Worm eggs unknowingly ingested by the Amber Snail hatch in the snails digestive track.
The larva then change into sporocysts, causing drastic mutations in the snail’s brain and physiology. Healthy snails seek darkness to hide from predators, but the infected Amber Snail moves itself into dangerous open space and light. It is also helpless to retract its newly swollen, pulsating tentacles.
The end result is that feeding birds mistake the exposed tentacles for a caterpillar or grub, and rip them off the snail’s defenseless head. The flatworm then grows to maturity inside the bird, laying eggs that are released in droppings for new snails to consume.
Here’s a clip of the zombie snail in action: