关于 Influenza D virus

Influenza D virus (IDV) belongs to the Orthomyxoviridae family, the genus Deltainfluenzavirus. It was first identified in pigs in the USA in 2011^[2]. IDV has since been documented globally, with confirmed cases in Europe, North America, South America, Africa, Asia, and recently Australia^{[3, 4]}.  Although IDV was initially detected in pigs, cattle are considered the main reservoir of the virus^[5]. The virus has been isolated from both cattle and pigs, while the presence of antibodies for IDV have been identified in various animal species. Seroprevalence of IDV has also been established in humans, particularly those in professional contact with cattle^[6].

IDV is transmitted through direct contact and aerosol over short distances. Experimental infection in seronegative calves has shown that the infection causes mild to moderate respiratory symptoms, including dry cough, unilateral or bilateral serous/mucous nasal discharge, serous ocular discharge, depression, and short breath. Clinical signs may vary depending on IDV co-infection. In pigs, IDV replicates in the upper respiratory tract with detectable shedding of the virus in nasal swabs, although no clinical signs of infection were observed. Other studies reported that IDV RNA was detected in pig lungs, implying IDV infection also in the lower respiratory tract.

IDV is an enveloped, spherical to pleomorphic virus with a diameter of approximately 100-120 nm, possessing a segmented genome consisting of seven negative-sense, single-stranded RNA segments^[2]. The genome organization is akin to that of Influenza C virus (ICV). The presence of one surface glycoprotein hemagglutinin-esterase-fusion (HEF) is characteristic of C- and D-type viruses. Phylogenetic analysis reveals a nucleotide homology of 50% for the HEF segment (the most variable) and 70% for PB1 (the most conserved segment) between ICV and IDV^[7].

IDV circulates globally in at least four distinct genetic lineages: D/swine/Oklahoma/1334/2011 (D/OK), detected in many European countries, the USA, Mexico, China, and Namibia; D/bovine/Oklahoma/660/2013 (D/660), identified in Italy, the USA, and Mexico; D/bovine/Yamagata/10710/2016 (D/Yama2016), found in Japan; and D/bovine/Yamagata/1/2019 (D/Yama2019), detected in Japan and China^[8]. Viral strains belonging to potentially new lineages were identified in California (D/CA2019), Brazil, and Turkey^[9]. Viral strains from lineages D/OK and D/660 were reported to frequently reassort with each other and exhibit antibody cross-reactivity. However, strains identified in the cattle population in Japan were distinct from those isolated in other countries, likely characterized by separate evolutionary paths^{[8, 10]}.

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