Pathogenicity and transmission of virulent Newcastle disease virus from the 2018-2019 California outbreak and related viruses in young and adult chickens.

In May of 2018, virulent Newcastle disease virus was detected in sick, backyard, exhibition chickens in southern California. Since, the virus has affected 401 backyard and four commercial flocks, and one live bird market in California, and one backyard flock in Utah. The pathogenesis and transmission potential of this virus, along with two genetically related and widely studied viruses, chicken/California/2002 and chicken/Belize/2008, were evaluated in both 3-week- and 62-week-old chickens given a low, medium, or high challenge dose. All three viruses were highly virulent causing clinical signs, killing all the chickens in the medium and high dose groups, and efficiently transmitting to contacts. The three viruses also replicated in the reproductive tract of the adult hens. Virus shedding for all viruses was detected 24 hours after challenge, peaking with high titers at day 4 post challenge. Although not genetically identical, the studied isolates were shown to be phenotypically very similar, which allows the utilization of the available literature in the control of the current outbreak.