abstract

Pseudomonas sp. are gram-negative rods that are important primary, opportunistic and nosocomial pathogens. They are ubiquitous in nature, inhabiting or infecting animals, water, plants, and soil, and are reported worldwide in a range of mammalian species, including marine mammals. Pseudomonas aeruginosa, a normal inhabitant of skin and mucous membranes, is a common opportunistic pathogen that causes disease secondary to extensive tissue injury, immunosuppression, and treatment with broad-spectrum antibiotics and corticosteroids. P. aeruginosa is a well-known nosocomial pathogen commonly recognized in respiratory infections in hospitalized human patients. Although P. aeruginosa has been associated with pneumonia and septicemia in California sea lions (CSL) and Pacific harbor seals (HS), the spectrum of lesions, potential predisposing factors, and associated bacterial genotypes have not been described.

To determine the role of Pseudomonas sp. in causing disease in pinnipeds, medical records, archived histology slides and pathology reports from stranded pinnipeds, managed clinically at The Marine Mammal Center (TMMC), Sausalito, California, and those that died during a HS mortality event, were reviewed. Between 1983 and 2007, 51 cases of documented or suspected Pseudomonas sp. infection were identified in 38 HS and 13 CSL, including 31 males, 19 females, one animal of unknown sex, 29 neonates/pups, 10 juveniles, 10 adults and two animals of unknown age. Animals spent an average of 11 days at TMMC before death, with a range of one to 53 days, and three HS spent no time at TMMC. Forty-five animals died and six were euthanized. Pure cultures of Pseudomonas aeruginosa and Pseudomonas sp. were isolated from 11 and two cases, respectively. Pseudomonas was isolated with other bacteria in 25 cases. Pseudomonas isolates from a subset of cases were genotyped via pulsed-field gel electrophoresis and several distinct genotypes were identified. Thirteen cases were not cultured; however, Pseudomonas infection was diagnosed presumptively based on characteristic histologic lesions. Lesions attributed to Pseudomonas infection included interstitial pneumonia, bronchopneumonia, lymphadenitis, cellulitis, and vasculitis, often with dissemination to numerous organs via bacteremia. Lesions were characterized by moderate to severe, acute, purulent to fibrinopurulent inflammation, accompanied by tissue necrosis, edema and hemorrhage, and containing characteristic, scant to myriad, predominantly perivascular, gram-negative bacterial rods. Pneumonia was present in 40 (78.4%) cases, considered the cause of death in 16 (31.4%) and contributory to death in 15 (29.4%). Septicemia was present in 44 (86.3%) cases, considered the cause of death in 15 (29.4%) and contributory to death in 26 (51.0%). Lesions most commonly associated with septicemia included meningoencephalitis, myocarditis, hepatitis, and splenitis. Twenty five deaths were considered multifactorial, with more than one contributing cause, including co-infections. Fourteen (36.8%) HS had concurrent phocine herpes virus-1 infection, 10 (76.9%) CSL had concurrent leptospirosis, one (7.7%) CSL had concurrent urogenital cancer, and one (7.7%) CSL had domoic acid intoxication. Cases clustered in the spring and summer of 1990, 1998, 2000, and 2006 in HS and in 2004-5 in CSL. These results indicate multiple genotypes of Pseudomonas sp. contribute to death in stranded pinnipeds, most commonly in neonates/pups, and frequently in animals with concurrent infections.

acknowledgements

The authors wish to thank the veterinary, technical and volunteer staff of The Marine Mammal Center, especially Judy Lawrence, and the pathology residents, and histology and microbiology laboratory staff of William R. Pritchard Veterinary Medical Teaching Hospital, School of Veterinary Medicine, University of California, Davis.