Cattle play a significant role in C. jejuni
epidemiology as an important host to campylobacter strains that are capable of causing disease in humans (31
). Based on data sets comprised of sample collections of C. jejuni
isolates from disparate animal sources, a number of MLST studies have provided a growing body of evidence for host specificity among C. jejuni
genotypes, including distinct pathogenic isolates associated with cattle (5
). Here, we present findings from the first longitudinal study and the largest to date MLST survey with respect to C. jejuni
populations in cattle, based on a set of epidemiologically linked isolates from dairy cattle farms within a defined geographical region over a temporally continuous period, with the primary objective of investigating the importance of cattle as a reservoir for human campylobacter infections.
Depending on a range of factors, including sampling type and size, recovery methods, herd type, season, and geography, wide discrepancies in the overall prevalence of C. jejuni
in cattle have been reported in past studies. Nonetheless, the prevalence rates of 39.5% (34
), 38.6% (3
), and 23.0% (22
) for thermophilic campylobacters described in several studies involving fecal samples of dairy cattle were generally in concordance with our findings (35.9%). Temporal fluctuations in prevalence were observed with a distinct peak in June (50.8%; Fig. ), coinciding with the seasonal peak traditionally seen in human infections in England and Wales (http://www.hpa.org.uk/infections
) and consistent with the pronounced late spring peak observed in the shedding numbers of thermophilic campylobacters in cattle in a previous study (32
). Although it is a widely held view that poultry is the predominant source of campylobacter infections, the findings from this study suggest that cattle may play a role in the seasonal peak in human infections or that the exposure to a common source of contamination may exist. Furthermore, a number of recent studies, including a 6-year survey, have shown evidence against a direct causal link between the seasonality of campylobacter prevalence in chickens and human infections (20
Several studies have delineated the C. jejuni
populations of isolates from various food-producing animals, which provided increasing evidence that members of the ST-61 complex are more restricted to cattle/bovine sources (8
). Our study further confirms the hypothesis by clearly demonstrating the dominance of the ST-61 complex in a large cattle data set. Interestingly, clonal complexes ST-21 and ST-42 were also frequently isolated, and clonal complexes ST-48, ST-22, and ST-257 were moderately common. Whereas the ST-42 complex was found to be strongly associated with ovine sources in previous studies (5
), this study provided the first description of a high prevalence in cattle. This finding raises the possibility that there may be similarities in the physiological features of the gastrointestinal tracts of cattle and sheep that cause ruminants to be more likely to carry similar C. jejuni
genotypes. Additionally, the previous report of cattle and sheep isolates sharing similar pulsed-field gel electrophoresis and flagellin genotypes fits well with this speculation (11
). Further investigation involving more ruminant isolates may well provide the key to explain certain aspects of the host specificity for C. jejuni
ST-61 and ST-42 clonal complexes. Indeed, there is evidence that the guts of cattle may provide a stable environment for bacterial strains, as there were instances where organisms were found to be genetically stable, dominant cattle strains. For example, certain pathogenic Salmonella
STs have been found to be associated exclusively with bovine sources (1
). In addition, C. coli
strains isolated from cattle were found to be highly clonal in a previous study, where more than 80% of the strains belonged to a single ST (21
). Incidentally, this group of C. coli
strains possessed the uncA
allele 17, which is found in the C. jejuni
genotypes belonging to the ST-61 clonal complex found in high numbers in this study.
To put into a public health context the contribution of the C. jejuni
genotypes identified in this study, we interrogated the Campylobacter
MLST database, which revealed that all of the C. jejuni
clonal complexes and at least 66% (27/41) of the STs identified in this study have been isolated from human disease in the past. In addition, all of the six most-common clonal complexes (ST-21, ST-45, ST-206, ST-61, ST-48, and ST-257) identified in the largest human data set to date (8
) have all been isolated from cattle in this study. Further, 13 of the clonal complexes reported here were present in an investigation of human campylobacter isolates conducted in the northwest of England in 2003, where the ST-21 complex was clearly the dominant genotype (29
). However, the genotypes identified in the present study are distinct from those recovered from environmental sources, where isolates belonging to the ST-45 complex and various uncommon genotypes were more frequently isolated (12
). In addition, it has been found that new and unassigned genotypes in humans were predominantly associated with swimming in natural bodies of water (17
). This suggests that the transmission pathway of campylobacters from cattle to humans may not necessarily include environmental sources or routes.
