Basic Parasite Lifecycle
Many types of equine parasites live in manure in the grass and are eaten as the horse grazes. Once these parasites enter the horse's system, they migrate, mature and lay eggs. A new generation of parasite eggs and larvae leaves the horse in manure, then waits in the grass to be ingested again.

Indirect Lifecycle
Parasites with indirect lifecycles depend on another organism to get into the horse. For example, tapeworm eggs develop in the oribatid grass mite and enter the horse when infected mites in the grass are ingested.

Bot Fly Lifecycle
Bots aren't worms, they're flies. Adult female bot flies lay eggs on the horse's legs, shoulder, chin, throat and lips. The eggs hatch and bot larvae enter the horse when licked or by burrowing under the horse's skin to the mouth.