BACKGROUND
campylobacter jejuni is the most common bacterial cause of human gastroenteritis worldwide  <cit> .
in many european countries, including finland, the number of laboratory confirmed c. jejuni infections doubled in the last decade  <cit> .
in finland, approximately  <dig> cases were reported in  <dig>  <cit> , with an incidence of 85/ <dig>  <dig> inhabitants.
campylobacter outbreaks are relatively uncommon in industrialized countries, and most of the cases occur sporadically  <cit> .
as a consequence, the sources of infection remain mostly unknown.
epidemiological studies in different countries indicate that eating improperly cooked meat and handling chicken carcasses are important risk factors for acquiring the illness  <cit> .
other risk factors highlighted in epidemiological studies include contact with pets  <cit> , drinking untreated water  <cit>  and swimming in natural water sources  <cit> .
outbreaks of campylobacteriosis are most commonly associated with drinking unpasteurized milk or contaminated water  <cit>  and eating improperly cooked poultry meat  <cit> .
c. jejuni has a wide distribution among different warm-blooded animals, including poultry, bovines, pigs, cats, dogs and various wild animals  <cit>  and birds.
as a consequence of faecal contamination, c. jejuni is also frequently isolated from natural waters  <cit> .
to estimate the proportion of human infections attributed to different sources of infection, various typing methods have been applied to distinguish between strains.
pulsed field gel electrophoresis  has been considered the method of choice due to its high discriminatory power; however, during the last decade - after its description for c. jejuni - multilocus sequence typing   <cit>  has generally been accepted as the most suitable method for population genetic analyses.
the major advantages of mlst compared to pfge are the standardized nomenclature and the ability to easily transfer and compare results between laboratories worldwide.
furthermore, different mathematical modelling approaches can readily be applied on the resulting sequence and allele data to facilitate source attribution.
for this purpose, different bayesian approaches, inferring the genetic population structure of c. jejuni, have garnered the most interest  <cit> .
bayesian analysis of population structure   <cit>  has recently been successfully applied in inferring population structures of e. coli  <cit>  and the s. mitis group streptococci  <cit> .
baps showed, in a simulation study, comparable power to other methods and was deemed also to be highly efficient from computational perspective  <cit> .
limited data exists on sequence types  present among bovine isolates in finland  <cit> , and estimating the proportion of human infections potentially linked to this source has been difficult.
to better understand the diversity of finnish bovine c. jejuni, we characterized  <dig> isolates using mlst.
we used baps v.  <dig>  for source attribution purposes and included additional mlst data obtained in our previous study  <cit>  from finnish bovines, retail poultry meat and human isolates from  <dig>
RESULTS
mlst of bovine isolates
genotypes of a total of  <dig> bovine c. jejuni isolates were identified by nucleotide sequences at all seven mlst loci.
ninety-three of these were assigned into nine previously described clonal complexes  .
the st- <dig> cc was predominant , followed by the st- <dig> cc , the st- <dig> cc , the st- <dig> cc  and the st- <dig> cc .
of the  <dig> sts observed among the isolates,  <dig>  were novel.
thirty-two isolates  had a unique st, and the most common sts among the isolates were st- <dig> , followed by st- <dig>  and st- <dig> .
numbers in parentheses after each st denote the number of isolates.
new sts and alleles are shown in boldface.
analyses of population structure of finnish bovine, poultry and human isolates
in our total set of  <dig> bovine, poultry and human isolates, including data from our previous study  <cit> ,  <dig> sts were found and included in the population structure analysis.
the  <dig> sts were found among  <dig> ccs and the most common ccs were the st- <dig> cc , the st- <dig> cc , the st- <dig> cc , the st- <dig> cc  and the st- <dig> cc .
the predominant sts were st- <dig> , st- <dig> , st- <dig> , st- <dig>  and st- <dig> .
overall,  <dig> % and  <dig> % of the sts found in bovine and poultry isolates, respectively, were also found in human isolates.
