the study population was defined as all human cases of laboratory - confirmed campylobacter infection with onset from april 2003 through march 2006 , reported by residents in 4 local authorities in northwestern england , as previously described ( 7 ) . water samples were collected at least each fortnight from october 2003 through december 2005 as 2-l grab samples from sampling points on 2 rivers associated with the study area ( river mersey and river wyre ) . water samples were transported to the food and environmental microbiology laboratory , royal preston hospital . campylobacter species were isolated by the addition of 10 ml of the water sample to 90 ml of warmed campylobacter enrichment broth ( product cm0983 , oxoid ltd , basingstoke , uk ) and incubated at 37c for 24 hours , followed by incubation at 42c for 24 hours . the enrichment broths were subcultured onto campylobacter blood - free selective agar ( charcoal cefoperazone deoxycholate agar product cm0739 , oxoid ltd ) at 37c for 48 hours microaerobically , by using a microaerobic gas generating kit ( product cn0025 , oxoid , ltd ) . campylobacter colonies were identified by morphologic features and confirmed by microaerobic and aerobic growth on blood agar . the colonies were then placed in amies transport and sent to the laboratory health protection agency regional laboratory in manchester , uk , for dna extraction and characterization . c. jejuni isolates were identified to species and typed by multilocus sequence typing as previously described ( 7 ) . statistical analysis was performed by using stata version 9.2 ( statacorp , college station , tx , usa ) . two - way tabulations and fisher exact test were used to estimate the direction and size of association of individual variables with sequence types . logistic regression was performed to estimate the significance of these findings and to examine them in a multivariate model . data were collected from 2 distinct locations , and , thus , area of residence was controlled for in analysis as a stratified variable . likelihood ratio tests were used to assess the significance of including or excluding variables from multivariate analysis . among the typed isolates of c. jejuni ( n = 1,104 ) , st-45 ( n = 49 ) was the third most prevalent sequence type reported in the study ( after st-257 and st-21 ) and highly restricted to cases reported between late april ( week 17 ) and early august ( week 32 ) ( figure 1 ) . when week of diagnosis was categorized into 2 seasons ( early summer [ weeks 1728 ] and all other weeks ) , logistic regression of season by sequence type confirmed that , when compared with all other typed cases of c. jejuni , cases of st-45 were more likely to be reported in early summer than during the rest of the year ( odds ratio 2.79 , confidence interval 1.564.99 , p = 0.001 ; 1,008 observations ) , and this relationship was not seen with other prevalent sequence types . stacked area charts showing the number of human case reports of the 4 most prevalent sequence types ( sts ) during the study , by 4-week intervals . seventy - four ( 37% ) of 198 river water samples were positive for c. jejuni , and among these , 28 sequence types ( table 1 ) were identified , 11 of which were also reported among human cases in the study population . the most prevalent sequence type identified was st-45 , and the seasonality of isolation appears to closely correlate with the seasonality of human disease onset caused by this type ( figure 2 ) . * st , sequence type ; ua , unassigned or newly identified sequence type at time of publication ;  , blank values . those sts also found in human cases of c. jejuni among the study population are indicated with a y and their prevalence indicated as a percentage of all typed cases of c. jejuni in the study population . numbers and sequence types ( sts ) of isolates of campylobacter jejuni from surface water samples compared with numbers of human cases of st-45 ( line ) for 2004 and 2005 , by 4-week interval . only the 4 most prevalent human types also identified in water samples are distinguished ( st-257 , st-45 , st-21 , and st-48 ) . category includes all other c. jejuni sequence types found both in human cases in the study and in water samples . the  other  category includes other c. jejuni sequence types found in water samples but not in human case - patients in the study . univariate logistic regression analysis identified that , in addition to illness during early summer , the statistically strongest associations ( p<0.1 ) with st-45 were being < 5 years of age and living in the more rural part of the study area . these 3 factors remained significantly and independently more associated with st-45 infection than with other sequence types when combined in a multivariate model ( tested by using likelihood ratio tests ) ( table 2 ) . when this model was used , subsequent estimation of the association of food and exposure variables showed that consumption of chicken at least once was less associated and consumption of home - delivered milk , and going fishing were more associated with st-45 than with other types . this finding was sustained in a multivariate model ( also including the demographic variables previously mentioned ) ( table 2 ) . however , information on exposure variables was not available for all 1,008 cases of c. jejuni , which resulted in a loss of statistical power . * obs , number of observations for each analysis ( i.e. , no . cases in the study with available information ) ; or , odds ratio , ci , confidence interval . initial analysis of basic demographics ( obs = 1,008 ) showed being ill in the early summer ( weeks 1728 ) and being < 5 y of age were independently associated with cases of infection