Salmonella Enteritidis: From The Chicken To The Egg

by Food and Drug Administration (FDA)

White, shining, unmarred a Grade A mystery now lies in the uncracked egg. Is it safe to eat? 9,999 times out of 10,000, yes. But ...

In May 1989, six nursing home patients in Pennsylvania died from Salmonella enteritidis poisoning after eating stuffing that contained undercooked eggs.

In July, 21 guests at a baby shower in New York became ill after eating a pasta dish made with a raw egg. One victim was 38 weeks pregnant and delivered her baby while ill. The newborn infant developed Salmonella enteritidis blood poisoning and required lengthy hospitalization.

Last August, a healthy 40-year-old man died, and 14 others were hospitalized, after eating egg-based custard pie contaminated with Salmonella enteritidis, which was served at a company party in Pennsylvania. The list goes on.

Public health officials are concerned. More than 49 outbreaks of Salmonella enteritidis poisoning took place in nine states and Puerto Rico last year, resulting in at least 13 deaths and more than 1,628 illnesses. According to the Jan. 5, 1990, issue of the Centers for Disease Control's Morbidity and Mortality Weekly Report, from January 1985 through October 1989, 189 Salmonella enteritidis outbreaks in the United States caused 6,604 illnesses and 43 deaths. Many more illnesses probably went unreported, says Joseph Madden, Ph.D., deputy director of FDA's division of microbiology.

Health investigators suspect that contaminated shell eggs caused nearly half of these outbreaks. The egg connection in these cases was determined by tracing the food eaten by the victims and taking cultures both from patients and foods.

Especially at risk for Salmonella poisoning are the elderly, the very young, pregnant women (because of risk to the fetus), and people already debilitated by serious illness, malnutrition, or weakened immune systems. Symptoms of Salmonella enteritidis infection usually include diarrhea, vomiting, abdominal pain, chills, fever, and headache. The bacteria can invade organs outside the gastrointestinal tract, causing complications that require lengthy hospitalization, even in healthy people.

Symptoms usually develop 12 to 36 hours after eating the contaminated food. The initial illness also can bring about serious chronic complications.

In 1985, in an incident in Chicago, more than 16,000 people contracted food poisoning from low-fat milk contaminated with Salmonella bacteria. Within two weeks, about 2 percent of these patients developed a chronic reactive arthritis condition linked to the infection. Although the Salmonella bacteria that made these people ill was not Salmonella enteritidis, researchers have found that rats infected with Salmonella enteritidis may develop the same arthritic condition. Researchers are concerned that Salmonella enteritidis may also cause this complication in humans.

Since 1976, says Robert Tauxe, M.D., a CDC expert on the spread of the disease, the reported rate for Salmonella enteritidis infections from food has increased more than sixfold in the northeastern part of the United States. First noted in the New England states, the infections also appeared in the mid-Atlantic region by 1983, and now have become a problem in the south Atlantic states as well. Recently, outbreaks were reported in Minnesota, Ohio and Nevada.

The problem also has become an international egg to crack. The U.S. Salmonella epidemic, says Tauxe, is dwarfed by dramatic increases that have been reported from Yugoslavia, Finland, Sweden, Norway, and the United Kingdom. In Britain alone, the number of confirmed Salmonella enteritidis cases reported for January through July 1988 (4,424 cases) was more than double the number (2,000) for the same period in 1987.

Source: Intact Eggs

At first, says Tauxe, we did not have an explanation for this striking increase. The first real clue that intact eggs were a source of the problem came in 1983, when CDC traced a large outbreak caused by Salmonella enteritidis to a commercial stuffed pasta product made with raw eggs.

Investigators then reviewed reports of past outbreaks and determined that at least since 1973, Salmonella enteritidis outbreaks appeared to be caused by the bacteria in clean, uncracked, Grade A eggs.

In the 1960s, Tauxe says, salmonellosis [the disease caused by the Salmonella bacteria] associated with chicken eggs was epidemic in the United States. At that time it was determined that eggs were being contaminated by Salmonella in chicken feces on the outside of the egg shell, which penetrated into the eggs through cracks in the shell. That led to strict rules, established and enforced by the U.S. Department of Agriculture, for washing and sanitizing shells of commercial eggs.

But this new epidemic is associated with Salmonella enteritidis in inspected, uncracked and sanitized Grade A eggs. The infected egg may appear normal, says Tauxe. The contamination comes from the inside, not the outside, of the egg.

How Does Contamination Occur?

No one knows how some intact eggs become contaminated with Salmonella enteritidis. Poultry researchers, however, suggest that the egg yolk becomes infected before the shell forms.

In fact, Charles Benson, Ph.D., of the University of Pennsylvania, says that in his experiments the bacteria were found not in the white, as when organisms penetrate the egg shell, but only in the yolk. This occurred even though Benson added iron to the white to encourage the bacteria to grow in the albumen, which has antibacterial properties.

Madden believes that in the past 10 years a new strain of Salmonella enteritidis that can live in chickens may have evolved. Other researchers are finding that Salmonella enteritidis bacteria migrate from the yolk to the white of the egg, where they can survive up to 12 hours. However, it is in the yolk where the bacteria multiply and thrive.

These and other findings, such as ovarian infections in egg-laying chickens, have led to the concept of transovarian transmission. According to this theory, the infection occurs first in the chicken and is transferred to the egg before the shell is formed.

Researchers also speculate that the infection may be passed from bird to bird in the same flock. For instance, Madden notes that several birds might pick up Salmonella enteritidis from the droppings of rodents and sparrows (known carriers of the organism) and spread it among the others. There are also reported cases, Madden adds, of workers picking up the bacteria on their clothing and transmitting Salmonella from one chicken house to another.

Only after scientists understand how Salmonella is transmitted will they know how to control it. Right now the proposed solution is a long-range plan to prevent spread of the disease by testing flocks and replacing infected ones.

About the Author

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