Strain Composition of the EhrlichiaAnaplasma marginalewithin Persistently Infected Cattle, a Mammalian Reservoir for Tick Transmission

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Abstract
Tick-borne ehrlichial pathogens of animals and humans require a mammalian reservoir of infection from which ticks acquire the organism for subsequent transmission. In the present study, we examined the strain structure ofAnaplasma marginale, a genogroup II ehrlichial pathogen, in both an acute outbreak and in persistently infected cattle that serve as a reservoir for tick transmission. Using themsp1α genotype as a stable strain marker, only a single genotype was detected in a disease outbreak in a previously uninfected herd. In contrast, a diverse set of genotypes was detected in a persistently infected reservoir herd within a region whereA. marginaleis endemic. Genotypic diversity did not appear to be rapidly generated within an individual animal, because only a single genotype, identical to that of the inoculating strain, was detected at time points up to 2 years after experimental infection, and only a single identical genotype was found in repeat sampling of individual naturally infected cattle. Similarly, only a single genotype, identical to that of the experimentally inoculated St. Maries or South Idaho strain, was identified in the bloodmeal taken byDermacentor andersoniticks, in the midgut and salivary glands of the infected ticks, and in the blood of acutely infected cattle following tick transmission. The results show that mammalian reservoirs harbor genetically heterogeneousA. marginaleand suggest that different genotypes are maintained by transmission within the reservoir population.