Scrub Typhus

Overview

Scrub typhus (also called tsutsugamushi fever or chigger fever) is an acute febrile zoonosis caused by Orientia tsutsugamushi, a Gram-negative obligate intracellular bacterium. It is transmitted by the bite of infected larval trombiculid mites (chiggers), specifically Leptotrombidium deliense and related species.

Pathogen and Unique Biology

O. tsutsugamushi is genetically and structurally distinct from other rickettsiae:

Its genome is 2.1 megabases with 42% repeated DNA - the highest proportion in the bacterial world
Lacks lipopolysaccharide (LPS) and lipid A - an important distinction from other Gram-negatives
Has minimal peptidoglycan
Attaches to host cells via the 56 kDa TSA56 surface protein binding to integrin α5β1, fibronectin, and syndecan-4
Escapes the endosomal vacuole, travels via microtubules to the perinuclear region, and multiplies there
Released by budding off a host-cell membrane rather than by lysis

The organism exhibits remarkable antigenic diversity across multiple serotypes (Kato, Karp, Gilliam, Kawasaki are most common), which explains why immunity wanes within 1-3 years and cross-protective immunity can be lost in as little as 1 month.

(Harrison's Principles of Internal Medicine 22E; Henry's Clinical Diagnosis and Management)

Epidemiology

Endemic in the "tsutsugamushi triangle" - bounded by Japan/Korea/Russia to the north, Australia/Indonesia to the south, and Pakistan to the west; includes India, China, Sri Lanka, Southeast Asia, Taiwan, Bangladesh, Nepal
Approximately 1 billion people at risk; estimated 1 million cases per year
Transmission occurs via chigger bite (larval stage only feeds on hosts; nymphs/adults live in soil)
Mites and their rodent hosts serve as the main reservoir; humans are dead-end, accidental hosts
Infected chiggers cluster in scrub vegetation - transitional terrain between forests and cleared land; common occupational disease in rural settings
Seasonal pattern: temperate zones - mainly autumn, with a lesser spring peak; tropical zones - wet season
Emerging reports from Chile, UAE, sub-Saharan Africa (Kenya, Cameroon, Congo, Tanzania), and serologic evidence from South America challenge the classic geographic boundary
In some endemic areas, >3% of the population may be infected or reinfected each month

A 2024 global meta-analysis (Dasgupta et al., 2024) and a concurrent systematic review (Wang et al., 2024) both confirmed the high global seroprevalence and widening geographic distribution of scrub typhus.

(Goldman-Cecil Medicine; Harrison's; Fitzpatrick's Dermatology)

Pathophysiology

After inoculation by a chigger bite, O. tsutsugamushi disseminates hematogenously and infects endothelial cells and macrophages throughout the body. The primary mechanism of injury is:

Vascular injury from intracellular infection of endothelium
This leads to increased vascular permeability, organ dysfunction, and the characteristic eschar at the bite site
In severe disease: encephalitis, interstitial pneumonia, ARDS, DIC

Clinical Features

Incubation period: 6-21 days (commonly around 10 days)

Classic Triad (seen in <50% of cases in endemic populations):

Eschar - pathognomonic when present; an erythematous papule at the bite site progresses to ulceration and black eschar; found in 7-97% of patients depending on geography and strain
Regional lymphadenopathy followed by generalized lymphadenopathy
Maculopapular rash - appears on day 4-6 of fever in ~35% of cases; begins on trunk, spreads centrifugally to extremities, fades within days

Typical Presentation:

Sudden onset high fever (up to 40-40.6°C), chills, severe headache
Myalgia, cough, gastrointestinal symptoms (nausea, diarrhea)
Relative bradycardia - a characteristic finding
Conjunctival injection, ocular pain

Severe Complications:

