Prolonged excretion of Listeria monocytogenes in a subclinical case of mastitis

As few pieces of literature are actually available on the course of listeri al udder infection a case of Listeria monocytogenes mediated mastitis in a 7 year old Simmental cow was intensively studied. Weekly investigations dem onstrated repeatedly the excretion of L. monocytogenes from the right hind udder quarter. Enumeration of listerial cells revealed excretion of 40000 c fu/ml during high lactation. PCR in clinical specimens showed positive resu lts in regularly investigated blood samples and an udder puncture but negat ive results in additional clinical specimens. Culture did not enable the is olation of suspicious colonies from these samples. After calving, listerial cells were easily redetectable from the affected quarter. Additionally, PC R gave L. monocytogenes specific signals in placenta tissue, umbilical cord tissue and ammnion fluid samples. Macrorestriction analysis followed by pulsed field gel electrophoresis demo nstrated identical restriction patterns in milk-borne isolates thus confirm ing a prolonged form of subclinical listerial mastitis for at least 15 mont hs. This fact gives evidence that L. monocytogenes survives, once adapted t o the udder, independent from the physiological status of the mammary gland . An acidification study using contaminated raw milk revealed that the lister ial cells were killed within a 3-day period when the pH value dropped prope rly.