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-yr 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 2 early
1. Huang CT. 1976. What is Pseudomonas pseudomallei? Elixir: 70-72
2. Mollaret HH. 1988. "L'affaire du Jardin des plantes" ou comment la melioidose fit son
apparition en France. Med. Mal. Infect. 18:643-654.
3. Trakulsomboon S, TL Pitt, DAB Dance. 1994. Molecular typing of Pseudomonas
pseudomallei from imported primates in Britain. Veterinary Record 135:65-66.
4. Wuthiekanun V, MD Smith, DAB Dance, NJ White. 1995. Isolation of Pseudomonas
pseudomallei from soil in Northeastern Thailand. Trans.Royal Soc.Trop. Med. Hyg. 89: 41-43.