This disease is caused by rickettsiae in the genus Anaplasma , with A. marginale the best known, and found throughout the tropical and subtropical regions of the world. It is an infectious, but not contagious, disease of ruminant animals that is transmitted mostly by ticks, but to a much lesser degree by tabanid fl ies. The host range of Anaplasma is considerable, including deer, Odocoileus spp.; elk, Cervus elaphus ; giraffe, Giraffa camelopardalis ; pronghorn, Antilocapra americana ; American bison, Bison bison ; cape buffalo, Syncerus caffer ; Asian water buffalo, Bubalus bubalis ; wildebeest, Connochaetes spp.; bighorn sheep, Ovis canadensis ; and many African antelope species in addition to domestic cattle, sheep, and goats.
Anaplasma infect erythrocytes (red blood cells) exclusively, causing anemia and reduced hemoglobin concentrations due to damage to the erythrocytes. However, although domesticated ruminants are quite susceptible to infection, wild ruminants are quite resistant. In either type of ruminant, survivors that have been exposed to acute anaplasmosis usually regain normal hematologic parameters, and possess antibodies for some time.
Cattle, and to a lesser degree sheep, are affected by anaplasmosis. Wild ruminants, though substantially immune, play a major role in maintenance and spread of the disease. As noted previously, ticks are largely responsible for inoculating domesticated livestock, and when wild and domesticated animals share the same pastures, the wild animals become a problem for ranchers. Only where effective tick vectors are absent is the problem diminished.
