Abstract

An adult, hermaphroditic Tridacna crocea ornamental clam imported from Vietnam into the United States of America for the purpose of home display, became moribund with sloughed byssal tissue and incomplete extension of the poorly responsive mantle. Necropsy findings included emaciation, visceral mass edema, and rare multifocal, 1-2 mm diameter, off-white to light tan gill nodules. Histopathology revealed marked inflammation and necrosis within the visceral mass and gills, with interstitial edema and atrophy of glandular, gonadal, and muscular tissues. Inflamed tissues contained large numbers of 10-15 µm extracellular, spherical organisms with a signet-ring morphology consistent with Perkinsus spp. trophozoites. The organisms often formed clusters of two to four cells and were surrounded by a host reaction consisting of a 1-4 µm rim of amorphous eosinophilic material and two to four host hemocytes. Incubation of infected host tissues in alternative Ray's fluid thioglycollate medium (ARFTM) confirmed the presence of Perkinsus sp. hypnospores that stained blue-black with Lugol's iodine. Polymerase chain reaction assays with sequencing of products revealed a high level of nucleotide similarity in the ITS and IGR regions of the RNA gene complex, but no exact match, to known P. olseni isolates. Perkinsus spp. organisms, including P. olseni and P. marinus, are highly pathogenic destructive protozoa which are internationally reportable. They are capable of disrupting American ecosystems populated by naive mollusks and negatively affecting both domestic and international shellfish industries.³ This is the first report of an exotic Perkinsus sp. pathogen in an imported ornamental clam maintained long term in a home aquarium. However, ongoing research indicates that T. crocea from Vietnam are commonly infected by such organisms.² Veterinarians, aquarium facility managers, and veterinary clients with hobby aquariums should use appropriate caution and responsible disposal practices for clam carcasses and water in which imported ornamental clams have been housed, to reduce the possibility of dispersing viable, exotic Perkinsus spp. organisms into domestic waters. Genetic characterization and advanced diagnostics for this organism are essential to assessing risk and environmental dissemination.

Acknowledgements

The authors wish to thank the University of Florida, College of Veterinary Medicine, Intramural funding program and the Florida SeaGrant program for funds used to complete this project. This work is published in the Journal of Zoo and Wildlife Medicine.¹

References

1.  Sheppard B.J., and C.F. Dungan. 2009. Exotic Perkinsus sp. protozoa in an imported Vietnamese ornamental clam (Tridacna crocea) maintained in a home aquarium. J Zoo Wildl Med 40: 140-146.

2.  Sheppard B.J., and A.C. Phillips. 2008. Perkinsus olseni detected in Vietnamese aquacultured reef clams, Tridacna crocea, imported to the U.S.A., following a mortality event. Dis Aquat Org 79: 229-235.

3.  Villalba A., K.S. Reece, M. Camino Ordas, S.M. Casa, and A. Figueras. 2004. Perkinsosis in mollusks: a review. Aquat Living Res 17: 411-432.