Introduction
Understanding the ecologic interactions among wild birds that facilitate
the maintenance, reassortment, and dispersal of influenza A viruses
(IAV) is key to forecasting global spread (Hall et al., 2014; Hill et
al., 2017; Hoye et al., 2011; Lang et al., 2016; The Global Consortium
for H5N8 and Related Influenza Viruses, 2016).
IAVs circulate endemically among
wild aquatic migratory bird populations, includingAnseriformes (waterfowl,
including ducks, geese, and swans) and Charadriiformes (seabirds,
including gulls, shorebirds, terns, and auks), many of which migrate
between breeding ranges in arctic and subarctic regions (ASRs) to
wintering grounds in southern latitudes (Dusek et al., 2014; G. A.
Gudmundsson, 1993; L. A. Gudmundsson, 1992; Hall et al., 2014; Hall et
al., 2015; Hall et al., 2013; Hénaux, Parmley, Soos, & Samuel, 2013;
Lang et al., 2016; Runstadler, Hill, Hussein, Puryear, & Keogh, 2013;
van Dijk et al., 2014).
During
migration, birds use staging and breeding grounds within the circumpolar
perimeter of the arctic circle, where populations from different
geographic origins aggregate at high densities, enabling direct and
environmental fecal-oral transmission of IAVs within and between species
(Causey & Edwards, 2008; R. Chen
& Holmes, 2009; Hill et al., 2016; Ito et al., 1995). The mass
migration of birds to ASRs each spring is considered an important
mechanism for inter-species transmission and global spread of IAVs,
although empirical data are often lacking from these remote, high
latitude regions where human populations are sparse (Causey & Edwards,
2008; Hoye, Munster, Nishiura, Klaassen, & Fouchier, 2010; Olsen et
al., 2006).
Previous studies support that migratory connectivity throughout ASRs
dictate the ecology and evolution of IAVs between hemispheres (Webster,
Marra, Haig, Bensch, & Holmes, 2002).
Northern latitudes are predicted
to have a high risk of IAV outbreaks among wild birds due to the
increased density of disparate bird populations during spring migration
and the persistence of IAVs in low temperature climate regimes (Herrick,
Huettmann, & Lindgren, 2013; A. M. Ramey et al., 2020; Runstadler et
al., 2013). Host movement patterns within ASRs also determine how
viruses disperse within the arctic circumpolar region and onward
transport to southern latitudes globally. During post-breeding
migration, wild birds such as shorebirds, gulls, skuas, and terns
prioritize short distance flights around the perimeter of the greater
polar arctic circle to avoid navigation challenges (Alerstam et al.,
2007), exemplifying the ecologic importance of stopover and staging
locations for migratory seabirds within ASRs. This great-circle
orientation of seabird migration through ASRs is likely to have
important but currently under-studied impacts on the directionality and
rate of movement of IAVs in space and time across the globe.
ASRs are also disproportionately
affected by global climate change, where observed and predicted annual
mean warming is more than twice the global mean (Arctic Monitoring and
Assessment Programme (AMAP), 2019). Alterations to climate regimes may
impact the distribution of wild birds and IAVs throughout the
circumpolar north due to geographic shifts in habitat use and trophic
food webs, changes in timing of migrations and breeding range locations,
and declines in reproductive success (Mckinney et al., 2015). Increased
surveillance in northern latitudes will enable researchers to gather
baseline data with which to measure future shifts in IAV dynamics
because of the rapidly changing arctic climate (Gilbert, Slingenbergh,
& Xiao, 2008; Jensen et al., 2008).
The migratory connectivity of ASRs with the rest of the globe is
particularly relevant given the expanding geographic range of highly
pathogenic avian influenza (HPAI) virus subtype H5Nx of clade 2.3.4.4
which was first detected outside of China in 2014 (Antigua, Choi, Baek,
& Song, 2019; Lee, Bertran, Kwon, & Swayne, 2017). HPAI clade 2.3.4.4
viruses have evolved by reassorting with low pathogenic viruses
throughout Eurasia and North America, and evidence of intercontinental
dissemination events through ASRs have been documented (Hill et al.,
2017; Lee et al., 2017). In 2014,
HPAI H5N8 of clade 2.3.4.4 was introduced by wild migratory birds from
East Asia to North America via the trans-Beringian route through Alaska,
from which H5N2 and H5N1 reassortant viruses emerged (Hill et al., 2017;
Lee et al., 2015; A. Ramey et al., 2016). H5N2 reassortant viruses
ultimately spread among domestic poultry throughout 15 states in the US,
which precipitated the culling of approximately 49 million chickens and
turkeys (Hill et al., 2017; Lee et al., 2016). Future introduction
events are anticipated in view of the large number of clade 2.3.4.4
outbreaks in areas adjacent to North America, although the exact route
of introduction remains elusive (Ramey et al., 2021).
