Lungworms (Dictyocaulus arnfieldi)

Lifecycle: Lungworm larvae go through intestinal walls into the circulatory system where they are carried to the lungs and mature. Eggs pass through the horse's system in manure.

How It Gets Into Your Horse: Lungworm larvae are swallowed as your horse eats infected grass.

Dangers If Left Untreated: Lungworm larvae irritate the small air sacs in the lungs, called the bronchioles, which can cause the horse to have a severe cough, difficulty breathing and loss of appetite.

Infection is usually light in older horses because they develop resistance to the parasite and usually have no signs. If foals are infected, they could die from a lungworm infection because they have less immunity. That's why it's a good idea to separate older horses from young foals and to maintain a regular deworming program.