First Detection and Genetic Characterization of Influenza D Virus in Cattle in Spain

Influenza D virus (IDV) and co-circulating respiratory pathogens were investigated using RT-qPCR in 316 samples collected from 210 Spanish cattle farms with bovine respiratory disease (BRD) outbreaks between July 2023 and September 2024. Thirty-eight IDV-positive samples, from 30 farms across 15 provinces throughout Spain, were identified. IDV was significantly more frequent in bronchoalveolar lavage samples (22.1%) and nasal swabs (13.5%) compared to lung tissues (5%) and other sample mixtures (5%). All IDV-positive specimens exhibited co-infections, with most (94.7%) harboring three to seven viral and/or bacterial pathogens, highlighting the complexity of BRD outbreaks. Cramer's V analysis revealed moderate IDV association with <i>Mycoplasma bovis</i> (V = 0.255) and <i>Pasteurella multocida</i> (V = 0.223), and a weaker IDV-bovine coronavirus association (V = 0.202), while IDV association with <i>Histophilus somni</i> (V = 0.025) and bovine herpesvirus 1 (V = 0.000) was negligible. Partial sequences of the hemagglutinin-esterase (<i>HEF</i>) gene from a subset of 13 IDV-positive samples (Ct < 32) were obtained. This confirmed the presence of the two major genetic lineages detected among cattle in Europe, D/OK and D/660, with D/660 exhibiting higher genetic diversity, as determined by DNAsp 6.12 software. This is the first report of IDV infection in Spanish cattle, confirming the circulation of the D/OK and D/660 lineages within the cattle population.