Campylobacter genotyping to determine the source of human infection

Abstract

Background: Campylobacter species cause a high proportion of bacterial gastroenteritis cases and are a significant burden on health care systems and economies worldwide; however, the relative contributions of the various possible sources of infection in humans are unclear.

Methods: National-scale genotyping of Campylobacter species was used to quantify the relative importance of various possible sources of human infection. Multilocus sequence types were determined for 5674 isolates obtained from cases of human campylobacteriosis in Scotland from July 2005 through September 2006 and from 999 Campylobacter species isolates from 3417 contemporaneous samples from potential human infection sources. These data were supplemented with 2420 sequence types from other studies, representing isolates from a variety of sources. The clinical isolates were attributed to possible sources on the basis of their sequence types with use of 2 population genetic models, STRUCTURE and an asymmetric island model.

Results: The STRUCTURE and the asymmetric island models attributed most clinical isolates to chicken meat (58% and 78% of Campylobacter jejuni and 40% and 56% of Campylobacter coli isolates, respectively), identifying it as the principal source of Campylobacter infection in humans. Both models attributed the majority of the remaining isolates to ruminant sources, with relatively few isolates attributed to wild bird, environment, swine, and turkey sources.

Conclusions: National-scale genotyping was a practical and efficient methodology for the quantification of the contributions of different sources to human Campylobacter infection. Combined with the knowledge that retail chicken is routinely contaminated with Campylobacter, these results are consistent with the view that the largest reductions in human campylobacteriosis in industrialized countries will come from interventions that focus on the poultry industry.

Figure 1. Experimental design.

Non-clinical isolates were from food, host animal, and environmental sources.

Figure 2. Assignment of Human clinical cases of campylobacteriosis to source using the Bayesian clustering algorithm STRUCTURE (A and B) and the asymmetric island model (C and D).

Each isolate is represented by a vertical bar, showing the estimated probability that it comes from each of the putative sources. Sources for Campylobacter jejuni were cattle (blue), chickens (yellow), wild birds (brown), the environment (green), and sheep (light gray). Sources for Campylobacter coli were cattle (blue), chickens (yellow), sheep (gray), swine (pink), and turkeys (black). Isolates are ordered by attributed source.

Figure 3. The origin of human campylobacteriosis in Scotland (2005–2006).

Probabilistic assignment of the source of human infection with Campylobacter jejuni (A and C) and Campylobacter coli (B and D) was determined using STRUCTURE and asymmetric island attribution models. Sequence types of disease-causing C. jejuni and C. coli were compared with data sets with isolates from cattle, chicken, and sheep. In addition, C. jejuni was compared with wild bird and environmental data sets and C. coli was compared with swine and turkey data sets. In each diagram, 5 equal-sized columns would be expected in the absence of any genetic differentiation by host species.