Cattle between 12-24 months of age which become infected are at increasing risk of becoming ill and all cattle over 2 years of age, when infected, become ill, and about 50% of cattle that are 2 years of age or older will die if not treated.

Once infected, these cattle will be carriers of A. marginale for many years if not life. These animals can serve as a source of infection for other cattle. These cattle are infected carriers and do not develop signs of disease.

Occasionally, however, some animals spontaneously clear them-selves completely of the infection and become as susceptible to the disease as they were originally.

Necropsies will generally show a markedly anemic and jaundiced animal with thin watery blood. The spleen usually is enlarged and soft with prominent follicles, while the liver appears mottled and yellow-orange. The gall bladder is often distended. The bile appears thick greenish brown. The hepatic and mediastinal lymph nodes are likely brown.