High morbidity and mortality occurred in a coral reef tank containing more than 100 giant clams including recent acquisitions (Tridacna crocea and Tridacna gigas) obtained from a commercial dealer in California. The T. crocea were from a Palau, Micronesia, government-sponsored aquaculture program. The T. gigas were from a government-run farm in the Kingdom of Tonga. Consistent mortality occurred in the newly acquired clams as well as in the resident population. The clinical signs included mantles pulling back into open (gaping) shell, the curving edges of mantles appearing pinched and contorted, and death occurring within one to three days. Water parameters/chemistries were within normal limits. Scrapings taken of mantle, gill, digestive gland, kidney, and gonad identified no common protozoans. Histopathological examination revealed interfollicular sinuses in the gonad containing numerous large, round (20-30 µm) organisms. These organisms contain a single large vacuole and are surrounded by external layers of abundant eosinophilic material, often displaying a serpiginous margin or fimbriated surface in section. The digestive gland was severely compromised by necrosis and sloughing of tubule epithelia, infiltration of numerous parasites in the sinuses, and degranulation of hemocytes. Individual trophozoites and clusters of daughter cells (schizonts) were present. Oocytes were rare and lysed. Muscle cells were atrophic. Gill epithelium had mostly sloughed off and its supporting connective tissue was swollen. Most zooxanthellae in mantle sinuses appeared in good condition; however, others were lysing and accumulations of pale golden brown granular debris (perhaps either lysed zooxanthellae or lipofuscin-like pigment granules) were present in the digestive gland and gonad. Therapy was attempted consisting of 30-minute freshwater dips of some of the exposed clams. The group of clams dipped included T. crocea (3 of 12 still alive), T. gigas (2 of 4 still alive), T. derasa (no mortalities in 13 clams), T. squamosa (no mortalities in 3 clams), and T. maxima (only one and it died). The tentative diagnosis is of a Perkinsus species, most likely of Indo-Pacific origin2,3,5,6 rather than domestic such as P. marinus or P. chesapeaki.7 Infections and mortalities associated with a Perkinsus sp., assumed at the time to be Perkinsus olseni (= Perkinsus atlanticus), have been identified in several species of Tridacnid clams on Great Barrier Reef islands from 1984-1987.1,4 However, until recently, this OIE-reportable organism had not been found in the Western Hemisphere. Identification of a Perkinsus infection with 96% genetic similarity to P. olseni in a dual infection with a minor component with P. olseni has been recently demonstrated in a shipment of T. crocea from Vietnam9 and in a single T. crocea housed long-term in a home aquarium.8 This is the first report of mortalities in shipments from Palau and Tonga and the first documented arrival of infected T. gigas into the USA. This expands the list of potential infected hosts and geographic sites sourcing animals into our country.10 Infections were not detected in T. derasa and T. squamosa which might be resistant to this pathogen.
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