Stochastic Transmission of Multiple Genotypically Distinct Anaplasma marginaleStrains in a Herd with High Prevalence of AnaplasmaInfection

ABSTRACTMultiple genotypically unique strains of the tick-borne pathogen Anaplasma marginaleoccur and are transmitted within regions where the organism is endemic. In this study, we tested the hypothesis that specific A. marginalestrains are preferentially transmitted. The study herd of cattle (n= 261) had an infection prevalence of 29% as determined by competitive inhibition enzyme-linked immunosorbent assay and PCR, with complete concordance between results of the two assays. Genotyping revealed the presence of 11 unique strains within the herd. Although the majority of the individuals (70 of 75) were infected with only a single A. marginalestrain, five animals each carried two strains with markedly distinct genotypes, indicating that superinfection does occur with distinct A. marginalestrains, as has been reported with A. marginaleand A. marginalesubsp. centralestrains. Identification of strains in animals born into and infected within the herd during the period from 1998 to 2003 revealed no significant difference from the overall strain prevalence in the herd, results that do not support the occurrence of preferential strain transmission within a population of persistently infected animals and are most consistent with pathogen strain transmission being stochastic.