The role of mammals in Avian Influenza: a review

Avian influenza (AI) is an infectious viral disease of birds, including domestic poultry, which has been  causing  outbreaks  worldwide,  leading  to  several  millions  of  dead  wild  birds  and  culled poultry. AI is mainly found in birds, but recently, there was an increase in reported infections in mammals,  ranging  from  no  symptoms  to  mass  mortality  events  and  some  human  cases. Epidemiologically of great concern, evidence of mammalian adaptations have been found, but the  transmission  routes  and  pathogenesis  in  mammals  are  still  to  be  defined.  Hence,  it  is paramount  to  address  all  facets  of  AI  viruses  epidemiology,  including  investigating  taxa  not customarily  thought  to  be  involved  in  the  transmission  and/or  trafficking  of  AI,  such  as  wild mammals.  The  scope  of  thisreport  was  to  assess  the  role  of  mammals  in  AI  epidemiology, virology  and  pathology,  i.e. AI  maintenance,  reservoir  role,  immunity,  role  of  mammals  in  a potential pandemic. To do so, we performed an all-encompassing review of the literature on the topic with a two-fold approach: a systematic review of the published AI cases in wild mammals and a narrative approach to provide an expert opinion on the role of mammals in AI spread. The final  number  of  peer-reviewed  papers  included  in  the  systematic  literature  review  was  76, resulting in 120 unique infection records with AI in wild mammal species. The most represented taxa were included in the order Carnivora. The risk of infection was identified mainly as predation (or feeding) upon infected birds or contact with avian species. Evidence of mammal-to-mammal transmission  in the  wild is only circumstantial  and  yet  to  be  confirmed. Cases  of  AI from  the systematic review of experimental findings were discussed concerning epidemiology, pathology and  virology.  Knowledge  gaps  and  potential  pandemic  drivers  were  identified.  In  summary, although a greater number of infections in wild mammals have been reported, there is no hard evidence for sustained mammal-to-mammal transmission in the wild. The factors contributing to the  increased  number  of  infections  found  in  wild  carnivores  are  not  clear  yet,  but  the unprecedented global spread of highly pathogenic avian influenza (HPAI) viruses creates ample opportunities for intense, mostly alimentary, contact between infected wildbirds and carnivores. Close  surveillance  of  circulating  strains  and  continued  assessment  of  new epidemiological situationsarecrucial to quickly identify strains with enhanced mammalian fitness.