The primary reservoir is currently cattle.

Article on the discovery of the strain (2013): "Isolation of a Novel Swine Influenza Virus from Oklahoma in 2011 Which Is Distantly Related to Human Influenza C Viruses" - https://journals.plos.org/plospathogens/article?id=10.1371/j...

Referenced in the article (re transmission to humans): "Efficient replication of influenza D virus in the human airway underscores zoonotic potential" - https://www.biorxiv.org/content/10.64898/2026.02.07.704474v1