Proposed life cycle of A. phagocytophilum. Proposed life cycle of A. phagocytophilum. There are diverse strains of A. phagocytophilum in nature, and the susceptibilities of mammalian species to A. phagocytophilum strains vary. A. phagocytophilum cannot be passed effectively from infected adult Ixodes sp. ticks to eggs. Thus, larvae are not infected. Ticks at the larval, nymphal, or adult stage acquire A. phagocytophilum strains through blood feeding on infected animals. Once infected at the larval or nymphal stage, A. phagocytophilum is maintained in ticks through metamorphosis and molting to the next life stage and transmitted to the animals via blood feeding when the animal host is susceptible to the particular strain. Humans are susceptible to only limited strains, are the dead-end host of A. phagocytophilum, and are not a normal part of the life cycle of A. phagocytophilum or ticks. The animal species susceptibility to putative Anaplasma strains shown is a proposal, most of which has not been proven experimentally. Yasuko Rikihisa Clin. Microbiol. Rev. 2011; doi:10.1128/CMR.00064-10