Emerging Influenza D Virus Threat: What We Know so Far!

Abstract

Influenza viruses, since time immemorial, have been the major respiratory pathogen known to infect a wide variety of animals, birds and reptiles with established lineages. They belong to the family Orthomyxoviridae and cause acute respiratory illness often during local outbreaks or seasonal epidemics and occasionally during pandemics. Recent studies have identified a new genus within the Orthomyxoviridae family. This newly identified pathogen, D/swine/Oklahoma/1334/2011 (D/OK), first identified in pigs with influenza-like illness was classified as the influenza D virus (IDV) which is distantly related to the previously characterized human influenza C virus. Several other back-to-back studies soon suggested cattle as the natural reservoir and possible involvement of IDV in the bovine respiratory disease complex was established. Not much is known about its likelihood to cause disease in humans, but it definitely poses a potential threat as an emerging pathogen in cattle-workers. Here, we review the evolution, epidemiology, virology and pathobiology of influenza D virus and the possibility of transmission among various hosts and potential to cause human disease.

Keywords: emerging pathogen; epidemic; influenza; influenza A virus (IAV); influenza B virus (IBV); influenza C virus (ICV); influenza D virus (IDV); influenza-like illness; pandemic; respiratory illness.

Figure 1

Summary of the wide host range of influenza viruses. Few host species are susceptible to infection by all the four known influenza viruses (influenza A virus—IAV, influenza B virus—IBV, influenza C virus—ICV and influenza D virus—IDV), while others are known to be infected by only specific influenza viruses. Cattles are the major known reservoir of influenza D virus; however, other small ruminants and humans have shown susceptibility to IDV infections in different conditions.

Figure 2

Schematic representation of influenza D virus. IDV is an enveloped virus with seven RNA segments. The virions are 80–120 nm in diameter. The glycoprotein hemagglutinin-esterase fusion (HEF) helps in viral entry into the host cells. The protein DM2 acts as the ion channel.