What is the etiology and what are the clinical features of scrub typhus?

Scrub Typhus: Etiology and Clinical Features

Etiology

Scrub typhus is caused by the obligate intracellular bacterium Orientia tsutsugamushi, which is transmitted to humans through the bite of infected larval trombiculid mites (chiggers). 1, 2

• The disease is primarily endemic in rural areas of South Asia, Southeast Asia, and the Western Pacific regions 3
• Transmission occurs when infected chigger mites feed on humans during their larval stage 3, 4
• O. tsutsugamushi has multiple strains, including Gilliam and Karp strains, which can vary in virulence 4
• The pathogen multiplies in endothelial cells, causing a vasculitis that is responsible for the clinical manifestations of the disease 5

Clinical Features

Common Presenting Symptoms

• Fever is the most consistent symptom, typically beginning 6-10 days after the chigger bite 5, 2
• Headache, myalgia, and malaise are frequently reported 5, 2
• Lymphadenopathy is a common finding on examination 2
• Eschar (a necrotic lesion with a black crust) at the site of the chigger bite is a characteristic finding, though not always present 5, 6

Characteristic Eschar

• The eschar represents the site of mite feeding and inoculation of O. tsutsugamushi 5
• It typically appears as a painless, necrotic lesion with a black crust surrounded by erythema 5
• The eschar can be easily overlooked during examination, especially in atypical locations such as the medial malleolus or other hidden areas 6

Laboratory Findings

• Mild leukocytosis is common 2
• Thrombocytopenia is frequently observed 2
• Elevated liver enzymes (SGOT, ALP) and bilirubin levels 2
• Elevated BUN and creatinine may indicate renal involvement 2
• Hypoalbuminemia is commonly noted 2
• Albuminuria is present in most cases 2

Imaging Findings

• Abnormal chest X-rays are common (77.8% in one study) 2
• Radiographic findings may include interstitial pneumonitis or evidence of acute respiratory distress syndrome 7

Complications

Scrub typhus can progress to severe disease with multi-organ involvement:

• Respiratory complications: interstitial pneumonitis, acute respiratory distress syndrome (ARDS) 2, 7
• Cardiovascular complications: myocarditis, pericarditis, arrhythmias (including atrial fibrillation) 1
• Neurological complications: meningoencephalitis, status epilepticus 7
• Hematological complications: disseminated intravascular coagulation (DIC), hemophagocytic lymphohistiocytosis (HLH) 2, 7
• Renal complications: acute renal failure 1, 2
• Hepatic complications: acute hepatic failure 1
• Septic shock, which can lead to significant mortality 2

Diagnostic Challenges

• The disease can mimic other infectious conditions, leading to misdiagnosis 6
• The classic triad of fever, rash, and eschar is not always present 6
• The eschar may be overlooked during physical examination 6
• Cross-reactivity with other infections can complicate serological diagnosis 6

Treatment

• Doxycycline is the drug of choice for all age groups, including children under 8 years 3
• For adults: doxycycline 100 mg twice daily for at least 3 days after fever subsides, with a minimum treatment course of 5-7 days 3
• For children under 45 kg: doxycycline 2.2 mg/kg body weight twice daily 3
• Treatment should be initiated immediately when scrub typhus is suspected, without waiting for laboratory confirmation 3
• Clinical response typically occurs within 24-48 hours of appropriate antibiotic therapy 3

Prognosis

• Mortality rates of up to 4% have been reported 3
• Early diagnosis and prompt treatment significantly improve outcomes 3
• Severe complications such as ARDS, septic shock, and multi-organ dysfunction are associated with higher mortality 2, 7