This is the first study to outline and compare the genotypic diversity of C. jejuni
in cattle herds with a set of spatially and temporally related isolates using MLST. We have demonstrated that C. jejuni
genotypes were highly diverse in cattle, with 90% of this data set represented by only 6 clonal complexes while the remaining 10% were represented by 11 complexes. These observations are in agreement with the notion that C. jejuni
has a weakly clonal population structure (9
). The similarities in genotypic diversity observed between different seasons according the Simpson's index of diversity (Table ) were somewhat surprising, as these similarities were in contrast to the hypothesis that the characteristic seasonal variation, which is the hallmark of C. jejuni
epidemiology, would be reflected to a certain extent by variations in the diversity of the genotypes. On the other hand, the apparent differences in the genotypic diversity of C. jejuni
between farms were consistent with the fact that there were geographical differences recorded for C. jejuni
clonal complexes, as discussed below. The evaluation of the differences in the genotypic diversity of C. jejuni
within and between various sources, regions, and time frames may provide additional insights into the ecological aspects of C. jejuni
populations and may, for example, assist in the identification of specific animals or environments that are potential drivers for generating novel genotypes.
To better define the C. jejuni epidemiology in cattle on dairy farms, C. jejuni clonal complexes were analyzed for differences in their distributions among farms, seasons, and cattle management groups.
We have demonstrated that the distribution of C. jejuni
genotypes was not random among farms (Tables and ), suggesting a strong geographical association among genotypes. Consequently, these findings led to the observation that two groups of farms which were 5 km apart (farms 3 and 5 and farms 1, 2, and 4) appeared to have maintained different genotypes, while farms within the same group (no more than 1 km apart) appeared to possess similar C. jejuni
genotypes. This observation suggests that campylobacters may be readily transmitted between distances of approximately 1 km, but transmission may be limited on a larger geographical scale. This is consistent with the previous report that C. jejuni
isolates from distances of less than 1 km were genetically more similar than isolates separated by greater distances, independent of sample or host types (12
Certain dynamics were also noted among clonal complexes between farms, in particular, the presence of the ST-61 complex and ST-42 complex genotypes were almost mutually exclusive; these strains were not present on any of the farms with comparable prevalence. This may be explained by a potential ecological competition between strains within the bovine gut, although further studies are required to corroborate this observation. In addition, although the results of this study and past studies have suggested that the ST-61 complex may be a cattle-adapted C. jejuni genotype, the apparent geographical associations shown in this study have revealed that there are indeed cattle farms with only a few ST-61 complex isolates. More importantly, this raises the question of how a strain that is well adapted to cattle can also exist in very low numbers in nearby groups of cattle within a defined geographic area. It is possible that the variations of genotypes observed were due to farm management factors, which may possibly be explained only by scrutinizing the particular differences in farming practices or the diets of animals on individual farms.
Surprisingly, a temporal pattern was detected only for one of the clonal complexes in the logistic regression model (Table ), where the probability of recovering ST-61 complex isolates was marginally increased in winter and significantly increased in spring, compared to summer. Consequently, this may suggest that no particular genotypes are exclusively responsible for the seasonal peak seen in summer. Nonetheless, the elucidation of the temporal characteristics of genotypes from different sources may help clarify the role of particular genotypes and their attribution to the rise of human infections during specific time frames.
Although there were no significant differences in the distributions of individual genotypes among cattle management groups, it appeared that the dominant genotypes (ST-61, ST-21, and ST-42 complexes) were more prevalent in younger cattle and gradually became relatively less prevalent in older cattle with higher proportions of uncommon genotypes. This observation agrees with a previous report which showed that adult animals carry a broader range of C. jejuni
serotypes than calves (22
We examined strain carriage over time in a small number of animals and observed no striking patterns, except for ST-61, which appeared to be more persistent in cattle and was carried on two sampling occasions in four animals, as well as on all three occasions from two animals. However, the number of animals involved was too small for this observation to be considered significant. Further investigations into this aspect of C. jejuni epidemiology could provide insights into the ecological dynamics of different genotypes within animal hosts.
This is the first report of a longitudinal study of C. jejuni in dairy cattle herds using MLST, and the results have provided important advances in knowledge in several key aspects of C. jejuni epidemiology, indicating that dairy cattle and their products may have a significant role as sources or transmission routes for human campylobacter infections.