| Summary: | <p>Abstract</p> <p>Background</p> <p>The occurrence of <it>Mannheimia </it>species in healthy sheep has only been investigated to a very limited extend since the genus and its five named species were established. The aim of the present study was to evaluate the occurrence of haemolytic <it>Mannheimia </it>species in apparently healthy sheep originating from four sheep flocks in South-Western Norway.</p> <p>Methods</p> <p>Typical <it>β</it>-haemolytic <it>Pasteurellaceae </it>were isolated from nasal swabs and subsequently subjected to bacteriological examination. A total of 57 <it>Mannheimia </it>isolates were obtained in pure culture. All isolates were genotyped by amplified fragment length polymorphisms (AFLP) analysis and compared to six reference strains. The 16S rRNA gene sequences of two isolates were also determined.</p> <p>Results</p> <p><it>β</it>-haemolytic <it>Mannheimia </it>species were isolated from 24% to 64% of the sheep in the four flocks. A total of 26 haemolytic <it>M. ruminalis</it>-like strains were isolated among which, a considerable genetic diversity was found. Eighteen <it>M. glucosida </it>isolates were obtained from three flocks, whereas <it>M. haemolytica </it>was only isolated from two flocks, 16 of them being from only one of the flocks.</p> <p>Conclusion</p> <p>We demonstrate that a relatively high number of apparently healthy sheep in Norway seem to carry the potentially pathogenic <it>M. haemolytica </it>and <it>M. glucosida </it>in the upper respiratory tract. An unexpectedly high number of haemolytic <it>M. ruminalis</it>-like organisms were also obtained in all four flocks. The usually non-haemolytic <it>M. ruminalis </it>are typically isolated from healthy ruminants. The significance of <it>β</it>-haemolytic <it>M. ruminalis</it>-like organisms is unknown and should be investigated in a future study.</p>
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