For over twenty years, a continuous geographical expansion of various USUV genetic lineages has been observed in Europe [1–3], accompanied by an increasing number of viral infections in hosts other than birds, i.e. humans and other mammals [3, 11–12]. Unlike other European countries, Poland does not conduct national surveillance of USUV in humans, animals, or mosquitos [11–12]. Nevertheless, given the confirmed occurrence of USUV in neighbouring countries such as Germany, Czech Republic, and Slovakia, it was hypothesized that the virus probably circulates actively also across Poland.
To verify this hypothesis, carcasses of various bird species susceptible to USUV infection were acquired for molecular testing, ensuring that the widest possible geographical area of Poland was covered. As a result, the USUV presence was confirmed in at least half of the voivodeships spanning spatially distant regions of Poland (Fig. 1), which most likely indicates the virus' widespread occurrence across the country rather than a concentration of viral infections limited only to particular areas, e.g. near the USUV-affected countries bordering Poland. Furthermore, as inferred from the phylogenetic analysis, circulating virus strains belong to the African USUV genetic lineages (Africa 2 and 3) (Fig. 2), whose prior emergence in other European countries, including France and Germany, was recorded already several years ago [1–2].
Since the virus is primarily transmitted by ornithophilic mosquitos from the genus Culex [13], multiple USUV detections at geographically distant sample collection spots in the country confirm the common and abundant presence of the virus’ competent vectors in Poland, as also recently reviewed [14]. Furthermore, the latest report demonstrating the ability of female Culex torrentium mosquitos to act as winter reservoirs for USUV in Poland [4], clearly highlights the urgent need to establish national surveillance activities toward arbovirus’ vectors.
In this study, the majority of USUV-infected species belonged to migratory birds that usually undertake seasonal long-distance journeys to non-breeding areas southward. The specified time points of accidental findings of the USUV-positive carcasses in Poland correspond to the fall migration period of almost all examined bird species (August-November), which in this respect does not exclude that some of the reported USUV infections might have occurred earlier abroad, i.e. before the flight route through Poland, possibly treated by migratory birds only as a short-term stopover site. Considering that the susceptibility and transmission potential of the northern house mosquitos to USUV has recently been confirmed in Sweden [15], such an epidemiological scenario does not seem entirely unlikely. In particular, it seems to be possible in the case of passerine birds found on the northern coast of Poland, which represents geographically the first land immediately after crossing the Baltic Sea.
Since most USUV-infected birds were found already dead, and we lack information on the likely clinical course of the viral infection, no conclusions can be drawn regarding differences in the pathogenicity of USUV among various bird species. Even in captive bar-headed geese showing neurological signs, the presence of USUV in their brains cannot be conclusively linked as the sole cause of death, because possible coinfections with other viral pathogens or environmental factors may have worsened the clinical manifestations. Nevertheless, lethal infection with USUV of African lineage has also been reported in anseriform birds [16].
To summarize, future studies conducted in Poland should focus on serological and molecular screening of USUV-susceptible animals and arbovirus’ vector surveillance, with particular emphasis on pursuing whole-genome sequencing of circulating USUV strains. In this manner, the countrywide tracking of the virus’ spread may contribute to understanding the global USUV epidemiology.