Although the organism is of less clinical importance than C. jejuni
, the incidence of C. coli
in the study population (5.75 cases per 100,000) is still a significant burden of disease. We present strong evidence that cases of C. coli
were more likely to be older and female than cases of C. jejuni
, and this has been described in other studies (10
), but the mechanism for this differential age distribution remains unclear. In fact, the epidemiologies of C. coli
and C. jejuni
, in terms of exposures recorded, appear remarkably similar in the population described.
Although C. coli
is prevalent in surface water samples in this study and there is some evidence that problems with drinking water supply are more associated with C. coli
infection than with C. jejuni
, sequence typing has demonstrated that the C. coli
isolates represent not only a different set of designated MLST types but also a genetically distinct (and in most cases distant) population from that described in human cases of disease. This is in contrast to results of other studies, where overlap of types between water and human cases has been reported for C. coli
) and C. jejuni
). Environmental waters can be considered a reservoir for Campylobacter
strains from both wildlife and livestock hosts and may also be contaminated with strains from human sewage effluent (13
). There is no epidemiological or microbiological evidence from this study linking human disease from C. coli
with direct exposure to environmental waters, but the population of bacteria identified in surface waters is of interest.
The separation of C. coli
strains from this study into three clades is in concordance with previous genetic analysis of all known C. coli
sequence types (25
), and the individual sequence types presented here align with these clades (S. Sheppard and N. McCarthy, personal communication). The close clustering of types from human cases in clade 1 relative to the other types presented (from water samples) may result in part from the intense sampling to date of this population in poultry, livestock, and humans. In comparison, relatively few types from environmental samples and other species have been described and, as this data set expands, similar clustering may emerge in other clades. However, clade 1 also represents a population with abundant opportunity for horizontal gene transfer (through close proximity of livestock and their contact with and consumption by humans), and we do not yet know enough about the ecology of C. coli
outside this domain to predict whether that emergence might happen.
It could be argued that the absence of human sequence types in clades 2 and 3 (water samples) in this study may simply reflect an incomplete picture of the full range of types identified in human samples and that as more sequence typing of C. coli
from human isolates is done, we may see a more diverse representation of clades in the clinical data. However, of the 17 new types identified in human samples in this study, all align with clade 1, suggesting a true separation of human and water populations. Of interest in this discussion are the intermediate types presented, all isolated from surface waters (including those most commonly isolated, ST-1766 and ST-1764) but aligned genetically more closely with clade 1 than the remaining water isolates. All these types are newly reported in this study, and although there is no evidence at present that they contribute to the burden of human disease, 7 of the 13 sequence types possess between one and three alleles that are also found in human isolates, in contrast to the other water samples, which share no alleles (data not shown). They may therefore represent avian or wildlife strains prevalent in the environment that have had greater opportunity to recombine in a common host with human/livestock strains than the other water sample sequence types in clades 2 and 3 and may yet be identified in livestock or food animals with further sampling of these populations. Recombination may occur where ecological and adaptive barriers are breached (25
), and types in clades 2 and 3 could represent populations of C. coli
that either predominate in species with little contact with livestock, domestic poultry, or humans or have reduced virulence for these species. Since these types were isolated from surface waters that are easily accessible to humans, companion animals, and livestock in both rural and suburban settings, a virulence barrier is a reasonable hypothesis, making these strains an interesting subject of further study.
There has been a suggestion that C. coli
infection may be associated with consumption of pâté and meat pies (10
), and C. coli
has been described as being found in offal in the United Kingdom with higher prevalence than C. jejuni
). United Kingdom food survey data suggest that the average quantity of bacon, ham, and pork products, liver, and meat pies purchased per individual over 65 years old is greater than for younger adults whereas there is no age difference for pâté (5
). Our data confirmed that study cases older than 65 years old, in addition to being more likely to have C. coli
than C. jejuni
, were significantly more likely (than all other age groups) to have consumed pork, ham, or bacon, offal, beef, lamb, and meat pies prior to illness (data not shown). Consumption of pâté, however, showed no association with either age or species of Campylobacter
However, despite C. coli
commonly being reported as the most prevalent species in swine and pork meat (20
) and the associations described above, individuals with C. coli
were less likely to have reported eating pork products prior to illness than those with C. jejuni
. Although reliable associations between food consumption and individual sequence types were not possible, it is clear that the majority of types identified in humans in this study and previously identified in swine have also been identified in poultry (3
), and given the very high reporting of poultry consumption in this study (28
), this route of infection seems the more likely. This is supported by a recent study of Danish isolates, demonstrating that only 9% of the C. coli
sequence types described for humans were also described for pigs compared with 38% that were also described for poultry (17
There is no strong evidence from this study of distinct exposures through food consumption or behavior reported by cases of C. jejuni and C. coli that could fully explain the different age and sex distribution of cases described. Where evidence of association is presented, the sample size of reported exposures cannot preclude association by chance, and the epidemiology and typing data together do not allow the confident proposal of a plausible hypothesis. The observed distribution may be mediated by factors related to host immunity or some mechanism of competitive colonization that is not yet understood.
New MLST sequence types of C. coli continue to be identified and published, and although the international MLST database is becoming more representative of the sources where types have been identified, it remains difficult to interpret epidemiology on the basis of sequence type alone. The demonstration in this study of the genetic difference between types commonly identified in humans and novel types in surface waters supports the continued application of genetic analysis to typing data in future studies as well as to existing data.