Profiling E. coli O157:H7 and Other STECs

Escherichia coli O157:H7 and other shiga toxin producing E. coli (STEC) can be found in meat and in a number of different fruits and vegetables, and may cause potentially deadly infections, especially in children and in the elderly. STEC also are referred to as verocytotoxic E. coli (VTEC) or enterohemorrhagic E. coli (EHEC)

Some history

Many decades ago, a run-of-the-mill strain of E. coli acquired the ability to produce a toxin known as stx2 – probably courtesy of a bacteriophage that transported it from a random Shigella microbe. After undergoing mutations that changed its antigenic structure and its ability to ferment certain carbohydrates, our E. coli acquired the capacity to produce a second toxin – stx1 – and shiga toxin-producing E. coli O157:H7 was born.

E. coli O157:H7 was isolated for the first time in 1970, from an Irish piglet that had developed enteritis. Five years later, the microbe was found in the stool of a California woman who was suffering from bloody diarrhea. The first confirmed foodborne disease outbreaks caused by E. coli O157:H7 took place in 1982, among patrons of McDonalds – the first on in Oregon, the second in Michigan.

E. coli O157:H7 is just one of several shiga toxin-producing E. coli strains, known collectively as STEC. Other strains include O104:H4 (the cause of a massive sprout-related outbreak in Germany in 2011), as well as O26, O45, O103, O111, O121, and O145.

What are shiga toxin-producing E. coli (STECs) and what is their natural habitat?

E. coli O157:H7 and other STECs are members of the family Enterobacteriaceae. They are motile, rod-shaped bacteria that are capable of growing either with or without oxygen. STECs are found in the large intestines of many animals, especially in ruminants such as cattle. They are introduced into the soil and surface water through contaminated feces or the use of raw manure as fertilizer.

How is STEC transmitted? What is the incubation period of the infection?

STEC food poisoning results when an individual eats food that is contaminated with the microbe. The infection also can be transmitted secondarily by contact with stool from an infected individual. It takes only 100-200 STEC microbes to cause an infection.The incubation period varies from one to eight days, depending on the dose, and on the susceptibility of the victim.

What are the symptoms of an infection with STEC? How long do they last?

Symptoms of STEC infections include severe – often bloody – diarrhea, abdominal pain, and vomiting. Unless complications arise, symptoms typically last for 5 to 10 days.

What is the prognosis?

STEC infections are self-limiting in most cases involving healthy adults. Young children and some elderly victims are at risk of developing hemolytic uremic syndrome (HUS), a severe and sometimes fatal complication. HUS strikes approximately 5-10% of STEC-infected children under the age of 10 years, affecting the function of the kidneys, digestive system and other organs. HUS is fatal in 3 to 5% of cases; 12-30% of HUS victims continue to suffer long-term consequences of their illness, including hypertension or impaired kidney function.

What foods carry STEC?

E. coli O157:H7 and other STEC bacteria can be found most commonly in raw beef, unpasteurized dairy products, and raw produce.

How can people protect themselves from STEC?

People acquire STEC infections by person-to-person transmission; by ingesting contaminated food, milk or water; by coming into contact with infected animals (for example at petting zoos); or by swimming or playing in contaminated water.

The US Centers for Disease Control and Prevention offer these suggestions for avoiding STEC infections:

WASH YOUR HANDS thoroughly after using the bathroom or changing diapers and before preparing or eating food.
WASH YOUR HANDS after contact with animals or their environments (at farms, petting zoos, fairs, even your own backyard).
COOK meats thoroughly. Ground beef and meat that has been needle-tenderized should be cooked to a temperature of at least 160°F/70˚C. It’s best to use a thermometer, as color is not a very reliable indicator of “doneness.”
AVOID raw milk, unpasteurized dairy products, and unpasteurized juices (like fresh apple cider).
AVOID swallowing water when swimming or playing in lakes, ponds, streams, swimming pools, and backyard “kiddie” pools.
PREVENT cross contamination in food preparation areas by thoroughly washing hands, counters, cutting boards, and utensils after they touch raw meat.

