Home News An emerging or silent zoonosis: infection by the Usutu virus
An emerging or silent zoonosis: infection by the Usutu virus Print E-mail

merleThe recently reported case of asymptomatic infection by the Usutu virus in a blood donor in Germany, and the demonstration of an important circulation of the virus in birds in the wild or in captivity (hundreds or thousands of cases among blackbirds, and not hundreds of thousands as erroneously interpreted by certain media), have brought this virus to greater prominence, and we think it is necessary to review the current knowledge on the subject.

Of African origin, the Usutu virus (USUV), thus called because it was identified near a river in Swaziland of the same name, is a flavivirus strongly related to the West Nile Virus. It emerged for the first time in Europe in 2001, in Vienna, Austria, causing "massive" mortality, in particular among blackbirds (Turdus Merula). In the course of the last decade, the introduction and/or circulation of this virus were gradually detected in other European countries either from abnormal mortality among birds, mainly blackbirds (Hungary, Switzerland, Italy, Spain, Germany), or by means of serology (Austria, United Kingdom, Italy, Hungary, Czech Republic, Poland). With the benefit of hindsight, it is currently assumed that the first major wave of infection with high blackbird mortality occurred in 1996 in Tuscany. But only some one hundred cases have been confirmed positive for the USUV at this time.

In Austria: Detection of the USUV on blackbirds in 2003 (92/177), 2004 (11/24), 2005 (4/103) and 2006, positive serology in several wild migrating bird species. The quasi non-existent mortality since 2006 could indicate the development of specific immunity. In Hungary: Introduction of passive surveillance on birds since 2003: 1 USUV positive case on blackbirds in 2005, and 6 detections in 2006.

In Switzerland: a "massive" mortality of more than one hundred Passeriformes and Strigiformes in the wild or in captivity has been reported, and the USUV has been identified in certain specimens. No massive mortality has been shown since 2008.

In Italy and Spain: some rare sources of infection have been reported.

Recently in Germany: In 2011, an important mortality of blackbirds occurred in the South-East of Germany (near Weinheim) where 72 cases of USUV were confirmed. Furthermore, important mortalities of wild birds in captivity in the Mannheim and Heidelberg zoos were reported. Analyses of mosquitoes in Germany have shown that the virus could survive there in winter and thus re-infect birds the following season. These data suggest that the USUV is currently endemic in certain regions of Germany. Research to determine if this sizeable morality rate noted among birds and a USUV infection could be linked is still being conducted.

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Their genomic diversity: The strains identified in Austria, Hungary and Switzerland show 99.9 % similarity, indicating a probable common origin with expansion into neighbouring countries. On the other hand, other USUVs identified in Spain and Italy are closer to the South-African strains.

Their ecology: Given the lack of ecological data, the USUV cycle is often assimilated with that of the WNV, but some ecological differences between these two viruses seem to emerge: whereas the WNV is essentially detected in mosquitoes of the Culex genus, the USUV has also been found in mosquitoes of the Aedes albopictus genus. On the other hand, unlike the WNV, the USUV is more prevalent in urban areas. Finally, the USUV targets mainly blackbirds. The symptoms include in particular the loss of feathers on the head and neck, apathy and loss of balance.

Zoonotic risk: The first human cases in Europe were reported at the end of 2011 in two immune suppressed Italian subjects with neurological disorders. These two persons had undergone a blood transfusion. More systematic research has since been conducted on the USUV in certain European countries among blood donors. As a result, this virus has just been identified for the first time in Germany in an asymptomatic blood donor (out of 4,200 blood samples analysed).

Risk for animal health: No case of transmission and a fortiori sickness in domestic animals has been reported.

And in Belgium...

Given the dispersal pattern noted in recent years for the WNV, CODA-CERVA, in cooperation with the Royal Institute of Natural Sciences (IRSNB), instructed by the Federal Agency for the Safety of the Food Chain (FASFC), has acquired tools for the rapid diagnosis of WNV infection and the epidemiological surveillance of this infection in wild birds since 2009. These broad-spectrum diagnosis tools can also detect the Usutu virus. Any positive case can be discriminated as WNV or USUV positive in a second step. No positive case has been detected among wild birds to date.

But the following question has to be asked: Are we dealing with an emerging infection in Europe or is this virus becoming increasingly more frequent in Europe because it is studied more? Only a reinforced surveillance of the USUV in Europe in the same vein as the WNV will make it possible to gauge the scope of this viral infection in humans, its zoonotic potential, and the role of reservoir played by wild birds.