conversely,  <dig> % and 69% of the sts found in human isolates were also found in bovine and poultry isolates, respectively.
furthermore,  <dig> % of the sts found in poultry were found in bovine isolates, but only  <dig> % of those in bovines were present in poultry isolates.
in analysing the relationships of clonal complexes with hosts, the st- <dig>  and st- <dig> ccs  were associated with bovine isolates, whereas the st- <dig> cc was associated with poultry  and human isolates .
bovine isolates were found in bovine-associated ccs in  <dig> % of the cases.
poultry and human isolates were found in the st- <dig> cc in  <dig> % and 36% of the cases, respectively.
the st- <dig> cc did not occur among poultry and human isolates.
the st- <dig> cc contained  <dig> % of all the poultry isolates,  <dig> % of the human isolates and  <dig> % of the bovine isolates.
st- <dig> , st- <dig> , st- <dig> , st- <dig>  and st- <dig>  were associated with the bovine host and contained  <dig> % of the bovine isolates.
none of the human or poultry isolates represented bovine-associated sts.
st- <dig> was associated with poultry  and human isolates  and was found in  <dig> % of the poultry isolates, 32% of the human isolates and  <dig> % of the bovine isolates.
st- <dig> was associated with human isolates  and was found in 34% of the human isolates,  <dig> % of the poultry isolates and  <dig> % of the bovine isolates.
st- <dig> was associated with the human isolates , but was absent from both other sources.
using baps, nearly all estimation runs converged to the same solution with five clusters having high posterior certainty in its vicinity according to the program output.
baps clusters  <dig> and  <dig> contained the majority of isolates .
baps cluster  <dig> contained all sts found in the st- <dig>  st- <dig>  st- <dig>  st- <dig>  and st- <dig> ccs in addition to two significantly admixed sts in the st- <dig> cc .
one st of the st- <dig>  and st- <dig> ccs  was admixed as well.
baps cluster  <dig> contained a total of three unassigned sts which were only found in human isolates.
in baps cluster  <dig> the st- <dig> cc was grouped together with two uncommon, unassigned sts.
baps cluster  <dig> comprised all, but two, sts of the st- <dig> cc, all sts from the st- <dig>  st- <dig>  st- <dig> and st- <dig> ccs and one st  from the st- <dig> cc, which was significantly admixed.
the remainder of the st- <dig> cc formed a distinct cluster , with no admixed sts and contained only bovine isolates.
sts shown in bold face were significantly admixed with at least one other cluster, but were put into the cluster with the highest posterior probability.
bold-faced underlined text shows number of isolates of each host in the specific baps cluster.
admixture was mainly found in clusters  <dig> and  <dig> for a total of nine sts  including a total of  <dig> isolates .
mainly novel sts in the st- <dig> complex , st- <dig> complex , st- <dig> complex , st- <dig> and st- <dig> were found to be admixed.
however, also st- <dig> , st- <dig>  and st- <dig>  were significantly admixed.
bovine isolates were found to be associated with admixture .
baps clusters  <dig> and  <dig> were associated with the bovine isolates , baps cluster  <dig> was associated with the poultry isolates and baps clusters  <dig> and  <dig> were not associated with any host.
bovine isolates were found in bovine-associated clusters in  <dig> % of cases.
of the poultry isolates  <dig> % were found in the poultry-associated cluster.
human isolates were found in the bovine-associated baps cluster  <dig> in  <dig> % of the cases and in  <dig> % of the cases found in the poultry-associated baps cluster  <dig>
the nj tree shown in figure  <dig> illustrates the molecular variation within and between the clusters estimated by baps from a phylogenetic perspective.
eburst analysis yielded seven groups containing two  to  <dig>  sts and  <dig> singletons.
table  <dig> shows the degree of similarity between the eburst groups and baps populations.
the biggest baps clusters  were made up of several eburst groups, while baps cluster  <dig> did not have an equivalent eburst group.