Additionally, always pay attention to recall notices, and return any recalled item to the store, or discard it in a sealed bag.

For more information on STECs and other food-borne pathogens, visit the CDC website or read Food Safety: Old Habits, New Perspectives.

E. coli O104:H4 – How Big Is The Iceberg?

“I believe the HUS cases are the real tip of the iceberg.”

– Dr. Michael T. Osterholm, PhD, MPH, Director of the University of Minnesota’s Center for Infectious Disease Research and Policy (CIDRAP), the publisher of CIDRAP News.

One puzzling aspect of the E. coli O104:H4 outbreak has been the very high number of cases of hemolytic uremic syndrome (HUS) and the large number of deaths in comparison to the total number of reported infections.

The World Health Organization, as of July 7th, reported a total of 3,941 cases worldwide, including 909 cases of HUS and 52 deaths. This tally actually dropped from the previously reported total (on July 1st) of 4,137 cases. The change was due to a redefinition of outbreak-related cases issued by the European Union.

Determining the actual number of outbreak victims in a foodborne disease investigation is rarely easy. I’ve wondered for some time how many mild and/or asymptomatic illnesses have gone unreported. On July 5th, I gave voice to my suspicion that many cases of E. coli O104:H4 have gone unreported, saying, “It’s likely that an unknown number of milder gastrointestinal illnesses also can be blamed on this outbreak strain.”

It is now clear that my hunch was well-founded. The European Centre for Disease Prevention and Control (ECDC) and the European Food Safety Authority (EFSA) have released an updated “Risk Assessment”which includes the following paragraph:

“The most significant development since June 29 relates to the preliminary results of the screening of children and staff in a school in Kreis Paderborn (Paderborn county), Germany, where three cases of HUS STEC and one case of non-HUS STEC infection have been identified since 13 June 2011 and onwards. In 22 of the 30 children tested (including the three HUS STEC cases, and the non-HUS STEC case), the epidemic strain of E. coli was isolated, indicating a significant level of asymptomatic infection. Asymptomatic carriage of STEC infection was also found in three kitchen workers at the school, in four guardians in four different child care centres in the district, and in three staff members of a supplying catering company. Further investigations of this cluster are ongoing.”

So, how big is the E. coli O104:H4 iceberg? Let’s do the math. First, we must decide on our multiplier. Let’s look at the two extreme cases.

The conservative estimate. The ECDC/EFSA report of asymptomatic cases documented a total of 22 children and 10 adults who were carriers of E. coli O104:H4 in its investigation of the Kreis Paderborn cluster of illnesses. Of these 32 carriers, only 4 developed active illnesses – including 3 cases of HUS. This gives us a multiplier of 8 (i.e., each reported case represents 8 actual infections).
The opposite extreme. In January 2011, CDC released its latest estimates of foodborne disease illness in the USA. The report details the multipliers that the authors developed for each pathogen to extrapolate actual illnesses from the numbers of reported illnesses. For non-O157 strains of enterohemorrhagic E. coli – the category into which E. coli O104:H4 falls – this multiplier is 106.8 (see Table 2 of report).

Taking these two multipliers and applying them to the most recent numbers reported by the World Health Organization, we arrive at the following:

The conservative estimate: 3,941 reported cases worldwide x 8 = 31,528 estimated symptomatic and asymptomatic infections.
The opposite extreme: 3,941 reported cases worldwide x 106.8 = 420,899 estimated symptomatic and asymptomatic infections.

The truth, no doubt, lies somewhere in between these two extremes.

Why is this important? Simply, the more people who have been infected – especially those who show no symptoms of illness – the more likely that E. coli O104:H4 will establish itself as thoroughly as E. coli O157:H7 already has done.

