Occurrence of Usutu virus in Southern Germany
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.