Abstract
BackgroundThe circulation of West Nile virus and Usutu virus was detected in the Emilia-Romagna region in 2008 and 2009. To evaluate the extent of circulation of both viruses, environmental surveillance, based on bird and mosquito testing, was conducted in 2008 and gradually improved over the years.MethodsIn February–March 2009–2011, 5,993 hibernating mosquitoes were manually sampled, out of which 80.1% were Culex pipiens; none tested positive for the viruses. From 2008 to 2011, 946,213 mosquitoes, sampled between May and October, were tested; 86.5% were Cx. pipiens. West Nile virus was detected in 32 Cx. pipiens pools, and Usutu virus was detected in 229 mosquito pools (217 Cx. pipiens, 10 Aedes albopictus, one Anopheles maculipennis s.l., and one Aedes caspius). From 2009 to 2011, of 4,546 birds collected, 42 tested positive for West Nile virus and 48 for Usutu virus. West Nile virus and Usutu virus showed different patterns of activity during the 2008–2011 surveillance period. West Nile virus was detected in 2008, 2009, and 2010, but not in 2011. Usutu virus, however, was continuously active throughout 2009, 2010, and 2011.ConclusionsThe data strongly suggest that both viruses overwinter in the surveyed area rather than being continually reintroduced every season. The lack of hibernating mosquitoes testing positive for the viruses and the presence of positive birds sampled early in the season support the hypothesis that the viruses overwinter in birds rather than in mosquitoes. Herd immunity in key bird species could explain the decline of West Nile virus observed in 2011, while the persistence of Usutu virus may be explained by not yet identified reservoirs. Reported results are comparable with a peri-Mediterranean circulation of the West Nile virus lineage 1 related strain, which became undetectable in the environment after two to three years of obvious circulation.
Highlights
West Nile virus (WNV) was first reported in the EmiliaRomagna region in 2008 [1], and evidence of circulation was obtained until 2010 [2]
Usutu virus (USUV) has been detected in the same area since 2009 [3], [5], and biomolecular data demonstrate that the virus has overwintered in Emilia-Romagna [2]
Wild birds are the most significant reservoir species for WNV, with different viremia duration and intensity according to the species [13], and a different role in virus transmission according to the species abundance
Summary
West Nile virus (WNV) was first reported in the EmiliaRomagna region in 2008 [1], and evidence of circulation was obtained until 2010 [2]. Usutu virus (USUV) has been detected in the same area since 2009 [3], [5], and biomolecular data demonstrate that the virus has overwintered in Emilia-Romagna [2] This flavivirus was first reported in Europe in Vienna in 2001 [6]; from there, the virus was reported in neighboring countries and has become endemic [7]. Similar to other arboviruses (vector-borne viruses), WNV and USUV exploit complex biotic and trophic interactions between vectors and reservoirs, to persist in a particular area These interactions are influenced by numerous ecological and environmental factors that can confer unique characteristics on the virus cycle in a particular ecosystem, including differential ways of transmission and permanence. To evaluate the extent of circulation of both viruses, environmental surveillance, based on bird and mosquito testing, was conducted in 2008 and gradually improved over the years
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