Why do ticks transmit borreliosis? - briefly
Ticks acquire Borrelia burgdorferi while feeding on infected mammals, and the bacterium persists in their midgut before moving to the salivary glands during subsequent bites. This internal migration enables the pathogen to be injected into new hosts, completing the transmission cycle.
Why do ticks transmit borreliosis? - in detail
Ticks serve as efficient vectors for the spirochete Borrelia burgdorferi because of a combination of biological, ecological, and physiological factors.
The life cycle of hard ticks (Ixodidae) includes larval, nymphal, and adult stages, each requiring a blood meal. During feeding, the tick’s mouthparts penetrate the host’s skin and create a feeding cavity lined with a cement-like substance that prevents blood leakage. This cavity provides a protected environment where Borrelia can be transferred from the tick’s salivary glands directly into the host’s bloodstream. The pathogen exploits the tick’s salivary proteins, which suppress host immune responses and facilitate pathogen survival.
Key mechanisms enabling transmission:
- Acquisition and maintenance: Larvae acquire Borrelia when feeding on infected reservoir hosts (typically small mammals). The spirochete survives the molt to the nymphal stage through transstadial persistence, remaining viable in the midgut.
- Migration to salivary glands: Upon the next blood meal, the bacterium migrates from the midgut to the salivary glands, a process driven by temperature rise and tick feeding cues.
- Saliva-assisted transmission: Tick saliva contains anti‑inflammatory, anti‑coagulant, and immunomodulatory compounds that dampen host defenses, allowing the spirochete to establish infection before the host can mount an effective response.
- Extended feeding duration: Nymphs and adults feed for several days, providing ample time for the pathogen to be released and for the host’s skin to become permeated.
- Host‑tick specificity: Many reservoir species (e.g., Peromyscus mice) maintain high pathogen loads without severe disease, ensuring a steady supply of infected blood meals for ticks.
Ecologically, ticks thrive in habitats that support dense populations of competent reservoirs, such as forested and suburban areas with abundant leaf litter. Seasonal activity patterns align with peak host activity, increasing encounter rates. Moreover, the low host specificity of certain tick species broadens the range of potential transmission events.
In summary, the combination of transstadial persistence, directed migration of Borrelia to salivary glands, immunosuppressive saliva, prolonged feeding, and ecological conditions that favor frequent contact with infected reservoirs makes ticks particularly adept at spreading Lyme disease.