Present address M.N. Skov, Research Unit for Clinical Microbiology, University of Southern Denmark, J.B. Winsløws Vej 21, DK-5000 Odense C, Denmark.
Transmission of Salmonella between wildlife and meat-production animals in Denmark
Article first published online: 15 OCT 2008
© 2008 The Authors. Journal compilation © 2008 The Society for Applied Microbiology
Journal of Applied Microbiology
Volume 105, Issue 5, pages 1558–1568, November 2008
How to Cite
Skov, M.N., Madsen, J.J., Rahbek, C., Lodal, J., Jespersen, J.B., Jørgensen, J.C., Dietz, H.H., Chriél, M. and Baggesen, D.L. (2008), Transmission of Salmonella between wildlife and meat-production animals in Denmark. Journal of Applied Microbiology, 105: 1558–1568. doi: 10.1111/j.1365-2672.2008.03914.x
- Issue published online: 15 OCT 2008
- Article first published online: 15 OCT 2008
- 2008/0018: received 5 January 2008, revised and accepted: 17 April 2008
- risk factors;
- Top of page
- Materials and methods
- Statistical analyses
Aims: To investigate the transmission of Salmonella spp. between production animals (pigs and cattle) and wildlife on production animal farms in Denmark.
Methods and Results: In the winter and summer of 2001 and 2002, 3622 samples were collected from Salmonella-infected and noninfected herds of pigs and cattle and surrounding wildlife. Salmonella was detected in wildlife on farms carrying Salmonella-positive production animals and only during the periods when Salmonella was detected in the production animals. The presence of Salmonella Typhimurium in wild birds significantly correlated to their migration pattern and food preference.
Conclusions: Salmonella was transmitted from infected herds of production animals (cattle and pigs) to wildlife that lived amongst or in close proximity to them.
Significance and Impact of the Study: Salmonella in animal food products is associated with the occurrence of Salmonella in primary animal production. Strategies to control the introduction and spread of infection should include wildlife management, as the nearby wildlife may act as reservoirs for Salmonella spp. and/or may be passive carriers of the bacteria.
- Top of page
- Materials and methods
- Statistical analyses
Many investigations have shown that contaminated food of animal origin is the main source of human infections (Bezanson et al. 1983; Holmberg et al. 1984; Tacket et al. 1985; Humphrey 2000). Salmonella is one of the most prevalent causes of human gastroenteritis in industrialized countries. Since the presence of Salmonella in animal food products is associated with the occurrence of Salmonella in primary animal production, strategies to control the introduction and spread of infection within herds are considered important (Wegener et al. 2003).
Epidemiological investigations have indicated that both trade of subclinically infected animals and the use of contaminated feed are the main risk factors for introducing Salmonella infections to herds (Stege et al. 1997; Stärk et al. 2002). However, Salmonella has a broad range of animal hosts and has been isolated from several wildlife species such as birds (Refsum et al. 2002a, 2002b), rodents (Healing 1991), hedgehogs (Nauerby et al. 2000; Handeland et al. 2002), and insects (Olsen and Hammack 2000; Mian et al. 2002). Transmission of Salmonella from infected wild birds to the environment, as well as to other animal species, has been reported (Kapperud et al. 1998). In some countries a relatively high prevalence of Salmonella in wildlife has been reported (Henzler and Opitz 1992; Refsum et al. 2002a) and it has been documented that wildlife may contribute to the horizontal transmission of Salmonella (Tauni and Österlund 2000; Warnick et al. 2001; Liebana et al. 2003).
In Denmark, however, routine wildlife disease surveillance has revealed a very low prevalence of Salmonella in wildlife (Dietz et al. 1998) relative to levels reported from other countries (Healing 1991; Hubalek et al. 1995; Hudson et al. 2000; Bailey et al. 2001; Millan et al. 2004). This suggests that there are fewer sources of Salmonella transmission in Denmark. Thus, evaluating the transmission of Salmonella between wildlife and production animals in Denmark is relatively more straightforward because there are fewer routes of transmission to consider.
