Novel avian metapneumovirus subtype C variant 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 laying Sheldrake ducks in China, which seriously endangers the healthy development of the duck industry. The causative agent of this disease has been traced to avian metapneumovirus subtype C (aMPV/C), known to cause acute upper respiratory tract infections and egg-drop in poultry. To date, no reports have been made to isolate and characterize aMPV/C infection in Sheldrake ducks in China. Here, a strain of virus, designated aMPV-FJ21, was successfully isolated from the diseased ducks exhibiting HFS. Transmission electron microscopy revealed that the virus is an enveloped particle exhibiting a spherical or pleomorphic morphology. Indirect immunofluorescence assays demonstrated that the aMPV-FJ21 strain had an obvious reactive activity with the ploy-antibody against aMPV/C F protein. The complete genome of aMPV-FJ21 was determined to be 14,149 nucleotides in length. Notably, the amino acid sequence of the G protein was only 55.6%-78.7% identical to those of other aMPV/C reference strains. Phylogenetic analysis indicated that aMPV-FJ21 forms a distinct lineage within the aMPV/C group and is genetically distant from the North American and Eurasian lineages, suggesting that it may represent a novel genetic lineage. In challenge experiments, laying Sheldrake ducks with aMPV-FJ21 reproduced the typical clinical symptoms and pathological lesions observed in the field cases. Altogether, we had isolated a novel aMPV/C variant from Sheldrake ducks exhibiting HFS, distinct from previously reported strains, and provided the first evidence confirming its role as the causative agent of HFS in ducks.