 The pathological and clinical signs are due to the direct development of large numbers of L3 larvae to adult worms over a relatively short period of time (approximately 3 weeks) in young animals with an immune system naïve to "Ostertagia" infections.The young adult worms then break out of the gastric glands, causing substantial damage to the abomasal wall.Mild cases result in reduced growth or production and severe cases can result in fulminating disease characterized by profuse watery diarrhoea, rapid weight loss, submandibular oedema ("bottle jaw"), anemia and death.