Transcontinental virus exchange is often detected in oceanic and coastal
regions at continental margins along migratory flyways (Alerstam et al.,
2007; Huang et al., 2014; Pearce et al., 2009). In the Pacific Rim,
Alaska has emerged as an ecologically important ‘hot spot’ through which
IAV carrier species migrate, connecting East Asia and North America for
virus exchange (Hill et al., 2017; Huang et al., 2014; Lee et al., 2015;
A. Ramey et al., 2015; A. Ramey et al., 2016).
Though significant westward virus
migration from Asia to Europe has been documented, far less is known
about the onward viral connectivity of mainland Europe with staging
locations within the North Atlantic and North America (Kilpatrick et
al., 2006; Olsen et al., 2006). Addressing how IAV dynamics may differ
between the North Atlantic and Pacific coastal regions relative to
global patterns of viral flow is critical for developing a well-rounded
understanding of influenza ecology. The current knowledge gap highlights
the potential for undetected virus spread between Europe and North
America, especially for HPAI clade 2.3.4.4 and other highly
consequential influenza viruses.
Iceland - at the confluence of the North Atlantic and Arctic oceans –
provides a unique study system for characterizing inter-hemispheric
viral flow between eastern and western hemispheres. The island is
situated within the East Atlantic flyway between Europe and North
America where resident and long-distance migratory birds interact
(Butler, 2012; Dusek et al., 2014; Hall et al., 2014; Hall et al., 2015;
Newton, 2008). Viruses of North
American and Eurasian ancestral lineages, as well as reassortant strains
between these lineages, have been isolated in Iceland, demonstrating the
North Atlantic as a potential corridor for the inter-hemispheric
movement of IAVs (Dusek et al., 2014).
Previous research has described
(a) high seroprevalence of IAVs among North Atlantic sea ducks (Hall et
al., 2015) and (b) genes from HPAI virus lineages in low pathogenic IAVs
among migrant birds in Iceland, revealing the importance of surveillance
in the North Atlantic for viruses with pandemic potential in humans
(Hall et al., 2014). There is very little evidence, however, describing
the directionality of virus movement and avian host transmission
dynamics within Iceland and throughout the North Atlantic region, which
may influence the spread of IAV globally (Dusek et al., 2014; Hall et
al., 2014; Hall et al., 2015; Kilpatrick et al., 2006; The Global
Consortium for H5N8 and Related Influenza Viruses, 2016).
We hypothesize that IAVs are transported via wild bird migration between
Europe and North America through Iceland and that Iceland plays an
important role in the inter-hemispheric transmission and reassortment of
IAVs in the North Atlantic, particularly with virus moving into North
America. This study examines (i)
viral connectivity of Iceland to other global regions, (ii)
inter-species transmission dynamics among wild aquatic birds in this
system, (iii) viral diffusion by wild aquatic birds within Iceland
compared to other global regions, and (iv) whether local reassortment
patterns may result in novel or HPAI reassortants with the potential for
inter-hemispheric transport, warranting public health concern.
These data reveal patterns of
inter-hemispheric virus movement and inform surveillance efforts related
to seasonal and emergent IAVs, in northern latitudes, particularly
throughout the North Atlantic region.
Materials and
Methods Field sample collection
From May 2010 through February 2018, we obtained IAV isolates from
various species of seabirds, shorebirds, and waterfowl as well as
environmental sampling of avian fecal material from locations throughout
Iceland (Dusek et al., 2022). Live sampled birds were captured using a
18m x 12m cannon-propelled capture net, noose pole, or hand capture.
Birds found dead or moribund were also sampled. Hunter-harvested
waterfowl and fisheries-bycatch seabirds were sampled as available. All
birds were identified to species and, for live birds, the majority were
individually marked with metal bands. Age characteristics were
determined and age was documented for each bird according to the
following schemes adapted from
U.S. Geological Survey (USGS) year classification codes: hatched in same
calendar year as sampling (1CY), hatched previous calendar year (2CY),
hatched previous calendar year or older exact age unknown (2CY+; i.e. at
least 2CY but could be any age older then 2CY as well), hatched three
calendar years prior to sampling (3CY), hatched four calendar years
prior to sampling (4CY), hatched more than four calendar years prior to
sampling (4CY+), or unknown if age could not be determined (U) (Olsen
KM, 2004; Prater, Marchant, & Vuorinen, 1977; USGS, 2020). Due to
species specific differences, not all aging categories could be applied
to all species sampled. All live birds were immediately released
following completion of sampling.
To sample for IAV, a single polyester-tipped swab was used to swab the
cloaca only (2010-2013) or to first swab the oral cavity then the cloaca
(2014-2017). Opportunistic environmental sampling of fecal material was
also conducted using a direct swabbing method (2018). Each swab sample
was immediately placed in individual cryovials containing 1.25 ml viral
transport media (Docherty & Slota, 1988). Vials were held on ice for up
to 5 hours prior to being stored in liquid nitrogen or liquid nitrogen
vapor. Samples were shipped on dry ice from Iceland to Madison,
Wisconsin, USA by private courier with dry ice replenishment during
shipping. Once received in the laboratory, samples were stored at
-80o C until analysis.