Ixodes scapularis ticks transmit several diseases, including those caused by Anaplasma phagocytophilum. A phagocytophilum is maintained in the tick transstadially through its different life stages. It is possible for I scapularis ticks to survive several seasons. I scapularis larvae that feed in midsummer overwinter as nymphs, while nymphs that engorge in early summer overwinter as adults. They must survive multiple stresses, including freezing temperatures, desiccation, and ice encasement. This study determined that ticks infected with A phagocytophilum demonstrate better survival in cold temperatures, and the bacterial organisms survive the winter in I scapularis tissues. Ixodid ticks are often active in winter months when other arthropods are not; this may be explained by the presence of antifreeze glycoproteins. A phagocytophilum induces expression of an antifreeze glycoprotein that enhances cold tolerance and helps tick survival.

Commentary: There are several examples of symbiotic relationships between bacteria and invertebrates. A tick-borne pathogen increased survival of its arthropod host and indirectly allowed increased likelihood of pathogen transfer to mammalian hosts in this example. A phagocytophilum is a pathogen of people and dogs, and this description of the way that this organism enhanced survival of its host underscores the importance of studying vector ecology. —Patricia Thomblison, DVM, MS

Anaplasma phagocytophilum induces Ixodes scapularis ticks to express an antifreeze glycoprotein gene that enhances their survival in the cold. Neelakanta G, Sultana H, Fish D, et al. J CLIN INVEST 120:3179-3190, 2010.