Molecular investigation of a listeriosis outbreak in goats caused by an unusual strain of Listeria monocytogenes.

During a 16-month period, 10 goats with listeriosis were identified in 2 herds that shared 3 bucks, including 1 that died of listeriosis. Using DNA fingerprinting, we determined that a single genetically unique Listeria monocytogenes strain had infected all goats from which isolates were available. All isolates were unable to metabolize rhamnose (rhamnose-negative), whereas as a species, L monocytogenes is considered to have a rhamnose-positive phenotype. Therefore, these isolates would have been characterized as a species other than L monocytogenes if any of a variety of commercial bacterial identification kits had been used for speciation. Silage was not fed to either herd, and L monocytogenes was not isolated from vaginal or rectal swab specimens obtained from healthy goats or from samples of feed. Because the 3 bucks were the only common elements between the 2 herds, our results suggest a venereal route of transmission for listeriosis.