Metapneumoviruses, members of the family Pneumoviridae, have been identified in birds (avian metapneumoviruses; AMPV’s) and humans (human metapneumoviruses; HMPV’s). AMPV and HMPV are closely related viruses with a similar genomic organization and cause respiratory tract illnesses in birds and humans, respectively. AMPV can be classified into four subgroups, A–D, and is the etiological agent of turkey rhinotracheitis and swollen head syndrome in chickens. Epidemiological studies have indicated that AMPV also circulates in wild bird species which may act as reservoir hosts for novel subtypes. HMPV was first discovered in 2001, but retrospective studies have shown that HMPV has been circulating in humans for at least 50 years. AMPV subgroup C is more closely related to HMPV than to any other AMPV subgroup, suggesting that HMPV has evolved from AMPV-C following zoonotic transfer. In this review, we present a historical perspective on the discovery of metapneumoviruses and discuss the host tropism, pathogenicity, and molecular characteristics of the different AMPV and HMPV subgroups to provide increased focus on the necessity to better understand the evolutionary pathways through which HMPV emerged as a seasonal endemic human respiratory virus.

Avian metapneumovirus (AMPV) represents a long-term threat to the poultry industry due to its etiological role in the induction of acute respiratory disease and/or egg drop syndrome in domestic turkeys, chickens, and ducks. Although this disease is commonly referred to as turkey rhinotracheitis, the host range of AMPV encompasses many avian species. We have screened 1323 oropharyngeal- and cloacal swab samples obtained from wild mallards in the Netherlands from 2017 to 2019 by RT-PCR using a degenerate primer pair to detect all members of the Paramyxoviridae and Pneumoviridae or an avian metapneumovirus subtype C (AMPV-C)-specific RT-qPCR assay. We identified a total of seven cases of AMPV-C infections in wild, healthy mallards (Anas platyrhynchos), of which two AMPV-C positive samples were further processed using next-generation sequencing. Phylogenetic analysis of the two complete genomes showed that the newly identified AMPV-C strains share closest sequence identity (97%) with Eurasian lineage AMPV-C strains identified in Muscovy ducks in China that presented with severe respiratory disease and egg production loss in 2011. Further analysis of G protein amino acid sequences showed a high degree of variability between the newly identified AMPV-C variants. PONDR scoring of the G protein has revealed the ectodomain of AMPV-C to be partitioned into a long intrinsically disordered and short ordered region, giving insights into AMPV G protein structural biology. In summary, we provide the first report of full-length AMPV-C genome sequences derived from wild birds in Europe. This emphasizes the need for further surveillance efforts to better characterize the host range, epidemiologic distribution, and pathogenicity of AMPV-C to determine the risk posed by cross-species jumps from wildfowl to domesticated avian species.

The Eurasian lynx (Lynx lynx), a widespread wild felid on the Eurasian continent, is currently classified as “critically endangered” in Germany. Understanding the impact of infectious agents is of particular importance for the continued conservation of these animals, especially regarding pathogens with broad host ranges and risk of interspecies transmission. Feline leukemia virus (FeLV) is known to infect wild and domestic felids worldwide, including several species of lynx, but it has not been reported thus far in the Eurasian lynx. In September 2020, a 16-month-old female Eurasian lynx from the Bavarian Forest, Germany, showed a sudden onset of gastrointestinal signs such as anorexia, diarrhea, and vomiting, and died within one week. Macroscopic and histologic examination revealed hemorrhagic-necrotizing enteritis and typhlocolitis, with the degeneration of crypts and crypt abscesses, as well as depleted Peyer’s patches. In addition, the animal showed lymphoid depletion (lymph nodes, thymus, and spleen) and hypocellularity of the bone marrow. FeLV infection was confirmed by immunohistochemistry and next generation sequencing. A secondary bacterial infection with hemolytic Escherichia coli and Clostridium perfringens type A was present in the intestine. This is the first report of FeLV-associated enteritis, lymphoid depletion and bone marrow suppression with associated secondary bacterial infection in a Eurasian lynx.