Abstract

The tiger-tail seahorse, Hippocampus comes, takes its name from its tail color of black or gray with cream or yellow bands. This striking appearance makes it a very popular species for home tanks and larger exhibits at commercial aquariums. In nature, this seahorse species is distributed mainly in the western central Pacific area of Malaysia, Singapore, Vietnam and the Philippines. Although not endangered, the tiger-tail seahorse may play an important role in the understanding of disease processes in this and threatened species of seahorses.

Between 6/11/00 and 8/12/00, 13 tiger-tail seahorses in quarantine at the National Aquarium in Baltimore, Maryland developed a fatal disease characterized clinically by fluid-filled vesicles around the abdominal pouch, cervical region and beneath the mandible. The affected seahorses were lethargic and anorectic. Despite treatments with trimethoprim/sulfamethoxazole baths (960 mg/10 gallons q 6–8 h SID), metronidazole baths (250 mg/10 gallons SID) or injectable ceftazidime (22 mg/kg IM q 72 h) the 13 seahorses died over a 6-week period. Necropsy findings on five animals showed soft, pale pouch skin containing grayish raised foci. The hepatopancreas had randomly distributed yellowish-white foci. Histopathologic examination of these tissues showed necrosis of the skin with ulceration, and necrosis with inflammation of both the hepatic and pancreatic components. Large basophilic intracytoplasmic inclusion bodies were evident within degenerated cells in the hepatopancreatic lesions. In several animals, similar necrotizing lesions were also found in the nasal passage, kidney, extrahepatic pancreatic tissue and coelomic fat. Pure cultures of Vibrio alginolyticus were isolated from the skin and liver lesions of one animal. In samples from the same animal, a cytopathic agent was detected in three fish cell lines in an attempt to isolate a virus; however, subcultivation produced no further cytopathic effect and identification of the agent was unsuccessful.

The nature of the lesions and the presence of large basophilic inclusion bodies suggest that a viral agent could be involved. The necrotic lesions and inclusion bodies resemble those in infectious pancreatic necrosis virus (IPNV), a birnavirus of fish. Vibrio alginolyticus is considered an opportunistic invader of already damaged tissues and a weak pathogen in stressed fish. Further studies including transmission electron microscopy of the inclusion bodies in the fixed tissues and development of seahorse cell lines are currently underway.