As for the “most probable source” of the outbreak, the Egyptian fenugreek seeds have been tracked forward to 25 countries so far – Austria, Belgium, Bulgaria, Cyprus, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Italy, Latvia, Lithuania, Luxembourg, the Netherlands, Norway, Poland, Portugal, Slovenia, Spain, Sweden, Switzerland and the United Kingdom.

As for the German outbreak investigation, the Federal Office of Consumer Protection and Food Safety has released its final Task Force report.

The German investigation may be complete, and the present outbreak winding down, but this is not the last we’ve heard from E. coli O104:H4.

Note to Anne: Home-Sprouting Seeds Is Not Safe Alternative

Whenever I speak with my husband’s 80-something Auntie Anne, I know that the subject will eventually turn to recipes and food safety. From grav lax to ganache, I’m her “go-to” gal. The last time we spoke, the topic was sprouts.

“I understand,” Anne said, “that sprouts in the supermarket are risky, and I don’t buy them anymore. But why can’t I buy the seeds at a health food store and sprout them myself?”

I explained that the seeds she finds in the health food store might be contaminated with Salmonella or E. coli.

“Then why,” she asked, ” can people eat the seeds without becoming sick?”

Putting aside the question as to how many people actually eat raw alfalfa seeds, arugula seeds, or mung beans, Anne’s question was a reasonable one. And the answer is in the arithmetic.

When a pathogenic microbe such as Salmonella, E. coli O157:H7 or E. coli O104:H4 is found in a batch of seeds, the level of contamination is typically very low. Researchers at the California Department of Health Services reported in 2001 (Journal of Food Protection, Vol. 64, No. 8, 2001, Pages 1240–1243) that they found Salmonella in contaminated batches of alfalfa seeds at levels as low as 0.07 per 100g – just a single live Salmonella in 3.15 pounds (1.4Kg) of seeds – and as high as 1.8 per 100g (or about one live Salmonella in 2 ounces of seeds).

If the level of pathogens in/on contaminated seeds is so low, why are sprouts such a problem? It’s all in the way the sprouts are produced.

Typical growing conditions for sprouts:- Moist, warm temperatures (typically, 70-80ºF)
Typical growing conditions for Salmonella and E. coli:- Moist, warm temperatures (optimally, 95-100ºF, but grow well in the 70-80ºF range)

And that’s the problem. The conditions required for sprouting seeds also encourage abundant growth of bacteria, including Salmonella and E. coli. A single Salmonella or E. coli cell can produce more than one million offspring during the first 10 hours of sprouting.

This risk is not just theoretical. Some of the German outbreak victims ate raw sprouts that they produced at home.

Another question I have been asked is how long pathogens such as Salmonella and E. coli can survive on or in seeds used for sprouting. The answer, in a word, is “years.”

The same study that documented the number of Salmonella present in the alfalfa seeds also mentioned that Salmonella had remained viable in the seeds after two years of storage at room temperature in the dark.

E. coli O157:H7 is just as rugged. In 1995, two Japanese sprouting facilities purchased radish seeds from the same US distributor. Between May and December 1996, more than 9,400 people in Japan became infected with a single outbreak strain of E. coli O157:H7, and 12 people died. Most of the outbreak victims were school children. The source of the infection was traced to radish sprouts produced in one of the two Japanese sprouting facilities.

Eight months later, in 1997, Japanese health authorities investigated another spurt of E. coli O157:H7 illnesses. The second series of illnesses was traced to radish sprouts produced in the second Japanese sprouting facility. The outbreak strain was identical to the 1996 outbreak strain. Clearly, this strain of E. coli O157:H7 survived on or in the US-produced radish seeds for more than a year.

It’s too soon for anyone to have determined the long-term survival of E. coli O104:H4; however, I would be astonished if it was very different from the others.

To summarize:

The ideal conditions for sprouting seeds and for growing bacteria overlap.
Bacteria will multiply into the millions per gram during the sprouting process.
Salmonella and E. coli can survive for 1-2 years – or longer – on or inside the dry seeds.

Finally, my advice to Auntie Anne – and to anyone else who is interested: Do not eat raw sprouts.