Virus isolation from tracheal explant cultures and oropharyngeal swabs in attempts to detect persistent Newcastle disease virus infections in chickens.

Three-to-seven-week-old broiler-type chickens were inoculated with Newcastle disease virus (NDV) by eye-drop (ED) or intratracheally (IT), and virus isolation was attempted from oropharyngeal (oral) swabs and medium harvested from tracheal explant cultures (TEC). The TEC were maintained in screw-capped tissue-culture flasks for at least 1 month, and medium harvested at regular feeding times was assayed for NDV and NDV antibody. The earliest and latest sample times were 3 and 21 days after NDV inoculation. The three experiments done were: Expt. 1, infection of nonvaccinates with NDV strain La Sota; Expt. 2, infection of NDV vaccinates and nonvaccinates with NDV strain Largo; and Expt. 3, infection of NDV vaccinates and nonvaccinates with NDV wild-type strain Kansas-Manhattan (KM) and two temperature-sensitive (ts) clones derived by J. S. Youngner from the KM strain. All experiments yielded similar results. On day 3 postinoculation (PI), most chickens were shedding virus recoverable by oral swabs and detectable in harvests from TEC prepared on that day. On day 7 PI, there was a sharp reduction in the frequency of virus-positive oral swabs, but there was no decline in the frequency of virus-positive TEC. On day 14 PI or later, all oral swabs and TEC were virus-negative, except for one chicken in Expt. 3 that was oral-swab-positive. There was no evidence of NDV persistence in the TEC of oral-swab-negative chickens on or after day 14 PI. The results of these experiments are in contrast with previous reports of the detection of latent NDV by virus isolation from harvests of TEC prepared 18 or more days PI. The ts clones of strain KM used in Expt. 3 induced a markedly poorer antibody response and were shed for a shorter time than the KM parental virus.