Other investigations have described the presence of Salmonella in wild birds, although most of these were screening studies (Kapperud and Rosef 1983; Handeland et al. 2002; Refsum et al. 2002a,b) that did not include additional data, i.e., contact of the birds with farm animals and their Salmonella status. Although some investigators have studied the presence of Salmonella in wild birds living near contaminated locations, such as refuse dumps (Cizek et al. 1994) or at Salmonella-infected animal production farms (Cizek et al. 1994; Kirk et al. 2002), they did not include longitudinal sampling. Thus, factors such as season, direction of transmission, and incidence of clinical disease among production animals have not been thoroughly evaluated.
The aim of the present study was to describe and analyse the transmission between production animals and wildlife living at or near Salmonella-infected production animal herds.
Materials and methods
- Top of page
- Materials and methods
- Statistical analyses
We studied the incidence of Salmonella in wildlife living at or near Salmonella-infected, as well as noninfected, cattle and pig farms in Denmark. A 2-year longitudinal study was conducted during the winter and summer months of 2001 and 2002. Samples were obtained from both cattle herds with clinical outbreaks and pig herds with persistent infections. In addition, samples were obtained from wildlife at locations geographically distant from production animals, such as urban areas, grain-producing farms, and from areas with a large concentration of migratory songbirds. The isolates from different animal species and sampling sites were characterized and compared using both serotyping and phage-typing techniques, as well as pulsed-field gel electrophoresis (PFGE).
Study design and selection of farms
A total of 13 production herds, five pig and eight cattle, were included in the study. The herds were geographically located within a radius of approximately 90 km in the central part of Denmark (west Zealand 2, Fun 1, southeast Jutland 3, and east Jutland 7). Each of the 13 herds was visited either once (two of the herds), twice (six herds), three times (two herds), or four times (three herds) (Table 1). The visitation frequency for a particular herd was determined by the influence of intervention measures against infection in the cattle herds, as well as by limitations in access to some herds due to a concurrent outbreak of Newcastle disease (Anon 2003). Eight controls were included: four farms without animal production, two houses in village areas, and two locations with a high concentration of migratory songbirds.
The selection of pig herds was based on data from the Danish Salmonella Surveillance Program, which encompasses serosurveillance of slaughter pigs (Nielsen et al. 2001; Wegener et al. 2003). The infected herds had been afflicted with a subclinical Salmonella infection for at least one year prior to the study. On the other hand, the noninfected herds had not been diagnosed seropositive for Salmonella for at least 2 years prior to the study. Selection of cattle herds was based on the diagnosis of acute clinical salmonellosis, while the selection of noninfected herds was based on information from local veterinarians. Prior to inclusion in the study, the Salmonella status of each herd was verified by bacteriological examination of 20 pooled, fecal samples. Each pool consisted of five replicate samples of 5 g of fecal material collected at different places in the stables. Control locations were selected by identifying habitats comparable to the other farms under investigation and ensuring that no animal production (i.e. pig, cattle or poultry) had taken place at that location during the previous year.
Collection of samples
Four sampling rounds were carried out in approximately 6-month intervals over 2 years (i.e. January to February and August to September). At each round, two farms at a time were sampled for 1 week. During each round, samples were collected from production animals, pets, birds, rodents and insects living at or near the farms. Controls were sampled only once. On the same day they were collected, the samples were transported at ambient temperature to the laboratory, refrigerated overnight, and cultured the following day.
Production and pet animals
Pooled fecal samples were collected each round from production animals and other domestic animals (if present) to determine their Salmonella status. Fecal samples were taken from as many places in the stable as possible. A sample was also taken from the slurry tank at the farm when possible.
Rats, mice and voles were trapped during the first sampling round at all farms. The highest priority was given to commensal rodents that commonly reside inside buildings. In general, 20 traps for rats and 50 traps for mice were set, both inside and outside, at each farm. A single-capture wire-mesh trap was used for rats and Longworth small mammal live traps were used for mice and voles. Both types of traps were made of metal. Trapped animals were killed with CO2 and placed in a plastic bag.
After the first round, the laborious sampling procedure was modified for subsequent rounds. Animals were trapped only on selected farms or were trapped occasionally by hand during the inspection of buildings. During some sampling periods, rodent feces were collected inside and around buildings, especially in places where traps were not set and also at control farms. Fecal samples were classified as originating from rats or mice based on morphology and size before referral to the laboratory.
