Abstract
In recent decades major declines in urban house sparrow (Passer domesticus) populations have been observed in north-western European cities, whereas suburban and rural house sparrow populations have remained relatively stable or are recovering from previous declines. Differential exposure to avian pathogens known to cause epidemics in house sparrows may in part explain this spatial pattern of declines. Here we investigate the potential effect of urbanization on the development of a bacterial pathogen reservoir in free-ranging house sparrows. This was achieved by comparing the prevalence of Salmonella enterica subspecies enterica serotype Typhimurium in 364 apparently healthy house sparrows captured in urban, suburban and rural regions across Flanders, Belgium between September 2013 and March 2014. In addition 12 dead birds, received from bird rescue centers, were necropsied. The apparent absence of Salmonella Typhimurium in fecal samples of healthy birds, and the identification of only one house sparrow seropositive for Salmonella spp., suggests that during the winter of 2013–2014 these birds did not represent any considerable Salmonella Typhimurium reservoir in Belgium and thus may be considered naïve hosts, susceptible to clinical infection. This susceptibility is demonstrated by the isolation of two different Salmonella Typhimurium strains from two of the deceased house sparrows: one DT99, typically associated with disease in pigeons, and one DT195, previously associated with a passerine decline. The apparent absence (prevalence: <1.3%) of a reservoir in healthy house sparrows and the association of infection with clinical disease suggests that the impact of Salmonella Typhimurium on house sparrows is largely driven by the risk of exogenous exposure to pathogenic Salmonella Typhimurium strains. However, no inference could be made on a causal relationship between Salmonella infection and the observed house sparrow population declines.
Highlights
Salmonella enterica subspecies enterica serotype Typhimurium has the potential to cause disease outbreaks in Passeriformes
None of which were conducted in Belgium, assessed the presence of pathogenic bacteria in moribund birds, or dead birds submitted for necropsy, whether or not related to epidemics in wild birds [1;2;5;11;15]
The house sparrows consisted of 42.6% female birds, 57.1% male birds and 1 undefined young house sparrow, which belonged to urban house sparrow populations (28.3%), suburban populations (21.15%), and to rural populations (50.55%)
Summary
Salmonella enterica subspecies enterica serotype Typhimurium has the potential to cause disease outbreaks in Passeriformes. Some phage types of Salmonella Typhimurium are considered host adapted, DT2 and DT99 in pigeons (Columba livia) [12], DT40 and DT56(v) in passerines [13], the latter two phage types have been isolated from captive birds and mammals and have been linked to disease in humans [1;3;14,15,16] In this perspective, most of the studies on prevalence and epidemiology of Salmonella spp. in free-living birds have been performed in the surroundings of farms in order to evaluate food safety and human health risks [7;9;17], or have been related to disease in animals or humans [3;14;15]. Since host-adapted Salmonella enterica strains could potentially reduce the reproduction success in their respective reservoir hosts [21;22], it is important to understand to what extent passerines are long term carriers of Salmonella Typhimurium, as birds in general have already been appointed as potential reservoirs for Salmonella enterica subspecies enterica [8;23;24]
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