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La Tour du Valat

Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Local differences in time and extent of mass mortality in Blackbirds (Turdus merula L.) after emergence of Usutu virus (USUV; Flaviviridae) in Vienna, Austria (2008)
Auteurs :	Hans M. Steiner, Auteur ; Thomas Holzer, Auteur
Type de document :	Article
Dans :	Vogelwarte (46(1) FEB 2008)
Article en page(s) :	25-35
Langues:	Allemand
Résumé :	USUV, previously known from Africa only, never before had been associated with fatal disease in vertebrates. Beginning with summer 2001, it caused mass mortality from mid-July to September in Blackbirds of Vienna and surroundings. From 2003 to 05 herd immunity developed and the proportion of USUV-positive dead Blackbirds decreased. As we have counted urban birds before and after summer 2001, we analysed population decrease of the species quantitatively. We used different methods: Steiner counted birds from September to June in established transects in six urban areas, beginning with 1993. Holzer in two districts recorded absolute numbers of birds in courts completely enclosed by buildings, in winters 2000/01 and 2003/04 respectively.For transect-counts density in January and February was considered to be typical for winter, in April for breeding time. To an unexpected scale, we found differences between subpopulations in time and extent of mortality: In six of the eight areas, with nearest distances from 0,2 to 2,9 km to river Danube, results are identical, regardless of method. In all months and subpopulations in 2002 decrease was between 58 and 94%, in one area after 2002 the species disappeared completely for four years, as late as 2007 two birds had been found again. As decrease in all months was very similar (Tab.4) it can be concluded, that the birds are strictly resident. Four of these subpopulations investigated at breeding time also, were unable for six years to increase their very low densities - in spite of herd immunity developing in Vienna after 2003. Reduced fitness of surviving individuals could explain this phenomenon.In remaining two areas, one being situated close to river Danube and to the six cited above, decrease was recorded (1) first in winter, in April in later years only, (2) decrease was not as dramatically, (3) increase to original densities occurred within one or two years.As USUV meanwhile emerged in Hungary, northern Italy and Switzerland, in case of mass mortality in summer, it is recommended to store dead birds in frozen condition or to bring it to a veterinary institution immediately.

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