A rapid virus neutralization assay for Newcastle disease virus with the swine testicular continuous cell line.

Five continuous cell lines, swine testicular (ST), human rectal tumor (HRT 18), fetal rhesus monkey kidney (MA104), bovine turbinate (BT), and quail tracheal (QT35), were evaluated and compared with chicken embryo fibroblasts (CEFs) for their ability to propagate B1 or Texas GB strains of Newcastle disease virus (NDV). The NDV Texas GB strain replicated in all the continuous cell lines used in this study. Only the ST and QT35 cells produced a cytopathic effect (CPE) similar to that produced in CEFs. However, the ST cell line remained attached while displaying CPE, whereas infected QT35 cells detached, as did the CEFs. The B1 strain of NDV replicated in ST cells, MA104 cells, and CEFs but with less CPE as compared with the Texas GB strain. Pretreatment with trypsin did not enhance CPE with either NDV strain at the level tested. Sera evaluated for neutralizing antibody titers to NDV were significantly higher in titer when the ST cell line was used and compared with CEFs. A high correlation was found between the microscopic examination and the tetrazolium dye (MTT) microassay methods for determining the viral neutralization endpoint, thus suggesting the ST cell line and MTT microassay could be used as an alternative to CEFs and microscopic examination for evaluating neutralizing antibodies titers to NDV.