Eggs (Stage 1) leave the horse via the feces and contaminate the ground, eggs develop into infective larvae in one week (Stage 2).

Larvae may live for several months, encased in a thin skin or sheath, which protects them from the environment. They crawl up on grass pasture forage infecting the horse when they are consumed (Stage 3).

After strongyle larvae are swallowed, further development occurs, but there are marked differences between large and small strongyles during this stage (Stage 4). Larvae of the large strongyle (Strongylus vulgaris) migrate extensively within the walls of arteries that supply blood to the gastro-intestinal tract. Hence they are commonly referred to as "bloodworms." Damage to the arterial walls by this parasite causes an aneurysm, an enlargement of the blood vessel.

Blood clots also form as a result of the bloodworms migrating in the arteries. These clots shut off the blood supply to the intestine and result in colic. Complete blockage of the arteries may occur, and when this happens the animal dies. S. vulgaris is the most harmful and dangerous of all the internal parasites.

Development of the large strongyle larvae requires about six months, before they become sexually mature worms. Adult worms (Stage 5) attach to the inner surface of the large intestine and are quite active bloodsuckers. The females lay eggs, which leave the body in the feces to complete the cycle.

Small strongyle larvae (Stage 4) penetrate the wall of the intestine. Their development causes nodule formations in the walls. This injury is much less serious than that caused by large strongyle larvae, but is still significant in interfering with digestion. Development of small strongyle larvae is also relatively rapid in comparison with the large strongyles, requiring about six weeks. Mature small strongyles (Stage 5) also live in the large intestine, but for the most part they are not attached to the lining and do not suck blood. The mature females lay eggs, which are also discharged in the feces like those of the large strongyles.

Initiated by the passage of eggs in the feces (Stage 1), the eggs develop to the infective stage (Stage 2) in two weeks. Larvae remains in a thick, tough egg shells, resistant to environmental conditions — pastures, paddocks and stables may remain infected for years.

Infection of the horse (Stage 3) occurs during grazing or drinking. Inside the stomach, the eggs hatch and the larvae penetrate the walls. They migrate (Stage 4) in the bloodstream to the liver and to the lungs. After a short period of development in the lungs, they are coughed up and swallowed. This brings them back to the small intestine, where they develop into mature worms (Stage 5) and start egg production in about 10 weeks to complete the cycle. The largest of the parasites impacting horses. They attain lengths of 10 to 12 inches. Masses of ascarids may rupture the small intestine and result in peritonitis, usually fatal.

Adult bot flies attack the horse and deposit eggs (Stage 1) on the hairs, principally on the legs, chest, neck, throat and around the mouth. Development of infective larvae in the eggs (Stage 2) takes one week. These eggs hatch and the larvae gain access to the horse's mouth (Stage 3) either by active migration or when the horse bites or chews at its legs, etc. Inside the mouth, the larvae invade the tongue or gums.

After a development period of three weeks, the larvae emerge, are swallowed, and attach themselves to the lining of the stomach. These larvae in the stomach are commonly referred to as "bots." They remain attached to the stomach for several months. In essence, this period spent in the stomach is a mechanism for overwintering.

In the spring and summer, the bots detach themselves from the stomach wall and are passed to the outside in the feces. The discharged bots burrow into the ground and pupate. After about three weeks, the adult flies emerge from the pupae to complete the cycle. In temperate regions, there is only one cycle completed each year.

Eggs leave the horse in feces and contaminate the pasture. Tapeworm eggs are eaten by free-living (non-parasitic) mites (oribatid mites) which are particularly prevalent on permanent pastures. The tapeworm undergoes an essential maturation stage inside the mite. Horses inadvertently ingest the oribatid mite when grazing.

Following digestion by the horse's intestinal tract, the tapeworm is able to infect the horse. It matures into an irritating adult at the junction of the small intestine with the large intestine. Heavy tapeworm infestation may cause colic, weight loss or ill-thrift.

Tapeworm problems may be increasing in frequency in recent years. Ivermectin (which is extremely effective at killing all other parasite groups) is not effective against tapeworms, and the widespread use of ivermectin may have promoted the number of tapeworms that are able to colonize the intestine.