#18,804
Until a decade ago, there were only 3 recognized types of influenza; A (IAV), B (IBV) and C ((CV), and of those A & B got most of our attention. In 2012, the CDC simply described Influenza C as:
Both Influenza B and C are considered unlikely to have much in the way of pandemic potential because they lack the multiple sub-types found with influenza A viruses (18 hemagglutinin and 11 neuraminidase subtypes) that allow for viral reassortment.Influenza Type C
Influenza type C viruses cause mild illness in humans and do not cause epidemics or pandemics. These viruses are not classified according to subtype.
Ears perked up, however, when in 2013 in PLoS Pathogens: A New Influenza C Virus Detected In Swine we learned of a new ICV displaying `only modest genetic similarity to human influenza C'.
A year later, in mBio: Characterizing A Novel Influenza C Virus In Bovines & Swine we looked at a second paper by these same researchers, which added cattle to the list of hosts found to carry this novel virus. While still calling it an Influenza C, they wrote:
Detailed genetic, biological, and antigenic characterization suggests that C/OK viruses in cattle and swine are distinct from human ICV and likely represent a new genus in the Orthomyxoviridae family, with C/OK virus as the type species of influenza D virus.Shortly thereafter, this new virus was formally recognized as Influenza D (IDV), and in 2016's JVI: Pathogenesis Of Influenza D in Cattle, we saw more evidence suggesting that cattle are its natural reservoir.
Somewhat more concerning, in 2016 we also saw evidence that humans might be susceptible to Influenza D infection (see Serological Evidence Of Influenza D Among Persons With & Without Cattle Exposure), although it wasn't at all clear if the virus produced any symptoms in humans.
In 2019, the growing host range of Influenza D was described in Current Opinion in Virology. But as we've seen with IAV, the deeper we look, the more we find.
Researchers tested 360 cats in northern China for IDV antibodies (181 domestic cats & 179 strays) and found overall (8 of 360) 2.22% had positive tests. Curiously, domestic cats showed significantly higher (3.31%) positives than strays (1.12%).
I've only posted the abstract and some excerpts, so follow the link to read the research letter in its entirety. I'll have a bit more after the break.
Influenza D Virus in Domestic and Stray Cats, Northern China, 2024
Mingshuai Shen1, Xinkun Zhao1, Jian Zhang1, Cun Liu, Chengyi Qi, Ruohan Wang, Jieshi Yu, Kezhou Wang, and Zhao Wang
Abstract
Influenza D virus infects primarily cattle, but infrequent reports of infections in cats occur. We detected influenza D virus antibodies in 8 of 360 cats in northern China. Domestic cats showed higher susceptibility than strays. Our results suggest a previously overlooked aspect of epidemiology of this virus in companion animals.
Influenza D virus (IDV) is a single-stranded, negative-sense RNA virus belonging to the genus Deltainfluenzavirus, family Orthomyxoviridae, and was first isolated from pigs in 2011 (1,2). IDV infects primarily cattle, but studies have documented the virus globally in a diverse range of animals, including small ruminants (eg, sheep), domestic animals (pigs, horses, camels), and wild ungulates (wild boars) (3,4).
Although there is no direct evidence that IDV can infect humans, an increasing number of studies have indicated that IDV has the potential for causing zoonotic infections (5). One recent study in the Puglia region of Italy identified 14 positive IDV antibody samples from 426 dogs sampled in 2016–2023, indicating that companion animals can be exposed to IDV (6). Our study aimed to investigate whether cats, a popular companion animal, are susceptible to IDV.
(SNIP)
Our findings demonstrated that, although overall IDV seroprevalence in cats in northern China was low (2.22%), domestic cats showed significantly higher exposure rates than strays. This difference might reflect increased human contact, potentially leading to viral exposure, considering stray cats’ independent lifestyles potentially limit such transmission opportunities.
As noted in similar studies of other species, the source of IDV infection in cats remains unclear. Potential transmission routes might include reverse zoonotic transmission or alternative pathways, such as raw milk exposure, as suggested by recent influenza A(H5N1) virus detections in cow’s milk (10). Although our serologic data cannot confirm active transmission or clinical impacts, the observed higher seroprevalence in domestic cats suggests that close human–animal interaction could potentially increase exposure risk.
IDV is not currently a major zoonotic threat, but its detection in cats highlights the need for vigilance. Domestic cats may potentially serve as reservoirs for IDV, which could contribute to viral adaptation. Our study emphasizes the importance of monitoring IDV in companion animals, especially considering recent findings demonstrating the potential of influenza viruses for cross-species transmission via unconventional routes (i.e., H5N1 virus in bovine milk).
Future research should prioritize molecular confirmation of active infections and assess milkborne transmission risks. Proactive surveillance in pets and their food sources is critical to understanding IDV’s evolving epidemiology and mitigating potential public health concerns.
Dr. Shen is a master’s degree student in the College of Laboratory Animals at Shandong First Medical University. His main research interest is the study of influenza virus pathogenicity factors.
IDV is not detected by routine human influenza surveillance programs, and there are no vaccines for animals or humans.
While the zoonotic potential of Influenza D appears to be low right now, that could change over time. Like influenza A viruses, influenza D and C viruses have segmented RNA genomes, which allows for reassortment.
The recent discovery of frequent reassortment between IDV clades D/660 and D/OK, along with spillovers into new hosts (like dogs and cats), makes influenza D a virus very much worthy of our attention.