Birds were caught by licensed ringers with mist-nets, Manson traps (a plastic-coated wire-metal maze trap with walk-in funnels), and by hand, as close to the stables as possible. The highest priority was given to species that live close to and in constant contact with production animals. The majority of birds were captured within the stables or within a 100 m radius from the stables. A few individuals were captured as far away as 500 m from stables.
Cloacal swab samples were collected on each farm over a 4- to 5-day period. All birds were released after samples were taken. To ensure that a sufficient number of birds were caught during the winter months, several feeding places were established at each farm using sterilized bird seeds (sunflower, millet and hemp). Birds at control locations were caught within a radius of 50–100 m. At these locations, the birds collected were chosen from a list of 13 species that are most closely associated with production animals. Cloacal swab samples were collected from all birds caught. However, birds that were recaptured within the same sampling round were only sampled once. The swabs were placed in a plastic tube containing 1 ml sterile water.
Insect species frequently found in close contact with production animals were collected from all production herds. Insects were not collected at control sites due to the limited occurrence of insects at these sites. Samples were collected from stables and, in the summer, among cattle on pastures. Insects were collected with a net placed on or around the production animals. They were then anesthetized with CO2 and kept in small plastic boxes. A pooled sample of insects typically consisted of several species, which were identified on the spot and noted. In the pasture, one or more cattle were tethered to enable a sufficient number of insects to be sampled.
Bacteriological examination and genotype characterization
Bacteriological examination was carried out by two laboratories at the Danish Food and Veterinary Research. Using accredited methods (The Danish Accreditation and Metrology Fund – DANAK no. 412 and 413), the Salmonella bacteria were isolated, identified, serotyped and phage-typed. Two slightly different standard culturing procedures were used at each of the two laboratories (Anon 1993; Baggesen et al. 1999; Feld et al. 2000). Both procedures are based on nonselective pre-enrichment (BPW, Merck 7228, Darmstadt, Germany) at 37°C for 16–20 h, followed by selective enrichment either in a Rappaport-Vassiliadis based medium (modified semi-solid Rappaport-Vassiliadis (MSRV, Oxoid CM910, Basingstoke, UK) or a Rappaport-Vassiliadis-soya peptone broth (Oxoid CM866), and in cysteine selenite broth (Merck 7709). All selective media were incubated overnight at 41·5–42·0°C. Salmonella bacteria were isolated on either Brilliant Green Agar (Oxoid CM329) or Rambach agar (Merck 73387) after incubation overnight at 37°C.
Fecal material (22 g) from production as well as pet animals was suspended in BPW to a total volume of 200 ml. Swab samples from birds and small pet animals were transferred to 1 ml of sterile water and mixed with BPW (to a total volume of 10 ml) prior to incubation. Rodents were necropsied, and both livers and 2–5 cm of intestines were transferred to 9 ml BPW. Approximately 1 g of the fecal samples from these animals was transferred to 9 ml BPW. Upon arrival to the laboratory, insects were killed at –20°C for 1-h prior to further processing. One gram of insects from each field sample was weighed in a test tube and then macerated. BPW was added to the sample to a final volume of 10 ml.
Ten single, presumptive, Salmonella-like colonies were collected from each plate and serotyped according to the Kauffmann–White classification scheme (Popoff and Le Minor 1997). All isolates of Salmonella Typhimurium and S. enteritidis were phage-typed. Phage-typing of isolates of S. Typhimurium was performed according to Callow (1959) as modified by Anderson et al. (1977) while phage-typing of isolates of S. enteritidis was performed as described by Ward et al. (1987).
PFGE was performed on three isolates per positive sample. Bacterial DNA for PFGE was prepared as prescribed by Olsen et al. (1994). Agarose plugs (chromosomal grade agarose from Biorad) containing DNA were digested with the restriction enzymes BlnI and XbaI (Amersham Life Science) for 4 h each. Electrophoresis was performed as described by the Centers for Disease Control (CDC) PulseNet (Swaminathan et al. 2001). The CDC standard H9812 was digested with XbaI and used as the molecular size marker. For S. Newport isolates, 200 μmol l−1 thiourea was added to the TBE running buffer, as described by Fawley and Wilcox (2002). Gels were stained in aqueous ethidium bromide (Sigma, 2 μg ml−1) and destained in distilled water for 15 min. Gels were then photographed (using 300-nm UV light), analysed, and interpreted by the use of BioNumerics (Applied Math, Siut-Martens-Latem, Belgium). All visible bands were included in the interpretation of PFGE patterns and in the creation of a dendrogram. Isolates were classified as different PFGE types if their patterns differed by more than one band.
