Listeria monocytogenes, a gram-positive rod-shaped bacterium, is ubiquitous within the environment and frequently isolated from mammal, bird, and fish species as well as soil, detritus and water. This bacteria may comprise part of the normal intestinal flora of some terrestrial mammals. In humans, infection can cause a severe inflammation of the central nervous system and more rarely a septicaemia or abortion in immuno-suppressed persons. In ruminants, infection causes listeriosis, commonly manifested as meningoencephalitis, abortion, neonatal death, and septicemia. Based on consultation with other diagnostic facilities and intensive review of published case accounts, no other documented cases of L. monocytogenes infection in cetaceans could be found. At Marineland of Canada, a captive-born 6-year old male killer whale (Orcinus orca) died from acute septicemia on 21 October 2004. Histopathology disclosed severe diffuse lymphoplasmacytic leptomeningitis, suggesting a neurotrophic virus or bacteria. Primary bacteriological culture of brain tissue revealed small numbers of Streptococcus equisimilis and Escherichia coli, while additional enrichment procedure revealed the presence of L. monocytogenes. Based on histopathological and bacteriological findings, it was concluded that the cause of death was listerial meningoencephalitis. The specific source of the bacterium could not be determined and infection may have been acquired by consumption of contaminated frozen fish, carrier hoofstock feces from park exhibits, wild bird feces, or from exposure to subclinically infected humans handlers or other cetaceans. Another possibility, although rare, is vertical transmission via human hands (public) that enter the petting zoo (deer) and then go directly to pet the orcas. This first occurrence of Listeriosis in a cetacean suggests that hygiene and biosecurity measures should be further enhanced at seaquaria where terrestrial mammals are kept in proximity of marine mammals. This may be especially significant in parks that allow the public to "feed and touch" their cetaceans, if in fact, as it is postulated, that human hands could possibly spread the terrestrial Listeria to cetaceans.
Thank you to Dr. Stephen Raverty, Veterinary Pathologist, The Animal Health Centre, Abbotsford, British Columbia, Canada, for his assistance and suggestions; and to the wonderful staff at Marineland of Canada.