Life Cycle of Scrub Typhus and Human Inoculation
The life cycle of scrub typhus involves a complex interaction between the causative bacterium Orientia tsutsugamushi, its vector (trombiculid mites), and mammalian hosts including humans, with transmission occurring primarily through the bite of infected larval mites (chiggers) that serve as both vector and reservoir. 1
Seven Key Points of Scrub Typhus Life Cycle and Human Inoculation
Vector and Reservoir: Trombiculid mites (particularly Leptotrombidium species) serve as both the vector and reservoir for Orientia tsutsugamushi. Only the larval stage (chigger) feeds on vertebrate hosts, while nymph and adult stages feed on plant material 2.
Transovarial Transmission: Orientia tsutsugamushi is maintained in mite populations through transovarial transmission (from female mite to offspring), allowing the bacteria to persist in mite colonies across generations without requiring mammalian hosts 1.
Human Inoculation: Infection occurs when an infected chigger feeds on human skin, creating a painless bite while injecting saliva containing Orientia tsutsugamushi. The bacteria are introduced intradermally during this feeding process 3.
Initial Infection Site: After inoculation, Orientia tsutsugamushi preferentially infects dendritic cells and monocytes at the bite site rather than endothelial cells, which was previously thought to be the primary target. This creates an eschar (necrotic lesion) in approximately 60-80% of patients 4.
Dissemination: The infected dendritic cells and monocytes serve as vehicles for bacterial dissemination from the initial infection site to regional lymph nodes and subsequently into the bloodstream, leading to systemic infection 4, 5.
Systemic Infection: Once in the bloodstream, Orientia tsutsugamushi infects various organs including lungs, liver, spleen, kidneys, and brain, with a particular tropism for endothelial cells in the microcirculation of these organs 6, 3.
Immune Response and Persistence: The host mounts a mixed Th1/Th2 immune response with production of various cytokines. Despite this immune response, Orientia tsutsugamushi can establish persistent infection in some tissues for up to 84 days, which may contribute to relapse or delayed clearance 3.
Clinical Implications
The understanding of this life cycle has important implications for diagnosis and management of scrub typhus. The disease presents with fever, headache, myalgia, and often an eschar at the bite site. If untreated, it can progress to severe complications including pneumonitis, meningoencephalitis, acute kidney injury, and multiple organ failure 1.
Early diagnosis and treatment with appropriate antibiotics (doxycycline is the treatment of choice) is crucial as mortality rates for untreated scrub typhus can be significant 1.
Prevention Strategies
Prevention strategies focus on avoiding mite bites in endemic areas through:
- Using insect repellents containing DEET
- Wearing protective clothing
- Avoiding sitting or lying on bare ground or grass
- Treating clothing with permethrin in high-risk areas 1
Understanding the life cycle of scrub typhus is essential for developing effective control measures, including potential vaccine strategies, which remain challenging due to the antigenic diversity of Orientia tsutsugamushi strains 2.