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- Materials and methods
- Statistical analyses
Due to the limited number of positive samples, only descriptive uni- and bivariate (chi-square) statistical analyses were performed (Dohoo et al. 2003) on S. Typhimurium-positive samples from wild birds. Behavioural patterns (e.g. distance to the herd, migration pattern, contact with slurry, foraging behaviour and foraging location) were analysed for biological association with S. Typhimurium using the odds ratio (OR) (Dohoo et al. 2003).
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- Materials and methods
- Statistical analyses
During the study, 2933 samples were collected from nonproduction animals and 689 samples were collected from production animals. Salmonella was not isolated from any of the nonproduction animals living at or near control locations, nor in herds without Salmonella infections. Salmonella was only detected in wildlife living at or near farms where Salmonella had been previously detected in the production animals and only during the periods when Salmonella was detected in the production animals (Table 1). In 72% (13 out of 18) of the sampling occasions where Salmonella was found in the production animals, one or more samples from the associated nonproduction animals were Salmonella-positive. At any given farm, the Salmonella type isolated from the nonproduction animals was, with only one exception, identical to the type in the production animals within the same sampling round. The exception was the S. Typhimurium phage type RDNC (routine dilution no conformity), isolated from a wild bird in the first sampling round. The type isolated from the production animals in the same sampling round was S. Typhimurium phage type U288 (Tables 1 and 2). PFGE typing showed that these isolates were not identical (XbaI PFGE type X5 and X6 in Fig. 1). However, during the next sampling round (6½ months later), S. Typhimurium phage type RDNC (phenotypically and genotypically identical to the type isolated previously from the wild bird) was isolated from production animals, along with phage types DT12 and U288 (Table 2).
|Winter 2001||Summer 2001||Winter 2002||Summer 2002|
|S. Typh.† phage type U288 (XbaI PFGE X6‡) in: 1 slurry sample (1) S. Typh. phage type RDNC§ (XbaI PFGE X5) in: 1 bird sample (29)||S. Typh. phage type U288 (XbaI PFGE X6) in: 1 pig sample (20) S. Typh. phage type RDNC (XbaI PFGE X5) in: 1 pig sample (20) 1 rodent sample (5) S. Typh. DT12¶ (XbaI PFGE X5) in: 1 pig sample (20)||S. Typh. phage type U288 (XbaI PFGE X6, X11, and X12**) in: 18 pig samples (20)** 1 slurry sample (1) 4 rodent samples (38) 1 insect sample (1) 1 bird sample (53)||S. Typh. phage type U288 (XbaI PFGE X6 and X12**) in: 8 pig samples (20) ** 1 slurry sample (1) 2 dog samples (2)|
Salmonella Typhimurium was the most frequently detected serotype isolated in all Salmonella-infected herds. Phage types such as DT12, DT66, DT120, DT135, U288, RDNC and NT (not typable) were present in the form of either single or mixed infections. 20 BlnI PFGE types (data not shown) and 20 XbaI PFGE types (Fig. 1) were identified. In Fig. 1, both the CDC-standardized XbaI PFGE profiles and the dendrogram for the 20 types are given.
In the pig herds, S. Typhimurium was found either alone, or together with either S. Newport or S. Derby. Salmonella infections in all pig herds appeared to be persistent as the bacteria were isolated from fecal samples (pooled fecal samples or slurry samples) in all sampling rounds (Table 1). S. Typhimurium was the only serotype detected in cattle herds. The cattle infections appeared to be self-limiting, as Salmonella was not detected more than once in a given herd, with one exception; S. Typhimurium was detected in both winter 2002 and summer 2002 in one of the cattle herds (Table 1).
