Why does a tick swell? - briefly
A tick expands as it engorges with the host's blood after it pierces the skin and begins feeding. The rapid increase in volume results from the accumulation of blood in its body cavity, replacing the previously empty or partially filled state.
Why does a tick swell? - in detail
Ticks expand dramatically after attaching to a host because they ingest large volumes of blood. During the feeding phase, the salivary glands produce anticoagulants and immunomodulatory proteins that keep the host’s blood fluid and suppress inflammatory responses. This allows the arthropod to draw blood continuously for several days. The ingested blood is stored in a highly extensible midgut, whose cuticle stretches without rupturing due to a flexible extracellular matrix composed of chitin and protein fibers. As the gut fills, the tick’s body length can increase three‑ to five‑fold, producing the characteristic engorged appearance.
Key physiological mechanisms include:
- Salivary secretion: Anticoagulants (e.g., apyrase, anticoagulant protein) prevent clot formation; anti‑inflammatory agents (e.g., prostaglandin E2) reduce host defenses.
- Midgut expansion: Muscle layers relax under neurohormonal control, permitting the gut wall to accommodate up to 200 µL of blood in adult females.
- Cuticular elasticity: The exoskeleton contains a soft, pliable region near the ventral side that expands while maintaining overall structural integrity.
- Osmoregulation: Aquaporins and ion transporters move water from the blood meal into the hemolymph, balancing internal pressure and preventing rupture.
Engorgement also triggers hormonal changes that promote egg development in females. The rapid increase in body mass is a direct outcome of these coordinated processes, enabling the tick to acquire sufficient nutrients for reproduction before detaching from the host.