discussion
our study revealed a high diversity of mlsts among  <dig> bovine c. jejuni isolates obtained from three major finnish slaughterhouses, representing  <dig> farms, in  <dig>  a total of  <dig> sts  were observed, nearly half of which were novel, emerging mostly from new combinations of known alleles and in two cases from new alleles carrying a one-nucleotide difference from alleles commonly found in cattle .
the emergence of a high number of novel sts could be explained by the life cycle of dairy cattle, providing a c. jejuni strain with the opportunity for long-lasting colonization and adaptation in the bovine host.
however, re-infection with a different strain or multiple strains, and thus the occurrence of recombination events, cannot be excluded.
the distribution of c. jejuni genotypes has previously been shown not to be random among farms, with farms no more than  <dig> km apart appearing to possess similar c. jejuni genotypes  <cit> , supporting the persistence of clones in cattle herds.
probably due to the disperse distribution of farms in finland, we found no clear evidence of regional differences in the distribution of bovine sts or ccs between different parts of the country.
this is in agreement with findings from scotland  <cit> .
in this study, as well as previous studies, the st- <dig> and st- <dig> ccs were shown to be common in cattle  <cit> .
the st- <dig> cc, in particular, is strongly associated with bovines and has been observed in cattle in other studies worldwide  <cit> .
we did not find members of the st- <dig> cc in poultry or humans  <cit> , and other studies have infrequently observed this cc in these hosts  <cit> .
also, st- <dig> was one of the most prevalent bovine sts  in our study, and sts that share five or more alleles with st- <dig> , have previously been reported only from cattle in the uk and ireland  <cit>  and scotland  <cit> .
in addition to sts in the st- <dig> cc, st- <dig> may represent another clonal lineage of c. jejuni adapted to the bovine gut.
source attribution is an important task in the risk assessment of the impact of different potential reservoirs for human infections caused by c. jejuni, and mlst has been shown to be an efficient method for assessing clusters of isolates with host specificity  <cit> .
on clonal complex level  <dig> % of the bovine isolates were found in bovine-associated ccs and  <dig> % of the poultry isolates were found in poultry-associated ccs.
however, on st level only  <dig> % of the bovine isolates were found in bovine-associated sts, reflecting the high diversity of sts found in bovine isolates within clonal complexes.
in addition, we used baps, a tool that has recently become popular for inferring population genetic structure  <cit>  to assign our isolates to genetically differentiated groups.
baps divided the  <dig> sts into five clusters such that clusters  <dig> and  <dig> contained all sts which baps identified as mosaics due to recombination.
of the bovine isolates  <dig> % were found in the bovine-associated baps clusters  <dig> and  <dig>  similarly, poultry isolates were found in  <dig> % of the cases in the poultry-associated baps cluster  <dig>  these results indicate that baps was useful for host assignment, even though our dataset was relatively small.
baps analysis showed comparable power to host assignment using clonal complexes but also reflected the phylogeny of our data.
baps clusters  <dig> and  <dig> contained most of the isolates and were substantially heterogeneous in the distribution of clonal complexes.
especially, the poultry-associated baps cluster  <dig> was very heterogeneous; the st- <dig> cc was most common and grouped together with several uncommon, unrelated clonal complexes, often not found in our poultry isolates.
in our previous study  <cit> , the st- <dig> cc found in our human isolates was associated with tasting of raw or undercooked meat as well as contact with dogs or cats.
also, the st- <dig> cc has been found from penguins on the antarctic  <cit> , implying that this cc has a wide host range and is environmentally well adapted.
the st- <dig>  st- <dig> and st- <dig> ccs, which were grouped together with the st- <dig> cc in baps cluster  <dig>  have been commonly found in companion animals in other studies  <cit> .
however, more studies are needed to establish the role of environmental contamination sources serving as c. jejuni vectors for both human infection and chicken colonization.
most admixture was found in clusters  <dig> and  <dig> with the majority of admixed sts being novel and associated with the bovine isolates.
