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Diseases of Aquatic Organisms

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DAO 58:127-142 (2004)  -  doi:10.3354/dao058127

Histological, ultrastructural, and in situ hybridization study on enlarged cells in grouper Epinephelus hybrids infected by grouper iridovirus in Taiwan (TGIV)

Chia-Ben Chao1,4,*, Chun-Yao Chen2, Yueh-Yen Lai3, Chan-Shing Lin3, Hung-Tu Huang4

1Institute for Animal Disease Prevention and Control, Kaohsiung, Taiwan 830, ROC
2Department of Life Science, Tzu-Chi University, Hualien, Taiwan 970, ROC
3Department of Marine Resources, National Sun Yat-Sen University, Kaohsiung, Taiwan 804, ROC
4Department of Biological Sciences, National Sun Yat-Sen University, Kaohsiung, Taiwan 804, ROC

ABSTRACT: Grouper iridovirus in Taiwan (TGIV) infection in the Epinephelus hybrid is a major problem in the grouper industry. ATPase gene sequences indicate that this virus is closely related to cell hypertrophy iridoviruses. Histologically, the appearance of basophilic or eosinophilic enlarged cells in internal organs is the most characteristic feature of this disease. These cells are acid-phosphatase positive and are able to phagocytose injected carbon particles. In our study, TGIV infection inhibited normal phagocytic ability in these cells in vivo after 4 d post-infection (p.i.) but not before 2 d p.i. Their staining properties and phagocytic ability suggested a monocyte origin of enlarged cells, which appeared in high numbers in the trunk kidney, head kidney, spleen and gill. After infection, the enlarged cells first appeared in the spleen, with an abundance peak at 64 h p.i. (Peak 1); at 120 h p.i., a second peak (Peak 2) occurred in the spleen, head kidney, trunk kidney and gill. Lower numbers of enlarged cells were observed in the liver, muscle, heart, eye, intestine, but no enlarged cells were found in the brain. A TGIV-specific DNA probe labeled most of the basophilic but not eosinophilic enlarged cells. Nuclei of infected cells were labeled during an early stage of the infection; at later stages, both nuclei and cytoplasms were labeled. Ultrastructurally, heterochromatins of the infected cells were marginated or aggregated to one side of the nuclei during the early stages of infection. Damage and rupture of the nuclear membrane started before formation of the viromatrix. Capsids were assembled in ring-shaped or disc-shaped structures. Bullet-shaped electron-dense material was present near the incomplete virus particles, and is speculated to be inserted into the capsids later.

KEY WORDS: In situ hybridization · Enlarged cell · TGIV · Morphogenesis · Cell hypertrophy iridovirus

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