Anaplasmosis, caused by the rickettsial hemoparasite Anaplasma marginale, is the most prevalent tick-transmitted disease of cattle worldwide and a major obstacle to profitable beef production in the continental United States.1"3 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 inclinically infected adult cattle.4 Furthermore, a study has shown that 16% of pregnant carrier cows will transmit anaplasmosis in utero producing persistently infected offspring.5 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 USA has been conservatively estimated to be over $400 per animal with some estimating the total cost to the beef industry to be over $300 million per year.3,4'6 In addition to the costs associated with clinical anaplasmosis, animals recovering from acute anaplasmosis, including those treated with recommended doses of tetracyclines, remain lifelong A marginale carriers. There are currently noantimicrobial compounds approved for elimination of persistent A. marginale infections in cattle, despite published reports of successful carrier clearance with tetracyclines. Carrier animals serve as reservoirs of infection for mechanical transmissionand infection of ticks.3'7'8 This restricts the export of cattle from endemic areas such as the United States to non-endemic territories such as Canada. Anaplasmosis is therefore a significant impediment to unrestricted international movement of cattle in North America. Successful measures to control and eradicate anaplasmosis are confounded by the absence of efficacious antimicrobial regimens to eliminate infections9, inadequate information regarding the usefulness of newer diagnostic tests in determining the success of disease eradication and ineffective vaccines to protect against new infections.
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