Samples were collected from numerous species of wildlife. A total of 2567 cloacal samples were collected from fifty-five different species of birds (Table 3). The house sparrow (Passer domesticus), tree sparrow (Passer montanus), barn swallow (Hirundo rustica), and blackbird (Turdus merula) were the most dominant species (Table 3). The dominant species among rodents (n = 225) were the Norway rat (Rattus norvegicus) and the house mouse (Mus musculus). The yellow-necked field mouse (Apodemus flavicollis), wood mouse (Apodemus sylvaticus), and bank vole (Clethrionomys glareolus) were trapped in lower numbers. Among the insect samples collected (n = 46) from the stables, houseflies (Musca domestica (L.)) and stable flies (Stomoxys calcitrans (L.)) were the predominant species. The sheep head fly (Hydrotaea irritans), the horn fly (Haematobia irritans (L.)), and the biting fly (Haematobosca stimulans (Mg.)) were the predominant species associated with cattle on pasture.
|English names||Latin names||Ecological guild||2001||2002||Total|
|Barn Swallow||Hirundo rustica||Aerial insectivorous||0/0||132/0||0/0||134/1||266/1|
|Blue Tit||Parus caeruleus||Parus||21/0||9/0||21/0||12/0||63/0|
|Dunnock||Prunella modularis||Terrestrial and low flycatching feeders||6/0||5/0||2/0||33/0||46/0|
|Great Tit||Parus major||Parus||63/0||24/0||44/0||27/1||158/1|
|Greenfinch||Carduelis chloris||Passerine Seedeaters||43/0||2/0||120/0||57/0||222/0|
|House Martin||Delichon urbica||Aerial insectivorous||0/0||78/0||0/0||41/0||119/0|
|House Sparrow||Passer domesticus||Passer||202/0||154/0||100/2||127/2||583/4|
|Pied Wagtail||Motacilla alba||Open-land insectivorous||0/0||8/0||0/0||22/3||30/3|
|Redpoll||Carduelis flammea||Passerine Seedeaters||3/0||0/0||18/0||0/0||21/0|
|Robin||Erithacus rubecula||Terrestrial and low flycatching feeders||16/0||3/0||11/0||53/0||83/0|
|Starling||Sturnus vulgaris||No guild||0/0||3/0||0/0||37/1||40/1|
|Tree Sparrow||Passer montanus||Passer||59/0||45/0||77/0||167/0||348/0|
|Willow Warbler||Phylloscopus trochilus||Foliage-gleaners||0/0||4/0||0/0||3/1||7/1|
|Wren||Troglodytes troglodytes||Terrestrial and low flycatching feeders||11/0||14/0||5/0||13/0||43/0|
|37 other species of wild birds‡||23/0||68/0||38/0||44/0||173/0|
Pooled samples from insects living with, or in close proximity, to the infected herds were 22·6% positive for Salmonella. Samples (some of which were pooled) from rodents were 5·2% positive, cats and dogs were 6·5% positive and from wild birds were 1·5% positive (Table 4).
|Infected herds||Noninfected herds||Locations without production animals||Total|
|n/pos (% pos)||n/pos||n/pos||(n)|
|Birds (single animal samples)||1285/20 (1·5%)||1004/0||278/0||2567|
|Rodents (pooled or single animal samples)||135/7 (5·2%)||68/0||22/0||225|
|Insects (pooled samples)||31/7 (22·6%)||15/0||Not sampled||46|
|Dogs and cats (pooled and single animal samples)||46/3 (6·5%)||34/0||Not sampled||80|
|Other animals* (pooled samples)||8/1 (12·5%)||5/0||2/0||15|
Salmonella (S. Typhimurium) was detected more often in wild birds exposed to cattle herds suffering clinical outbreaks, than in wild birds exposed to pig herds with persistent infections. Samples from wild birds located near infected pigs were found to be Salmonella-positive in only 2 of 10 sampling occasions. Salmonella-positive birds were detected in 4 of the 10 sampling occasions where Salmonella was detected in cattle at the same time. The Salmonella-positive birds near infected pig herds were exclusively blackbirds caught at the same farm, one during each winter sampling (Table 2). The Salmonella-positive birds near cattle herds were composed of various species. Near one heard, there was a single blackbird. Near another herd, there were three blackbirds, one house sparrow, and one chaffinch (Fringilla coelebs). A single house sparrow was near a third herd. Finally, there were three pied wagtails (Motacilla alba), two blackbirds, two house sparrows, one willow warbler (Phylloscopus trochilus), one great tit (Parus major), one starling (Sturnus vulgaris), and one barn swallow near the last herd. All Salmonella-positive birds were caught during the two winter sampling rounds (Table 3), with only one exception. Salmonella Typhimurium was isolated from 11 wild birds in summer 2002 (Table 3) in one of the cattle herds.
