Abstract
Three adult lined seahorses, Hippocampus erectus, were close to completing their scheduled quarantine when two were unexpectedly found dead three days prior to the end of treatments. The seahorses had originally been net caught by a shrimping boat in the Gulf of Mexico just north of Tampa, approximately 15 miles offshore. Upon arrival at The Florida Aquarium they were placed together in a 30 gallon quarantine tank. Daily husbandry included a 50% water change and a diet of live brine shrimp and/or frozen mysids. Their quarantine consisted of: a six-day course of an oxolinic acid bath at 1 ppm, a chloroquine (10ppm for 24 hours) bath, live Artemia soaked in praziquantel (soaked 15-20 minutes in 400mg/100ml solution, fed for three consecutive days, then repeated in two weeks), live Artemia soaked in fenbendazole (soaked 15-20 minutes in 400mg/ml solution, fed for three consecutive days, then repeated in two weeks), and finally a 2 ppm praziquantel bath for five days. The seahorses died within 24 hours of adding the praziquantel to the tank.
Prior to their death the seahorses looked normal on external exam and all were eating well. Water quality parameters were all within acceptable limits. The Florida Aquarium had previously used praziquantel baths on syngnathids many times with no adverse reactions. The praziquantel dosage calculations were rechecked and proved to be accurate.
On gross necropsy both seahorses had numerous multifocal raised white to clear nodules within the liver and raised white nodules adhered to the coelomic wall and the serosal surfaces of the GI tract. One of the seahorses also had a single large white/yellow nodule within the posterior kidney. Wet mounts of the liver revealed several unidentified encysted metazoan parasites, occasional cysts with microsporidians, and a single large trematode. On histopathology, microsporidians at varying stages of development were noted within bile ducts in liver tissue. A wet mount of the large renal nodule revealed a single large trematode. The white coelomic nodules appeared to be metazoan metacercariae on wet mount. Many of these metazoan parasites were observed in cross section on histopathology. Final identification is pending.
There are several possibilities as to why these seahorses died. One theory is that the praziquantel bath treatment killed the visceral and coelomic trematodes/cestodes and triggered an acute anaphylactic reaction. Anaphylactic-type reactions have been previously described in fish following inoculation with pre-sensitized protein antigens or non-specific mitogens. The anaphylactic reaction is presumed to be caused by degranulation of the eosinophilic granular cells (EGCs), releasing histamine, and resulting in widespread vasodilation.1,2 It is also possible that the dying trematodes released toxins which caused the mortalities; however in Mitchell's study3 that assumed this scenario there were necrotic foci surrounding the metacercaria which we did not see in our seahorses. The hepatic/biliary microsporidians may have contributed to the mortality. There were three reports of sudden death in spiny seahorses (H. barbouri) associated with biliary microsporidiosis.4
This case emphasizes the importance of doing thorough necropsies on all fish that die. There is very little information available on metazoan parasites (identification, life cycle, prevalence, and pathogenicity) in syngnathids; we feel that this is an area that warrants more research.
References
1. Goven BA, et al. In vivo and in vitro anaphylactic type reactions in fish. Dev Comp Immunol, 1980. Winter; 4(1): 55-64.
2. Ellis AE. Eosinophilic granular cells (EGC) and histamine responses to Aeromonas salmoncida toxins in rainbow trout. Dev Comp Immunol, 1985. Spring; 9(2): 251-60.
3. Mitchell AJ. Importance of Treatment Duration for Praziquantel Used against Larval Digenetic Trematodes in Sunshine Bass. Jour Aquatic Animal Health, 1995. 7: 327-330
4. Bull C, ed. Seahorse Husbandry in Public Aquaria. Project Seahorse Manual. 2002