There are four types of influenza viruses: A, B, C and D. Influenza D a new orthomyxovirus distantly related to influenza C virus, was described in pigs with respiratory symptoms in 2011 in US (1). These viruses primarily  implicated in bovine respiratory disease complex (1-3) and are not known to infect or cause illness in human. Though Ben Hause isolated the virus from a diseased pig in 2011, he later found that cattle were the primary reservoir for influenza D. Hause identified and characterized the new virus as part of his doctoral research under Li’s tutelage. International Committee of Taxonomy of Viruses made it official by naming a new  virus  –  “Influenza  D”  as  proposed  by  South  Dakota  State  University (SDSU). Further investigations showed that this newly emerged virus was prevalent in samples from cattle diagnosed with bovine respiratory disease complex (BRDC) (4-6). Recent serosurveys in Italy showed extremely high seroprevalence rates in cattle (92.4% seropositive) (7) and a low but increasing seroprevalence in swine, from 0.6% in 2009 to 11.7% in 2015 (8). Influenza D Virus was also reported in Cattle, from Ireland (9). This virus has been detected in bovine samples in several other countries, including France (10), Japan, and China (11). In 2015, during routine diagnostic investigations of respiratory disease outbreaks in swine herds in Northern of Italy, the circulation of IDV was demonstrated both by molecular detection of viral genome and virus isolation (12). Dr.  Li  (under  whom  Dr.  Hause  did  his PhD)  and  Dr. Radhey  Kaushik,  Professor  and  Assistant Head  at  Microbiology  Department were funded by National Institute of Health (NIH) with $400,000 to study this virus, especially about its biology, genetics and evolution.

Similar to Influenza C virus (ICV), Influenza D virus (IDV) also has a genome consisting of seven segments, only one surface glycoprotein, the hemagglutinin-esterase fusion (HEF) that exhibits receptor binding, receptor destroying and membrane fusion activities, thus combining the functions of HA and NA of influenza A and B viruses. Viral RNA Polymerase (PB1, PB2 and PA) transcribes  each  mRNA  segments  and  is  primed  by  a  method  called  cap snatching (Fig. 1). ICV appears to be more antigenically stable and slower to evolve compared to other influenza viruses. Re-assortment does not occur between ICV and IAV strains. IDV strains isolated from swine and cattle have been found to frequently reassort with each other (Table 1)

In a study it was observed that IDV replication occurs mainly in the upper respiratory tract and nasal swabs are the preferred sample for diagnosis. In this attempt a similar quantities of lower (lungs) and upper (nasal swabs) respiratory tract samples (361 to 350) were examined, the majority of IDV-positive samples were nasal swabs (14 against 3). This finding was found to be in in agreement with the previous reports of experimental infections of pigs with IDV conducted by Hause and coworkers (1), in which virus was only detected in the upper respiratory tract. On the other hand, IDV was also detected in three lung samples, suggesting that infection can also reach the lower respiratory tract either by active virus replication, or possibly passively by muco-ciliary transportation.

The presence and circulation of IDV in Italian pig farms is further highlighted by the serological results presented in the study.  30 of the 74 positive farms showed sera with positive titers not higher than 20, which is in agreement with the findings of Hause and coworkers (1, 13). This low seroconversion was confirmed by the HI test results in farms known to be infected by IDV, with 57% of the positive sera showing a titer of 20. In this study, co-infections with other swine respiratory pathogens were identified in eight out of nine herds IDV-positive herds, while in one farm, otherwise healthy gilts were found positive for IDV alone. To offer a definite answer to this question, more pathogenesis and transmission experiments in pigs must be conducted, including infections with IDV alone or combined with other pathogens to investigate possible synergy. While IDV is a novel virus, its importance and potential for interspecies transmission should not be underestimated.