Statistical analysis of factors correlated with occurrence of S. Typhimurium in birds (Table 3) revealed that both the behaviour and the migration pattern were correlated with the occurrence of S. Typhimurium. Thus, partially migratory (OR = 3·0) or short-to-medium distance migratory (OR = 5·8) birds were at higher risk of contracting Salmonella infections than nonmigrating (resident) birds. Long-distance migrants (OR = 0·9) were at a somewhat lower risk compared to nonmigrating birds.
The detection of S. Typhimurium in birds was significantly associated with the food preferences of the birds. In the summer, birds feeding on insects and invertebrates were at a higher risk of infection compared to birds feeding on seeds and grains (OR = 3·6). There were no statistical differences in the detection of S. Typhimurium (P = 0·08) among birds foraging predominately on the ground, in vegetation, or in the air. However, a slightly higher risk of S. Typhimurium detection was detected for birds foraging on the ground in comparison to aerial foraging or foraging in the vegetation (OR = 3·2). Wild birds that were at some point in contact with slurry were not significantly associated with positive samples (P = 0·09). No significant difference was observed between species living close to or inside stables compared to species living peripherally to the buildings (summer: P = 0·18; winter: P = 0·22).
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- Materials and methods
- Statistical analyses
Salmonella was isolated from wildlife at farms where Salmonella was also detected in the production animals and only during the periods when Salmonella was detected in the production animals. Salmonella was isolated from insects (22·6%), rodents (5·2%), cats and dogs (6·5%), and wild birds (1·5%) living close to the infected herds. The percentage of infected animals found in each herd in this study is not directly comparable with results from other studies (Barber et al. 2002; Hilton et al. 2002; Refsum et al. 2002a; Veling et al. 2002) because the actual prevalence depends on the sampling location and methods applied. Thus, the frequency of Salmonella isolated from 22 g of feces from production animals was compared to the isolation frequencies from 1 g of rodents and from cloacal swab samples from birds. This may result in an unrealistically low prevalence of Salmonella, e.g. in birds. However, previous Danish investigations based on bird carcasses have also revealed a low prevalence of Salmonella in wild birds (Jørgensen 2002). Barber et al. (2002) showed that only 6% of samples from flies were Salmonella-positive, whereas Bailey et al. (2001) found a prevalence (18·7%) more closely resembling the results of this study (22·6%). Salmonella infections in rodents were investigated in the United Kingdom by Healing (1991), who found that rarely more than 10% of the animals carried Salmonella. Other investigations of Salmonella in birds (Fenlon 1981; Palmgren et al. 1997; Edel et al. 2002; Refsum et al. 2002a) reported a higher prevalence of Salmonella-positive birds than reported in this study. Blackbirds, pied wagtails, and house sparrows are the species most frequently infected with Salmonella according to this study. Other studies showed that (Fenlon 1981; Kapperud and Rosef 1983; Palmgren et al. 1997; Morishita et al. 1999; Bonnedahl et al. 2002; Palmgren et al. 2002; Refsum et al. 2002a) Salmonella was predominantly found in other bird species such as starling (Morishita et al. 1999), gulls (Larus spp.) (Fenlon 1981; Edel et al. 2002) and bullfinch (Pyrrhula pyrrhula) (Refsum et al. 2002a) with a 7·1%, 12·9% and 54% prevalence, respectively. However, this investigation included post-mortem records (Refsum et al. 2002a), which is not comparable to the methods we used. Kapperud and Rosef (1983) discovered a prevalence (0·8%) of Salmonella-positive birds among birds lacking clinical disease, a value comparable to that presented in the present study. In contrast to our study, Cizek et al. (1994) discovered a far higher prevalence of Salmonella-positive house sparrows in the vicinity of Salmonella-infected cattle farms (23%, as opposed to 1·5%); however, they did not find any Salmonella-positive blackbirds or pied wagtails (Cizek et al. 1994). Kirk et al. (2002) found the prevalence of Salmonella-positive cowbirds (Molothus ater) to be 1·2% and house sparrows to be 3·2%. In conclusion, the prevalence of Salmonella in wildlife varies a great deal between different studies, and it is not possible to determine whether the variation is a result of different sampling methods and study designs applied, or whether it reflects a significant difference in disease prevalence among countries.
