FIG 9.

Histopathology of sections of trachea and brain of 2-week-old chickens infected oculonasally with parental and chimeras of NDV strains BC and GBT, involving swaps of the polymerase-associated protein genes. (Trachea) Severe inflammation in the tracheal mucosa (black arrows) and submucosa (double black arrows) was observed with parental rGBT, rBC-GBT(L), and rBC-GBT(N+P+L), whereas inflammation was reduced (red arrows) with rBC, rGBT-BC(L), and rBC-GBT(N+P+L). (Brain) Severe inflammation and marked perivascular aggregates of lymphocytes and fewer plasma cells were observed in meninges in response to rGBT and rBC-GBT(N+P+L) (black arrows), whereas inflammation and lesions were reduced with rBC and rGBT-BC(N+P+L) (red arrows). The trachea and the brain lesions indicate that rGBT L alone or together with N and P increased the virulence of rBC, and the converse swap increased the virulence of rBC.