Date Published: October 31, 2006
Publisher: BioMed Central
Author(s): Louise L Poulsen, Turið M Reinert, Rikke L Sand, Magne Bisgaard, Henrik Christensen, John E Olsen, Snorre Stuen, Anders M Bojesen.
The occurrence of Mannheimia 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 Mannheimia species in apparently healthy sheep originating from four sheep flocks in South-Western Norway.
Typical β-haemolytic Pasteurellaceae were isolated from nasal swabs and subsequently subjected to bacteriological examination. A total of 57 Mannheimia 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.
β-haemolytic Mannheimia species were isolated from 24% to 64% of the sheep in the four flocks. A total of 26 haemolytic M. ruminalis-like strains were isolated among which, a considerable genetic diversity was found. Eighteen M. glucosida isolates were obtained from three flocks, whereas M. haemolytica was only isolated from two flocks, 16 of them being from only one of the flocks.
We demonstrate that a relatively high number of apparently healthy sheep in Norway seem to carry the potentially pathogenic M. haemolytica and M. glucosida in the upper respiratory tract. An unexpectedly high number of haemolytic M. ruminalis-like organisms were also obtained in all four flocks. The usually non-haemolytic M. ruminalis are typically isolated from healthy ruminants. The significance of β-haemolytic M. ruminalis-like organisms is unknown and should be investigated in a future study.
Mannheimia was proposed for the trehalose-negative [Pasteurella] haemolytica complex including at least seven species, five of which were named . [P.] haemolytica was reclassified as M. haemolytica including the former [P.] haemolytica biogroup 1 and serovars 1, 2, 5, 6, 7, 8, 9, 12, 13, 14 and 16 while M. glucosida was proposed to include [P.] haemolytica serovar 11, biogroups 3A-H and the β-glucosidase and meso-inositol positive strains of biogroup 9. M. varigena includes the former [P.] haemolytica biogroup 6 and Bisgaard taxon 15 and taxon 36. The former [P.] granulomatis and Bisgaard taxon 20 and [P.] haemolytica biogroup 3J were combined in M. granulomatis. Finally, M. ruminalis was proposed to include the former Bisgaard taxon 18 and [P.] haemolytica biogroup 8D . A novel Mannheimia species was subsequently reported from Australian feedlot cattle . The specificity of serotyping as a diagnostic tool was subsequently investigated by Angen et al. [3,4] who demonstrated that serotyping does not represent a reliable method for identification. The same authors emphasized that extended phenotypic and genetic characterization is necessary for proper identification of these organisms making it difficult to decide which of the present taxa are dealt with in previous studies, based only upon phenotyping and serotyping [5-11].
Based on the phenotypic characters outlined for each Mannheima species in Table 1, the species distribution of isolates obtained from the present investigation is summarized in Table 2. There were found no discrepancies between the phenotypic character demonstrated for present isolates and the ones outlined for the different Mannheimia species by Angen et al. . However, given the considerable genetic diversity among the isolates classifying with M. ruminalis will be referred too, as M. ruminalis-like organisms. Interestingly, all isolates classified with M. ruminalis were β-haemolytic on ovine blood, although this species generally is regarded as non-haemolytic. The M. ruminalis-like organisms were the most prevalent Mannheimia species present in all four flocks.
Previous investigations on the prevalence of [P.] haemolytica have shown a considerable variation. A range between 8.9% and 96.2% of healthy sheep that carry these organisms in the nasal cavity have been reported [6,7,22]. The variation is likely to be caused by several factors including different isolation techniques, misidentification, and seasonal variation. Swabbing of the tonsils and nasal cavity of slaughtered sheep showed that [P.] haemolytica could be isolated from 95% of the tonsils and 64% of the nasopharyngeal swabs . Furthermore, it has been found that the prevalence of [P.] haemolytica in temperate climates varies seasonally with a higher prevalence in spring and early summer .
The present study demonstrates that M. haemolytica, M. glucosida and M. ruminalis-like organisms are commonly found in the upper respiratory tract of healthy sheep. Interestingly, the occurrence of haemolytic M. ruminalis-like isolates was unexpectedly high. This organism has usually been isolated in a non-haemolytic form. The importance of this finding will have to be investigated in future studies, in order to conclude on the biological significance of the different haemolytic/non-haemolytic phenotypes.
AFLP: Amplified Fragment Length Polymorphisms
The author(s) declare that they have no competing interests.
LLP, RLS and TRM took part in all aspects of the investigation including planning, sampling, phenotypic and genotypic characterization and drafting of the manuscript. SS participated in the planning, sampling and initial characterization of the bacterial isolates. MB participated in the planning, phenotypical characterization and drafting of the manuscript. HC did the 16S rRNA sequence alignments. JEO participated in the planning and drafting of the investigation. AMB participated in the genotypical characterization, drafting and revision of the manuscript. All authors read and approved the final manuscript.