Poxvirus Infections in Common Swifts (Apus apus)

Members of the Genus Avipoxvirus have been reported in more than 374 avian species worldwide. Generally, poxviruses are considered to have a limited host range; however, canarypox-like viruses appear to infect a broader host range. In 2024, four juvenile common swifts in an age of 30 to 35 days from three different breeding sites of Lower Saxony, Germany, showed nodular, verrucous and crusted skin lesions of the wings, especially of the dorsal surface of the elbow and forearm region and the skin between the outer primary flight feathers. The skin on the head or the feet as well as the mucous membrane of the oral cavity was not affected. Two swifts of these cases had to be euthanized due to pox-related alterations of feather follicles of the primary flight feathers and associated feather growth disorders and loos. The other two swifts were released into the wild after recovery and successful hand-rearing. The diagnosis of a poxvirus infection is based in our cases on a combination of clinical examination, histology, transmission electron microscopy, and molecular biological investigations. The histological examination of skin lesions displayed severe heterophilic and histiocytic dermatitis with severe epithelial hyperplasia, mild orthokeratotic hyperkeratosis and cytoplasmic eosinophilic inclusion bodies (Bollinger's inclusion bodies) within frequently swollen keratinocytes (ballooning degeneration). Using transmission electron microscopy, virions showing typical brick-shaped poxvirus morphology were visualized. Avipoxvirus-specific DNA was detected by PCR. The sequence analysis of the 176 bp amplicon revealed 100% identity to a canarypox virus and a shearwaterpox virus 2 strain. Herpesvirus- or papillomavirus-specific DNA was not detected by PCR. To our knowledge, this is the first report of an avipoxvirus infection in a swift species. Notably, in all swift cases, the disease was limited to the skin of the wings. The origin of the virus and its transmission to swifts are unknown. However, transmission from other wild birds via biting insects appears to be possible, as has been described for canarypox-like viruses.