In vitro transmission of Chlamydia using naturally infected koala (Phascolarctos cinereus) semen.

Transmission of Chlamydia pecorum infection has generally been assumed to be via the urogenital route and in an attempt to confirm this we investigated an in vitro method of Chlamydia infection using naturally infected koala semen to inoculate a cell line and attempt to estimate C. pecorum infectious load. A total of 57% of 122 koala semen samples had low C. pecorum copy number or no burden, while 18% of semen samples contained >10000 inclusion-forming units/mL, as determined by quantitative polymerase chain reaction. In vitro inoculation of a McCoy cell line resulted in successful infection from 4% of semen samples where C. pecorum burden was >105 inclusion-forming units/mL. Our preliminary study suggests that transmission of C. pecorum infectious dose may be restricted to peak bacterial shedding in semen associated with recent infection. Here, we report venereal transmission of C. pecorum in koala semen is possible; however, we speculate that antimicrobial factors and innate immune function receptors associated with semen may inhibit chlamydial growth. These mechanisms have yet to be reported in marsupial semen.