Campylobacter cells (electron micrograph by courtesy of the USDA)

A recent report published by EFSA reveals that in 2005 Campylobacter overtook Salmonella as the main cause of food poisoning in Europe. What is it about this comparatively little known bacterium that has made it such a common cause of illness in European consumers?

In December 2006 the European Food Safety Authority (EFSA) published a report entitled The Community Summary Report on Trends and Sources of Zoonoses, Zoonotic Agents, Antimicrobial Resistance and Foodborne Outbreaks in the European Union. By way of explanation, zoonoses are diseases that can be transmitted from animals to humans, either directly, or more often through contaminated food. In other words, the report is the closest thing we have to a survey of food poisoning in the EU. It is only the second such report that EFSA has produced, but it includes one rather important difference from the 2004 report. The number one zoonoses and cause of food poisoning in the EU is no longer Salmonella, but is now Campylobacter. This has been the case in the UK and the USA for some years, but the latest figures have focused attention on the new number one bug.

Cases of campylobacteriosis in humans 2001-2005
Country	2005	2004	2003	2002	2001
Austria	5,065	5,365	3,926	4,446	3,919
Belgium	6,879	6,716	6,556	7,354	7,357
Czech Republic	30,268	25,492	-	-	-
Denmark	3,677	3,724	3,537	4,385	4,620
Finland	4,002	3,583	3,190	3,738	3,969
France	2,049	2,127	1,997	1,353	203
Germany	62,114	55,796	47,876	56,350	54,410
Hungary	8,293	9,087	-	-	-
The Netherlands	3,761	3,273	2,805	3,421	3,682
Spain	5,513	5,958	6,048	5,051	6,149
Sweden	6,811	6,169	7,149	7,137	7,845
United Kingdom	52,686	50,388	52,126	54,372	62,052
Others	6,244	5,802	4,371	2,725	3,425
Total	197,363	183,480	139,581	150,332	157,631

According to the report, cases of campylobacteriosis (the disease caused by Campylobacter) reported in the EU rose by almost 8% in 2005 to a total of just over 197,000 -- in the same year, cases of salmonellosis fell by 9.5% to 176,000. By far the majority of cases were recorded in Germany and the UK, with the Czech Republic close behind (see table). Other countries reported far fewer cases, but this is probably at least partly due to differences in microbiological testing methods and reporting procedures. Bear in mind that most forms of food poisoning are probably under-reported and it becomes apparent that the real number of cases in Europe is probably much higher. Some experts estimate that it could be ten times higher. Whatever the true figure, the underlying trend in the incidence of campylobacteriosis in Europe seems to be upward. The underlying reasons for this are still uncertain, but if the trend is to be reversed, then both industry and consumer need to understand Campylobacter a little better.

An unlikely cause of food poisoning

There are 16 different species of Campylobacter, but only a few affect humans. Campylobacter jejuni is the most common in cases of human illness in Europe. You don't need to ingest a lot of Campylobacter cells to become ill -- just a few hundred is probably enough. Once this has happened symptoms usually take 2-5 days to develop, sometimes even longer. The main symptoms are diarrhoea (sometimes bloody) and abdominal pain, which is occasionally so severe that it can be mistaken for appendicitis. Most people recover within a week, but complications can sometimes set in. Most serious of these is a condition called Guillain-Barré syndrome -- a form of paralysis rather like polio that can occasionally be fatal.

Most cases of campylobacteriosis are sporadic, but large outbreaks do occur. For example, in 2005 about 80 people at company offices in Copenhagen were made ill by contaminated chicken salad in canteen meals. The disease is largely unpleasant, rather than dangerous, but the economic cost of 200,000 plus cases a year is considerable.

If you wanted to design a bacterium that would become the commonest cause of food poisoning in Europe, you probably would not come up with anything remotely like Campylobacter. It is fragile, doesn't grow in food, is killed easily by cooking and is sensitive to salt, drying, acid pH, oxygen and freezing. You would think that this would make it simple to control, but this doesn't seem to be the case at all. Despite appearing ill equipped as a foodborne pathogen, Campylobacter is a health problem not just in Europe, but worldwide. Nobody is entirely sure why this should be, but enough is known about the bacterium to provide some clues and to suggest how the situation might be improved.

The finger points at poultry

The most important point to recognise is that Campylobacter is very common in the environment and in the food chain. It can be found in a wide range of animals and birds, including most food animals and pets, in raw milk and in untreated water. The problem is that we don't know for sure how important these various sources are as contributors to human disease, mainly because the sporadic nature of most cases of campylobacteriosis makes it very difficult to trace the source. As yet there is no universally agreed method for typing Campylobacter, as there is with Salmonella. This makes tracing cases of illness back to their real source doubly difficult. It is also possible that there are other sources of Campylobacter that haven't yet been identified.