all admixed sts with the highest posterior probability in cluster  <dig>  were admixed with cluster  <dig>  and most of these sts were found only in bovine isolates.
in contrast, most admixed sts with the highest posterior probability in cluster  <dig> were admixed with clusters  <dig> and  <dig>  in which only human isolates were assigned to and mostly contained uncommon, unassigned sts.
these findings could imply that recombination is more common in sts specific to bovines, which is supported by the high diversity of our bovine isolates.
bovines have a longer life-span than poultry and persistence of c. jejuni clones in herds and specific bovine-associated lineages imply that these strains can adapt to long-lasting colonization, thereby increasing the chance of horizontal transfer of genetic material and recombination.
the st- <dig> cc was found as a separate cluster  by baps, with the exception of st- <dig> .
this finding was not surprising since the st- <dig> cc is known to have imported c. coli alleles  and therefore is phylogenetically less related to other c. jejuni clonal complexes  <cit> .
both st- <dig> and st- <dig> do not possess the unca <dig> allele, but st- <dig> carries the unca <dig> allele.
this allele is common in both the st- <dig> cc and the c. coli related st- <dig> cc and likely the presence of this allele caused st- <dig> to be included in baps cluster  <dig>  st- <dig>  however, carries the unca <dig> allele, which is commonly found in both the st- <dig> cc  and the st- <dig> cc.
this explains why this particular st was grouped together with the st- <dig> cc and at the same time admixed with cluster  <dig>  these results demonstrate that the import of c. coli dna can have a large impact on the mlst analysis of c. jejuni strains and this should be taken into account in source attribution studies.
studies from the uk  <cit>  and new zealand  <cit> ; have indicated that chicken  could attribute to 57-80%, cattle  to 18-39%, and other, mainly environmental, sources to 1-4% of all c. jejuni infections.
compared to these studies we found a lower source attribution for chickens  and a higher source attribution for bovines .
this could be the result of limited sampling of c. jejuni isolates from chicken meat in our study and the fact that c. jejuni is more difficult to detect by cultivation from meat compared to faecal samples.
the meat samples, however, represented all three major chicken meat producers and were collected during the summer peak  <cit> , when most human c. jejuni infections occur in finland  <cit> .
the national low prevalence of campylobacter spp.
in finnish chicken flocks   <cit>  in comparison to other eu countries could lead to a different source attribution when compared to studies from other countries.
in a finnish slaughterhouse study, c. jejuni was detected in  <dig> % of the faecal samples and  <dig> % of bovine carcasses  <cit> .
however, none of the c. jejuni isolates from carcasses represented pfge types similar to human isolates  <cit> .
bovines could be an underestimated route for campylobacter infections in finland, although foodborne transmission would be least likely.
however, transmission could occur through either direct contact or environmental transmission by shared reservoirs for human patients and bovine c. jejuni strains.
a large proportion of our isolates  could not be attributed to any source .
more than half of these isolates represented the st- <dig> cc, which has been detected in various hosts, including starlings  <cit> , rabbits, environmental waters, wild birds and cattle  <cit> .
in our previous study this cc was related to drinking non-chlorinated water from a small water plant or from natural water sources  <cit> .
faecal contamination from wild animals and birds into natural water sources is common and could be hypothesized to have a pronounced role in human infections in summer in our finnish study region uusimaa.
this is also supported by the finnish case-control study that identified swimming and drinking from dug wells as important risk factors for infection during summertime  <cit> .
therefore the role of different water-associated transmission routes should not be underestimated in future attribution studies of finnish domestically acquired c. jejuni human infections.
CONCLUSIONS
due to the wide distribution and occurrence of some c. jejuni ccs and sts among different hosts, source attribution is a complicated issue and bayesian methods are considered useful for quantitative probabilistic assignment of sts to genetically related clusters.
in our study  <dig> % of the bovine isolates and  <dig> % of the poultry isolates were found in clusters associated with each host.
of the human isolates  <dig> % was found in the bovine-associated baps cluster  <dig> and  <dig> % was found in the poultry-associated cluster.
inclusion of mlst data in detailed epidemiological case-control studies and parallel extensive regional sampling schemes would greatly improve the attribution of human infections to the source and help develop specific control schemes to limit the numbers of human infections.

