This is a disease of cattle caused by an organism called Anaplasma marginale. This organism is a rickettsia, half way between the viruses and the bacteria. It is susceptible to tetracyclines, unlike viruses. The disease, anaplasmosis, is caused when the infected cattle react to the agent and destroy their own infected red blood cells. This reaction causes a severe anemia.
All cattle are susceptible to infection by A. marginale.
Also, deer, elk, and other ruminants are susceptible to becoming infected and can act as natural reservoirs of the agent. A number of ruminants, such as cattle, deer, and elk can be carriers of the anaplasmosis agent. These species can carry the agent all or most of their lives and serve as a reservoir for infection of other animals. The transfer of the agent from a carrier animal to a susceptible animal can occur by a number of routes. One of the most common ways is via ticks.