Changes in peripheral blood leucocytes of sheep experimentally infected with Mycoplasma agalactiae

- We monitored leucocyte frequencies during Mycoplasma agalactiae infection in sheep.
- We observed a prolonged leucopenia involving granulocytes, monocytes and T cells.
- The kinetics of leucocyte depletion was unique for each cell population studied.

Contagious agalactia is a serious disease of small ruminants affecting mainly mammary glands, joints and eyes. In sheep, the main aetiological agent is Mycoplasma agalactiae (Ma) whose abilities to persist in the target organs are known. Since there is no information on the effect of acute and chronic Ma infection on circulating leucocytes, the present study was designed to monitor granulocytes, monocytes, T and B lymphocytes, by flow cytometry, in female lactating sheep nasally infected with Ma. A profound depletion of leucocytes was observed from day 5 to day 34 post infection (p.i.). In particular, while the granulocytes returned to baseline levels by day 12 p.i., the monocytes remained significantly low until day 20 p.i. The infection caused a prolonged depletion of peripheral T lymphocytes (both CD4+ and CD8+) while B lymphocytes remained unaltered throughout the study. Mycoplasma agalactiae was detected by real-time PCR in several anatomical sites (ear, nose and milk) from day 2-5 p.i. until the end of the study (i.e., day 50 p.i.) while a transient bacteraemia was observed from day 5 to day 12 p.i. The leucopenia observed following intranasal Ma infection is likely due to leucocyte infiltration within the target organs.