Anaplasmosis, caused by the rickettsial hemoparasite Anaplasma marginale (Am), is the most prevalent tick-transmitted disease of cattle worldwide and a major obstacle to profitable beef production in the continental United States (US). Anaplasmosis is readily transmitted through biological and mechanical vectors, such as ticks and biting flies, and iatrogenically through needles and equipment contaminated with infected blood. Clinical anaplasmosis, characterized by anemia, icterus and fever, is associated with significant production losses, abortions, and mortalities in cattle. It is estimated that the introduction of anaplasmosis into a previously naive herd can result in a 3.6% reduction in calf crop, a 30% increase in cull rate, and a 30% mortality rate in clinically infected adult cattle. Furthermore, a study has shown that 16% of pregnant carrier cows will transmit anaplasmosis in utero producing persistently infected offspring. The existence of both horizontal and vertical anaplasmosis transmission has important implications for disease control in endemic areas. The cost of a clinical case of anaplasmosis in the US has been conservatively estimated at more than $400 per animal, with some estimating the total cost to the beef industry at more than $300 million per year.