Etiology and histopathology of the white pox disease of the Caribbean elkhorn coral Acropora palmata
Sutherland, Kathryn Patterson
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White pox is a lethal disease of the Caribbean elkhorn coral, Acropora palmata. White pox was first reported in 1996 in the Florida Keys, and has since been observed throughout the Caribbean. Annual photographic monitoring at eight coral reef sites in the Florida Keys showed that A. palmata populations with white pox disease signs sustained losses averaging 87% between 1996 and 2002. Rate of tissue loss associated with white pox is rapid, averaging 2.5 cm2'day"l, and is greatest during periods of seasonally elevated temperature and rainfall. The etiology of white pox was determined t rough examination of surface mucopolysaccharide layers (SML) from white pox- affected and unaffected corals collected on reefs throughout the Caribbean. Metabolic profiles of pure bacterial strains isolated from SML were used to identify potential pathogens. Potential pathogens were experimentally inoculated onto healthy A. palmata. Corals inoculated with isolate PDL l00 showed white pox signs, and PDL lOO was reisolated from these corals (satisfying Koch's postulates). 16S rDNA gene sequence analyses of PDL 100 demonstrated a 100% identity to the enterobacterium Serratia marcescens. The source of the S. marcescens strain that causes white pox is uncertain. This study begins the search for the origin of the pathogen through the screening of seawater collected in the Florida Keys using biochemical and restriction analyses. Thirteen bacterial isolates were identified as putatively belonging to the genus Serratia, and the majority of these isolates were from stations where human fecal contamination of seawater is probable. White pox-affected and apparently healthy oral tissues were collected from white pox- diseased colonies of Acropora palmata in the Florida Keys. Tissues were processed for histopathology with light microscopy. White pox i associated with rounding of granular gland cells, necrosis, and atrophy. Most lesions were, concentrated in the coenosarc tissue. Cellular and tissue degeneration, however, was observed in other apparently healthy and diseased coral tissues, and there was no significant difference ,in the number or type of abnormalities present in diseased versus apparently healthy specimens. 'The e similarities may indicate that colonies affected by white pox sustain a whole-colony reaction to infection.