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J. Clin. Microbiol. doi:10.1128/JCM.02068-07
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Dynamic transmission of numerous Anaplasma phagocytophilum genotypes among lambs in an endemically infected sheep flock

Department of Veterinary Pathology, Faculty of Veterinary Science, University of Liverpool, Leahurst, Chester High Rd, Neston, Cheshire, CH64 7TE, United Kingdom; Department of Production Animal Clinical Sciences, Norwegian School of Veterinary Science, Kyrkjevegen 332/334, N-4325 Sandnes, Norway; Department of Bacteriology, National Veterinary Institute, SE-751 89, Uppsala, Sweden

^* To whom correspondence should be addressed. Email: rjbirt{at}liv.ac.uk.

	Abstract

The transmission dynamics of A. phagocytophilum strains circulating within juvenile members of a sheep flock grazing on Ixodes ricinus-infested pasture in southern Norway were monitored. PCR-based detection the bacterial p44 fragments in the blood of 16 lambs sampled weekly for 16 weeks following their release into pasture revealed rickettsemia in all animals, with an increasing proportion of infected animals as the survey progressed. Comparison of partial msp4 sequences obtained from infected blood samples revealed 24 distinct genotypes, some of which were repeatedly encountered, occurring in up to six sheep over a 14 week period, whereas others were only observed once. Individual sheep were infected by up to five distinct genotypes, with a specific genotype being encountered for between one and three consecutive weeks, and in some sheep, genotypes detected early in the study were also present in later samples. In general, detection of A. phagocytophilum by PCR correlated well with the observation of infected neutrophils in blood smears. Together these results reveal a previously unrecognized diversity of A. phagocytophilum strains simultaneously circulating within an endemically-infected population and are consistent with a remarkably dynamic transmission of strains among infected animals.