Encephalitis / meningoencephalitis
Interstitial pneumonia / ARDS
Myocarditis, pericarditis
DIC (disseminated intravascular coagulation)
Hemophagocytic syndrome
Acute renal failure
Hepatitis (elevated transaminases common)
Retinal vein occlusion
Leukopenia, thrombocytopenia

Special Notes:

The eschar is often absent in intertriginous areas (axilla, groin, behind ears) - examine carefully
HIV-infected patients do not appear to have more severe disease
Pregnancy: abortion is common; a 2024 systematic review (Sengupta et al., 2024) documented significant adverse fetal outcomes with rickettsial infections in pregnancy
Movement disorders have been reported - a 2026 systematic review (Garg et al., 2026) documented this as a recognized neurological complication

(Fitzpatrick's Dermatology; Goldman-Cecil Medicine; Harrison's)

Diagnosis

Diagnosis is clinically challenging, especially when the eschar is absent. Epidemiologic context is critical.

Test	Notes
IFA (Indirect Fluorescent Antibody)	Gold standard; cutoff titer ≥1:400 in endemic areas (96% specific, 48% sensitive); fourfold rise between paired samples is preferred criterion
IgM ELISA	High diagnostic accuracy; may replace IgM IFA as reference standard
PCR (47 kDa, 56 kDa, groEL, 16S rRNA)	Best on eschar biopsy specimens; less sensitive on blood; most effective in acute phase
LAMP assay	Doesn't require thermocycler; useful for point-of-care; sensitivity 67-100%
Rapid ICT (lateral flow)	Variable sensitivity (20-100%); useful in field settings
Weil-Felix (OX-K agglutination)	Proteus mirabilis OX-K; historically used but insensitive and nonspecific - not recommended as sole test

Key diagnostic note: Antibodies typically appear only in the second week of illness, so serology is often negative early. PCR from eschar swabs is the most sensitive test in the first week.

(Henry's Clinical Diagnosis and Management; Goldman-Cecil Medicine)

Treatment

Drug of choice: Doxycycline

Severity	Regimen
Mild-moderate	Doxycycline 100 mg PO twice daily for 7-15 days
Severe	Doxycycline 200 mg IV daily for 7 days
Pregnancy / pediatrics	Azithromycin 500 mg/day for 3 days (preferred)
Doxycycline-resistant cases	Azithromycin, rifampin 600 mg/day, or chloramphenicol 500 mg QID
Severe combined regimen	Doxycycline + azithromycin (superior to monotherapy)

Important caveats:

Single-dose or very short courses (1-2 days) lead to relapses
Some strains in Thailand are poorly responsive to doxycycline and chloramphenicol but respond to azithromycin and rifampin
Quinolones should be avoided
A 2023 meta-analysis (Kabir et al., 2023) found macrolides (azithromycin) to be effective alternatives to doxycycline in pediatric scrub typhus

(Harrison's; Goldman-Cecil Medicine; Fitzpatrick's Dermatology)

Prognosis

Untreated: case fatality rate 6% (range 0-30%) depending on geographic region and strain
With appropriate doxycycline treatment: mortality drops to <1%
Relapses may occur even after treatment
Mild cases may resolve spontaneously

Prevention

Repellents (DEET-containing) applied to skin and clothing
Permethrin-treated clothing
Avoid scrub vegetation in endemic areas
No licensed vaccine is available; development is complicated by the extreme antigenic diversity of O. tsutsugamushi
Prophylaxis with doxycycline (weekly) has been used in military personnel in high-risk situations

Differential Diagnosis

Other rickettsial infections (murine typhus, epidemic typhus, spotted fevers)
Leptospirosis
Dengue fever
Typhoid fever
Viral hemorrhagic fevers
Infectious mononucleosis
Meningococcemia

Sources: Harrison's Principles of Internal Medicine 22E (2025), Goldman-Cecil Medicine, Fitzpatrick's Dermatology Vol 1, Henry's Clinical Diagnosis and Management by Laboratory Methods; supplemented with PubMed systematic reviews 2023-2026.

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