maintenance hosts for the bacteria. Humans are considered accidental hosts, becoming infected when they enter these “mite islands” or “typhus islands”—small, localized areas with high concentrations of infected mites.
Global Distribution
Scrub typhus is widely distributed across a geographical area historically called the “tsutsugamushi triangle”. This region spans over 8 million square kilometers and extends from far eastern Russia and Japan in the north to northern Australia in the south, and westward to Pakistan and Afghanistan. Countries like Korea, China, Taiwan, India, Malaysia, Thailand, and Indonesia are included within this endemic zone.
Understanding these endemic regions is important for both residents and travelers, as over one billion people are estimated to be at risk, with approximately one million cases occurring annually. While the disease is primarily associated with rural and suburban environments, increased global travel has led to cases being identified in non-endemic areas when infected individuals return home. The presence of scrub typhus is closely linked to suitable climates with sufficient moisture and scrub vegetation, which supports the mite populations.