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Differences in Clinical Manifestations and Hematological and Serological Responses after Experimental Infection with Genetic Variants of Anaplasma phagocytophilum in Sheep

Department of Sheep and Goat Research, Norwegian School of Veterinary Science, Sandnes, Norway,¹ National Veterinary Institute, Uppsala, Sweden,² Institute of Microbiology and Immunology, Ljubljana, Slovenia,³ Research Laboratory for Infectious Diseases, National Institute of Public Health and the Environment, Bilthoven, The Netherlands⁴

Received 3 January 2003/ Returned for modification 17 March 2003/ Accepted 24 April 2003

Five-month-old lambs were experimentally infected with two 16S rRNA genetic variants of Anaplasma phagocytophilum, variants 1 (GenBank accession no. M73220) and 2 (GenBank accession no. AF336220). Additional sequencing of the groESL heat shock operon gene indicated that these variants differ in three nucleotides at positions 782, 824, and 890. The variants were obtained by blood sampling of A. phagocytophilum-infected lambs from one sheep flock in Norway and were stored at -70°C with 10% dimethyl sulfoxide as a cryoprotectant before being inoculated intravenously into susceptible lambs. The infectious blood contained, per ml, approximately 0.5 x 10⁶ neutrophils infected with either of the variants. Six weeks after the primary inoculation, the lambs were challenged with the same infectious dose of the heterologous variant. The results of the study indicate a marked difference in clinical manifestation, neutropenia, antibody response, and cross-protection after experimental infection with the two variants of A. phagocytophilum.

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