with st-45 , when controlled for the stratified variable of residential area . further analysis of exposures ( controlled for the initial model ) showed going fishing , consuming chicken , and consuming home - delivered milk to be independently associated with cases of infection with st-45 , and these associations remained in the final multivariate model , including all variables listed in the table . for the full regression model , only 580 of the possible 1,008 cases had information on all the variables of interest . we have shown that a single prevalent human strain of c. jejuni , st-45 , is strongly associated with the early summer seasonal peak of campylobacteriosis incidence described previously in northwestern england ( 7 ) . this strain is also frequently isolated from recreational surface waters adjacent to the populations studied , from which other prevalent strains have rarely been isolated . the concordance between period of st-45 isolation in water and reported incidence in humans is striking and suggests a relationship between the presence of this strain in the environment and human infection . when compared with case - patients infected by other sequence types of c. jejuni , persons infected with st-45 are more likely to live in rural areas , to be < 5 years of age , to have gone fishing before illness , or to have consumed home - delivered milk , and were much less likely to have eaten any chicken in the 2 weeks before illness . the coincident seasonal presence of st-45 in both surface water and in case - patients may arise simply through seasonal excretion of st-45 , resulting from well - characterized seasonal infection in humans ( 1,3,7 ) or livestock . the river systems sampled were both urban and rural in character , and contamination through human sewage discharge or animal feces from adjoining pastures is possible . however , other common human and animal campylobacter sequence types were largely absent from the water sampled . although ingestion of untreated surface waters has previously been shown to be a risk factor for campylobacteriosis in a uk - wide case - control study ( 9 ) , these new data are not sufficiently robust to demonstrate a causal link for st-45 infection . although data were collected on proxy exposures to water , reported exposures were very low . furthermore , systematic water sampling would be required to confirm the apparent seasonal positivity of st-45 in these data . the potential role of pets , and in particular dogs , in bridging the gap between exposure to surface water ( for example , while being exercised ) and domestic exposure settings remains to be investigated . although cases of infection caused by st-45 were no more associated with owning dogs than were cases of other sequence types ( data not shown ) , the exposures of those dogs were not recorded . in a recent study , cases of st-45 clonal complex were more associated with contact with pet dogs and cats than were other clonal complexes identified ( 10 ) ; pet - mediated transmission of st-45 might be supported in this study by the observed association of st-45 with young children . despite the evidence presented of a potential environmental transmission route for st-45 , it is also a type well recognized to colonize poultry ( 1012 ) , and the incidence among humans may be a result of consumption of seasonally contaminated poultry ( 2 ) . however , the absence of strong early summer seasonality among other recognized chicken - adapted sequence types suggests that this is not the case ( data not shown ) . also , the evidence from our study is strong that consumption of chicken ( a common human exposure ) is less associated with human st-45 infection than with other types . an explanation for these observations may be that st-45 represents a strain of campylobacter that is comparatively well adapted to survival outside an animal host , as has been hypothesized for some strains of c. coli ( 13 ) . other studies have reported that st-45 is more widely distributed in terms of host and ecologic niche , including water , than other common sequence types ( 12,14 ) . evidence also has indicated that st-45 is more resilient to physical stress than other sequence types ( 15 ) . this would certainly support the hypothesis that st-45 was more available to infect humans through transmission routes other than food , either through direct exposure to water or the countryside through outdoor activities or by indirect exposure through pets . the hypothesis is further supported by the association of human st-45 with more rural area of residence in these data . the availability of st-45 to humans due to its hypothesized adaptive survival outside animal hosts would apply even more so to poultry , because of their increased environmental exposure , and 1 study has demonstrated that contamination of a new flock with st-45 arose from an isolate in a puddle outside a chicken house ( 12 ) . thus , through an as - yet - uncharacterized adaptation , st-45 may be a strain of c. jejuni that is able to bridge the various recognized environmental , livestock , and human transmission settings for disease , making it a key target for intervention in reducing campylobacter prevalence . it may also be a key driver for the early summer rise in human incidence , both through nonfoodborne exposure and contamination of food animals .
in a study of campylobacter infection in northwestern england , 20032006 , c. jejuni multilocus sequence type ( st)45 was associated with early summer onset and was the most prevalent c. jejuni type in surface waters . 
 st-45 is likely more adapted to survival outside a host , making it a key driver of transmission between livestock , environmental , and human settings .