Virus strain influenced the interspecies transmission of influenza D virus between calves and pigs.

Influenza D viruses (IDV) belong to a new genus in the family Orthomyxoviridae. IDV is the aetiologic agent of acute, mild respiratory disease in ungulate species with agricultural importance (cattle, pigs, sheep, goats, camels, etc.). Despite the initial isolate being of porcine origin, serological data suggest cattle to be the primary host of IDV. The study aims were twofold: elucidating species-specific replication kinetics of IDV in bovine and porcine hosts and defining the interspecies potential with two different IDV strains. Three calves and three pigs were intranasally inoculated with the prototypic strain D/swine/Oklahoma/1334/2017 or a genetically distinct cattle isolate, D/bovine/Texas/72/2017. Two days following infection, three naïve pigs and three naïve calves were co-housed with inoculated calves and pigs, respectively. The species of IDV origin had no effect on virus replication kinetics in the upper respiratory tract of inoculated calves and pigs; similar shedding profiles were observed for each species and virus. However, interspecies transmission was found to be associated with virus origin species; D/bovine/Texas/72/2017 and D/swine/Oklahoma/1334/2017 were directly transmitted only to contact calves or pigs, respectively. Even so, transmission efficiency was higher for calves compared to pigs. Together, these data show that cattle and pigs are permissive for IDV replication, but IDV transmission may be species dependent. Host-specific mutations likely influenced transmission efficiencies between agriculturally important mammalian species.