Researchers have cautioned over the potential public health risks of influenza C and D viruses, with their seroprevalence recorded in cattle, pigs, camels, small ruminants, horses, and humans.
Influenza viruses were first documented in the late 16th century. They have evolved into four major virus types — alphainfluenza virus (influenza A), betainfluenza (influenza B), gammainfluenza (influenza C), and deltainfluenza (influenza D). The A and B types are responsible for the seasonal flu epidemics in humans.
Within each type, there are subtypes, clades, lineages, and sub-lineages.
Influenza C and D viruses have seven-segmented genomes, unlike the eight-segmented influenza A and B viruses.
Influenza C emerged nearly 74 years ago, with humans as primary hosts. Influenza D virus, a distant relative of the C virus, was isolated in 2011, with cattle as the primary host.
Despite its initial emergence in swine, the influenza D virus has turned out to be a transboundary bovine pathogen and a broader host range, similar to influenza A viruses.
Chithra Sreenivasan and her fellow researchers, writing in the journal Pathogens, noted that while influenza A virus has established a host range that included most of the terrestrial and marine mammals over the last century, it could not breach the avian–bovine or human–bovine, species barriers.
“Hence it can be inferred that influenza A evolution witnessed an unfavorable stance in bovine species.”
Interestingly, ruminant species have proved quite susceptible to the seven-segmented influenza viruses, C and D, with cattle considered the primary reservoir for influenza D viruses.
In their review, researchers with the Maxwell H. Gluck Equine Research Center at the University of Kentucky, and the Aaron Diamond AIDS Research Center at Columbia University, set out to describe the ecology of the influenza C and D viruses, looking at cross-species transmission, host range, phylodynamics, virus biology, species specificity and receptor preferences.
The authors said the fact that these two seven-segmented genome viruses have several lineages, capable of undergoing recombinations and reassortments, coupled with their active co-existence in bovine and swine species, common genomic composition, and receptor specificities, could potentially enable these viruses to broaden the host range by species spillover.
Furthermore, the detection of the influenza D virus in all continents and its seroprevalence in cattle, swine, camels, small ruminants, horses, and more importantly in humans, suggests the potential means for bidirectional transmission and public health risks.
Evidence suggests that the influenza D virus can replicate in human respiratory tissue, they said.
The review team comprised Sreenivasan, Dan Wang and Feng Li, with the Gluck Center; and Zizhang Sheng, with the Aaron Diamond AIDS Research Center.
Sreenivasan, C.C.; Sheng, Z.; Wang, D.; Li, F. Host Range, Biology, and Species Specificity of Seven-Segmented Influenza Viruses—A Comparative Review on Influenza C and D. Pathogens 2021, 10, 1583. https://doi.org/10.3390/pathogens10121583