Persistence of Anaplasma ovis infection and conservation of the msp-2 and msp-3 multigene families within the genus Anaplasma.

Goats which have recovered from acute Anaplasma ovis infection remain seropositive, although infected erythrocytes cannot be detected by microscopic examination. Persistence of A. ovis 17 to 21 months following experimental infection was demonstrated by PCR detection of the msp-5 gene. Quantitative analysis of persistent rickettsemia over time showed that all levels were below the limit of microscopic detection and ranged from a low of 10(2) organisms/ml to peaks of 10(6) organisms/ml. Two patterns of persistent rickettsemia were observed: the first was characterized by cyclic fluctuations at 6- to 9-week intervals, similar to the pattern described for A. marginale-infected cattle, while in the second pattern, repetitive cycles did not occur and the rickettsemia levels were relatively constant. The msp-2 and msp-3 multigene families, which provide the genetic capacity for outer membrane protein antigenic variation during persistent A. marginale rickettsemia, were identified in the A. ovis genome by Southern blot analysis, and expression of an MSP-2 homologue was confirmed by using immunoblots.