 campylobacter jejuni is the most common bacterial cause of human gastroenteritis worldwide. in many european countries, including finland, the number of laboratory confirmed c. jejuni infections doubled in the last decade. in finland, approximately cases were reported in, with an incidence of 85/ inhabitants. campylobacter outbreaks are relatively uncommon in industrialized countries, and most of the cases occur sporadically. as a consequence, the sources of infection remain mostly unknown. epidemiological studies in different countries indicate that eating improperly cooked meat and handling chicken carcasses are important risk factors for acquiring the illness. other risk factors highlighted in epidemiological studies include contact with pets, drinking untreated water and swimming in natural water sources. outbreaks of campylobacteriosis are most commonly associated with drinking unpasteurized milk or contaminated water and eating improperly cooked poultry meat. c. jejuni has a wide distribution among different warm-blooded animals, including poultry, bovines, pigs, cats, dogs and various wild animals and birds. as a consequence of faecal contamination, c. jejuni is also frequently isolated from natural waters. to estimate the proportion of human infections attributed to different sources of infection, various typing methods have been applied to distinguish between strains. pulsed field gel electrophoresis has been considered the method of choice due to its high discriminatory power; however, during the last decade - after its description for c. jejuni - multilocus sequence typing has generally been accepted as the most suitable method for population genetic analyses. the major advantages of mlst compared to pfge are the standardized nomenclature and the ability to easily transfer and compare results between laboratories worldwide. furthermore, different mathematical modelling approaches can readily be applied on the resulting sequence and allele data to facilitate source attribution. for this purpose, different bayesian approaches, inferring the genetic population structure of c. jejuni, have garnered the most interest. bayesian analysis of population structure has recently been successfully applied in inferring population structures of e. coli and the s. mitis group streptococci. baps showed, in a simulation study, comparable power to other methods and was deemed also to be highly efficient from computational perspective. limited data exists on sequence types present among bovine isolates in finland, and estimating the proportion of human infections potentially linked to this source has been difficult. to better understand the diversity of finnish bovine c. jejuni, we characterized isolates using mlst. we used baps v. for source attribution purposes and included additional mlst data obtained in our previous study from finnish bovines, retail poultry meat and human isolates from  mlst of bovine isolates genotypes of a total of bovine c. jejuni isolates were identified by nucleotide sequences at all seven mlst loci. ninety-three of these were assigned into nine previously described clonal complexes. the st- cc was predominant, followed by the st- cc, the st- cc, the st- cc and the st- cc. of the sts observed among the isolates, were novel. thirty-two isolates had a unique st, and the most common sts among the isolates were st-, followed by st- and st-. numbers in parentheses after each st denote the number of isolates. new sts and alleles are shown in boldface. analyses of population structure of finnish bovine, poultry and human isolates in our total set of bovine, poultry and human isolates, including data from our previous study, sts were found and included in the population structure analysis. the sts were found among ccs and the most common ccs were the st- cc, the st- cc, the st- cc, the st- cc and the st- cc. the predominant sts were st-, st-, st-, st- and st-. overall, % and % of the sts found in bovine and poultry isolates, respectively, were also found in human isolates. conversely, % and 69% of the sts found in human isolates were also found in bovine and poultry isolates, respectively. furthermore, % of the sts found in poultry were found in bovine isolates, but only % of those in bovines were present in poultry isolates. in analysing the relationships of clonal complexes with hosts, the st- and st- ccs were associated with bovine isolates, whereas the st- cc was associated with poultry and human isolates. bovine isolates were found in bovine-associated ccs in % of the cases. poultry and human isolates were found in the st- cc in % and 36% of the cases, respectively. the st- cc did not occur among poultry and human isolates. the st- cc contained % of all the poultry isolates, % of the human isolates and % of the bovine isolates. st-, st-, st-, st- and st- were associated with the bovine host and contained % of the bovine isolates. none of the human or poultry isolates represented bovine-associated sts. st- was associated with poultry and human isolates and