Parasite Lifecycles

All horses get internal parasites, no matter what their breed, where they live or what they're fed. Parasite control is a necessary part of horse health care. Left untreated, internal parasites can cause serious problems, including weight loss, lethargy, poor coat condition, digestive disturbances, diarrhea, colic and even death. In fact, before ivermectin was introduced over 20 years ago, one of the causes of death in horses was verminous colic caused by migrating large strongyle larvae. Parasite control is a necessary part of horse health care.

Parasites infect a horse after the horse unintentionally ingests them. Equine parasites have three different lifecycles:

Basic Parasite Lifecycle
Many types of equine parasites spend part of their lives as infective larvae in the grass. They're eaten as the horse grazes. Once inside the horse, these parasites migrate in the horse's body, mature to adults and lay eggs. A new generation of parasite eggs leaves the horse in manure, ending up in the grass to be eaten again.

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

Bot Fly Lifecycle
Bots aren't worms, they're flies. Mature 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 skin of the chin to emerge into the horse's mouth.