Unusual Listeria Monocytogenes Causes Outbreak

US - A listeriosis outbreak, in dairy cattle, with high case mortality and acute death after an onset of symptoms has been investigated using gross pathology and bacteriologic approaches, including molecular characterisation of a clinical Listeria monocytogenes isolate.
calendar icon 17 January 2011
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In a herd of 315 animals, nine animals showed clinical symptoms consistent with listeriosis, including three animals that died within two to four days after acute onset of clinical signs, four animals that were euthanised, and two that survived.

Initial EcoRI ribotyping and serotyping indicated that this outbreak was caused by an unusual L. monocytogenes serotype 4b strain, which was classified into lineage III.

Further characterization of this isolate by DNA sequencing–based subtyping methods indicated that the strain responsible for this outbreak represented a unique genotype as supported by its classification into a new sigB allelic type, which has not been identified previously among >290 isolates, and by compelling phylogenetic evidence. While lineage III isolates are generally rare, they seem to be more common among L. monocytogenes isolates from animals with clinical signs of listeriosis.

This is the first report of a particularly severe clinical course of disease associated with infection by a lineage III strain. The high prevalence of Listeria spp., including L. monocytogenes, in the farm environments may favour emergence and evolution of novel, and possibly more virulent, L. monocytogenes strains. Continued monitoring of animal listeriosis cases and outbreaks may not only improve animal health but also aid in the early discovery of newly emerging L. monocytogenes strains.

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