was found in % of the poultry isolates, 32% of the human isolates and % of the bovine isolates. st- was associated with human isolates and was found in 34% of the human isolates, % of the poultry isolates and % of the bovine isolates. st- was associated with the human isolates, but was absent from both other sources. using baps, nearly all estimation runs converged to the same solution with five clusters having high posterior certainty in its vicinity according to the program output. baps clusters and contained the majority of isolates. baps cluster contained all sts found in the st- st- st- st- and st- ccs in addition to two significantly admixed sts in the st- cc. one st of the st- and st- ccs was admixed as well. baps cluster contained a total of three unassigned sts which were only found in human isolates. in baps cluster the st- cc was grouped together with two uncommon, unassigned sts. baps cluster comprised all, but two, sts of the st- cc, all sts from the st- st- st- and st- ccs and one st from the st- cc, which was significantly admixed. the remainder of the st- cc formed a distinct cluster, with no admixed sts and contained only bovine isolates. sts shown in bold face were significantly admixed with at least one other cluster, but were put into the cluster with the highest posterior probability. bold-faced underlined text shows number of isolates of each host in the specific baps cluster. admixture was mainly found in clusters and for a total of nine sts including a total of isolates. mainly novel sts in the st- complex, st- complex, st- complex, st- and st- were found to be admixed. however, also st-, st- and st- were significantly admixed. bovine isolates were found to be associated with admixture. baps clusters and were associated with the bovine isolates, baps cluster was associated with the poultry isolates and baps clusters and were not associated with any host. bovine isolates were found in bovine-associated clusters in % of cases. of the poultry isolates % were found in the poultry-associated cluster. human isolates were found in the bovine-associated baps cluster in % of the cases and in % of the cases found in the poultry-associated baps cluster the nj tree shown in figure illustrates the molecular variation within and between the clusters estimated by baps from a phylogenetic perspective. eburst analysis yielded seven groups containing two to sts and singletons. table shows the degree of similarity between the eburst groups and baps populations. the biggest baps clusters were made up of several eburst groups, while baps cluster did not have an equivalent eburst group. discussion our study revealed a high diversity of mlsts among bovine c. jejuni isolates obtained from three major finnish slaughterhouses, representing farms, in a total of sts were observed, nearly half of which were novel, emerging mostly from new combinations of known alleles and in two cases from new alleles carrying a one-nucleotide difference from alleles commonly found in cattle. the emergence of a high number of novel sts could be explained by the life cycle of dairy cattle, providing a c. jejuni strain with the opportunity for long-lasting colonization and adaptation in the bovine host. however, re-infection with a different strain or multiple strains, and thus the occurrence of recombination events, cannot be excluded. the distribution of c. jejuni genotypes has previously been shown not to be random among farms, with farms no more than km apart appearing to possess similar c. jejuni genotypes, supporting the persistence of clones in cattle herds. probably due to the disperse distribution of farms in finland, we found no clear evidence of regional differences in the distribution of bovine sts or ccs between different parts of the country. this is in agreement with findings from scotland. in this study, as well as previous studies, the st- and st- ccs were shown to be common in cattle. the st- cc, in particular, is strongly associated with bovines and has been observed in cattle in other studies worldwide. we did not find members of the st- cc in poultry or humans, and other studies have infrequently observed this cc in these hosts. also, st- was one of the most prevalent bovine sts in our study, and sts that share five or more alleles with st-, have previously been reported only from cattle in the uk and ireland and scotland. in addition to sts in the st- cc, st- may represent another clonal lineage of c. jejuni adapted to the bovine gut. source attribution is an important task in the risk assessment of the impact of different potential reservoirs for human infections caused by c. jejuni, and mlst has been shown to be an efficient method for assessing clusters of isolates with host specificity. on clonal complex level % of the bovine isolates were found in bovine-associated ccs and % of the poultry isolates were found in poultry-associated ccs. however, on st level only % of the bovine isolates were found in bovine-associated sts, reflecting the high diversity of sts found in bovine