A review of studies on the transmission of Anaplasma phagocytophilum from sheep: implications for the force of infection in endemic cycles.

We review the findings of a longitudinal study of transmission of the intracellular tick-borne bacterium Anaplasma phagocytophilum from sheep to Ixodes ricinus ticks under natural conditions of tick attachment in the UK. In this study, sheep-to-tick transmission efficiency varied in a quadratic relationship with the number of adult ticks that were feeding on the sheep. We raise the hypothesis that this relationship may be due to conflicting effects of the density of ticks on bacterial survival and target cell (neutrophil) fluxes at the tick-host interface: in the same sheep at the same time, resistance to ticks was progressively inhibited with increasing number of feeding adult ticks, and investigation of serological responses to tick antigens suggesting loss of resistance may be associated with polarisation of host Th1 to Th2 type responses to ticks. We also raise the hypothesis that these properties, with superimposed effects on tick survival, may mean that variation in tick density is an important causal factor of observed variations in the force of A. phagocytophilum infection amongst different geographic foci.