An Epidemiological Study of Akoya Oyster Disease Using Polymerase Chain Reaction Targeting Spirochaetes Genes

In a previous study, a member of Spirochaeta was presumed to cause Akoya oyster disease (AOD), and Candidatus Maribrachyspira akoyae was proposed as the putative causative agent. With the aim of performing an epidemiological study of AOD, polymerase chain reaction (PCR) assays were developed using the primers designed from shotgun metagenomic sequences identified as having high homology to Spirochaetes genes by a BLAST search. PCR assays with six primer sets produced specific target amplicons (n = 63–67) in pearl oysters collected from five AOD-endemic areas (n = 88), but did not in pearl oysters (n = 36) collected from three AOD-free areas. All individuals (n = 17) with the color index a-value above 3.0, which is an index of AOD development, were positive for all 6 primer sets in PCR. There was a positive correlation between the a-value and the results of PCR. These results show that the previous identification of Spirochaeta as an etiological agent of AOD is reasonable. Sequencing of amplification products confirmed only one nucleotide substitution in the PCR products, suggesting that the pathogenic causative agent of AOD has limited genetic diversity.