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... jatkoa (3) (paradigman muutos vai huomaamatta jäänyt evoluutio

 

Influenza viruses spill over periodically from their primordial reservoirs (aquatic fowls) to the intermediate/secondary hosts to facilitate better adaptation and transmission and some of these hosts must remain as permanent niches for sustained IAV transmission. Other than birds, influenza A affects diverse mammalian populations such as pigs, seals, horses, dogs, cats, wild cats, minks, whales, and humans.
 
 The global pandemic of 2009 caused by swine-origin H1N1 was reported in swine, turkey, dogs, and cat [9,10,11,12,13,14].
 
 Over the last few years, influenza infection landscape has widened to include new mammalian hosts such as bats, seals, and whales [6,15,16,17,18].
 
 Humans are the intermediate hosts for many diseases and zoonotic infections can occur in two ways: (1) isolated, dead-end infections which fail to establish and adapt as in the case of Ebola and hantaviruses (2) virus adapts and establishes in the intermediate or secondary hosts, and also sustain horizontal transmission, as in influenza [19]. 
 
 Such stable host-switch events lead to strong adaptations (ex. H5N1 and H9N2) which can resist the evolutionary pressure or the antagonistic environment posed by the novel hosts [20,21,22]. 
 
The factors that govern the virulence, pathogenicity and transmission of influenza viruses could be multifactorial including both viral as well as host factors. Host factors such as availability of the receptors, the presence of host innate immune and other cellular factors, population size and its interconnectivity all govern the sustainability of influenza transmission [23]. 
 
Influenza viral determinants undergo adaptive mutations, to expand or to limit the host range. Among the viral factors, HA glycoprotein is the primary factor determining the host range and interspecies transmission. Other viral proteins such as NP, PB2, and NS1 have also been involved in host range restriction and adaptation [24]. 
 
 For example, avian influenza polymerase possesses a limited function in human hosts and hence host-specific genetic changes have occurred to the polymerase subunits and NP during natural evolution. 
 
Though uncommon in recent times, IAV has been reported in ruminant species in the past. However, a tight host genetic bottleneck might have played a major role in the evolution, preventing the adaptive mutations necessary for the sustained transmission cycles in a novel host.
 
 Interestingly, the recently emerged influenza D, for which cattle are considered to be the primary reservoir, is widespread in cattle herds across the world. 
 
In this review, we conducted a comprehensive search of the available scientific reports/journal articles on influenza over the last century, with reference to bovine species, to understand the timeline of bovine IAV incidences with respect to human pandemics and epidemics, natural and experimental infections, seroepidemiological studies, and the role of bovine cellular and host factors in the evolution of influenza.

2. Literature Search Strategy:  Katso lähde artikkeli!

2019 Jun 17;11(6):561. doi: 10.3390/v11060561

PMCID: PMC6631717  PMID: 31213032Abstract

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