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Anaplasmosis, also known as yellow bag or yellow fever, is an infectious parasitic disease of cattle caused by the microorganism anaplasma marginale. This parasite infects the red blood cells and causes severe anemia, weakness, fever, lack of appetite, depression, constipation, decreased milk production, jaundice, abortion, and sometimes death. The incubation time for the disease to develop varies from 2 weeks to over 3 months, but averages 3 to 4 weeks. Adult cattle are more susceptible to infection than calves. The disease is generally mild in calves under a year of age, rarely fatal in cattle up to 2 years of age, sometimes fatal in animals up to 3 years of age, and often fatal in older cattle. Once an animal recovers from infection, either naturally or with normal therapy, it will usually remain a carrier of the disease for life. Carriers show no sign of the disease but act as sources of infection for other susceptible cattle. Occasionally, however, some animals will spontaneously clear themselves completely of the infection and become as susceptible to the disease as they were originally.