Both phenotypical and genotypical typing of the isolates show that the Salmonella types isolated from wildlife were identical (with the exception of one case) to at least one of the types isolated from production animals on the same sampling occasion. This suggests that the population of Salmonella may be shared among different animal species at each farm. In cases where the phenotype did not allow us to characterize the clonal relationship between isolates (e.g. identification of inconclusive phage types such as RDNC or NT), we used genotypic typing. The use of both typing methods has previously been used to strengthen conclusions regarding the transmission of isolates between different sources (Bonnedahl et al. 2002; Refsum et al. 2002b; Liebana et al. 2003). Salmonella-positive birds near pig herds were found only during the winter. Five Salmonella-positive rodent samples were found during the winter and only two positive samples during the summer. Although the numbers are small, our results suggest that in herds where Salmonella infection has a more persistent character, the transmission between wildlife and production animals is more likely to take place during the humid winter season than during the dry summer. This may be due to a change in behaviour in the winter as wildlife may move closer to farms in search of food and shelter. In Denmark, house mice commonly move in the spring from human settlements, such as farms, to the open countryside and back again in the autumn (Carlsen 1993).
In cattle herds, both the presence of clinical disease and the high prevalence of Salmonella among the production animals (Table 1), appear to be the most important factors for the transmission of Salmonella to nonproduction animals. Refsum et al. (2002a) found that outbreaks of salmonellosis among birds peak in February and March, although their study did not investigate the potential contact between wildlife and production animals.
Both migratory behaviour and food preference of birds were factors significantly associated with the distribution of S. Typhimurium. This association was, however, not evident with respect to their foraging activities relative to the location of food sources (i.e. on the ground, in air or in vegetation). In accordance with our study, other investigations identified the source of food as a possible risk factor for bacterial infections in birds. Accordingly, Brittingham et al. (1998) investigated whether the prevalence of six different genera of bacteria was related to diet. None of the samples were Salmonella-positive. However, the results showed that omnivorous species had a higher prevalence of Streptococcus spp. than herbivorous species. In addition, Waldenström et al. (2002) showed that both birds feeding on invertebrates and opportunistic feeders were more commonly infected with Campylobacter spp. than other species. In agreement with our Salmonella results, Waldenström et al. (2002) also showed that the prevalence of Campylobacter spp. was higher in short-distance migrants than in long-distance migrants.
In conclusion, our results indicate that there is a transmission of Salmonella between production animals and nonproduction animals. Similar results have been reported by other workers (Kapperud et al. 1998; Warnick et al. 2001). However, in the present study the interaction seems to be most pronounced in relation to outbreaks of clinical disease among the production animals. Both the design of this study, as well as the low number of Salmonella-positive samples, does not allow us to draw firm conclusions about the direction of the transmission. Nonetheless, the results strongly indicate that Salmonella is most likely transmitted from production animals to nonproduction animals living near to infected herds, especially during acute outbreaks with concomitant high concentrations of bacteria in the surroundings. Our results do not support the hypothesis that nonproduction animals are the source of infection in production animals. However, it must be pointed out that nonproduction animals may act as a reservoir for pathogens, and/or as passive carriers, and may pose a potential risk of transmission to other herds, or reintroduction to the original herd.
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We would like to thank the farmers and their families for their kindness and collaboration, Klaus B. Fries, Henning Heldbjerg, Per A. Kjær, Jan Bolding Kristensen, Søren H. Nielsen, Lars U. Rasmussen, Folmer Jensen, Iver M. Skadborg, Aase Borges, and Claus Dahl for collection of samples. We also thank the laboratory technicians at the National Food Institute and National Veterinary Institute for their technical support. This work was supported by the Danish Ministry of Food, Fisheries and Agriculture (Grant No FØSI00-6).
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