The roundworm, or ascarid, is a prolific egg layer. Each female can lay from 100,000 to 200,000 eggs each day. The eggs pass out of the horse with the feces. Infective larvae develop within the eggs, which are triple-coated and are not affected by adverse weather conditions. Therefore, they remain viable for up to 10 years. It is important to remember that fecal tests do not detect migration of parasite larvae within the horse.
Ascarid Eggs
This is an ascarid egg, which may have been lying on the ground for 10 years. When it is ingested, it begins an amazing journey. The egg’s coating is digested in the stomach. As the eggs reach the small intestine, they hatch, and the larvae immediately penetrate the lining of the intestinal tract, beginning a 30-day migration.
Liver With Ascarid Larvae Damage
The eggs travel via the hepatic vein to the liver, where they eat their way around the liver for seven to 10 days. The real damage takes place here and in the lungs. Fortunately, the liver is a very resilient organ and can regenerate itself. We seldom see any permanent damage to the liver from ascarid larval migration.
Small Hemorrhages From Ascarid Larvae Migration
The larvae then go to the lungs and continue their migration for 14-21 days, again eating their way around lung tissue. Damage done to the lungs is a different story than that of the liver, because the lungs, which heal by scarring, do not regenerate. This damage is permanent. After the ascarids mature and are ready to complete migration, they burrow from the blood side of the lung into the air side.
When migrating ascarid larvae are present, the immune system reacts violently to the foreign protein and destroys the alveoli. Such damage predisposes foals to pneumonia and may result in pulmonary hemorrhaging in a horse that becomes an athlete. When a horse is just a few months old, it has all the lung tissue it is ever going to have. Because lung tissue heals by scarring, damage to these sensitive structures is permanent. There will be less functional lung for the horse to use.
Horses whose lungs have been damaged by ascarid larval migration may have to breathe harder and faster to meet their oxygen demands as they develop and are asked to perform.
The worms then crawl from the alveoli into the bronchioles, to the bronchi, and into the trachea. They cause enough irritation to elicit a cough, so they are coughed to the back of the throat and re-swallowed as mature larvae. As adults, they swim upstream in the small intestine, robbing the horse of nutrition. These parasites have a very efficient migration. When the larvae reach the small intestine for the second time, their presence causes little consequence to the horse.
Normal Lung Tissue, This microscopic look at normal lung tissue shows empty air spaces – the alveoli – where oxygen is exchanged for CO2. 
Damaged Lung Tissue, This image shows lung tissue damaged by ascarid larval migration. Ascarid Larval Migration
Although horses may develop an immunity to ascarids, that immunity does not prevent the migration and damage these parasites may cause.
Ascarid larval migration can lead to other diseases. It reduces overall thriftiness in foals and can be related to foalhood pneumonia. Ascarid larvae may have an immunosuppressive effect in the lungs, which means they can reduce the immune system’s ability to respond to foreign invaders like bacteria and viruses. Remember, fecal tests can’t detect these migrating larvae.
Horses Live in Contaminated Areas
Think for a moment about how these roundworm survival attributes could affect your horses. The ascarids in your horses are still producing hundreds of millions of eggs. The eggs are not affected by climatic conditions, and they survive for up to 10 years. The only difference between your horses and horses in a natural setting is that your horses don’t migrate 25 miles a day away from these eggs. Instead, they are contained in small grazing areas, which ensures that they ingest parasites. Parasitism has become a chronic disease that horses are faced with daily.