Table 2.

The results of in vivo reversion of virulence examination of the attenuated viruses in day-old specific pathogen free (SPF) chicks.

Virusa	Passage no.
	P1					P5					P10
	C. signs	Lesions score		PCR detection		C. signs	Lesions score		PCR detection		C. signs	Lesions score		PCR detection
		Trachea	kidneys	Trachea	kidneys		Trachea	kidneys	Trachea	kidneys		Trachea	kidneys	Trachea	kidneys
att-IBM41	None	0	0.67	+	+	None	0	0	+	+	None	0	0	+	+
att-IB2	None	0	0	+	+	None	0	0	+	+	None	0	0	+	+
att-IBvar2	None	0.67	1.3	+	+	None	0.67	1.0	+	+	None	1	1.3	+	+

^aDay-old SPF chicks received 10⁴ EID₅₀/bird of the attenuated viruses separately via the ocular route. Birds were observed daily for clinical signs for 5 d. At 5 days post-infection, the virus detection was confirmed by RT-PCR in kidneys and tracheal homogenates. The tissue homogenates were then used to inoculate a second set of birds. At the 1st, 5th, and 10th passages; tracheal and kidney lesions were scored.

WT, wild-type