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
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