Characterization of brown spot disease of gulf coast shrimp

Adult penaeid shrimp were collected from mariculture raceways, holding tanks, and near-shore waters of Galveston Island, Texas. Those with darkly pigmented lesions were chosen for study. When observed by scanning electron microscopy, diseased cuticle shows dense populations of bacteria within a lesion, whereas normal cuticle shows a relatively low number of bacteria. Some lesions exhibited a morphologically homogeneous population of bacteria while others revealed a heterogeneous or mixed infection. Bacteriological analysis of a homogenate of each lesion revealed the presence of a wide variety of organisms which produced extracellular lipases, proteases, and chitinases. Lipolytic and proteolytic organisms were consistently isolated from all 20 lesions examined, while chitinoclastic bacteria were found in all but two lesions. According to the presumptive isolation scheme used, 10 distinct phenotypes of bacteria were differentiated among the isolates from 20 lesions. These phenotypes were classified into four genera: Vibrio, Alteromonas, Spirillum, and Flavobacterium. Groups of 10 shrimp were abraded and inoculated with representative isolates from each of the 10 phenotypes. Four isolates were able to initiate lesion formation. These were two Vibrio species, one Alteromonas species, and one Spirillum species. All of these four pathogens were lipolytic, while only two were chitinoclastic. This evidence suggests that there is not one specific chitinoclastic bacterium which causes brown spot symptoms, but that a variety of bacteria acting alone or in groups may be the causal agents.