Most experts consider that the most important source of Campylobacter in relation to human disease is poultry. A recent Danish study of the main risk factors for human campylobacteriosis identified the main domestic factor as "…eating fresh, unfrozen chicken." The EFSA report identifies chicken broiler meat as the most common food involved in outbreaks and states that up to 66% of some poultry meat samples were contaminated with the pathogen. Even higher rates have been found in other surveys. The reason for this seems to be that poultry can pick up Campylobacter easily, but without becoming ill. They seem to do this mainly from other infected birds or from the environment, rather than via the egg. Free ranging poultry flocks are almost always infected, probably because they are likely to come in contact with other birds and animals. The situation is compounded during slaughtering and processing. Research has shown that chickens can even pick up infection from other birds during transport and that the carcasses of slaughtered birds often become contaminated during processing, especially at the defeathering, eviscerating and scalding stages.

The result of this is that any fresh poultry meat purchased by the consumer stands a very good chance of carrying Campylobacter, so the bacterium is constantly being introduced into domestic kitchens. Once this happens, the possibility of cross contamination when food is being prepared rises dramatically. Prepare a salad, or a similar uncooked dish, immediately after handling chicken and without washing your hands properly first, and it is clear that your risk of infection is high. The authors of the Danish study mentioned above said "The marked increase in consumption of fresh, unfrozen poultry in Denmark in the 1990s likely contributed substantially to the increasing incidence of human campylobacteriosis in this period." The evidence for this looks strong, but many microbiologists doubt that it tells the whole story.

Campylobacteriosis is more common in the summer than in the winter, especially in northern Europe. It is not immediately apparent why this should be, as unlike Salmonella, the bacterium will not multiply in food no matter what the temperature. One commonly held theory among microbiologists is that this might be explained by the growing popularity of summer barbecues, where the risky combination of fresh chicken and poor hygiene is quite likely to occur.

No quick fix?

There are big gaps in our knowledge about Campylobacter and how it causes so many cases of food poisoning. But it does seem clear that it would help if it were possible to lower the rate of contamination in fresh chicken, and this is the solution that is most commonly proposed. In the early 1990s Europe had a similar problem with a particular type of Salmonella (Salmonella Enteritidis) mainly caused by contaminated eggs. Many EU countries, notably the UK and Scandinavia, have had considerable success in tackling this by vaccination programmes and improving biosecurity and hygiene on poultry farms. This is almost certainly one of the main reasons why the EFSA report shows a falling incidence of salmonellosis.

Sadly, the same approach cannot be applied to Campylobacter at present, simply because there isn't a practical vaccine available that would protect birds from infection. It is also difficult to test birds to identify the ones that are already infected, because the traditional test methods are difficult and time consuming. Fortunately, rapid new PCR-based methods are already in use in Denmark and may provide the answer to this problem.

The Danish poultry industry has taken something of a lead in combating Campylobacter infection and in 2002 introduced "Campylobacter-free" poultry meat onto the domestic market at a premium price. This is defined in Danish law and requires that "the flock shall be controlled to give a 95% guarantee that less than 1% of birds are infected with Campylobacter." The scheme operates through screening flocks for infection, by testing samples from 300 birds in each flock, often using the PCR method. Early indications were that consumers were unwilling to pay extra for Campylobacter-free meat, but this may change if the public become more aware of the risks.

Improving biosecurity on poultry farms has also been looked at as a means of preventing birds becoming infected with Campylobacter in the first place. This means a combination of physical and procedural hygiene measures designed to prevent birds from coming into contact with sources of infection, especially wild birds and animals and contaminated environments. The Danish experience suggests that this can only be partially successful, especially in the summer months, and it is clearly an unrealistic option for free-range flocks. According to a UK Food Standards Agency report "…. the evidence for the effectiveness of biosecurity measures to control campylobacters in poultry flocks is sparse and there are many gaps in our knowledge." Nevertheless, the same report concluded that some biosecurity measures are better than none in the absence of better information as to exactly how poultry flocks become infected.

Interventions further along the food chain may also help. Improved hygiene in processing plants and better design of equipment like defeathering machines could help to lower contamination rates in poultry meat. The sensitivity of Campylobacter cells to freezing has also been suggested as a control. Although frozen poultry can still be contaminated, the number of bacteria tends to be much lower. Research suggests that fresh poultry carries a much higher risk of infection than frozen. This approach has been tried in Iceland with some success and has also been suggested as one solution to the very high rates of campylobacteriosis recorded in New Zealand. Decontamination techniques might also be useful, EU legislation permitting, especially steam treatment of carcasses during processing.

Current knowledge of Campylobacter, where it comes from and how it infects poultry, is just not sufficient to identify a single measure that could halt the steady increase in rates of human infection in Europe. More research is clearly necessary to fill some of the gaps in that knowledge. The best hope at present is probably to adopt a series of measures throughout the food chain that will work together to limit rates of contamination in poultry -- the Danes have shown that this is possible. European consumers can also help by taking some responsibility for their own health and ensuring that they cook chicken properly and practice better hygiene when preparing food, especially when barbecue season arrives.

References

The Community Summary Report on Trends and Sources of Zoonoses, Zoonotic Agents, Antimicrobial Resistance and Foodborne Outbreaks in the European Union in 2005. The EFSA Journal (2006), 94

Wingstrand, et al. Fresh chicken as main risk factor for campylobacteriosis, Denmark. Emerging Infectious Diseases (2006), Vol 12, No. 2