Life Cycle of Orientia tsutsugamushi in the Mite
Orientia tsutsugamushi is maintained in mite populations primarily through transovarial transmission (from female mite to offspring), allowing the bacteria to persist across generations without requiring mammalian hosts. 1
Transmission Cycle in Mites
Transovarial Transmission
- The bacterium passes from infected female mites directly to their eggs
- In Leptotrombidium chiangraiensis, transovarial and filial infection rates can reach 100% across generations 2
- In Leptotrombidium imphalum, transovarial infection rates may start at 100% but can decline in subsequent generations (to 93.3% in F1 and lower in F2) 2
Transstadial Transmission
- Once infected, the bacterium persists through all developmental stages of the mite:
- Egg → Larva (chigger) → Deutonymph → Adult 3
- The bacterial load varies significantly between life stages, with the highest concentration occurring during the larval (chigger) stage 4
- This strategic concentration during the larval stage maximizes transmission potential, as only larvae are parasitic and feed on vertebrate hosts 4
Bacterial Development Within Mites
- O. tsutsugamushi exists in different developmental forms within the mite:
- Intracellular bacteria (IB): Both round and elongated forms with higher peptidoglycan levels
- Extracellular bacteria (EB): Primarily round forms 5
- These forms have different physical properties and express different proteins involved in bacterial physiology and host-pathogen interactions 5
Mite-to-Human Transmission
- Transmission to humans occurs exclusively through the bite of infected larval mites (chiggers) 1
- During feeding, the chigger creates a stylostome (feeding tube) in the host's skin
- The bacterium is injected with the mite's saliva during this feeding process 1
- After infection, the bacteria can spread systemically in humans, potentially causing scrub typhus
Maintenance in Nature
- O. tsutsugamushi can be maintained in mite colonies for multiple generations without requiring mammalian hosts 1
- In some mite species, the bacterium may co-exist with other bacteria such as novel species of Amoebophilaceae, which might influence transmission efficiency 3
- Multiple genotypes of O. tsutsugamushi can co-exist within a single mite and be maintained through generations, though their proportions may vary across different life stages 4
Clinical Relevance
- Understanding this life cycle is crucial for developing prevention strategies in endemic areas
- The high efficiency of transovarial transmission explains why the disease persists in nature despite treatment of human cases
- Prevention strategies must focus on avoiding mite bites through appropriate protective measures in endemic regions 1