Large roundworm also known as ascarids are very long worms up to 40cm when mature and produce large numbers of tough coated adhesive eggs which can survive for several years, and can stick to any surrounding environment. The mature worms are white in colour and much thicker than other equine worms. The eggs can stick to the coat and udders of the mare and even to the walls and stable floors. The eggs have very thick shells and therefore can survive on pastures over the winter months and perhaps for many years. These eggs then develop into larvae (young worms) which migrate through the liver and lungs and are eventually coughed up to then be ingested, subsequently maturing into egg laying adults in the small intestine. This complex lifestyle creates great potential for disease and can stunt growth and development. Respiratory obstruction is common as a result of the presence of larval stages in the lungs. Intestinal blockage and impaction colic is also common in foals due to the sheer physical size of the adult worms. Their presence in the gut can block the passage of food material as well as leading to nutritional deficiencies. A heavy burden of mature worms in the intestine may well give the classic signs of ill thrift, a pot-bellied appearance and or sluggishness. Such a burden has the potential for fatal colic. Large roundworm mainly affects youngsters up to 18 months old but horses with a neglected treatment history are also at risk.