The virus was first identified in US pigs with influenza-like illness in 2011, and an increase was detected in 2014 in cattle (US, China and France). Based on recent serological evidence it is likely that the virus was already present in US cattle (USA, Nebraska) around 2003. The genome of the virus identified by the French shows a 94-99% match with the American counterpart, suggesting an intercontinental spillover. The virus is now also present in Africa. In addition to pigs and cattle, it is likely to infect other species such as small ruminants and wild boar, but its primary reservoir is cattle.
The virus has also been detected in several European countries. The influenza D virus (IDV) is not included in the OIE list of notifiable diseases as well not included in the Animal Disease Notification System (ADNS) of the European Commission. Therefore, most Member States have not implemented a specific surveillance.
The significance of the virus is unclear as its clinical impact is unknown. Conclusive evidence for human infection is not yet available.
In Ireland, the virus was first identified in pigs and then in cattle during 2014-2016. According to an Irish study IDV is widespread in Irish cattle. Results showed a high prevalence of IDV in cattle sampled at slaughter (94.6%) or for diagnostic reasons (64.9%), whereas prevalence in samples taken for diagnostic reasons from sheep (4.5%) and pigs (5.8%) was much lower.
A serological study was conducted to assess the prevalence of antibodies against influenza D virus in human serum samples collected in Italy from 2005 to 2017. Serum samples were analysed by haemagglutination inhibition and virus neutralization assays. The results showed that the prevalence of antibodies against the virus increased in the human population in Italy during the time period, with a trend characterized by a sharp increase in some years, followed by a decline in subsequent years. The virus showed the ability to infect and elicit an immune response in humans. However, prevalence peaks in humans appear to follow epidemics in animals and not to persist in the human population.
A UK study presents the results of molecular testing of cattle submitted for post-mortem examination where respiratory disease signs were present during the winter and spring of 2017/2018. Influenza D virus was detected in 8.7% of the cases, often as the sole viral agent and always in conjunction with bacterial co-infection with one or more agents. Viral RNA was present in both the upper and lower respiratory tract and pathological changes in lung tissues were observed alongside signs of concurrent bacterial infections. Sequencing of one UK isolate revealed that it is similar to viruses from the Republic of Ireland and Italy.
According to the EFSA risk assessment, the virus is enzootic in the European cattle population, but there are regional differences in prevalence: while in France the seroprevalence was 36.3%, in Luxembourg and Italy it was 81.8%-84.2%. The virus prevalence in the Italian cattle herd ranged from 6.3% to 9.5%. In pigs, the virus is much less prevalent.
The significance of the virus and its zoonotic potential are not yet clear, but serological and virological studies suggest that the virus is also capable of infecting humans. Despite the absence of clinical signs, the virus was able to replicate in the upper respiratory tract of ferrets and direct contact between ferrets was also observed.
A study from 2019 confirmed that the virus replicates efficiently in vitro in a model that replaces the respiratory epithelium at temperatures corresponding to the human upper and lower respiratory tract.
EFSA's simplified quantitative IDV risk assessment exposure model indicated a possible infection of human by IDV through aerosols in cattle farms.
Further studies are needed to fully assess the risk of IDV for both animal and human health.
It would be also important to know the prevalence of the virus in the Hungarian cattle and pig herds.
Mazzetto E, Bortolami A, Fusaro A, Mazzacan E, Maniero S, Vascellari M, Beato MS, Schiavon E, Chiapponi C, Terregino C, Monne I and Bonfante F (2020) Replication of Influenza D Viruses of Bovine and Swine Origin in Ovine Respiratory Explants and Their Attachment to the Respiratory Tract of Bovine, Sheep, Goat, Horse, and Swine. Front. Microbiol. 11:1136. doi: 10.3389/fmicb.2020.01136