Usutu virus (USUV) appeared in Europe for the first time in 1996, as confirmed by retrospective Studies performed in Italy. Further virus detections were recorded in Austria (2001), Hungary (2005), Switzerland (2006), and Italy (2009). In 2010, the virus was isolated for the first time in Germany in a pool of mosquitoes. The subsequent year was marked by mass-mortality events in the avifauna in the North of the Upper Rhine valley and the neighbouring regions of the Pfalz and the Neckar valley. Since then repeated, regionally confined mortality events have particularly affected local blackbird populations but infections in zoo birds, especially in owls kept in aviaries, are also very common.
Through the close cooperation with the veterinary state investigation offices, avian clinics of the veterinary medicine faculties, the Bernhard Nocht Institute for Tropical Medicine (BNITM), the German Mosquito Control Association (KABS), the Nature and Biodiversity Conservation Union (NABU), several avian clinics/practices, bird sanctuaries, and numerous honorary ornithologists it was possible to detect USUV in 230 wild and captive birds in the Southwest of Germany from 2011 to 2015. An upsurge in the number of USUV cases was observed in 2016 in Germany and neighbouring countries to the West, followed by a prominent northward spread in 2017. Since 2018, USUV circulates nationwide, covering nearly all federal states. Thus, starting in the Rhine-Main region in 2011, the virus initially spread in a northerly direction along the Rhine valley and gradually radiated throughout the entire country in the subsequent years. Blackbirds in particular have experienced regional mass die-offs, but other songbird species and owls have also been severely affected by this disease. The extent of bird mortalities in 2018 by far exceeded that of the previous years. Similarly, in 2019, regionally confined, cumulative diseases cases were detected in birds and later confirmed in the laboratory. For 2020, a decline in the number of confirmed cases is becoming apparent. However, conclusive number sets and distribution charts for both 2019 and 2020 are not yet available.
Origin and significance
USUV has its origin in Africa (named after a river in Swaziland) and is transmitted by mosquitoes. The main hosts for the virus are wild birds, which usually do not become ill. However, highly susceptible bird species such as black birds can develop an USUV-associated disease. These birds become clinically apparent through apathy and central nervous system disorders including a staggering gait or head/neck turning (opisthotonus), possibly leading to mass-mortality events of certain bird species.
So far only a few USUV transmissions to humans (zoonotic pathogen) have occurred in endemic countries (i.e. where the virus is permanently present), especially in patients with a weakened immune system. Until today, Germany has not reported any clinical USUV cases in humans, not even in immunocompromised patients. However, recent studies have detected the virus in healthy blood donors in Germany, Austria, France, and Italy.
Occurrence of Usutu virus in Germany 2012, 27 July 2012
On July 24, 2012 the National Reference Laboratory for West Nile virus at the Friedrich-Loeffler-Institut (FLI) confirmed the detection of Usutu virus (USUV) in two dead blackbirds from Rhineland-Palatinate. The sample was sent in by the State Diagnostic Agency Koblenz. Some days earlier, the Bernhard-Nocht-Institute (BNI) and the Action group for mosquito control (KABS) had already reported detection of the virus in several blackbirds. BNI und FLI closely cooperate in the surveillance of these Usutu virus cases.
Independently, both partners have been conducting monitoring activities for zoonotic pathogens (e.g. West Nile virus): in wild birds (Seidowski et al., 2010; Ziegler et al., 2010) and in mosquitoes (Jöst et. al. 2011).
We ask the State Veterinary Agencies to send samples of dead birds without delay to the Institute of Novel and Emerging Infectious Diseases of the FLI (address see below). If requested, the FLI will assist in the establishment of USUV diagnostics at the diagnostic agencies (quantitative real-time RT-PCR).
Occurrence of Usutu virus in Southern Germany, 16. September 2011
Usutu virus (USUV) was identified in a blackbird found dead in Southern Germany (Southern Hesse). The virus was detected on September 13, 2011 at the Bernhard Nocht Institute for Tropical Medicine (BNI) in Hamburg. On September 14, 2011 USUV was also detected by the Friedrich-Loeffler-Institut in 4 dead blackbirds from the Mannheim area. Investigations to clarify whether this infection is associated with the increased mortality of blackbirds observed in this region are ongoing. The detection of USUV in blackbirds is not completely surprising. Tropical USUV was first isolated in Germany last year in mosquitoes from Baden-Württemberg by scientists of the German Mosquito Control Association (KABS) and the BNI in Hamburg (Jöst et al. 2011).
USUV has its origin in Africa (named after a river in Swaziland). Main hosts for the virus are wild birds which as a rule do not become ill. It is however known that some wild bird species, e.g. blackbirds, are also very susceptible to the virus. Clinically, these animals often show apathy and central nervous disorders such as staggered movements or unnatural turning movement of the head. The mortality may be considerable. So far, the virus is transmitted in Europe by mosquitoes (Culicidae).
USUV is closely related with the West Nile virus and the Japanese Encephalitis virus within the family of Flaviviridae.
Outside Africa, USUV was first detected in 2001 in Austria, where it caused a die-off of birds, particularly blackbirds, in the Eastern part of the country between 2001 and 2003. Since 2003, the number of infected wild birds has decreased, at present only sporadic cases are reported. Antibodies against the virus could still be detected in wild birds from 2005 to 2007. Furthermore, USUV was found in Hungary, Switzerland, Italy, and Spain. In the two latter countries no increased mortalities were observed. However, infection was also detected in sparrows, titmice and owls.
Antibodies against USUV have been detected in wild birds from numerous European countries, e.g. Hungary, Italy, Great Britain, Poland, and the Czech Republic, which indicates a widespread occurrence of the pathogen in Central Europe.
In Germany, too, the Robert Koch-Institute detected low antibody titres against USUV in 3 out of 3,300 investigated wild birds (without exception migratory birds) between 2002 and 2005 (Linke et al. 2007). Another investigation of wild birds (> 3,000 samples) focussing on West Nile virus is being carried out at the FLI since 2007. To identify cross-reactions, the FLI also carries out tests for USUV-specific antibodies, so far with a negative result.
Although it is assumed that USUV has a zoonotic potential, human infections have so far been very rare. One case was observed in Africa (in 1982 in Dakar) and two cases were reported in Italy (2009) in immunosuppressed elderly patients. In the latter, the infection was accompanied by fever, headaches and skin rashes; in principle, an inflammation of the brain (encephalitis) may occur. At present, there is no evidence in Germany for a transmission of the virus to humans (more information on human medicine can be found on the website of the Bernhard-Nocht-Institute under www.bnitm.de). For hygienic reasons, it is generally recommended not to touch dead birds with bare hands.
The further development or a possible spread of USUV among wild birds in Germany is monitored. Investigations carried out so far in Central Europe show a spread rate of approx. 50 kilometres per year.
In the strongly affected areas, targeted surveillance programmes in wild birds (animals found dead) and mosquitoes will provide information on the spread of the virus.
These investigations are also necessary with regard to a possible introduction of West Nile virus which was first observed in Austria after the occurrence of USUV. West Nile virus is also transmitted by mosquitoes and occurs in wild birds and horses. Humans can also become infected. Recently, human infections have been registered in Northern Greece, Italy, Romania, and Russia.