Introduction: Listeria (L.) monocytogenes cause orally acquired infections and are of major importance in animal health. Although silage has been incriminated to be a major infection source in the past, few is known about the transmission of L. monocytogenes between the farm environment and ruminants. In order to determine potential infection sources, this study investigated the distribution of genetic subtypes in a sheep farm during a listeriosis outbreak.
Material and Methods: L. monocytogenes were isolated from a lamb with septicemia and the brainstem of 3 sheep with encephalitis. Samples from the farm environment (faeces, feeds including silage, swabs from feed bunk, soil, water tank and floor) were screened for presence of L. monocytogenes during and 4 weeks after the listeriosis outbreak by 1 to 2 selective One-Broth Listeria enrichment followed by incubation on Brilliance Listeria agar plates. Colonies confirmed to be L. monocytogenes by Matrix-Assisted-Laser-Desorption/Ionization-Time-Of-Flight-Mass-Spectrometry (MALDI-TOF-MS) were subtyped with the multilocus sequence typing method (MLST). The obtained PCR products were sequenced in order to determine sequence types (ST) and clonal complexes (CC).
Results: L. monocytogenes were identified only in soil and water tank swabs during the outbreak. No Listeria were detected in faeces. Four weeks later, following thorough cleaning of barns, L. monocytogenes was absent in environmental samples. All environmental and clinical STs belonged to CC 4, lineage I. Whereas ST 4 was present in the environment and brains; ST 57 was isolated from the lamb with septicemia.
Conclusion: Our results indicate that sheep do not act as an amplification host for L. monocytogenes and that L. monocytogenes does not persist in the sheep environment for an extended time period. Soil and water tanks, but not silage, were likely infection sources in this listeriosis outbreak and farm management appears to be a crucial factor for the occurrence of listeriosis outbreaks.