What is Scrub Typhus?
Scrub typhus is an acute febrile illness caused by the bacterium Orientia tsutsugamushi, a member of the rickettsial group of pathogens. It is transmitted to humans through the bite of infected chigger mites (larval trombiculid mites) that feed on rodents, the natural reservoir of the organism. Scrub typhus is found across a vast “tsutsugamushi triangle” stretching from northern Japan and eastern Russia in the north, through southern China and the Indian subcontinent, to northeastern Australia and the western Pacific islands. It is particularly endemic in Southeast Asia, including Thailand, where it is a recognized cause of acute undifferentiated febrile illness, especially in rural and agricultural areas.
Epidemiology in Thailand
Scrub typhus is a significant cause of acute undifferentiated fever in Thailand, particularly in the northern and northeastern regions where rice paddies, forests, and brushy areas provide ideal habitat for the chigger mites. Cases peak during the cool, dry season from November to April, when agricultural activities such as rice harvesting bring people into close contact with chigger-infested areas.
The disease is increasingly recognized as one of the most common causes of acute encephalitis and febrile illness with multiorgan involvement in Southeast Asia. In Thailand, scrub typhus is often underdiagnosed because its symptoms overlap with dengue, leptospirosis, malaria, and typhoid fever — all of which are prevalent in the same geographic areas.
Symptoms: The Classic Triad and Beyond
The incubation period is typically 1 to 3 weeks after a chigger bite. The disease has an abrupt onset with the following characteristic features:
• Sudden high fever: Often reaching 39°C to 40°C (102°F to 104°F), with an acute, dramatic onset
• Severe headache: Often frontal, persistent, and refractory to analgesics
• Myalgias: Muscle aches, particularly of the back and shoulders
• The eschar: At the site of the chigger bite, a papule develops that becomes necrotic and forms a characteristic black crusty lesion called an eschar. The groin and abdomen are the most common sites, followed by the chest and axillae. The eschar is typically painless with an erythematous halo and may have lymphangitis radiating from it. While this finding is highly suggestive of scrub typhus, it was only described in approximately 19% of a South Korean study series, and it is often located in hidden areas (groin, axillae, scalp) and can be missed on examination.
• Rash: A maculopapular rash that typically appears around days 5 to 7 of illness, beginning on the trunk and spreading to the extremities. The rash may be subtle or absent in some cases.
• Lymphadenopathy: Regional lymph nodes near the eschar are commonly enlarged and tender
• Splenomegaly: Enlargement of the spleen
Severe scrub typhus can involve multiple organ systems and may be fatal without treatment:
• Encephalitis: Confusion, altered mental status, seizures
• Pneumonitis: Respiratory distress, acute respiratory distress syndrome (ARDS)
• Myocarditis: Cardiac inflammation, arrhythmias
• Meningitis: Nuchal rigidity, photophobia
• Hepatitis: Elevated liver enzymes, jaundice
• Acute renal failure
Without treatment, fever typically subsides spontaneously after about 2 weeks, but the mortality rate may be 10% to 30%. With appropriate antibiotic treatment, mortality drops below 1%.
Diagnosis
Diagnosis is primarily clinical, supported by laboratory and serological findings:
• Serological assays: Indirect fluorescent antibody (IFA), enzyme-linked immunosorbent assay (ELISA), and rapid diagnostic tests using lateral flow chromatography are the mainstays of laboratory diagnosis. IFA is considered the gold standard but results take days to weeks.
• PCR: Polymerase chain reaction amplification of Orientia genes from eschar tissue is highly effective for early diagnosis. Blood PCR is less sensitive but useful in eschar-negative cases.
• Blood tests: Typical findings include elevated C-reactive protein, mild transaminitis (elevated AST/ALT), thrombocytopenia, and hyponatremia. Lymphocytosis and an elevated white blood cell count may be present.
• Chest X-ray: May show interstitial infiltrates in cases with pneumonitis
Scrub typhus must be differentiated from dengue, leptospirosis, typhoid fever, malaria, rickettsialpox, murine typhus, and other causes of acute undifferentiated fever. The presence of an eschar, lymphocytosis, and elevated C-reactive protein may help distinguish scrub typhus from dengue.
Treatment
Scrub typhus responds dramatically to antibiotic therapy, and treatment should never be delayed pending laboratory confirmation when clinical suspicion is high:
• First-line treatment: Doxycycline 100 mg orally twice daily. Treatment should continue until the patient has been afebrile for at least 3 days (typically 7 to 14 days total). A rapid defervescence — often within 24 to 48 hours — is characteristic of successful treatment and serves as a useful diagnostic clue.
• Alternative for mild disease: Azithromycin 500 mg once daily for 3 days. Shorter courses are associated with higher relapse rates.
• Severe disease: Minocycline 100 mg intravenously twice daily, or doxycycline if IV is not available. Chloramphenicol (500 mg four times daily) is an alternative but carries a risk of relapse and serious adverse effects including aplastic anemia.
Relapse occurs in up to 20% of cases, especially with short-course therapy or in patients who stop antibiotics too soon after fever resolves. Patients should be instructed to complete the full course of treatment and return if fever recurs.
Prevention
• Avoid chigger habitats: Chiggers thrive in grassy, brushy, and wooded areas. Avoid sitting on the ground in tall grass, brush, or forest floor, especially during the cool, dry season.
• Use insect repellent: DEET-based or permethrin-treated clothing provides effective protection against chigger bites
• Wear protective clothing: Long sleeves, long pants tucked into socks, and closed shoes when working or hiking in endemic areas
• Shower immediately: Showering and changing clothes after potential exposure can wash off unengaged chiggers before they bite
• Clear vegetation: Keep grass and brush trimmed around homes, especially in rural areas
• Chemoprophylaxis: Daily doxycycline 100 mg has been used for military personnel and others with prolonged high-risk exposure, but is not generally recommended for tourists or the general public
Dr. Kijakarn notes that scrub typhus is frequently misdiagnosed as dengue in Thailand. “Both diseases cause high fever in the same geographic areas, but the treatments are completely different. If a patient has been hiking, farming, or spending time in grassy areas and presents with fever, we specifically look for the eschar. Missing the diagnosis and delaying doxycycline can be fatal, especially in children and elderly patients. We always ask about outdoor exposure before assuming dengue.”
This article is for general information only and does not replace professional medical advice. If you are experiencing symptoms, please consult a healthcare provider.
Leave a Reply