How do ticks become infected with Borrelia? - briefly
Ticks acquire Borrelia spirochetes while feeding on infected rodents, birds, or other vertebrate hosts, and the bacteria survive the molt to the next developmental stage. Larvae become infected during this initial blood meal, and the pathogen is retained through subsequent stages, allowing transmission to new hosts.
How do ticks become infected with Borrelia? - in detail
Ticks acquire Borrelia bacteria primarily during blood meals taken from infected vertebrate hosts. The acquisition process follows a sequence of biological events:
- Host attachment – Larval or nymphal ticks attach to a mammal, bird, or reptile that harbors spirochetes in its bloodstream or skin.
- Ingestion of bacteria – While feeding, the tick ingests blood containing Borrelia. The spirochetes enter the tick’s midgut lumen.
- Midgut colonization – Borrelia adhere to the midgut epithelium using outer surface proteins (e.g., OspA). The bacteria proliferate within the midgut, avoiding the tick’s immune defenses.
- Transstadial persistence – The pathogen survives the molting process from larva to nymph or from nymph to adult, maintaining infection across developmental stages.
- Migration to salivary glands – Upon the next feeding, environmental cues trigger expression of OspC, facilitating spirochete migration from the midgut to the hemocoel and ultimately to the salivary glands.
- Transmission to new host – During saliva secretion, Borrelia are expelled into the feeding site, entering the new host’s tissue and establishing infection.
Additional pathways contribute to the overall infection dynamics:
- Co‑feeding transmission – Adjacent ticks feeding on the same host can exchange Borrelia without the host developing systemic infection, due to localized skin infection.
- Rare transovarial passage – Female ticks occasionally transmit spirochetes to their offspring, though this route accounts for a minimal proportion of infected larvae.
Reservoir species—such as white‑footed mice in North America and certain rodents and birds in Europe—maintain high levels of bacteremia, providing a continuous source of infection for questing ticks. Environmental factors, including temperature, humidity, and habitat composition, influence tick activity and host‑contact rates, thereby modulating the prevalence of Borrelia within tick populations.