By Eric Sideman
A very interesting and biologically correct way of looking at your body is that the contents of your digestive tract are outside of your body. The tract is a plumbing system passing through in which food is put and digested and only nutrients are absorbed through the stomach and intestinal walls into your body. This view is important for understanding that you can eat a lot of very bad things and get away with it, if they are not absorbed into your body. This system of external defense does not always work, however, because some chemicals can be absorbed. Some of these chemicals are produced by organisms that can live in the digestive system.
The digestive system is not clean. Many organisms live there very happily feeding on the vast amount of good things pushed through on a regular basis. Most of these are harmless, many are actually beneficial. The most common inhabitant of the digestive tract of warm blooded animals is a bacterium called Escherichia coli. About 100 strains of E. coli are carried around in animals. Most are benign. In fact, the normal ones metabolize nutrients in the intestine and produce many beneficial metabolites that are released into the intestinal contents and absorbed and contribute to the host’s health. For example, E. coli produces vitamin K and colicins, which inhibit other microorganisms and protect the host from other infections.
But all strains of E. coli are not benign. A very dangerous strain, first found in a human in 1975, is now becoming more prevalent. It is called E. coli 0157:H7 and accounts for a great deal of illness and even some deaths each year. E. coli 0157:H7 can live without causing any symptoms in otherwise healthy animals, such as cows, but if humans ingest the organism they usually regret it.
The first outbreak of disease caused by E. coli 0157:H7, in 1982, was associated with consumption of ground beef. Since then the pathogen has been transmitted via all sorts of food (see table). The majority of outbreaks have been via foods of bovine origin, most commonly ground beef. However, recent large outbreaks have implicated apple cider, lettuce and other salad greens, and contaminated water.
Tracing the implicated produce to their point of origin and finding the site of contamination is often a very difficult task taken on by the Federal Center for Disease Control and state departments of health. It is particularly difficult when the items are purchased from large retail chains. Potential points of contamination become numerous and include the farm, transportation, warehouses, distributors, retail stores and home. By the time investigators reach a suspected point of contamination, no contaminated food may remain. The large number of outbreaks and the varying points of contamination highlight the need to emphasize food safety at each step in the food handling chain, including the farm.
E. coli 0157H:7 produces one or both of two antigenically distinct toxins referred to as verotoxins 1 and 2 and also as shiga-like toxins I and II. As the bacteria grow on substrate, the toxin builds up and people can either become ill from ingesting toxin on food or from ingesting the bacteria, which then live and grow in the digestive tract and produce toxin.
Symptoms are diarrhea, often bloody, and abdominal cramps. Fever is low grade or absent. Symptoms usually appear two to eight days after exposure and usually resolve on their own in several days. Children, the elderly and the immune compromised can suffer much worse. They can develop hemolytic uremic syndrome, HUS, which is a serious disease that affects the kidneys and blood clotting system. Most people recover but a prolonged hospital stay is often required. Some people die. There is no evidence that antibiotic treatment is helpful for infections.
Where Is It, How Long Does It Survive, How Does It Spread?
E. coli 0157:H7 lives in the digestive tract of animals. It has been shown to be carried by cows, sheep, chickens, pigs and deer. I have even seen reports that wild birds can carry it, but I can find no one who knows the source of those reports. The animals usually appear normal, healthy. Mahmoud El-Begearmi, Food Safety Specialist with Cooperative Extension, told me that since the normal strains of E.coli live in all warm blooded animals, the assumption should be that 0157:H7 can too, so I would add horses to the list of farm animals, but I can find no report of horses as carriers. I asked Mahmoud about healthy people as carriers and he said, “That is a good question.”
Food becomes contaminated by coming in contact with manure. If intestinal contents contact meat during slaughter, contamination occurs. Grinding meat mixes surface contaminated sections with the rest so that whole lots may become contaminated. Anything that contacts the manure can become contaminated and anything that contacts contaminated meat can in turn become contaminated. Such cross contamination during food preparation occurred in the Georgia/Tennessee outbreak. Meat in a fast food chain was the point source, even though it was well cooked. It had indirectly contaminated other items, such as hamburger buns, when raw through employees handling food. Infected people can spread the disease by not washing their hands well after going to the bathroom.
The bacteria can be carried wherever the manure goes. Infected livestock intermittently shed E. coli 0157:H7 in their manure. The manure can contaminate food items such as milk, vegetables, fruit and water when it comes in contact. Manure deposited in pastures, orchards or crop fields by grazing livestock may contaminate produce. Manure used as a soil amendment may contaminate crops in contact with it.
