Recurrent bacteraemia in sheep infected persistently with Anaplasma phagocytophilum

Following experimental or natural infection with Anaplasma phagocytophilum, the causative agent of tick-borne fever (TBF), sheep may be infected persistently for several months or years. In the present study, quantitative real-time polymerase chain reaction was used to investigate the duration and magnitude of primary bacteraemia and to establish whether the organism is present continuously in the peripheral blood after the period of primary bacteraemia and the cessation of clinical signs. Persistent infection was characterized by a clearly defined period of primary bacteraemia followed by recurrent cycles of bacteraemia, usually lasting a few days and of lower magnitude, interspersed by negative periods of variable duration in which bacterial DNA could not be detected. During a 150-day period of consecutive sampling of four sheep, A. phagocytophilum was detected on 64.25 ± 4.9 occasions, which means that on average bacterial DNA was detected in 42.8 ± 3.3 percent of all samples, with the positive days falling into 15-20 distinct cycles. Primary bacteraemia lasted for 15.5 ± 2.33 days, but secondary and subsequent cycles of bacteraemia were short-lived, with 61% of the cycles lasting only 1-2 days and 39% lasting for 3 or more days. Secondary and subsequent cycles of bacteraemia were not accompanied by febrile responses or other clinical features of TBF. For three animals, bacterial DNA was detected at 311, 318 and 358 days post infection, indicating the long-term persistence of A. phagocytophilum within peripheral blood.