The Disease

Introduction
Since 1963 when Pasteurellosis was first detected in striped bass and white perch in the United States, this disease has become a worldwide concern in aquaculture. Photobacterium damsela subsp. piscicida was known until recently as Pasteurella piscicida. It is a gram-negative rod which causes a disease in fish known either as Pseudotuberculosis or Fish Pasteurellosis.

This serious problem in Japanese yellowtail culture can result in losses on individual farms of up to 50%. The bacterium's taxonomic position as a bonafide Pasteurella has been questioned for a number of years and based on small subunit ribosomal ribonucleic acid (rRNA) sequences; whole deoxyribonucleic acid (DNA) relatedness and biochemical characterization, Gauthier et al. (1995) have reassigned the bacterium to a subspecies of P. damsela. Some fish farmers and scientists may still refer to the disease as Pasteurellosis.

Etiology
Photobacterium damsela subsp. piscicida is a Gram-negative, pleomorphic, non-motile bacterium that grows slowly on NaCl-supplemented media. 

Serologically, the strains found all over the world are highly homogeneous and there seems to be only one serotype. However, ribotyping revealed genetic variation of different geographical isolates.