Studies of survival of the bacteria in manure show the importance of proper manure management in crop production. A study of the fate of E. coli 0157:H7 in bovine feces showed that populations increase during the first three days and then decrease. The decrease is more rapid at higher temperature, perhaps associated with dehydration of the feces. The pathogen survived seven to eight weeks at 37° C, eight to nine weeks at 22° C, and 10 to11 weeks at 5° C. This study has important implications for the use of manure and manure tea in the production of vegetables and fruit (see recommendations below).
Farm studies have shown varying frequencies of isolation of E. coli 0157:H7 from livestock. A recent study in 14 states in the United States revealed 22% of control herds carried it while 50% of herds implicated in cases carried it. Studies in England showed 4% of herds carrying E. coli 0157:H7. The prevalence increases in the summer in both sheep and cattle, as does a correlated higher incidence of human infection.
Poorly managed cattle that are subjected to dietary stress may carry unusually high numbers of E. coli 0157:H7. When animals are deprived of feed or feed is offered only intermittently, such as during transport to the butcher, the normal rumen microorganisms are not as active as they would be under everyday conditions of digestion. Normal microbial activity keeps the bad bugs at bay, according to Mark Rasmussen, an Agricultural Research Service Veterinary Medical Officer.
The survival and growth of E. coli has been studied on many substrates. Store surveys have isolated E. coli 0157:H7 in all sorts of food, including beef, chicken, turkey, pork, apple cider, melons, salad vegetables and milk. Oddly enough it was shown to be unable to survive in mayonnaise, but it has been shown to survive in acidic foods, such apple cider.
Evidence is mounting that populations quickly adapt to acidic conditions. Cultures for one to two doublings at a pH of 5 showed increased survival in acidic foods such as salami (pH 5.0) or apple cider (pH 3.4) compared with the original populations. Such laboratory studies highlight the need to consider adaptation when questioning the survival of the bacteria in what may be considered stressful environments.
Many recommendations are surfacing. Food safety concerns us all, especially farmers with a commitment to organic production. All farmers must help to alleviate the problem before harsh regulations such as pasteurization are imposed and ruin some foods. Some people are calling for pasteurization of apple cider, which would leave it tasting more like sugar water than a natural fruit juice. Even though only a very small proportion of food has been shown to be contaminated, the seriousness of the disease highlights the need for farmers and consumers to take precautions to avoid spreading the pathogen.
Regulations should be directed toward keeping the produce clean and toward cleaning rather than pasteurizing. Pasteurization changes the flavor and reduces nutrition, and the high cost of the equipment is likely to put many farms and orchards out of business. Michael Phillips of Lost Nation Cider Mill in Lancaster, N.H., says the little orchards aren’t going to be able to afford it. Jim Schupp at Highmoore Farm has instructions for cleaning apple drops before cider is made. You can get them from him or me or any Extension office.
Using tree harvested apples, as opposed to drops, may help to ensure more confidence in the safety of fresh apple cider. On the other hand, most orchards make cider from their drops. If drops are used, harvesting should be done with careful inspection to ensure that only clean fruit are picked up. Cornell Extension recommends that if orchards are frequented by large flocks of starlings or other roosting birds, soiled fruit should not be used in unpasteurized cider. All fruit should be subjected to extensive brush washing to ensure removal of any adhering material. Fruit should be spray rinsed with high pressure, clean water sprays.
Growers should avoid washing methods that may increase contamination. Crisping lettuce, a common practice in retail stores, is an example of a scary practice. It involves submerging lettuce in a basin of tepid water followed by refrigeration. The water is infrequently changed or recirculated and numerous cartons of lettuce are bathed in the same water. This practice is likely to spread rather than remove the pathogen. Lynn Byczynski, editor of Growing for Market, reported that growers are developing new washing recommendation for salad growers. She will report them in GFM and I will pass them on to you.
Any growers who use manure on their farms should take extra precautions. Pay attention to the guidelines in MOFGA’s Organic Certification Program. We do not permit manure or manure tea on root crops within 120 days of harvest and within 60 days of harvest of any crop.
The safest way to handle manure is to compost it. Studies have shown that E. coli 0157:H7 does not survive the composting process, but it is very important to note that the compost must be managed to get hot. In studies, at least two turnings were necessary to kill all of the pathogen. Piles that did not heat or that were not mixed well still carried the pathogen. If a grower uses manure in a compost pile it should be managed to heat, or the compost should not be used for at least 120 days, or the compost should be used only were there is no risk of contamination of food.
James Martin at the Maine Department of Health and Human Services points out that washing fruits and vegetables is very important and always has been. E.coli 0157:H7 is only one of many food-borne pathogens, such as Shigella, Salmonella and Listeria. All produce should be washed, including a rinse with fresh running water, whether a known risk of E. coli 0157:H7 contamination exists or not.
Eric is MOFGA’s director of technical services. You can direct your questions to him at the MOFGA office.