isolates within clonal complexes. in addition, we used baps, a tool that has recently become popular for inferring population genetic structure to assign our isolates to genetically differentiated groups. baps divided the sts into five clusters such that clusters and contained all sts which baps identified as mosaics due to recombination. of the bovine isolates % were found in the bovine-associated baps clusters and similarly, poultry isolates were found in % of the cases in the poultry-associated baps cluster these results indicate that baps was useful for host assignment, even though our dataset was relatively small. baps analysis showed comparable power to host assignment using clonal complexes but also reflected the phylogeny of our data. baps clusters and contained most of the isolates and were substantially heterogeneous in the distribution of clonal complexes. especially, the poultry-associated baps cluster was very heterogeneous; the st- cc was most common and grouped together with several uncommon, unrelated clonal complexes, often not found in our poultry isolates. in our previous study, the st- cc found in our human isolates was associated with tasting of raw or undercooked meat as well as contact with dogs or cats. also, the st- cc has been found from penguins on the antarctic, implying that this cc has a wide host range and is environmentally well adapted. the st- st- and st- ccs, which were grouped together with the st- cc in baps cluster have been commonly found in companion animals in other studies. however, more studies are needed to establish the role of environmental contamination sources serving as c. jejuni vectors for both human infection and chicken colonization. most admixture was found in clusters and with the majority of admixed sts being novel and associated with the bovine isolates. all admixed sts with the highest posterior probability in cluster were admixed with cluster and most of these sts were found only in bovine isolates. in contrast, most admixed sts with the highest posterior probability in cluster were admixed with clusters and in which only human isolates were assigned to and mostly contained uncommon, unassigned sts. these findings could imply that recombination is more common in sts specific to bovines, which is supported by the high diversity of our bovine isolates. bovines have a longer life-span than poultry and persistence of c. jejuni clones in herds and specific bovine-associated lineages imply that these strains can adapt to long-lasting colonization, thereby increasing the chance of horizontal transfer of genetic material and recombination. the st- cc was found as a separate cluster by baps, with the exception of st-. this finding was not surprising since the st- cc is known to have imported c. coli alleles and therefore is phylogenetically less related to other c. jejuni clonal complexes. both st- and st- do not possess the unca allele, but st- carries the unca allele. this allele is common in both the st- cc and the c. coli related st- cc and likely the presence of this allele caused st- to be included in baps cluster st- however, carries the unca allele, which is commonly found in both the st- cc and the st- cc. this explains why this particular st was grouped together with the st- cc and at the same time admixed with cluster these results demonstrate that the import of c. coli dna can have a large impact on the mlst analysis of c. jejuni strains and this should be taken into account in source attribution studies. studies from the uk and new zealand; have indicated that chicken could attribute to 57-80%, cattle to 18-39%, and other, mainly environmental, sources to 1-4% of all c. jejuni infections. compared to these studies we found a lower source attribution for chickens and a higher source attribution for bovines. this could be the result of limited sampling of c. jejuni isolates from chicken meat in our study and the fact that c. jejuni is more difficult to detect by cultivation from meat compared to faecal samples. the meat samples, however, represented all three major chicken meat producers and were collected during the summer peak, when most human c. jejuni infections occur in finland. the national low prevalence of campylobacter spp. in finnish chicken flocks in comparison to other eu countries could lead to a different source attribution when compared to studies from other countries. in a finnish slaughterhouse study, c. jejuni was detected in % of the faecal samples and % of bovine carcasses. however, none of the c. jejuni isolates from carcasses represented pfge types similar to human isolates. bovines could be an underestimated route for campylobacter infections in finland, although foodborne transmission would be least likely. however, transmission could occur through either direct contact or environmental transmission by shared reservoirs for human patients and bovine c. jejuni strains. a large proportion of our isolates could not be attributed to any source. more than half of these isolates represented the