the spring of 1985, some 16,000 people in and around Chicago were reported to have acquired Salmonella-associated diarrhea and vomiting after a small technical mix-up in a dairy processing plant. After an intensive investigation, the estimate of casualties was raised to almost 200,000. Only a few months before, the plant had been hailed as one of the safest and most modern in the United States (and by implication, of course, the world). In this case the cause appeared to be a structural flaw in the technology itself, which, as in Prince Edward Island, had allowed some unpasteurized milk to slip into the system. After the epidemic the plant, the largest in U.S. history until then, went bankrupt.
In 1994 an estimated 224,000 people got sick from Salmonella typhimurium. Tanker trucks in Minnesota that had been carrying liquid raw eggs were subsequently used to haul ice cream premix. The tanks had apparently not been well cleaned out. All the eggs were in one basket and guess what? Somebody dropped it.
When I’m teaching, I like to tell the parallel story, which also took place in 1994, of the old Mennonite couple at the St. Jacobs farmers’ market who sold a homemade delicacy called cook cheese. Apparently, they didn’t properly clean out a barrel that had been used to store chickens. Eighty-two people got sick. It was big news locally. It was sad for the old couple, but the problem could be handled locally and provided an excellent opportunity for education. There are still many such small outbreaks around the world. The major advantage, from a public health point of view, is that you can identify the farms, talk to the farmers, and improve the situation with a few simple recommendations. Trace-backs, responses, and regulations are much more difficult at economies of scale; they require more sophisticated (and expensive) molecular laboratory techniques and tend to evoke industrial-type solutions, like food irradiation, which mostly increase problems rather than solve them.
Several other trends have emerged in recent years. First, even if they are not large, outbreaks of salmonellosis and other foodborne bacteria are becoming more widespread as they cross borders and oceans. We are in the midst of a Salmonella pandemic. Because of mass distribution of food, and because food from many sources gets mixed up, relabeled, and redistributed at various points in the system, outbreaks are more difficult to trace back to where they started. Second, even though the bacteria involved are considered to be adapted to animals, they are also being connected with fresh produce. The sh*t is everywhere: fresh sprouts of all sorts, cantaloupe, chip dips, minced beef, powdered milk, lettuce, tomatoes, and pigs’ ears (fed as treats to dogs that then infect people) are some of the sources of human infections. Finally, some of the newer strains of Salmonella are resistant to a wide variety of antibiotics.
Salmonella typhimurium DT104, which sounds like the name of a small warship, first emerged in cattle in the United Kingdom in the early 1980s and then went pandemic in the next couple of decades. This organism is more likely to kill both people and animals than other members of the Salmonella extended family and is resistant to most of the antibacterial drugs one might wish to launch against it. Fortunately, although it has become widespread in North America and Europe, it does not (yet) appear to be common.
Trying to understand the emergence and spread of Salmonella is a lesson in the complex dynamics of social-ecological systems we think we control. Although a few of them prefer one host (typhi in people, cholerae-suis in pigs), most Salmonella are both liberated and cosmopolitan. S. panama came into the United Kingdom by way of dried eggs during World War II and from there migrated into pig feed and from there into people. S. eastbourne rode the cocoa bean boats from Africa into eastern Canada and brought its sweet tenesmus dances to children all over Canada and the United States in contaminated candy. Among the many we might reflect upon, the story of Salmonella enteritidis may be one of the more instructive.
Chickens and turkeys often carry a few Salmonella in their intestines or on their feathers, without any apparent ill effects. Tens of millions of people have gotten sick from similarly few Salmonella over the past few decades, and many have died. We are currently on the slowly declining tail (we hope) of a global pandemic of salmonellosis, mostly from chickens and mostly S.enteritidis. But bacteria are, from an evolutionary point of view, considerably more “fit” than the rest of us, and the emergence of S. enteritidis furnishes a cautionary tale.
In the early twentieth century, two serologically related Salmonellas—S. gallinarum, which causes diarrhea in chicks, and S. pullorum, or fowl typhoid, were quite common in poultry flocks in Europe and North America. Veterinarians noticed two important things about these organisms: they were adapted to domestic chickens and waterfowl, and they made the birds (but not people) sick. The first characteristic made the disease vulnerable to a “test and slaughter” method of eradication—a kind of mass-slaughter/napalm operation that many animal disease control people seem to find attractive (someone should do a psychological study on that); the second characteristic was strong motivation to carry out such a program. The eradication of fowl typhoid has been a success story; the disease is rare in any country that boasts a “modern” poultry industry.
About the same time as this Salmonella was eliminated from poultry, another Salmonella—S. enteritidis—wandered over from its natural home in rodents and took up residence in the vacated ecological niche. Not many disease specialists know much about ecology, so this shift in bacterial ecology was not widely investigated. The notion that various species of all types and sizes in the world are interconnected, and that ecological niches are not really vacated but just filled with other species, makes disease treatment and control seem, well, complicated.
In any case, the vets didn’t worry too much about it, since enteritidis doesn’t make chickens sick. It does make people sick, however, but veterinarians and physicians have a long history of not talking much to each other. By the late 1980s, there was a global pandemic—in people, not chickens. Enteritidis was even more clever than scientists imagined, since it lived inside the ovaries of the birds laying the eggs, and they got it from their closely guarded and very valuable parents, the so-called breeder flocks. These flocks are the source of most of the world’s commercial chickens. People didn’t even have to be dirty to get sick. All that hand washing and all those chemicals for naught! All those great genetics! How could a bacterium be so vile and anarchistic?
In 1988, Edwina Currie, then a junior health minister in the British government, drew attention to a problem with S. enteritidis in eggs. After Edwina made her announcement, sales of eggs in the UK fell by 60 percent overnight, and many egg producers went out of business; Edwina herself was relieved of her job. By 1994, Edwina was back in the news, helping to launch a celebrity-chef cookbook called My Perfect Omelette and claiming that British eggs were now the safest in the world. That may well be, but the global pandemic of salmonellosis is not over yet, and the organisms keep coming up with new chemical-evading stratagems as fast as those chemicals can be devised.
S. enteritidis continues to adapt. An epidemic in Canada and the United States of an unusual molecular strain, PT 30, was traced to raw whole almonds in 2000 and 2001. (The PT refers to “phage type”; phages are viruses that infect bacteria and can be used to trace them.) PT 30 is an uncommon strain in foodborne diseases, and almonds are an uncommon vehicle for infection. Investigators looked everywhere for an animal source, since Salmonella are not supposed to live out there in the wild without an animal host. They couldn’t find any animals near the nut farms. One of the researchers has suggested that the bacteria have been growing in the soil, which, because the almond growers have been growing trees at much higher densities than was once thought possible, is richer in nutrients than was once thought possible. If this supposition is true, then people have pushed the bacteria down new, interesting, and (for consumers) dangerous evolutionary paths.
Most Salmonella do not travel first class, as the ones inside the eggs have. Usually, they hang around in the dust and feces in the chicken barn, cling to the outside of the eggshell, and only get into the eggs after the eggshells have been washed. A 2006 survey in Europe found that in some countries,
