Inter-species transmission dynamics
Anseriformes and Charadriiformes
were responsible for 59% and 41% of IAV introductions to Iceland,
respectively (Supplementary figure 5). Local host-specific phylodynamic
analyses within Iceland demonstrated that ducks (BF= 6.05) and
shorebirds (BF= 5.57) drive transmission as sources of IAVs to gulls,
while inter-species virus exchange occurs bi-directionally between
shorebirds and ducks (shorebirds to ducks, BF= 5.16; ducks to
shorebirds, BF= 3.27) (Figure 4a). Although geese in Iceland carry IAVs,
there is no quantifiable inter-species transmission between geese and
other species, which suggests a limited interface for geese-derived
transmission to occur in Iceland.
The proportion of time viruses are
hosted by a particular species, as derived from host-specific Markov
rewards analyses, revealed that gulls host IAVs for the greatest
proportion of time in Iceland (62.5% versus 22.8% (geese), 8.4%
(shorebirds), 6.3% (ducks)) (Supplementary Table 3b). This differs from
global analyses where Anseriformes host IAVs for the greatest proportion
of time as compared with other host orders (Supplementary Table 3c).
Additionally, Anseriformes are the most significant drivers of
transmission to other avian orders globally, including to
Charadriiformes (BF= 29,353) and Galliformes (BF= 29,353) (Supplementary
figure 6). Median diffusion rates, a measure of how quickly hosts move
viruses in space and time, were similar among shorebirds (577 km/year
(95% HPD= 36.7-1844.2)), ducks (604 km/year (95% HPD= 20.1-2768.6)),
gulls (483 km/year (95% HPD= 286-734.9)), and geese (386 km/year (95%
HPD= 194.6-640.9)) within Iceland, with some extreme outliers among
ducks and shorebirds (Figure 4b).