st- cc, which has been detected in various hosts, including starlings, rabbits, environmental waters, wild birds and cattle. in our previous study this cc was related to drinking non-chlorinated water from a small water plant or from natural water sources. faecal contamination from wild animals and birds into natural water sources is common and could be hypothesized to have a pronounced role in human infections in summer in our finnish study region uusimaa. this is also supported by the finnish case-control study that identified swimming and drinking from dug wells as important risk factors for infection during summertime. therefore the role of different water-associated transmission routes should not be underestimated in future attribution studies of finnish domestically acquired c. jejuni human infections.  due to the wide distribution and occurrence of some c. jejuni ccs and sts among different hosts, source attribution is a complicated issue and bayesian methods are considered useful for quantitative probabilistic assignment of sts to genetically related clusters. in our study % of the bovine isolates and % of the poultry isolates were found in clusters associated with each host. of the human isolates % was found in the bovine-associated baps cluster and % was found in the poultry-associated cluster. inclusion of mlst data in detailed epidemiological case-control studies and parallel extensive regional sampling schemes would greatly improve the attribution of human infections to the source and help develop specific control schemes to limit the numbers of human infections. 
in the baps analysis % of the human isolates were found in bovine-associated baps clusters and % of the human isolates were found in the poultry-associated baps cluster. these findings suggest that bovines and poultry were equally important as reservoirs for human c. jejuni infections in finland in our results differ from those obtained in other countries where poultry has been identified as the most important source for human infections. of the human isolates % were found in clusters not associated with any host which warrants further investigation, with particular focus on waterborne transmission routes and companion animals. campylobacter jejuni is the most common bacterial cause of human gastroenteritis worldwide. multilocus sequence typing has been successfully applied to population genetics of campylobacter jejuni and mathematical modelling can be applied to the sequence data. here, we analysed the population structure of a total of finnish c. jejuni isolates from bovines, poultry meat and humans collected in using a combination of bayesian clustering and phylogenetic analysis. in the first phase we analysed sequence types of finnish bovine c. jejuni isolates by mlst and found a high diversity totalling sts of which nearly half were novel. in the second phase we included mlst data from domestic human isolates as well as poultry c. jejuni isolates from the same time period. the low prevalence of c. jejuni in poultry flocks in finland could explain the lower attribution of human infection to poultry. between the human and bovine isolates we found an overlap of %, while 69% of the human isolates were overlapping with the chicken isolates.
 campylobacter jejuni is the most common bacterial cause of human gastroenteritis worldwide. due to the sporadic nature of infection, sources often remain unknown. multilocus sequence typing has been successfully applied to population genetics of campylobacter jejuni and mathematical modelling can be applied to the sequence data. here, we analysed the population structure of a total of finnish c. jejuni isolates from bovines, poultry meat and humans collected in using a combination of bayesian clustering and phylogenetic analysis.  in the first phase we analysed sequence types of finnish bovine c. jejuni isolates by mlst and found a high diversity totalling sts of which nearly half were novel. in the second phase we included mlst data from domestic human isolates as well as poultry c. jejuni isolates from the same time period. between the human and bovine isolates we found an overlap of %, while 69% of the human isolates were overlapping with the chicken isolates. in the baps analysis % of the human isolates were found in bovine-associated baps clusters and % of the human isolates were found in the poultry-associated baps cluster. baps reflected the phylogeny of our data very well.  these findings suggest that bovines and poultry were equally important as reservoirs for human c. jejuni infections in finland in our results differ from those obtained in other countries where poultry has been identified as the most important source for human infections. the low prevalence of c. jejuni in poultry flocks in finland could explain the lower attribution of human infection to poultry. of the human isolates % were found in clusters not associated with any host which warrants further investigation, with particular focus on waterborne transmission routes and companion animals. 