TABLE 1.

Summary of pre-S/S- and S-DNA vaccination in 3-week-old ducks

DNA vaccine	Prechallenge			Postchallenge
	Total anti-DHBs titera	Neutralization		Virus removal rate (min)d	Viral replicatione	Histological changesf
		In vivob	In vitroc
S-DNA	(2–5) × 104 (4/4)	3/3 neutralized by >5 μl of anti-S	90–95% reduction in viral DNA by 35 and 70 μl of anti-S	<5–15 (3/4)	Not detected (3/4)	Mild (3/4)
				45 (1/4)	∼2% hepatocytes (1/4)	Marked (1/4)
Pre-S/S-DNA	(1–4) × 104 (4/4)	1/3 neutralized by 80 μl of anti-preS/S, no neutralization by 20 and 40 μl of serum	50% reduction in viral DNA by 70 μl of anti-pre-S/S; no reduction by 35 μl	45–90 (4/4)	10–40% hepatocytes (4/4)	Mild to moderate (4/4)

^aThe range of total anti-DHBs antibody titers as measured by ELISA after the third vaccination from four ducks/group. Numbers in parentheses show the number of ducks. 

^bNeutralizing ability of serum from vaccinated ducks when preincubated with 10⁶ DHBV DNA genomes (10⁶ ID₅₀) prior to inoculation into 1-day-old ducklings (three animals/group). Different volumes of neat anti-S (5, 10, and 20 μl) or anti-pre-S/S (20, 40, and 80 μl) sera from ducks R81 & R76, respectively, were used. 

^cNeutralizing ability of serum when preincubated with 7 × 10⁷ DHBV DNA genomes prior to inoculation of PDH cultures. 

^dThe time taken for removal of 90% of virus inoculum from the bloodstream PC as determined by Southern blot hybridization. Numbers in parentheses show the number of ducks. 

^eViral replication in the liver 4 days p.c., determined by the presence of total DNA and cccDNA, as well as by the presence of viral antigen (DHBsAg)-positive hepatocytes. Numbers in parentheses show the number of ducks. 

^fHistological changes seen in the liver tissue samples taken 4 days p.c., in relation to the presence of periportal necrosis, intralobular degeneration and focal necrosis, portal inflammation, and fibrosis (23). Numbers in parentheses show the number of ducks.