106 Serial Evaluation ofA. Marginale Positivity Rate Within a Single Herd of Mature Beef Cows
Authors: Rogers Lauren R , Robbins Grayson , Clark Grace , McDermott Emily , Reif Kathryn E , Krumpelman Bernard W , Galloway Doug L , Powell Jeremy G 
Published: May 2023

Abstract
Anaplasmosis is an economically devastating disease in cattle that is caused by the rickettsial pathogen Anaplasma marginale. Infected cattle can serve as biological reservoirs for the organism, thus enabling further transmission of the disease to naive individuals via ticks, biting flies and contaminated needles. It is estimated that this disease causes over $300 million in expenses for the U.S. cattle industry annually. As a result, a better understanding of this disease and its prevalence is needed. As part of an ongoing collaborative effort between the University of Arkansas and the Kansas State University (KSU) College of Veterinary Medicine, general surveillance testing of beef cattle herds in Washington county, Arkansas previously indicated a possible 30% positivity rate for A. marginale in 2019. The objective of this study was to utilize both quantitative polymerase chain reaction (qPCR) and competitive enzyme-linked immunosorbent assay (cELISA) tests to determine the whole-herd prevalence of A. marginale within the University of Arkansas Washington county research herd. In the fall of 2020, the entire fall-calving herd (168, apparently healthy, mature beef cows) was sampled and tested. Both whole blood and serum samples were collected from each animal. qPCR testing was completed on each whole blood sample submitted at the KSU College of Veterinary Medicine, while cELISA testing was completed on each serum sample by the University of Arkansas. Of the68 cows tested, 20 cows tested seropositive, with a % Inhibition of 30 or above (mean = 45%). Confirmatory qPCR testing on all seropositive samples, however, indicated that all cows were negative for active infection with A. marginale. Subsequently, in the spring of 2022, all available cows that had previously tested seropositive for A. marginale were retested using the same protocol as previously described, except that all qPCR testing was completed by University of Arkansas Veterinary Entomology Lab. Of the 17 mature cows still in inventory, that had previously tested seropositive for A. marginale, only 7 were still seropositive with a % inhibition of 30 or above (mean = 39%). However, confirmatory qPCR testing on all 7 seropositive samples indicated that all cows were negative for active infections with A. marginale. Because of its lower cost, compared with qPCR testing, ELISA testing is commonly recommended to and utilized by producers and veterinarians as a first-line diagnostic for individual and herd health evaluation and management. Information gathered by this study indicates that cELISA testing alone may not provide an accurate depiction of actual infection rates within a herd, and that further investigation into possible causes of false positivity are required. As part of this study, additional assays investigating the specificity of the commercial Anaplasmosis cELISA and possible cross-reaction with other endemic rickettsial pathogens will be performed.