Influenza D Virus Infection in Feral Swine Populations, United States

Influenza D Virus was first discovered in domesticated pigs in the US in 2011. Later evidence arose that cattle were the primary reservoir for influenza D Virus. But, as with all Influenza-like viruses, the Influenza D Virus was unpredictable.

At the moment Influenza D Virus has been identified in domestic cattle, swine, camelid, and small ruminant populations across North America, Europe, Asia, South America, and Africa.

[Feral swine: not cuddly at all]

A new study investigated the seroprevalence and transmissibility of Influenza D Virus in feral swine[1]. Swine were introduced into what is now the United States in the 15th century. Since that time, populations of free-ranging swine have spread to about 40 states. During 2012-2013, scientists evaluated feral swine populations in four US states.

Of 256 swine tested, 57 (19.1%) were seropositive for Influenza D Virus, indicating a previous infection. Previous studies have suggested that domestic swine are major sources of Influenza A Virus exposure for feral swine[2]. Among 96 archived influenza A virus-seropositive feral swine samples collected from 16 US states during 2010-2013, 41 (42.7%) were also Influenza D Virus seropositive.

While Influenza D Virus was shown not to cross-react with Influenza A Virus (yet), the our study showed that 42.3% of the IAV-seropositive feral swine also had exposure to IDV. The rate of seroprevalence of Influenza D Virus in Influenza A Virus-seropositive feral swine was more than twice that observed among Influenza A Virus-negative feral swine.

The question is, therefore, why are feral swine, that are infected with Influenza A Virus, more susceptible to an infection with Influenza D Virus. Or vice versa.

[1] Ferguson et al: Influenza D Virus Infection in Feral Swine Populations, United States in Emerging Infectious Diseases – 2018. See here.
[2] Martin et al: Feral Swine in the United States Have Been Exposed to both Avian and Swine Influenza A Viruses in Applied and Environmental Microbiology – 2017