Novel avian metapneumovirus subtype C is a newly emerged pathogen causing hydrosalpinx fluid syndrome in Sheldrake ducks in China

Since 2021, an epidemic disease characterized with hydrosalpinx fluid syndrome (HFS) has been circulating in the breeding and laying Sheldrake ducks in China, which seriously endangers the healthy development of the duck industry. The pathogen of the disease was tract to avian metapneumovirus subtype C (aMPV/C) causing egg-drop and acute upper respiratory diseases in poultry. To date, no further reports have been made to isolate and characterize aMPV/C infection in Sheldrake ducks in China. In this study, a strain of virus (named as aMPV-FJ21) was isolated from diseased Sheldrake ducks exhibiting HFS using Vero cell line. Transmission electron microscopy showed that the strain was an enveloped virus with spherical or multiple morphologies. The immunofluorescence assay revealed that the virus strain aMPV-FJ21 had an obvious reactive activity with the ploy-antibody against aMPV/C F protein. The genome sequence of aMPV-FJ21 was 14149 nucleotides (nt) in length, sharing 87.1%-95% nt similarity to that other aMPV/C strains, and yet the G protein of the strain was only 55.6%-78.8% identical to that of aMPV/C strains. Phylogenetic analysis showed that aMPV-FJ21 formed an independent branch of aMPV/C and had a distant genetic relationship with other aMPV/C strains, suggesting that it might represent a new genetic lineage. In vivo challenge experiments demonstrated that aMPV-FJ21 induced clinical symptoms similar to those of natural cases in laying Sheldrake ducks, with a reduction of approximately 15% in the average of daily egg production. In addition, aMPV-FJ21-infected ducks shed progeny virions via tears for the longest period (22 days). Detection of tissue distribution of viral RNA showed that the viral RNA exhibited higher expression in non-parenchymatous tissues than in parenchymatous tissues. Altogether, we isolated a new lineage of aMPV/C from Sheldrake ducks with HFS and confirmed that the virus is the real agent responsible for HFS in Sheldrake ducks for the first time, which provides basic data for further study of the pathogenic mechanism of the virus.