A new genus of influenza virus - Influenza D virus (review)

Influenza D virus (IDV) discovered in swine in 2011 and then in cattle and other animals was subsequently classified as a separate genus Influenza D virus ( Orthomyxoviridae , Deltainfluenzavirus ) (B.M. Hause et al., 2014). It is assumed that influenza D virus descended from human influenza C virus (ICV) from 300 to 1,500 years ago (Z. Sheng et al., 2014; S. Su et al., 2017). Its virion contains seven segments of RNA. The IDV genome sequence is 50 % different from ICV, no recombinants are formed between IDV and ICV, and no cross-reactivity of the antibodies occurs as well (B.M. Hause et al., 2011). Retrospective analysis showed that the virus has been circulating in North America since 2002 at the latest (M. Quast et al., 2015). Cattle is the main reservoir of the pathogen (L. Ferguson et al., 2015) but it also infects small ruminants, horses (H. Nedland et al., 2018), camels (E. Salem et al., 2017), and pigs (Z. Yan et al., 2018), including in wildlife (L. Ferguson et al., 2018). The virus provokes bacterial infections which affect the lung parenchyma, slowing growth, decreasing milk yields, and causing reproductive delay. In severe acute disease, IDV can move into the bloodstream in cattle and goats via penetration through capillaries lining respiratory tract. Calves possess passive immunity due to natural feeding that weakens in the 6-8-month-old animals, making them susceptible to the infection. Small ruminants serve as a reservoir for IDV and can transmit infection to other livestock. Wild boars can also be dangerous as IDV vectors between wild and domestic animals. IDV has not yet been found in poultry. At present, three types of influenza D virus are circulating simultaneously. The experiments have shown that the virus infects polecats (B.M. Hause et al., 2011) and guinea pigs (C. Sreenivasan et al., 2015). IDV successfully replicates in human’s respiratory epithelial cell culture at 33 to 37°C (M. Holwerda et al., 2019). The selection pressure for IDV is higher in pigs than in cattle and goats, so IDV, if successfully adapted, can spread widely among pigs. Therefore, a new public health risk could arise given the similarity in receptors between pig and human. The accumulated data on the ability of IDV to infect humans are ambiguous and require further in-deep study. Particular attention should be paid to persons involved in the management of farm animals susceptible to IDV. The pathogen is widespread across the planet and poses a potential threat to agriculture in countries where the breeding of cattle, small ruminants and pigs is of great importance to the economy. The fact that the virus is capable of infecting a wide range of hosts makes it potentially harmful to humans too.