Leptospira interrogans serovar hardjo (LH) is an important infectious agent that may cause abortions, stillbirths, infertility and reduction of milk production in cattle (Ellis et al, 1985), with a possible zoonotic role (Levett, 2001). Conflicting and outdated data is available regarding the survival of LH in raw milk: infected cows were reported to shed viable leptospires in their milk (Ellis et al, 1976), and milk was supposed to be the mean of LH transmission in one human-to-human case (Bolin & Koellner, 1988); on the other hand undiluted cow milk was proven to rapidly kill LH (Kirschner & Maguire, 1955). A further investigation on the serovar Pomona confirmed the lytic antileptospiral effect (Stalheim, 1965), probably due to crude milk phospholipids.
The aims of this study were to evaluate the DNA persistence and the LH survival in experimentally contaminated raw milk samples stored at 4±3°C during a ten-day study period. For that reason, a negative fresh tank milk sample was infected with a LH strain (108 cell/ml grown in EMJH liquid medium) and examined with an rrs16S gene targeting real time PCR to quantify the detectability of LH DNA in 1 ml aliquots extracted at days 1, 2, 3, 4, 8, 9 and 10. Seven aliquots at the same time intervals were inoculated into liquid and semisolid EMJH media to attempt the culture (OIE Manual procedures). Leptospiral viability and growth were optically evaluated by means of a dark field microscopy.
All the 7 aliquots resulted PCR positive, demonstrating that raw milk did not alter the detectability of the LH DNA during the study time. Moreover, all the attempted cultures were successful, showing the capability of LH to survive and grow in raw milk until day 10.