Two Outbreaks of Pigeon Paramyxovirus 1 With High Mortality in Captive Pigeons ( <i>Columbia livia</i> ) in Denmark, 2022–2023

This study describes the first outbreaks with virulent avian paramyxovirus 1 (APMV-1) since 2005 in Denmark. Both outbreaks were caused by pigeon specific variants, denoted pigeon paramyxovirus 1 (PPMV-1). The first outbreak was in June 2022 and affected captive pigeons near Næstved, in southeast Denmark, where 1053 captive birds were housed, and hereof 851 pigeons. A second and separate outbreak occurred in June 2023 in Aalborg, northwest Denmark, involving 1851 captive birds, of which 40 were pigeons. In both outbreaks, pigeons were predominantly affected, characterised by high mortality, and presented with neurological signs, along with thin-shelled eggs. Pathological and virological assessment revealed multi-systemic infections in pigeons, including neuronal and vascular endothelial tropism. Chickens were affected only in the 2022 outbreak, with reported extended hatching periods and chicks dead at hatching, and with no apparent lesions detected at both macro- and microscopic investigations. Fusion protein (F) gene sequence classified the 2022 virus isolate as genotype VI.2.1.1.2.2 and the 2023 virus as genotype XXI.1.1, with polybasic cleavage sites ¹¹²RRQKRF¹¹⁷ (2022) and ¹¹²KRQKRF¹¹⁷ (2023). However, mean death time (MDT) tests categorised both virus isolates as mesogenic, and an intracerebral pathogenicity index (ICPI) test of the 2022 virus isolate showed an ICPI index of 0.65, categorising the virus as lentogenic. This is the first report of PPMV-1 isolates with polybasic cleavage site and associated mortality in captive pigeons in Denmark. The sudden resurgence of outbreaks in Denmark after nearly two decades without similar incidents highlight the potential threat posed by circulating viruses in wild birds, such as feral pigeons, and emphasise the importance of surveillance in wild bird populations for improved risk recognition and early detection of emerging threats.