Listeria monocytogenes strains from ruminant rhombencephalitis constitute a genetically homogeneous group related to human outbreak strains.

Listeriosis is a disease which causes significant economic losses at the farm level due to high morbidity and mortality in ruminants. This study was performed to investigate the role of ruminants in the epidemiology of listeriosis in northern Italy and the possible association of animal-adapted strains of L. monocytogenes with strains associated with human disease. Twenty ruminant rhombencephalitis isolates, previously confirmed as Listeria monocytogenes by bacteriology and PCR, were characterized by serotyping, pulsed-field gel electrophoresis (PFGE), multi-virulence-locus sequence typing (MVLST) and multiplex single nucleotide polymorphism typing (mSNP-typing) for the detection of epidemic clones (ECs). Subtyping results were subsequently compared with those obtained from human, food and environmental isolates of L. monocytogenes, including 311 isolates from the University of Turin, Italy, and 165 isolates representing major listeriosis human outbreaks worldwide, in addition to other unrelated isolates. Both mSNP-typing and MVLST showed that 60% of the analyzed isolates belonged to epidemic clone I (ECI), which has been epidemiologically linked to several human outbreaks of listeriosis. In particular, the 1981 Canada outbreak was linked to the use of sheep manure and the 1985 California outbreak to the use of raw cow's milk. In our study, ECI isolates were collected from different ruminant species on geographically and temporally distinct occasions for the last 13 years. Our results support the hypothesis that ruminants represent possible natural reservoirs for L. monocytogenes strains capable of causing epidemics of listeriosis in humans.