Karlsson et al. Acta Veterinaria Scandinavica 2010, 52:2 http://www.actavetscand.com/content/52/1/2 Results All 21 semen and 43 cauda epididymidis samples were negative in the PCR. The internal amplification control (mimic) worked well for all samples analysed. None of the 21 and 43 paired-sera from dairy and beef bulls, respectively, were positive in the antibody detec- tion assay. Most samples were clustered well below the cut-off value 100. Only six samples had S/P% above 20, where 42 was the highest value. Discussion In this study we found no presence of chlamydial agent in any semen or cauda epididymidis tissue samples, i.e. all samples were negative by real-time PCR. This is in concordance with an Austrian study [31] where neither Cp. abortus nor Cp. pecorum was detected in 273 semen samples from bulls at five AI centres. On the other hand, the results contradict those reported from other investigations performed in apparently healthy bulls. In Lithuania as much as 29.8% of 47 tested bulls had chla- mydial agent in their semen, as judged by PCR [13], and chlamydiae were detected by immunofluorescence in 14.3% of 42 bovine ejaculates from the Czech republic [32]. In German and Swiss investigations of semen sam- ples, 9.2% and 6.6%, respectively, were found positive by PCR [22,23]. The sensitivity of the PCR assay was estimated to 1 IFU per PCR with no indication of potential inhibitory factors. In a previous investigation of cows from dairy herds with reproductive disorders we identified positive specimens, including vaginal swabs, placenta and milk when using the same PCR assay [33]. Moreover, several positive specimens from different organs in pigs and placentae in sheep (unpublished data) as well as con- junctival and nasal swabs from cats [34] have been demonstrated by the same PCR at our laboratory. Those samples were handled and stored in a similar way as in the present study. Therefore, the test is considered robust and to have a high sensitivity and specificity. All sera were negative in the Cp. abortus ELISA assay with values far below the cut-off value. The specificity of the test has been reported to be 100% when used to analyse Scottish sheep documented free of Cp. abortus [35] and 90% when sera from New Zealand, a country free from Cp. abortus, were analysed [36]. The sensitiv- ity were estimated to 91% and 80%, respectively, when analysing sera from experimentally Cp. abortus infected sheep [35,36]., and it can, hence, not be excluded that some of our sera were positives but not detected by the test. However, the fact that all the beef bulls, which came from as many as 124 different herds from all over Sweden, were still seronegative after they had been housed together for six months, in adjacent pens under Page 3 of 4 the same roof, indicates that Cp. abortus is not present in Swedish beef cattle herds. Moreover, the absence of seropositives among the analysed dairy bulls indicates that Cp. abortus is very rare, or absent, in Swedish bulls. These results are in agreement with a previous study in Swedish dairy cows where only 2 out of 525 sera were positive in the same ELISA and only Cp. pecorum were confirmed in vaginal swabs [33]. Conclusions This study suggest the risk for venereal transmission of chlamydial infection through Swedish bull semen is low. However, because Chlamydophila spp. infection rates seem to differ throughout the world, it is essential to clarify the relative importance of transmission of the infection through semen on cattle fertility. Acknowledgements The authors wish to thank Maj Hjort at the National Veterinary Institute (SVA), for performing the serological analyses and for assistance with the DNA preparations. This study was supported by the Swedish Farmer’s Foundation for Agricultural Research and the Programme for Infection Biology at the Faculty of Veterinary Medicine and Animal Science, Swedish University of Agricultural Sciences. It was part of the EU research collaboration COST 855. Author details 1Division of Ruminant Medicine and Veterinary Epidemiology, Department of Clinical Sciences, Faculty of Veterinary Medicine and Animal Science, Swedish University of Agricultural Sciences (SLU), Box 7054, SE-750 07, Uppsala, Sweden. 2Department of Animal Health, Section for Farm Animal Health and Welfare, National Veterinary Institute, Pb 750 Sentrum, N-0106 Oslo, Norway. 3Department of Animal Health and Antimicrobial Strategies, Section of Farm animals, National Veterinary Institute (SVA), SE-751 89, Uppsala, Sweden. 4Department of Animal Health and Antimicrobial Strategies, Section of Antibiotics, National Veterinary Institute (SVA), SE-751 89, Uppsala, Sweden. Authors’ contributions ACK participated in the design and coordination of the study, drafted and rewrote the manuscript, carried out the PCR and interpreted the results. SE implemented the PCR systems, constructed the mimic and interpreted the results. CB and SA participated in the design and coordination of the study. YP sampled and wrote about the beef bulls. All authors have been involved in revising the manuscript. Competing interests The authors declare that they have no competing interests. Received: 14 December 2009 Accepted: 13 January 2010 Published: 13 January 2010 References 1. Storz J: Overview of animal diseases induced by chlamydial infections. Microbiology of Chlamydia Barron, AL. Florida: CRC Press, Inc 1988, 167-192. 2. Wehrend A, Failing K, Hauser B, Jager C, Bostedt H: Production, reproductive, and metabolic factors associated with chlamydial seropositivity and reproductive tract antigens in dairy herds with fertility disorders. Theriogenology 2005, 63:923-930. 3. Wittenbrink MM, Schoon HA, Bisping W, Binder A: Infection of the bovine female genital tract with Chlamydia psittaci as a possible cause of infertility. Reprod Dom Anim 1993, 28:129-136. 4. Holliman A, Daniel RG, Parr JG, Griffiths PC, Bevan BJ, Martin TC, Hewinson RG, Dawson M, Munro R: Chlamydiosis and abortion in a dairy herd. Vet Rec 1994, 134:500-502.