An Environmental and Molecular Epidemiologic Study of Melioidosis at an Oceanarium in Hong Kong
Melioidosis is a disease caused by the bacterium Burkholderia (formerly Pseudomonas) pseudomallei and is endemic to South East Asia and Northern Australia. The organism is a free-living environmental saprophyte and can be isolated from soil and/or water. The disease can be severe resulting in acute septicaemia and death and affects a wide range of animal species including man. It has been responsible for outbreaks in zoo animals with a large outbreak at the Jardin des Plantes in Paris in 1975–76, which was introduced through an imported animal and resulted in multiple mortalities.2 Melioidosis was first reported in Hong Kong in 1975 after some dolphins died suddenly at a local oceanarium, Ocean Park.1 Other species that have succumbed to melioidosis at the park, include pinnipeds, birds (including a penguin), a lesser panda and a llama. Cetaceans appear to be the most susceptible with at least 35% of all mortalities in this species being attributed to this disease.
Deaths of animals from melioidosis have occurred sporadically throughout the 26-year history of the park. An epidemiologic investigation to identify the probable source(s) and/or reservoir of B. pseudomallei and the relatedness of the strains isolated from the park was undertaken to inform on suitable preventive measures against the disease. Soil and water samples, rain, and wind with rain during typhoons were collected from all areas of the oceanarium focusing around the facilities where cases of melioidosis had occurred. The samples were screened for the presence of B. pseudomallei using enrichment broths and media selective for the organism.4 B. pseudomallei was isolated from 3/15 typhoon samples and 3/220 soil samples only. A total of 30 environmental and clinical isolates from various species, were characterized by a molecular method that indexes variation in infrequently occurring restriction sites in the bacterial DNA using pulse field gel electrophoresis (PFGE).3 PFGE demonstrated that all isolates from the park belonged to two strain types with one strain predominating and representing 87% of all isolates.
The results suggest that a single strain of B. pseudomallei has been responsible for the vast majority of clinical cases of infection at the park including the original outbreak in dolphins in the mid 1970s. Two possible explanations for these findings are that there is a reservoir of the organism, which has survived in the soil over many years, or that infected animals have contaminated their environment and created a cycle of persistence of this particular strain. Another feasible postulation is that the predominant strain, found in the three soil and two of three typhoon samples, was carried from the soil during strong winds thereby increasing exposure of susceptible animals to the organism during typhoons. The strains from the park should be compared with isolates from outside the park to provide a wider epidemiologic picture.
The authors wish to thank the staff of Ocean Park Laboratory and the LHI at the CPHL for their support and assistance and Ocean Park Corporation who supported this work. Also, to Dr. P.L. Ho who provided two early cetacean strains.
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