Large Roundworm (Parascaris equorum)
Roundworm or ascarids are usually only problematic for foals and younger horses.
Roundworm eggs can remain viable in the soil for many years and horses usually ingest as they graze. The eggs hatch inside the intestine and the emerging larvae migrates through the walls of the small intestine and into the veins.
Larvae are then transported via the blood to the lungs where they are coughed up, re-swallowed and passed into the small intestine. Once in the small intestine the larvae develop into adult roundworm.
The adults, which can grow up to about 50cm long, lay eggs which are passed out in the faeces.