Isolation and identification of Salmonella spp. from red foxes (Vulpes vulpes) and badgers (Meles meles) in northern Italy

Background Salmonella spp. have been isolated from a wide range of wild animals. Opportunistic wild carnivores such as red foxes (Vulpes vulpes) and badgers (Meles meles) may act as environmental indicators or as potential sources of salmonellosis in humans. The present study characterizes Salmonella spp. isolated from the intestinal contents of hunted or dead red foxes (n = 509) and badgers (n = 17) in northern Italy. Findings Thirty-one strains of Salmonella belonging to 3 Salmonella enterica subspecies were isolated. Fourteen different serovars of S. enterica subsp. enterica were identified, among which were serovars often associated with human illness. Conclusions Wild opportunistic predators can influence the probability of infection of both domestic animals and humans through active shedding of the pathogen to the environment. The epidemiological role of wild carnivores in the spread of salmonellosis needs to be further studied.


Findings
Salmonellae are enteric bacteria capable of infecting humans and both domestic and wild animals [1]. The Center for Disease Control and Prevention and the European Food Safety Authority reports that salmonellosis is the most important foodborne infection in humans in industrialized countries [2]. Although Salmonella infections in humans are generally transmitted through food of animal origin, such as eggs, chicken, pork or beef meat [3], the role of wildlife in the maintenance of salmonellosis is of increasing interest. Salmonella spp. can be shed in faeces from healthy animals for a long period of time and can be isolated at virtually every step of the game meat chain [4]. In addition, wildlife can be involved in human salmonellosis taking part in the ecology of these bacteria and thereby contributing to the persistence of bacteria in the environment [5].
In the last two decades, the populations of badgers (Meles meles) and, in particular, red foxes (Vulpes vulpes) have increased and the animals have adapted to peri-urban and urban environments across Europe. Ecological factors have been attributed to the increase in fox population, in particular the abundant availability of anthropogenic food sources [6,7]. Red foxes and badgers can be considered as indicators and spreaders of zoonotic infections due to their feeding habits [8]. Indeed, they have an omnivorous diet that includes prey and plants and they also scavenge around human waste disposal sites and dustbins. Thus, they are exposed to many potential sources of Salmonella [9], considering also the fact that Salmonella spp. can survive for long periods in the soil, water, and on a variety of surfaces [10,11]. Presence of Salmonella spp. in the intestinal contents of red foxes and badgers may reflect true intestinal infections and potential excretion of bacteria thus suggesting the role of these species in the ecology of Salmonella spp. [12].
Plenty of literature on the prevalence of Salmonella spp. in humans, livestock and more recently, wild ungulates [13][14][15] is available, while on the contrary little is known and scarce data are available about the natural occurrence and spread of this pathogen in small carnivores.
In order to investigate the prevalence of Salmonella infections in the red fox and badger populations in northern Italy, a total of 509 hunted or found dead (including road kills) red foxes and 17 found dead (including road kills) badgers were examined. The sampling was carried out within a health monitoring program on free-ranging animals in the Lombardy Region in northern Italy from 2009 to 2010 and included both rural and peri-urban habitats. Out of the 509 foxes examined, age was reported in114, while the gender was recorded in 182 (98 adults and 16 un-weaned cubs; 100 males and 82 females).
No lesions related to salmonellosis as expected were found at necropsy neither in foxes nor in badgers as salmonellosis is often subclinical [12].
Contents of the large intestine were sampled and cultured according to the International Organization for  results of the antigen determination were used for the final serological characterization using the White -Kauffmann -Le Minos scheme.
The prevalence of enteric Salmonella infection was calculated for each species. The exact Fisher's test was performed to evaluate the association between sex, age and the isolation of bacteria in foxes. All confidence intervals (CI) were implemented using the Clopper-Pearson method setting 1-α = 0.95 as the confidence level.
The observed prevalence of Salmonella infections in red foxes and badgers were unexpectedly low considering their omnivore feeding behaviour and the high prevalence found in wild boars living in the same areas (24.82%) [15]. Salmonella spp. have also been found in prevalences ranging from 5.5% to 24.82% in wild boars in other areas of Europe [1,4,15,18]. The difference in prevalence observed between foxes and wild boars may be explained by the different social behavior of these species. Indeed, the effect of animal social structure at the local scale is recognized to influence the transmission of infections between individuals [19]. Since Salmonella spp. are transmitted via a fecal-oral route, the maintenance of the infection in wild boars may be favored by large social groups that amplified the chance of contact between different individuals [20]. On the contrary foxes are characterized by small familiar groups defending their territories thus making contact between individuals less intimate.
Infection of red foxes with S. Typhimurium has been described as the result of ingestion of Salmonella infected, sick or dead small passerines during winter [12]. Moreover, carriage of S. Enteritidis by foxes near poultry farms has been related to their predatory and scavenging habits [21].
Five serovars of Salmonella subsp. enterica (i.e. S. Enteritidis, S. Typhimurium, S. Infantis, S. Derby, S. Mbandaka) represented 45.1% of the isolates. It is noteworthy that these serovars are among the 10 serovars responsible for most human cases of salmonellosis in Europe during 2010 [2]. This finding may reflect that foxes have extended their  habitats into urban and peri-urban areas and therefore may be exposed to the same serovars as humans [6].
In conclusion, Salmonella spp. were isolated from the large intestine of 5.70% of the red foxes and 11.76% of the badgers in the Italian Lombardy region. Serovars representing 45.1% of the isolates were among the 10 serovars responsible for most cases of human salmonellosis in Europe in 2010. Consequently, wild predators, through an active shedding of the pathogen in the environment, can indirectly increase the probability of infection of both domestic animals and humans.