NEW YORK (GenomeWeb News) – Almost all campylobacteriosis food poisoning cases in a UK community were caused by Campylobacter jejuni bacteria originating in farm animals — particularly chickens and cattle, new research suggests.
In a paper appearing online today in PLoS Genetics, UK researchers analyzed bacterial samples from roughly 1,200 human campylobacteriosis cases, comparing them to the C. jejuni found in livestock, wild animals, and the environment. Their analysis suggests that nearly 97 percent of the C. jejuni populations in infected humans originated in chickens and livestock. In contrast, populations in wild animals and other environmental sources seem to have a minor role in human campylobacteriosis.
“The idea was that we wanted to know where this thing comes from,” lead author Daniel Wilson, a post-doctoral researcher at the University of Chicago who completed the work as a research associate in genetics at Lancaster University, told GenomeWeb Daily News.
Although most people may think of Escherichia coli and other bugs as food poisoning culprits, Wilson said, C. jejuni is actually the leading cause of bacterial gastroenteritis in developed countries. In the US, for example, C. jejuni afflicts two to three million people a year. In severe cases, such infections can lead to a neurological disease called Guillain-Barre syndrome or reactive arthritis.
There have been a number of different hypotheses about where disease-causing C. jejuni come from. Because the bug causes gastroenteritis, it’s easy to blame food sources. But that’s not necessarily the case, Wilson said, noting that some aspects of Campylobacter’s epidemiology are consistent with waterborne infections.
In an effort to determine where human-infecting C. jejuni were coming from, Wilson and his colleagues compared the genetic sequences found in human campylobacteriosis cases with those from potential source sites.
They obtained 1,231 C. jejuni isolates from human samples taken in Lancashire, UK, over two years and sequenced seven housekeeping genes in the C. jejuni found in these samples. They then compared these with C. jejuni isolated from livestock (chicken, cattle, sheep, and pigs), wild animals (birds and rabbits), and environmental sources (water and sand).
Because Campylobacter is very diverse and there’s overlap between some C. jejuni populations, Wilson said, they couldn’t use the presence or absence of specific alleles to distinguish bacterial populations. Instead, the researchers compared the frequency of these alleles in different populations.
Using this “evolutionary approach,” they traced the 256 different C. jejuni genotypes back to their likely sources. Their results suggest that 96.6 percent of human campylobacteriosis cases are caused by C. jejuni populations carried by chickens, cattle, sheep, and pigs.
Bacterial populations associated with chickens caused more than half of all cases tested. Although it was more difficult to distinguish C. jejuni found in cattle from those found in sheep, the results suggest that cattle populations caused just over a third of cases, while sheep populations caused around four percent. Meanwhile, the researchers detected Campylobacter from pigs in less than one percent of campylobacteriosis cases.
In contrast, populations carried by wild animals appear to be responsible for just 2.3 percent of cases, while other environmental isolates were linked to 1.1 percent of cases.
These results differ from research published in 2005 suggesting livestock played a minor role in campylobacteriosis — a discrepancy that may be explained by differences in samples sizes or in analytical approaches, Wilson said. “Of course, there could be regional differences,” he added.
To address this possibility, his colleagues are currently collecting samples in Scotland and New Zealand. They are also in discussions with the US Centers for Disease Control and Prevention in Atlanta, Wilson said, though it’s unclear whether the agency will ultimately participate in the studies.
The work also provides clues about the bug’s transmission route, though that is usually more difficult to discern than source information. While it is unlikely that the environment is the main transmission route in the community tested, it is unclear whether C. jejuni is transmitted to humans through infected food or via another route.
For instance, because Lancashire contains a mixture of rural and urban areas, it’s possible that humans were exposed to C. jejuni from livestock without eating or handling tainted meat. Wilson noted that the team is currently looking at the disease epidemiology in more detail to understand the effect that of urban-rural factors, if any.
If it turns out, as the researchers suspect, that food is the transmission route from livestock to humans, there are three stages at which you could prevent Campylobacter infections, Wilson explained — first at the source, by reducing the incidence of C. jejuni in farm animals. It may also be possible to reduce C. jejuni in meat during food processing stages as well as during food preparation. By educating the public about good food hygiene, Wilson said, it may be possible to curb Campylobacter infections.