A comparative study of clinical manifestations, haematological and serological responses after experimental infection with Anaplasma phagocytophilum in two Norwegian sheep breeds

Summary

Background: It has been questioned if the old native Norwegian sheep breed, Old Norse Sheep (also called
Norwegian Feral Sheep), normally distributed on coastal areas where ticks are abundant, is more protected against
tick-borne infections than other Norwegian breeds due to a continuously high selection pressure on pasture. The
aim of the present study was to test this hypothesis in an experimental infection study.
Methods: Five-months-old lambs of two Norwegian sheep breeds, Norwegian White (NW) sheep and Old Norse
(ON) sheep, were experimentally infected with a 16S rRNA genetic variant of Anaplasma phagocytophilum (similar
to GenBank accession number M73220). The experiment was repeated for two subsequent years, 2008 and 2009,
with the use of 16 lambs of each breed annually. Ten lambs of each breed were inoculated intravenously each
year with 0.4 ml A. phagocytophilum-infected blood containing approximately 0.5 × 106 infected neutrophils/ml. Six lambs of each breed were used as uninfected controls. Half of the primary inoculated lambs in each breed were
re-challenged with the same infectious dose at nine (2008) and twelve (2009) weeks after the first challenge. The
clinical, haematological and serological responses to A. phagocytophilum infection were compared in the two
sheep breeds.
Results: The present study indicates a difference in fever response and infection rate between breeds of
Norwegian sheep after experimental infection with A. phagocytophilum.
Conclusion: Although clinical response seems to be less in ON-lambs compared to NW-lambs, further studies
including more animals are needed to evaluate if the ON-breed is more protected against tick-borne infections
than other Norwegian breeds.