Swine infected with ICV and IDV showed different symptoms. Experimentally infected Swine with ICV may exhibit normal temperature up to eight days postexposure and typically fail to exhibit traditional influenza-like symptoms. Several pigs showed slight dyspnea and increased nasal secretion after intranasal inoculation, clearing quickly or persisting for up to ten days. Whereas IDV infection may be more pathogenic in swine than ICV. Influenza-like illness was first observed in 15-week-old pigs naturally infected with IDV in April 2011. A retrospective serological analysis of samples from United States swine farms (2010–2012) identified four additional positives via reverse transcriptase polymerase chain reaction (RT-PCR). However, experimental infection in 10-week-old pigs resulted in no clinical signs.

IDV can be readily isolated and cultured in swine testicle (ST) cells. Whereas Nasal, tonsil, oropharyngeal, pharyngolaryngeal, and tracheal swabs; bronchoalveolar or nasal lavage; and nasopharyngeal aspirates have all been described specifically for ICV isolation. Enzyme-linked immunosorbent assays (ELISA), HI, and serum neutralization (SN) are used for the diagnosis. Hemagglutination inhibition (HI) assays are often performed with chicken or turkey erythrocytes following virus isolation to identify unknown influenza viruses by antigenic cross-reactivity. The use of polyclonal serum demonstrates cross-reactivity among all examined human ICV isolates, although the same level of cross-reactivity is not observed between isolates from pigs and humans or between ICV and IDV. Conventional and real-time RT-PCR assays are useful for detection of low viral loads in infected tissue or supernatant. Real-time RT-PCR has been used in the detection of all genera of influenza. Serological assays can detect antibodies to specific virion proteins, and are used in the typing of newly identified influenza viruses. Vaccination, a primary method for controlling pathogenic influenza viruses in swine is not currently applicable to IDV. To prevent and limit influenza infections in swine, common industry biosecurity practices should be in place.

According to Swine Health Information Center By the Center for Food Security and Public Health, College of Veterinary Medicine, Iowa State University (August 2015), information is lacking on the recently discovered IDV in swine and cattle. Especially the current understanding is limited by the paucity of similar isolates available for study. Analyses of the evolutionary rate will be critical in understanding the evolution and host range of this novel virus. Influenza D has 50% similarity to C and can cause problems in human as well. However it has not yet shown to be pathogenic to humans and  research  on  this  virus  is  still  underway.

References

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  2. Ferguson  L, Olivier  AK, Genova  S, Epperson  WB, Smith  DR, Schneider  L, et al. Pathogenesis of influenza D virus in cattle. J Virol. 2016;90:5636–42.
  3. Ng  TF, Kondov  NO, Deng  X, Van Eenennaam  A, Neibergs  HL, Delwart  E. A metagenomics and case-control study to identify viruses associated with bovine respiratory disease. J Virol. 2015;89:5340–9.
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  5. Mitra, N., Cernicchiaro, N., Torres, S., Li, F. & Hause, B. M. Metagenomic characterization of the virome associated with bovine respiratory disease in feedlot cattle identified novel viruses and suggests an etiologic role for influenza D virus. Gen. Virol. 2016, Aug;97(8):1771-84.
  6. Ng TF, et al. A metagenomics and case-control study to identify viruses associated with bovine respiratory disease. Virol. 2015;89:5340–5349. doi: 10.1128/JVI.00064-15.
  7. Rosignoli C, Faccini  S, Merenda  M, Chiapponi  C, De Mattia  A, Bufalo  G, et al. Influenza D virus infection in cattle in Italy [in Italian]. Large Animal Review. 2017;23:123–8.
  8. Foni E, Chiapponi  C, Baioni  L, Zanni  I, Merenda  M, Rosignoli  C, et al. Influenza D in Italy: towards a better understanding of an emerging viral infection in swine. Sci Rep. 2017;7:11660.
  9. Flynn O, Gallagher C, Mooney J, Irvine C, Ducatez M, Hause B, McGrath G, Ryan E. Influenza D Virus in Cattle, Ireland. Emerg Infect Dis. 2018 Feb;24(2):389-391.
  10. Ducatez MF, Pelletier C, Meyer G. Influenza D virus in cattle, France, 2011-2014.Emerg Infect Dis. 2015;21:368–71.
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